**Part 1**

## **Integrated Cultural Weed Management**

**1** 

*Lithuania* 

**Intercropping of Pea and Spring Cereals for** 

Lina Šarūnaitė1, Aušra Arlauskienė2, Irena Deveikytė1, Stanislava Maikštėnienė2 and Žydrė Kadžiulienė<sup>1</sup>

*2Joniškėlis Experimental Station of the Lithuanian Research* 

*Centre for Agriculture and Forestry,* 

**Weed Control in an Organic Farming System** 

*1Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry,* 

Organic or sustainable management systems is focused on the creation of greater crop spatial and temporal diversification in crop rotation, and a reduction in the negative effects for food quality and environment, specifically a reduction in synthetic pesticide use (Lazauskas, 1990; Anderson, 2010). The relationship and competition beetween crop and weed populations is determined by the practical application of basic ecological principles in such management systems (Liebman & Davis, 2000; Singh et al., 2007). Crop diversification, which alters the composition of weed communities and influences their density, helps stabilize agricultural crop and weed communities (Barbery, 2002). Different seasonal types of agricultural crops (e.g. winter or spring crops) with different growth cycles and agronomic requirements provide unfavourable conditions for segetal plant life cycles. This prevents weed spread, germination, growth and seed ripening (Liebman & Dyck, 1993; Koocheki et al., 2009). In organic farming systems, an important role is assigned to a crop rotation (plant sequence diversification), catch crops and intercrops (Liebman & Davis, 2009; Anderson, 2010), and crop potential usage for

Intercropping is the simultaneous production of more than one crop species in the same field (Willey & Rao, 1980). Intercrops can be combinations of two or more species, including both annuals and perennials or a mixture (Anil et al., 1998). When two or more crops are growing together, each must have adequate space to maximize synergism and minimize intercrop competition and decrease weed competition. Therefore, before implementing specific intercropping systems, it should be taken into account: spatial arrangement (Malezieux et al., 2009); plant density (Neumann et al., 2007; Andersen et al., 2007); maturity dates of the crops being grown (Anil et al., 1998); and plant architecture (Brisson et al., 2004). One of the most commonly used intercropping mixtures is the legume/nonlegume (usually cereals) combination (Ofori & Stern, 1987; Anil et al., 1998; Hauggaard-Nilsen et al., 2008). Biologically fixed nitrogen (N2) of legumes is the most common plant growth stimulating factor and improved crop competition with respect to weed species in organic or sustainable farming systems (Berry et al., 2002). Studies in the literature have demonstrated that grain legumes are weak suppressors of weeds, but mixing species in a cropping system becomes a

suppressing and tolerating segetal plants (Liebman & Dyck, 1993).

**1. Introduction** 

## **Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System**

Lina Šarūnaitė1, Aušra Arlauskienė2, Irena Deveikytė1, Stanislava Maikštėnienė2 and Žydrė Kadžiulienė<sup>1</sup> *1Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry, 2Joniškėlis Experimental Station of the Lithuanian Research Centre for Agriculture and Forestry, Lithuania* 

#### **1. Introduction**

Organic or sustainable management systems is focused on the creation of greater crop spatial and temporal diversification in crop rotation, and a reduction in the negative effects for food quality and environment, specifically a reduction in synthetic pesticide use (Lazauskas, 1990; Anderson, 2010). The relationship and competition beetween crop and weed populations is determined by the practical application of basic ecological principles in such management systems (Liebman & Davis, 2000; Singh et al., 2007). Crop diversification, which alters the composition of weed communities and influences their density, helps stabilize agricultural crop and weed communities (Barbery, 2002). Different seasonal types of agricultural crops (e.g. winter or spring crops) with different growth cycles and agronomic requirements provide unfavourable conditions for segetal plant life cycles. This prevents weed spread, germination, growth and seed ripening (Liebman & Dyck, 1993; Koocheki et al., 2009). In organic farming systems, an important role is assigned to a crop rotation (plant sequence diversification), catch crops and intercrops (Liebman & Davis, 2009; Anderson, 2010), and crop potential usage for suppressing and tolerating segetal plants (Liebman & Dyck, 1993).

Intercropping is the simultaneous production of more than one crop species in the same field (Willey & Rao, 1980). Intercrops can be combinations of two or more species, including both annuals and perennials or a mixture (Anil et al., 1998). When two or more crops are growing together, each must have adequate space to maximize synergism and minimize intercrop competition and decrease weed competition. Therefore, before implementing specific intercropping systems, it should be taken into account: spatial arrangement (Malezieux et al., 2009); plant density (Neumann et al., 2007; Andersen et al., 2007); maturity dates of the crops being grown (Anil et al., 1998); and plant architecture (Brisson et al., 2004).

One of the most commonly used intercropping mixtures is the legume/nonlegume (usually cereals) combination (Ofori & Stern, 1987; Anil et al., 1998; Hauggaard-Nilsen et al., 2008). Biologically fixed nitrogen (N2) of legumes is the most common plant growth stimulating factor and improved crop competition with respect to weed species in organic or sustainable farming systems (Berry et al., 2002). Studies in the literature have demonstrated that grain legumes are weak suppressors of weeds, but mixing species in a cropping system becomes a

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 5

nonlegume (Anil et al., 1998). In intercropping the risk of nitrogen losses through leaching is substantially reduced in comparison to sole cropped pea (Neumann et al., 2007). Urbatzka et al. (2009) suggest, when pea is cultivated in a mixture with cereals, the N utilization effect was higher than in sole pea crop. In Danish and German experiments, the accumulation of phosphorous (P), potassium (K) and sulphur (S) was 20% higher in the intercrop (50:50) than in the respective sole crops (Hauggaard-Nielsen et al., 2009). The concentration of nitrogen is one of the most important criteria for grain quality evaluation. Pea intercropped with spring cereal increased the nitrogen concentration in intercrops compared with sole cereal (Ghaley et. al., 2005; Mariotti et al., 2006). Thus, better nutrition conditions are created

Results from our study conducted in Dotnuva suggests that intercrops were less productive than sole pea crop (except for pea intercropped with wheat). However, the pea / barley and pea/triticale intercrops were slightly more productive than the sole cereal crops (Table 1). At Dotnuva, according to productivity, the dual-component intercrops were ranked in the following order: pea / wheat > pea / triticale; pea / barley > pea / oats. The data from the Joniskelis site show that in a heavy loam *Cambisol*, crop productivity was on average 20.5%

Ps pea 2936.5 37.2 108.8 2896.6 33.6 83.8 P+SWi pea 550.4 37.3 20.5 795.3 34.5 23.5

P+SBi pea 565.7 36.9 20.6 649.0 33.1 18.2

P+Oi pea 445.2 37.0 16.6 432.9 33.3 12.2

P+STi pea 520.7 34.2 19.2 1240.8 33.4 35.5

SWs wheat 3002.9 19.2 59.8 3387.9 18.7 53.9 SBs barley 2583.3 16.6 46.4 2995.8 17.7 45.4 Os oat 2897.0 17.6 48.3 3955.1 16.6 56.0 STs triticale 2717.9 19.5 55.1 3220.5 21.7 59.9 LSD05 743.12 5.70 20.06 611.8 6.32 9.42

Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi –

in dual-component intercrops data averaged over 2007-2010

Table 1. Grain yield and nitrogen content of pea and spring cereals grown as sole crops and

Loamy soil (Dotnuva) Clay loam soil (Joniskelis)

yield (kg ha-1)

mg kg-1 kg ha-1 mg kg-1 kg ha-1

Nitrogen

Nitrogen Grain

wheat 2401.3 22.2 54.8 2473.2 20.0 42.3 total 2951.9 24.9 75.3 3268.5 23.5 65.7

barley 2184.5 19.6 44.9 2386.4 18.7 38.4 total 2750.1 24.2 65.5 3035.4 21.8 56.6

oat 2109.9 18.5 39.3 3837.5 17.4 57.1 total 2555.4 22.4 55.9 4270.4 19.0 69.4

triticale 2214.6 26.6 48.9 2000.6 23.4 39.3 total 2735.3 23.3 68.2 3241.4 27.2 74.8


atmospheric dinitrogen (N2), and thereby reducing competition for soil NO3

in intercrops, therefore crops have a higher competitive ability against weeds.

higher, compared with that of crops grown in the Dotnuva site (Table 1).

Grain yield (kg ha-1)

Crop component

Sole crops and intercrop

pea and triticale.

way to improve the ability of the crop itself to suppress weeds (Lemerle et al., 2001; Mohler, 2001; Jensen et al., 2006). Therefore, intercropping of cereals and grain legumes: pea (*Pisum sativum* L. (Partim), lupin (*Lupinus angustifolius* L.), bean (*Vicia faba* L.), vetch (*Vicia sativa* L.) *et ctr* is a neglected theme in agricultural research. Weeds continue to play a major limiting role in agricultural production. The control of weeds using classical pesticides raises serious concerns about food safety and environmental quality, which have dictated the need for alternative weed management techniques.

The field experiments were carried out in 2007–2010 at the Institute of Agriculture (Dotnuva, loamy soil) and the Joniskelis Experimental Station (Joniskelis, clay loam soil,) of the Lithuanian Research Centre for Agriculture and Forestry. The aim of this study was to determine the effect of intercropping pea with spring cereals on crop competition, yield performance and weed control in organic farming conditions. The following trial design was used for intercrops and sole crops: 1) pea (cv. 'Pinochio'), Ps, 2) pea/spring wheat (*Triticum aestivum* L. emend. Fiori et Paol., cv. 'Estrad'), PWi, 3) pea/spring barley (*Hordeum vulgare* L., cv. 'Aura'), PBi, 4) pea/oats (*Avena sativa* L., cv. 'Migla'), POi, 5) pea/spring triticale (× *Triticosecale* Wittm., cv. 'Nilex'), PTi, 6) spring wheat, Ws, 7) sprig barley, Bs, 8) oats, Os, 9) spring triticale, Ts. The experimental plots were laid out in a complete one–factor randomised block design in three replicates. Individual plot size was 2.5 × 12 m. The intercrop design was based on the proportional replacement principle, with mixed pea grain and spring cereals grain at the same depth in the same rows at relative frequencies (50:50 –a relative proportion of grain legume and spring cereals seeds). Wheat seeds rate were 5.5, barley 4.7, oat 6.0, triticale 4.5 and pea 1.0 mln seeds ha-1 for sole crop. Weeds were assessed twice: at stem elongation growth stage (BBCH 32–36) and at development of grain filling growth stage (BBCH 73). Mass of weeds and botanical composition was determined in 0.25 m2 at 4 settled places of each treatment. The experimental data were processed by the analysis of variance and correlationregression analysis methods using a software package "Selekcija". Weed number and mass data were transformed to *x* + 1 .

#### **2. Benefits of intercropping of cereal and grain legume**

#### **2.1 Yield and quality of intercrops**

Cereal and legume intercropping systems are one of the important agronomic practices, wherein usually the productivity of the system as a whole is higher in comparison with that of their performance individually. Intercropping of cereals with grain legumes has been a common cropping system in rain-fed areas and especially in the Mediterranean countries (Anil et al., 1998; Lithourgidis et al., 2006). Grain legumes extensively used in intercropping with cereal include pea, vetch, lupin and bean (Hauggaard-Nielsen et al., 2001; Andersen et al., 2005; Ghaley et al., 2005; Berk et al., 2008). A number of different cereal crops have been proposed to be appropriate for intercropping with grain legumes such as barley oat, triticale, and wheat (Thomson et al., 1992; Berk et al., 2008). Intercropping advantages include improved soil conservation (Anil et al., 1998), yield stability (Hauggaard-Nielsen et al., 2003; Lithourgidis et al., 2006) and favours weed control (Banik et al., 2006). This leads to improved utilisation of environmental resources, light, water, and nutrients, in a multiple plant species community (Brisson et al., 2004; Corre-Hellou et al., 2007). The legume can provide N benefits to the nonlegume directly through mycorrizal links, root exudates, or decay of roots and nodules; or indirectly through a spring effect, where the legume fixes

way to improve the ability of the crop itself to suppress weeds (Lemerle et al., 2001; Mohler, 2001; Jensen et al., 2006). Therefore, intercropping of cereals and grain legumes: pea (*Pisum sativum* L. (Partim), lupin (*Lupinus angustifolius* L.), bean (*Vicia faba* L.), vetch (*Vicia sativa* L.) *et ctr* is a neglected theme in agricultural research. Weeds continue to play a major limiting role in agricultural production. The control of weeds using classical pesticides raises serious concerns about food safety and environmental quality, which have dictated the need for

The field experiments were carried out in 2007–2010 at the Institute of Agriculture (Dotnuva, loamy soil) and the Joniskelis Experimental Station (Joniskelis, clay loam soil,) of the Lithuanian Research Centre for Agriculture and Forestry. The aim of this study was to determine the effect of intercropping pea with spring cereals on crop competition, yield performance and weed control in organic farming conditions. The following trial design was used for intercrops and sole crops: 1) pea (cv. 'Pinochio'), Ps, 2) pea/spring wheat (*Triticum aestivum* L. emend. Fiori et Paol., cv. 'Estrad'), PWi, 3) pea/spring barley (*Hordeum vulgare* L., cv. 'Aura'), PBi, 4) pea/oats (*Avena sativa* L., cv. 'Migla'), POi, 5) pea/spring triticale (× *Triticosecale* Wittm., cv. 'Nilex'), PTi, 6) spring wheat, Ws, 7) sprig barley, Bs, 8) oats, Os, 9) spring triticale, Ts. The experimental plots were laid out in a complete one–factor randomised block design in three replicates. Individual plot size was 2.5 × 12 m. The intercrop design was based on the proportional replacement principle, with mixed pea grain and spring cereals grain at the same depth in the same rows at relative frequencies (50:50 –a relative proportion of grain legume and spring cereals seeds). Wheat seeds rate were 5.5, barley 4.7, oat 6.0, triticale 4.5 and pea 1.0 mln seeds ha-1 for sole crop. Weeds were assessed twice: at stem elongation growth stage (BBCH 32–36) and at development of grain filling growth stage (BBCH 73). Mass of weeds and botanical composition was determined in 0.25 m2 at 4 settled places of each treatment. The experimental data were processed by the analysis of variance and correlationregression analysis methods using a software package "Selekcija". Weed number and mass

Cereal and legume intercropping systems are one of the important agronomic practices, wherein usually the productivity of the system as a whole is higher in comparison with that of their performance individually. Intercropping of cereals with grain legumes has been a common cropping system in rain-fed areas and especially in the Mediterranean countries (Anil et al., 1998; Lithourgidis et al., 2006). Grain legumes extensively used in intercropping with cereal include pea, vetch, lupin and bean (Hauggaard-Nielsen et al., 2001; Andersen et al., 2005; Ghaley et al., 2005; Berk et al., 2008). A number of different cereal crops have been proposed to be appropriate for intercropping with grain legumes such as barley oat, triticale, and wheat (Thomson et al., 1992; Berk et al., 2008). Intercropping advantages include improved soil conservation (Anil et al., 1998), yield stability (Hauggaard-Nielsen et al., 2003; Lithourgidis et al., 2006) and favours weed control (Banik et al., 2006). This leads to improved utilisation of environmental resources, light, water, and nutrients, in a multiple plant species community (Brisson et al., 2004; Corre-Hellou et al., 2007). The legume can provide N benefits to the nonlegume directly through mycorrizal links, root exudates, or decay of roots and nodules; or indirectly through a spring effect, where the legume fixes

alternative weed management techniques.

data were transformed to *x* + 1 .

**2.1 Yield and quality of intercrops** 

**2. Benefits of intercropping of cereal and grain legume** 

atmospheric dinitrogen (N2), and thereby reducing competition for soil NO3 - with the nonlegume (Anil et al., 1998). In intercropping the risk of nitrogen losses through leaching is substantially reduced in comparison to sole cropped pea (Neumann et al., 2007). Urbatzka et al. (2009) suggest, when pea is cultivated in a mixture with cereals, the N utilization effect was higher than in sole pea crop. In Danish and German experiments, the accumulation of phosphorous (P), potassium (K) and sulphur (S) was 20% higher in the intercrop (50:50) than in the respective sole crops (Hauggaard-Nielsen et al., 2009). The concentration of nitrogen is one of the most important criteria for grain quality evaluation. Pea intercropped with spring cereal increased the nitrogen concentration in intercrops compared with sole cereal (Ghaley et. al., 2005; Mariotti et al., 2006). Thus, better nutrition conditions are created in intercrops, therefore crops have a higher competitive ability against weeds.

Results from our study conducted in Dotnuva suggests that intercrops were less productive than sole pea crop (except for pea intercropped with wheat). However, the pea / barley and pea/triticale intercrops were slightly more productive than the sole cereal crops (Table 1). At Dotnuva, according to productivity, the dual-component intercrops were ranked in the following order: pea / wheat > pea / triticale; pea / barley > pea / oats. The data from the Joniskelis site show that in a heavy loam *Cambisol*, crop productivity was on average 20.5% higher, compared with that of crops grown in the Dotnuva site (Table 1).


Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 1. Grain yield and nitrogen content of pea and spring cereals grown as sole crops and in dual-component intercrops data averaged over 2007-2010

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 7

According to literature, annual weeds, whose short vegetation period coincides with the cereal growth season, are most common in spring cereals (Rassmusen, 2002; Barbery, 2002). Weed seed germination is influenced by many factors such as field history and seed bank, soil properties, tillage practices and crop technologies. Annual weed seeds germinate at different times. *Thlaspi arvense* L. germinated at the earliest time, other weeds germinated when soil temperature had warmed up. *Fallopia convolvulus* (L.) A. Löve germinated a little later (Špokienė, 1995). In general, the intensive weed germination period is when soil warms up to 10-15 ° C. Therefore, the most intensive competitive interaction between weeds and crops occurs in the second half of May until mid June and late July to mid August in Eastern

Our results revealed that annuals dominated the weed flora composition (7-18 species) while there were fewer perennials (2-8 weed species). The most common annual weed species on fertile soils were: *Chenopodium album* L., *Veronica arvensis* L., *Stellaria media* (L.) Vill., *Galium aparine* L., *Fallopia convolvulus*, *Thlaspi arvense*, *Viola arvensis* Murray , *Lamium purpureum* L., *Polygonum aviculare L*., *Polygonum persicaria* L., *Fumaria officinalis* L., *Tripleurospermum perforatum* (Merat) M. Lainz. The dominance of these weed species can be explained by a very good adaptation to the existing soil and climatic conditions and soil tillage regime (Protasov, 1995). Such species are characterized by higher soil nutrient assimilation compared to agricultural crop plants. The following perennial weed species were identified: *Cirsium arvense* (L.) Scop., *Sonchus arvensis* L., *Taraxacum officinale* F.H. Wigg., *Equisetum arvense* L., *Tussilago farfara* L. Both experimental sites were similar in weed

In a loamy soil (Dotnuva), plant diversity in crop rotation was higher (at cereal stem elongation growth stage BBCH 32-36) *Chenopodium album* was the dominant weed species from the 12-13 species identified. This species accounted for 61.7-77.2% of the total weeds documented. Whereas, in clay loam soil (Joniskelis), there were fewer (7-9) weed species of which the most frequent were *Stellaria media* (16.1-26.9 %), *Veronica arvensis* (9.8–16.8 %), *Galium aparine* (7.6-13.3%), *Chenopodium album* (7.3-12.9 %), *and Fallopia convolvulus* (6.9-9.9 %). Based on Špokienė and Povilionienė's (2003) findings, according to weed harmfulness reduction, the species can be ranked as follows: *Cirsium arvense* (10) > *Sonchus arvensis* (9) > *Taraxacum officinale (8)* > *Chenopodium album, Stellaria media (7) > Galium aparine, Fallopia convolvulus (6)> Polygonum* sp. (5)> *Thlapsi arvense (4).* According to Lithuanian researchers' data, weed species such as *Viola arvensis, Veronica arvensis, and Lamium purpureum* are less harmful; however, the number of weed species in a crop (weed harmfulness threshold) is of

Our research data (Joniskelis) revealed that weed germination was significantly lower in pea / barely intercrop, spring wheat and oat sole crops compared to the pea sole crop. At Dotnuva and Joniskelis experimental sites the weed number tended to decrease 3.6-19.5 % and 3.9-19.5 %, respectively. In Joniskelis, all intercrops and sole crops had good suppression of *Thlaspi arvense*. Slightly fewer weeds germinated in cereal sole crops compared to intercrops. The germination of *Galium aparine* was significantly lower, and the number of *Fallopia convolvulus* tended to decrease in Joniskelis' cereal sole crop. Different crops (intercrop and sole crop) had little effect on the variation of perennial weed number.

**2.2.1 Weed species composition, germination time and conditions** 

Europe (Špokienė, 1995).

species and number (Table 2).

great importance (Špokienė & Povilionienė, 2003).

Clay loam soils have high capillary water capacity, therefore plants are not so readily affected by lack of soil moisture (Maikštėnienė et al., 2006). The data from the Joniskelis site evidenced that all intercrops were more productive than sole pea crop. Moreover, the sole spring cereal was lower yielding than cereal intercropped with pea (except for pea intercropped with wheat). The rough structure of these soils was more favourable for cereals than for peas. At Joniskelis, according to productivity, intercrops were ranked in the following order: pea / oats > pea / wheat, pea / triticale > pea / barley. In dual-component intercrops with increasing productive density of cereals and their share in the yield, the total yield of the intercrops increased in Dotnuva (r = 0.650; *P*<0.05; r = 0.969; *P*<0.01, respectively) and in Joniskelis (r = 0.576; *P*<0.05; r = 0.916; *P*<0.01, respectively). Results obtained in various soils showed that when peas were grown mixed with oats or barley, their productivity was directly influenced only by cereals yield (r = 0.991; *P*<0.01; r = 0.971; *P*<0.01, respectively), whereas the productivity of peas grown in mixed crop with wheat or triticale was influenced by both components: yields of cereals (r = 0.825; *P*<0.01 and r = 0.984; *P*<0.01, respectively) and pea (r = 0.637; *P*<0.05 and r = 0.842; *P*<0.01, respectively) of the intercrop.

The accumulation of nitrogen in cereal grain is an indicator of different crop species competitive power. The findings from Dotnuva site showed that pea grown in sole crop accumulated 2.2% more nitrogen than pea intercropped with cereal. However, the grain nitrogen concentration of cereal intercropped with pea averaged 19.2% higher than that in sole cereal crop. The nitrogen concentration in pea grain was slightly lower in Joniskelis compared with Dotnuva. The amount of grain nitrogen did not differ between sole pea crops and pea intercrops. The grain nitrogen concentration of cereal intercropped with pea averaged 6.4% higher than that in sole crop. At both experimental sites, the highest grain nitrogen concentration was in spring wheat and triticale intercropped with pea.

In loamy soil (Dotnuva), sole pea produced a higher yield, therefore the nitrogen content was 29.8% higher compared to the corresponding data in clay loam soil (Joniskelis). The intercrops accumulated similar nitrogen concentrations in the total grain yield in both experimental sites. The amount of nitrogen in total grain yield of intercrops was greater by 26.3% in Dotnuva and by 23.8% in Joniskelis compared to the averaged amount of nitrogen in grain of sole cereal crop in corresponding experimental sites.

#### **2.2 Intercropping for weed management**

Weed management is a key issue in organic farming system (Bond & Grundy, 2001). Improvement of crop competition with weeds has been emphasised as the benefit of the increased sowing density of sole crops or intercropping (Auskalniene & Auskalnis, 2008; Liebman & Davis, 2000). Individual cereal species vary in their competitiveness against weeds. Weed suppression has been found to be greater in intercrops compared with sole crops, indicating synergism among crops within intercrops (Liebman & Dyck, 1993; Bulson et al., 1997; Szumigalski & van Acker, 2005; Deveikytė et al., 2008, 2009). In an organic farming weeds are controlled not only by direct means (manually or mechanacilly) and preventive measures (appropriate crop rotation, tillage, crop management) but also by increasing crop tolerance of weeds (choice of genotypes, sowing method, fertilization strategy) (Barbery, 2002; Anderson, 2010).

#### **2.2.1 Weed species composition, germination time and conditions**

6 Weed Control

Clay loam soils have high capillary water capacity, therefore plants are not so readily affected by lack of soil moisture (Maikštėnienė et al., 2006). The data from the Joniskelis site evidenced that all intercrops were more productive than sole pea crop. Moreover, the sole spring cereal was lower yielding than cereal intercropped with pea (except for pea intercropped with wheat). The rough structure of these soils was more favourable for cereals than for peas. At Joniskelis, according to productivity, intercrops were ranked in the following order: pea / oats > pea / wheat, pea / triticale > pea / barley. In dual-component intercrops with increasing productive density of cereals and their share in the yield, the total yield of the intercrops increased in Dotnuva (r = 0.650; *P*<0.05; r = 0.969; *P*<0.01, respectively) and in Joniskelis (r = 0.576; *P*<0.05; r = 0.916; *P*<0.01, respectively). Results obtained in various soils showed that when peas were grown mixed with oats or barley, their productivity was directly influenced only by cereals yield (r = 0.991; *P*<0.01; r = 0.971; *P*<0.01, respectively), whereas the productivity of peas grown in mixed crop with wheat or triticale was influenced by both components: yields of cereals (r = 0.825; *P*<0.01 and r = 0.984; *P*<0.01, respectively) and pea (r = 0.637; *P*<0.05 and r = 0.842; *P*<0.01, respectively) of

The accumulation of nitrogen in cereal grain is an indicator of different crop species competitive power. The findings from Dotnuva site showed that pea grown in sole crop accumulated 2.2% more nitrogen than pea intercropped with cereal. However, the grain nitrogen concentration of cereal intercropped with pea averaged 19.2% higher than that in sole cereal crop. The nitrogen concentration in pea grain was slightly lower in Joniskelis compared with Dotnuva. The amount of grain nitrogen did not differ between sole pea crops and pea intercrops. The grain nitrogen concentration of cereal intercropped with pea averaged 6.4% higher than that in sole crop. At both experimental sites, the highest grain

In loamy soil (Dotnuva), sole pea produced a higher yield, therefore the nitrogen content was 29.8% higher compared to the corresponding data in clay loam soil (Joniskelis). The intercrops accumulated similar nitrogen concentrations in the total grain yield in both experimental sites. The amount of nitrogen in total grain yield of intercrops was greater by 26.3% in Dotnuva and by 23.8% in Joniskelis compared to the averaged amount of nitrogen

Weed management is a key issue in organic farming system (Bond & Grundy, 2001). Improvement of crop competition with weeds has been emphasised as the benefit of the increased sowing density of sole crops or intercropping (Auskalniene & Auskalnis, 2008; Liebman & Davis, 2000). Individual cereal species vary in their competitiveness against weeds. Weed suppression has been found to be greater in intercrops compared with sole crops, indicating synergism among crops within intercrops (Liebman & Dyck, 1993; Bulson et al., 1997; Szumigalski & van Acker, 2005; Deveikytė et al., 2008, 2009). In an organic farming weeds are controlled not only by direct means (manually or mechanacilly) and preventive measures (appropriate crop rotation, tillage, crop management) but also by increasing crop tolerance of weeds (choice of genotypes, sowing method, fertilization

nitrogen concentration was in spring wheat and triticale intercropped with pea.

in grain of sole cereal crop in corresponding experimental sites.

**2.2 Intercropping for weed management** 

strategy) (Barbery, 2002; Anderson, 2010).

the intercrop.

According to literature, annual weeds, whose short vegetation period coincides with the cereal growth season, are most common in spring cereals (Rassmusen, 2002; Barbery, 2002). Weed seed germination is influenced by many factors such as field history and seed bank, soil properties, tillage practices and crop technologies. Annual weed seeds germinate at different times. *Thlaspi arvense* L. germinated at the earliest time, other weeds germinated when soil temperature had warmed up. *Fallopia convolvulus* (L.) A. Löve germinated a little later (Špokienė, 1995). In general, the intensive weed germination period is when soil warms up to 10-15 ° C. Therefore, the most intensive competitive interaction between weeds and crops occurs in the second half of May until mid June and late July to mid August in Eastern Europe (Špokienė, 1995).

Our results revealed that annuals dominated the weed flora composition (7-18 species) while there were fewer perennials (2-8 weed species). The most common annual weed species on fertile soils were: *Chenopodium album* L., *Veronica arvensis* L., *Stellaria media* (L.) Vill., *Galium aparine* L., *Fallopia convolvulus*, *Thlaspi arvense*, *Viola arvensis* Murray , *Lamium purpureum* L., *Polygonum aviculare L*., *Polygonum persicaria* L., *Fumaria officinalis* L., *Tripleurospermum perforatum* (Merat) M. Lainz. The dominance of these weed species can be explained by a very good adaptation to the existing soil and climatic conditions and soil tillage regime (Protasov, 1995). Such species are characterized by higher soil nutrient assimilation compared to agricultural crop plants. The following perennial weed species were identified: *Cirsium arvense* (L.) Scop., *Sonchus arvensis* L., *Taraxacum officinale* F.H. Wigg., *Equisetum arvense* L., *Tussilago farfara* L. Both experimental sites were similar in weed species and number (Table 2).

In a loamy soil (Dotnuva), plant diversity in crop rotation was higher (at cereal stem elongation growth stage BBCH 32-36) *Chenopodium album* was the dominant weed species from the 12-13 species identified. This species accounted for 61.7-77.2% of the total weeds documented. Whereas, in clay loam soil (Joniskelis), there were fewer (7-9) weed species of which the most frequent were *Stellaria media* (16.1-26.9 %), *Veronica arvensis* (9.8–16.8 %), *Galium aparine* (7.6-13.3%), *Chenopodium album* (7.3-12.9 %), *and Fallopia convolvulus* (6.9-9.9 %). Based on Špokienė and Povilionienė's (2003) findings, according to weed harmfulness reduction, the species can be ranked as follows: *Cirsium arvense* (10) > *Sonchus arvensis* (9) > *Taraxacum officinale (8)* > *Chenopodium album, Stellaria media (7) > Galium aparine, Fallopia convolvulus (6)> Polygonum* sp. (5)> *Thlapsi arvense (4).* According to Lithuanian researchers' data, weed species such as *Viola arvensis, Veronica arvensis, and Lamium purpureum* are less harmful; however, the number of weed species in a crop (weed harmfulness threshold) is of great importance (Špokienė & Povilionienė, 2003).

Our research data (Joniskelis) revealed that weed germination was significantly lower in pea / barely intercrop, spring wheat and oat sole crops compared to the pea sole crop. At Dotnuva and Joniskelis experimental sites the weed number tended to decrease 3.6-19.5 % and 3.9-19.5 %, respectively. In Joniskelis, all intercrops and sole crops had good suppression of *Thlaspi arvense*. Slightly fewer weeds germinated in cereal sole crops compared to intercrops. The germination of *Galium aparine* was significantly lower, and the number of *Fallopia convolvulus* tended to decrease in Joniskelis' cereal sole crop. Different crops (intercrop and sole crop) had little effect on the variation of perennial weed number.

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 9

**2.2.2 The competitive ability of pea intercropped with different spring cereal species**  Intercropping advantages may be influenced by both plant density and relative frequency of the intercrop components (Subkowicz & Tendziagolska, 2005). The density of plants in intercrops varied between different experimental location, soil and cultivation conditions in our research. According to crop density data, pea plant accounted for 27.2% of barley intercrop and 29.7% of wheat intercrop at Dotnuva site. The greater density of pea was observed in intercrop with oat and triticale (35.2 and 34.7%, respectively). In Joniskelis, the number of pea plants was lower (20.3-24.6 %) in intercrops, except for pea intercropped with

The highest productive density of pea in sole crop and intercrop was obtained in a loamy soil (Dotnuva) while a lower density was observed in clay loam soil (Joniskelis). Productive stem density of pea in crop structure was similar: 12.0-18.4 % (40-58 stems per m2) in loamy soil, 10.2-20.4% (28-43 stems per m2) in clay loam soil (Table 3). The more stable productive densities of intercrop were obtained in a loam soil (286-346 stems per m2) compared to a clay loam soil (211-275 stems per m2). This crop density in intercrop structure on a clay loam soil was determined by the specific properties of the soil (high clay content) and weather conditions. The weather conditions are essential on the formation of intercrop productivity and weed germination. They influence the optimal plant density and create the basis for competition between the components during crop germination period. The comparison between the different intercrops showed that the highest productive density was in pea intercropped with spring wheat (346 stems per m2) and with barley (332 stems per m2) in a loam soil, and peas with oats (275 stems per m2) and with wheat (268 stems per m2) in a clay

pea 109 48 40 58 48

total 346 332 316 286

pea 81 34 37 28 43

total 268 231 275 211

Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi –

Table 3. The productive density of sole crop and intercrop data averaged over 2007-2010

According to the literature, cereal has a stronger ability for weed suppression than pea (Andersen et al., 2007). German researchers note that crowding coefficients for semi-leafless pea cultivars were smaller than for conventional leafed types, therefore plant height of pea appears to be more important than plant leaf type for weed suppression (Rauber et al., 2001). The clay loam soil (Joniskelis) was more favourable for cereal growth: pea plants were

Sole crops and intercrop (BBCH 73) Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs Productive stems per m-2

cereal 298 292 258 238 478 398 442 368

cereal 235 195 247 168 355 307 343 334

triticale (34.7%).

loam soil.

Dotnuva

Joniskelis

pea and triticale.

Place Crop

component

Further suppression of weeds depends on the crop's ability to impede weed growth. It is widely accepted that the competitive interaction between weeds and crops does not occur only at early stages of plant development (Lazauskas, 1990).


Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 2. Weed emergence and density of the most important species in sole crop and intercrop data averaged over 2007-2010

Further suppression of weeds depends on the crop's ability to impede weed growth. It is widely accepted that the competitive interaction between weeds and crops does not occur

> Sole crops and intercrop (BBCH 32–36) Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs Weed m2

Dotnuva 2.3 1.7 1.9 2.3 1.9 2.6 2.2 1.6 1.7 Joniskelis 5.8 3.5 3.3 2.8\* 5.2 3.5 4.7 2.5\* 5.8

Dotnuva 1.1 0.6 0.6 0.3 1.2 0.6 0.4 0.4 0.7 Joniskelis 8.7 8.8 7.7 5.5 10.0 8.7 10.0 6.8 7.3

Dotnuva 3.7 1.7 0.9 0.9 0.8 1.0 0.7\* 1.6 0.4\* Joniskelis 8.0 3.8\* 2.8\*\* 4.2 4.7 3.3\*\* 3.3\* 2.5\*\* 3.3\*\*

Dotnuva 0 0 0 0 0 0 0 0 0 Joniskelis 8.8 6.0 5.8\* 7.0 6.7 5.0\* 5.2\* 4.0\*\* 4.8\*

Dotnuva 1.9 1.1 1.6 1.3 0.7 2.1 1.3 2.0 1.2 Joniskelis 5.3 4.2 4.7 5.5 6.3 3.5 3.0 3.8 5.5

Dotnuva 2.7 1.9 2.2 3.2 2.3 2.4 2.4 1.8 2.2 Joniskelis 10.7 12.2 11.0 12.5 9.3 10.5 14.7 9.5 17.0

Dotnuva 35.7 35.6 35.7 33.8 36.6 32.8 37.2 41.9 35.7 Joniskelis 8.5 7.8 5.3 5.8 4.7\* 4.2\* 5.8 4.5\* 4.8

Dotnuva 1.2 0.7 0.6 0.7 0.2\*\* 0.1\*\* 0.2\*\* 0.7 0.3\*\* Joniskelis 0 0 0 0 0 0 0 0 0

Dotnuva 1.0 0.1 1.4 0.6 0.7 0.7 0.6 0.8 1.7 Joniskelis 0 0 0 0 0 0 0 0 0

Dotnuva 1.9 2.2 1.8 2.1 1.9 2.9 1.2 2.3 2.2 Joniskelis 0 0 0 0 0 0 0 0 0

Dotnuva 2.9 0.8 3.6 3.0 2.3 0.2 0.5 0.1 3.2 Joniskelis 0.2 3.7\* 1.3 2.7 3.0 2.5 1.7 0.5 4.2\*\*

Dotnuva 1.8 0.7 0.1 0.9 1.0 0.3 1.0 1.4 0.4 Joniskelis 0.7 0.3 0.5 1.5 2.2 0.7 0.3 0.5 0.3

Dotnuva 57.9 48.1 51.7 51.3 50.9 46.6 48.2 55.8 51.0 Joniskelis 66.3 60.5 53.2\* 55.9 63.7 51.7\* 60.0 44.0\*\* 63.2

Dotnuva 12 13 13 13 13 13 13 13 13 Joniskelis 8 8 7 8 9 8 8 8 7

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and

Table 2. Weed emergence and density of the most important species in sole crop and

only at early stages of plant development (Lazauskas, 1990).

Species Place

*Viola arvensis* 

*Veronica arvensis* 

*Thlapsi arvense* 

*Galium aparine* 

*Fallopia convolvulus* 

> *Stellaria media*

*Chenopodium album* 

*Polygonum persicaria* 

*Polygonum aviculare*

*Chaenorrhinum minus* 

> *Cirsium arvense*

*Sonchus arvensis* 

Total number of weeds

Number of weeds species

triticale.

intercrop data averaged over 2007-2010

#### **2.2.2 The competitive ability of pea intercropped with different spring cereal species**

Intercropping advantages may be influenced by both plant density and relative frequency of the intercrop components (Subkowicz & Tendziagolska, 2005). The density of plants in intercrops varied between different experimental location, soil and cultivation conditions in our research. According to crop density data, pea plant accounted for 27.2% of barley intercrop and 29.7% of wheat intercrop at Dotnuva site. The greater density of pea was observed in intercrop with oat and triticale (35.2 and 34.7%, respectively). In Joniskelis, the number of pea plants was lower (20.3-24.6 %) in intercrops, except for pea intercropped with triticale (34.7%).

The highest productive density of pea in sole crop and intercrop was obtained in a loamy soil (Dotnuva) while a lower density was observed in clay loam soil (Joniskelis). Productive stem density of pea in crop structure was similar: 12.0-18.4 % (40-58 stems per m2) in loamy soil, 10.2-20.4% (28-43 stems per m2) in clay loam soil (Table 3). The more stable productive densities of intercrop were obtained in a loam soil (286-346 stems per m2) compared to a clay loam soil (211-275 stems per m2). This crop density in intercrop structure on a clay loam soil was determined by the specific properties of the soil (high clay content) and weather conditions. The weather conditions are essential on the formation of intercrop productivity and weed germination. They influence the optimal plant density and create the basis for competition between the components during crop germination period. The comparison between the different intercrops showed that the highest productive density was in pea intercropped with spring wheat (346 stems per m2) and with barley (332 stems per m2) in a loam soil, and peas with oats (275 stems per m2) and with wheat (268 stems per m2) in a clay loam soil.


Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 3. The productive density of sole crop and intercrop data averaged over 2007-2010

According to the literature, cereal has a stronger ability for weed suppression than pea (Andersen et al., 2007). German researchers note that crowding coefficients for semi-leafless pea cultivars were smaller than for conventional leafed types, therefore plant height of pea appears to be more important than plant leaf type for weed suppression (Rauber et al., 2001). The clay loam soil (Joniskelis) was more favourable for cereal growth: pea plants were

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 11

3.08 2.41 3.59 3.55

Table 5. The aboveground mass of crop during vegetation period in sole crop and intercrop

The pea was suppressed in intercrops, where the productive density of pea stems was 40-58 (Dotnuva) and 28–43 stems m-2 (Joniskelis), the productive density of cereal was 238-298 and 168-247 stems m-2, respectively. This indicates that the mass per pea plant in intercrops was 1.6-2.6 times lower compared to pea sole crop. Therefore, at Joniskelis site, the aboveground mass of crops during the growing season (BBCH 51) was lower for pea /wheat by 14.9%, pea/barley by 8.8%, and pea/triticale by 29.4% compared to the respective cereal sole crop. Only oat grown in intercrop produced more dry matter (9.9%) in aboveground mass

Indices allow researchers to quantify and express several attributes of plant competition, including competition intensity and importance, competitive effects and responses, and the outcome of competition (Weigelt & Jolliffe, 2003). An aggresivity value of zero indicates that component crops are equally competitive. If aggressivity value is higher than zero the species in the crop dominates, if this value is lower than zero the species is being chocked (Willey, Rao, 1980). Spring cereal has been dominant in intercrops due to the higher rate of aggression (Ac), the competitiveness ratio (CRc) in spring cereals. In most cases, oat was

The cultivation conditions were less favourable for crop growth in 2008 (in loamy soil) and 2009 (in clay loam soil), therefore weed density increased by up to 1.5-2 times until the harvesting period. The weakest competitive ability of cereal was obtained during 2008 and 2009. The study showed that the role of intercropped pea in weed suppression was limited. H. Hauggaard-Nielsen et al. (2008) indicate that a relative proportion of pea intercrop

The ability of pea intercropped with cereal to suppress weed species was revealed only at the development of the grain at filling growth stage (BBCH 73) and during favorable crop growing conditions. The total number of weeds in intercrops was significantly reduced compared to pea sole crop at maturity stage (BBCH 73) at both experimental sites (Table 7).

characterised as the strongest weed suppresser in intercropping system (Table 6).

around 40-50% is needed in order to achieve a level of intraspecific competition.

**2.2.3 Weed suppression in sole crops and intercrops** 

pea 521.4 104.1 117.4 73.5 184.6

total 824.9 551.6 986.8 713.9 Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi –

pea 6.76 3.09 3.12 2.59 4.20

Sole crops and intercrop (BBCH 51) Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs

cereal 3.07 2.22 3.67 3.30 2.73 1.97 2.62 3.12

cereal 720.8 434.2 913.3 529.3 969.5 605.0 897.8 1010.9

Indicators Crop

Dry matter of one stem (g)

> Dry matter (g m-2)

pea and triticale.

compared to oat sole crop.

component

weighted average

(BBCH 51), Joniskelis 2007-2010 averaged data

shorter (13.2%), and cereals taller (2.6-4.9%, except for spring barley) compared with respective crops in a loamy soil (Dotnuva) (Table 4). The pea plants were 22.1-29.9 % shorter (Dotnuva) and 34.1-42.0% (Joniskelis) compared to oat, spring wheat and tricicale. The height of spring cereals ranked as follows: oat > triticale > wheat > barley. According to the study, the height of pea plants declined by 20.1-24.7% in higher density intercrops (Dotnuva), and in lower density intercrops (Joniskelis) by 11.0-25.0% compared to pea sole crops. The height of intercropped cereals was not significantly different than cereal sole crops. Pea plants intercropped with oat, in some cases with barely and triticale were taller than those of sole crops.


Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 4. The plant height in sole crop and intercrop data averaged over 2007-2010

The weed suppression depended on the growth intensity of the crop aboveground biomass during the growing season. The mass per pea plant and per cereal stem at the beginning of cereal heading (BBCH 51) showed that the intercrops produced more biomass (0.18–1.05 g) compared to the cereal sole crops (Table 5). Comparison of different cereal species showed the lowest aboveground biomass per cereal stem was both in spring barley sole crop and intercropped with pea. Oat intercropped with pea accumulated the highest dry matter yield in the aboveground part. Here we identified the lowest aboveground mass per pea plant. The data of the aboveground mass suggested that pea grew slowly in intercrops until start of heading of cereals and poorly competed with cereals. During the experimental period, the aboveground mass was influenced by productive plant density but not by mass per stem. Peas produced more aboveground biomass in the second half of the vegetation period, in contrast to cereals, which already holds a dominant position in the first stages of growth. Weeds are suppressed for the durationof the vegetation period when the pea intercropped with cereal is established at appropriate densities. During the main crop growing period, when the development rate of the intercropped plant species do not coincide, favourable weather conditions for one or the other intercropped species can influence the degree of competition. The Joniskelis' experimental data indicated that the productive plant density in intercrops was lower for peas, which require higher nutrition area.

shorter (13.2%), and cereals taller (2.6-4.9%, except for spring barley) compared with respective crops in a loamy soil (Dotnuva) (Table 4). The pea plants were 22.1-29.9 % shorter (Dotnuva) and 34.1-42.0% (Joniskelis) compared to oat, spring wheat and tricicale. The height of spring cereals ranked as follows: oat > triticale > wheat > barley. According to the study, the height of pea plants declined by 20.1-24.7% in higher density intercrops (Dotnuva), and in lower density intercrops (Joniskelis) by 11.0-25.0% compared to pea sole crops. The height of intercropped cereals was not significantly different than cereal sole crops. Pea plants intercropped with oat, in some cases with barely and triticale were taller

pea 56.7 42.9 43.6 42.7 45.3

pea 49.2 36.9 38.2 39.7 43.8

Sole crops and intercrop (BBCH 73) Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs Height of crop (cm)

cereal 70.5 54.7 82.5 80.9 72.8 57.8 80.9 79.4

cereal 72.4 58.7 89.4 79.6 74.7 56.6 84.9 82.8

66.5 53.6 75.1 74.8

67.9 55.5 84.4 72.2

The weed suppression depended on the growth intensity of the crop aboveground biomass during the growing season. The mass per pea plant and per cereal stem at the beginning of cereal heading (BBCH 51) showed that the intercrops produced more biomass (0.18–1.05 g) compared to the cereal sole crops (Table 5). Comparison of different cereal species showed the lowest aboveground biomass per cereal stem was both in spring barley sole crop and intercropped with pea. Oat intercropped with pea accumulated the highest dry matter yield in the aboveground part. Here we identified the lowest aboveground mass per pea plant. The data of the aboveground mass suggested that pea grew slowly in intercrops until start of heading of cereals and poorly competed with cereals. During the experimental period, the aboveground mass was influenced by productive plant density but not by mass per stem. Peas produced more aboveground biomass in the second half of the vegetation period, in contrast to cereals, which already holds a dominant position in the first stages of growth. Weeds are suppressed for the durationof the vegetation period when the pea intercropped with cereal is established at appropriate densities. During the main crop growing period, when the development rate of the intercropped plant species do not coincide, favourable weather conditions for one or the other intercropped species can influence the degree of competition. The Joniskelis' experimental data indicated that the productive plant density in

Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi –

Table 4. The plant height in sole crop and intercrop data averaged over 2007-2010

intercrops was lower for peas, which require higher nutrition area.

than those of sole crops.

Place Crop

Dotnuva

Joniskelis

pea and triticale.

component

weighted average

weighted average


Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 5. The aboveground mass of crop during vegetation period in sole crop and intercrop (BBCH 51), Joniskelis 2007-2010 averaged data

The pea was suppressed in intercrops, where the productive density of pea stems was 40-58 (Dotnuva) and 28–43 stems m-2 (Joniskelis), the productive density of cereal was 238-298 and 168-247 stems m-2, respectively. This indicates that the mass per pea plant in intercrops was 1.6-2.6 times lower compared to pea sole crop. Therefore, at Joniskelis site, the aboveground mass of crops during the growing season (BBCH 51) was lower for pea /wheat by 14.9%, pea/barley by 8.8%, and pea/triticale by 29.4% compared to the respective cereal sole crop. Only oat grown in intercrop produced more dry matter (9.9%) in aboveground mass compared to oat sole crop.

Indices allow researchers to quantify and express several attributes of plant competition, including competition intensity and importance, competitive effects and responses, and the outcome of competition (Weigelt & Jolliffe, 2003). An aggresivity value of zero indicates that component crops are equally competitive. If aggressivity value is higher than zero the species in the crop dominates, if this value is lower than zero the species is being chocked (Willey, Rao, 1980). Spring cereal has been dominant in intercrops due to the higher rate of aggression (Ac), the competitiveness ratio (CRc) in spring cereals. In most cases, oat was characterised as the strongest weed suppresser in intercropping system (Table 6).

The cultivation conditions were less favourable for crop growth in 2008 (in loamy soil) and 2009 (in clay loam soil), therefore weed density increased by up to 1.5-2 times until the harvesting period. The weakest competitive ability of cereal was obtained during 2008 and 2009. The study showed that the role of intercropped pea in weed suppression was limited. H. Hauggaard-Nielsen et al. (2008) indicate that a relative proportion of pea intercrop around 40-50% is needed in order to achieve a level of intraspecific competition.

#### **2.2.3 Weed suppression in sole crops and intercrops**

The ability of pea intercropped with cereal to suppress weed species was revealed only at the development of the grain at filling growth stage (BBCH 73) and during favorable crop growing conditions. The total number of weeds in intercrops was significantly reduced compared to pea sole crop at maturity stage (BBCH 73) at both experimental sites (Table 7).

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 13

was in pea / barley intercrop and wheat and barley sole crops. Pea intercropped with wheat or triticale and triticale sole crop exhibited similar weed suppression; the number of weeds per m2 decreased by 9.8, 12.7 and 14.0, respectively. The best ability to suppress weeds was shown by oat sole crop and oat intercropped with pea with a decrease in weeds per m2 by

*Viola arvensis* Dotnuva 1.3 1.4 0.7 0.4 1.0 0.6 1 0.0\* 0.7

*Thlaspi arvense* Dotnuva 1.1 0.2\*\* 0.0\*\* 0.0\*\* 0.0\*\* 0.0\*\* 0.0\*\* 0.0\*\* 0.0\*\*

*Galium aparine* Dotnuva 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

*Stellaria media* Dotnuva 2.3 1.7 1.8 1.4 1.7 2.0 1.0 0.4\* 1.0

*Sonchus arvensis* Dotnuva 2.2 0.3 0.7 0.9 0.7 0.3 0.4 1.4 0.2

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and

important species in sole crops and intercrops, data averaged over 2007-2010

Table 7. Weed density (weed m-2) of the grain at filling growth stage (BBCH 73) of the most

Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs

Joniskelis 5.6 4.0 3.0 1.6\*\* 4.2 4.1 5.4 1.2\*\* 4.6

Dotnuva 0.2 0.3 0.0 0.1 0.0 0.2 0.2 0.0 0.0 Joniskelis 15.3 10.2 10.8 2.9\*\* 8.2\* 11.1 12.3 5.0\*\* 8.6\*

Joniskelis 3.7 1.3\*\* 0.2\*\* 0.0\*\* 0.8\*\* 0.5\*\* 0.2\*\* 0.0\*\* 0.5\*\*

Dotnuva 1.7 1.0 0.2\*\* 1.1 0.7\* 0.6\* 0.4\* 0.3\*\* 0.1\*\* Joniskelis 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Dotnuva 2.0 0.7\* 2.2 0.4\* 0.9 0.2\*\* 0.7 0.4\* 0.7 Joniskelis 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Joniskelis 4.4 2.2\*\* 3.5 1.7\*\* 3.4 3.1 2.5 1.3\*\* 3.1

Dotnuva 1.2 0.8 0.8 0.6 1.1 1.7 0.2 1.8 1.3 Joniskelis 7.8 5.2 7.1 4.2\* 6.6 5.3 5.9 4.2\* 5.8

Joniskelis 11.3 8.4 9.4 3.3\*\* 5.9\*\* 6.0\*\* 8.2 3.0\*\* 5.4\*\*

Dotnuva 37.6 27.0\* 23.4\*\* 22.0\*\* 28.0\* 22.9\*\* 24.2\*\* 21.9\*\* 26.0\*\* Joniskelis 7.2 7.0 6.1 5.1 9.3 6.1 8.1 1.7\*\* 6.2

Joniskelis 6.1 3.4 5.3 1.4\* 3.6 1.6\* 3.9 0.5\*\* 4.1

Dotnuva 2.8 0.7 3.2 3.1 2.9 0.2 1.1 0.4 3.6 Joniskelis 0.8 4.3 2.2 2.8 3.2 6.2\* 3.0 0.7 4.8\*

Dotnuva 54.7 35.7\*\* 33.2\*\* 30.3\*\* 37.6\*\* 29.4\*\* 30.0\*\* 27.0\*\* 34.3\*\* Joniskelis 71.3 50.7\* 54.2\* 25.8\*\* 51.0\* 49.4\*\* 55.3\* 19.6\*\* 49.2\*\*

Dotnuva 6 5 3\*\* 4\*\* 5\* 4\*\* 3\*\* 3\*\* 4\*\* Joniskelis 9 7\* 7\* 5\*\* 8 7\* 8\* 4\*\* 8\*

Species Place Sole crops and intercrops (BBCH 73)

24.4 and 30.1, respectively.

*Veronica arvensis*

*Polygonum persicaria*

*Polygonum aviculare* 

*Fallopia convolvulus*

*Chenopodium album*

*Cirsium arvense*

Total number of weeds

Number of weeds species

triticale.

The strongest weed suppression was observed in higher plant density intercrop and sole crop in Dotnuva. However, the number of weeds was 31.3-50.6% lower in intercrop compared to pea sole crop. In lower density crops (Joniskelis), the number of weeds in intercrop was 22.4-31.0% lower except for the oat sole crop and oat intercropped with pea. The oat displayed strong weed suppression capabilitieswith the number of weeds 72.5% lower in oat sole crop and 63.8% in oat/pea intercrop compared to pea sole crop. Comparison between cereal sole crops and intercrops showed a reduction in weed numbers in intercrop by an average 37.5%, in sole crops by 44.8% at Dotnuva, and by 36.3 and 39.1%, respectively in Joniskelis compared to pea sole crop. The number of weed species also significantly decreased except for pea/wheat (Dotnuva) and pea/triticale (Joniskelis) intercrops.


Note. Intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 6. Plant aggressivety (Ac) and competition rate (CRc,) in organically grown intercrops

The number of weeds observed during the cereal maturity stage (BBCH 73) varied compared to the weed number in spring (BBCH 32-36) (Table 8). Thus, weed population dynamics was influenced not only by the crop suppression ability, but also by the total weediness of crop and weed species. In Joniskelis, in the lower density pea sole crop, the number of weeds increased by 4.9 m-2, and in Dotnuva decreased by 3.1 m-2 during the period from emergence to cereal grain-filling growth stage. At Dotnuva, in the higher density crop, the total number of weeds decreased by 12.4–28.8 m-2 compared to the corresponding data in the spring. The variation of weed numbers during the growing season differed little between sole crop and intercrop (except oat sole crop and intercrop) with a decrease of 28.8 and 21.0 m-2, respectively. At Joniskelis, more marked differences between crops were determined; however, the suppression of weeds was weaker compared to the Dotnuva data. According to the spring weed density, the lowest suppression of weeds

The strongest weed suppression was observed in higher plant density intercrop and sole crop in Dotnuva. However, the number of weeds was 31.3-50.6% lower in intercrop compared to pea sole crop. In lower density crops (Joniskelis), the number of weeds in intercrop was 22.4-31.0% lower except for the oat sole crop and oat intercropped with pea. The oat displayed strong weed suppression capabilitieswith the number of weeds 72.5% lower in oat sole crop and 63.8% in oat/pea intercrop compared to pea sole crop. Comparison between cereal sole crops and intercrops showed a reduction in weed numbers in intercrop by an average 37.5%, in sole crops by 44.8% at Dotnuva, and by 36.3 and 39.1%, respectively in Joniskelis compared to pea sole crop. The number of weed species also significantly decreased except for pea/wheat (Dotnuva) and pea/triticale (Joniskelis)

Intercrop Year Loamy soil (Dotnuva) Clay loam soil (Joniskelis)

Ac CRc Ac CRc

2007 1.06 4.75 0.56 3.37 2008 0.80 2.99 0.40 2.89 2009 1.10 5.22 1.09 2.13 2010 - - 0.37 2.63

2007 1.09 6.52 0.96 7.84 2008 0.20 1.51 0.91 3.88 2009 0.92 7.90 1.50 3.09 2010 0.57 2.28

2007 1.25 4.89 1.22 15.63 2008 1.17 2.90 1.15 5.45 2009 1.41 9.89 1.26 4.67 2010 - - 0.85 5.25

2007 0.78 3.65 0.38 2.44 2008 0.70 4.05 1.52 1.33 2009 1.99 7.92 -0.17 0.54 2010 - - 0.26 1.82

Note. Intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat,

Table 6. Plant aggressivety (Ac) and competition rate (CRc,) in organically grown intercrops

The number of weeds observed during the cereal maturity stage (BBCH 73) varied compared to the weed number in spring (BBCH 32-36) (Table 8). Thus, weed population dynamics was influenced not only by the crop suppression ability, but also by the total weediness of crop and weed species. In Joniskelis, in the lower density pea sole crop, the number of weeds increased by 4.9 m-2, and in Dotnuva decreased by 3.1 m-2 during the period from emergence to cereal grain-filling growth stage. At Dotnuva, in the higher density crop, the total number of weeds decreased by 12.4–28.8 m-2 compared to the corresponding data in the spring. The variation of weed numbers during the growing season differed little between sole crop and intercrop (except oat sole crop and intercrop) with a decrease of 28.8 and 21.0 m-2, respectively. At Joniskelis, more marked differences between crops were determined; however, the suppression of weeds was weaker compared to the Dotnuva data. According to the spring weed density, the lowest suppression of weeds

intercrops.

P+SWi

P+SBi

P+Oi

P+STi

P+STi – pea and triticale.

was in pea / barley intercrop and wheat and barley sole crops. Pea intercropped with wheat or triticale and triticale sole crop exhibited similar weed suppression; the number of weeds per m2 decreased by 9.8, 12.7 and 14.0, respectively. The best ability to suppress weeds was shown by oat sole crop and oat intercropped with pea with a decrease in weeds per m2 by 24.4 and 30.1, respectively.


Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale.

Table 7. Weed density (weed m-2) of the grain at filling growth stage (BBCH 73) of the most important species in sole crops and intercrops, data averaged over 2007-2010

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 15

considered less harmful for agricultural crops (Špokienė & Povilionienė, 2003). The suppressing effect for *Viola arvensis, Veronica arvensis* was more pronounced in the lower density crop in Joniskelis. The number of these weed species slightly increased in the cereal sole crop compared to the pea intercropped with cereal. The number of these weeds significantly decreased in oat sole crop and intercropped with pea, and *Veronica arvensis* decreased even in triticale sole crop and intercropped with pea. Although the presence of *Thlaspi arvense* was low, the number of weeds was significantly reduced in all cereal sole crops and intercrops regardless of the site. At Dotnuva site, where *Polygonum persicaria* and *Polygonum aviculare* were found, the cereal sole crop suppressed these weeds slightly sronger compared to the pea intercrop. There were significantly fewer *Polygonum aviculare* plants in wheat, oat sole crops and intercropped with pea compared to the pea sole crop, whereas, *Polygonum persicaria* was suppressed by all cereal sole crops and some of their

The number of *Viola arvensis* was low at Dotnuva during cereal maturity stage compared to the findings in spring, (BBCH 32-36); the weed numbers were reduced. However, at Joniskelis, where the incidence of these weed species was higher, the number of weeds was reduced only in oat and triticale sole crops and intercropped with pea (by 1.3, 1.3 and 1.3, 1.0 weed m-2, respectively). The number of *Viola arvensis* increased in pea/wheat and wheat, barley sole crop. At Dotnuva, the number of *Veronica arvensis* during the gropwing season was reduced in all crops 0.2-1.2 weed m-2; however, at Joniskelis, the weed number increased in the majority of the crops. The number of *Viola arvensis* was reduced in oat sole crop (1.8 weed m-2) and pea intercropped with oat (2.8 weeds m-2), triticale (1.8 weed m-2). The number of *Thlaspi arvense* was reduced in all crops (0.4–1.6 weed m-2) at Dotnuva and at Joniskelis (2.5–4.3 weed m-2). The number of *Polygonum persicaria* and *Polygonum aviculare*  increased in the majority of crops. The number of weeds most consistently decreased in oat and triticale sole crops. *Viola arvensis, Veronica arvensis, Thlaspi arvense, Polygonum persicaria, Polygonum aviculare* are less harmful, the mass of the weeds was low and significantly

*Galium aparine* and *Fallopia convolvulus* are common in crops. Their numbers significantly decreased in oat sole crop and intercropped with pea; *Galium aparine* was also decreased in pea intercropped with wheat compared to pea sole crop. Cleare advantages of intercrops compared to sole crops were not detected against these two harmful species of weeds.

A strong suppressive effect of crops on *Galium aparine* was identified during cereal maturity stage when the number of weeds declined by 1.9–5.3 weed m-2 compared to the findings in spring. The advantages of intercrops were clear with oat, wheat and triticale intercrops reducing the number of *Galium aparine* by 5.3, 3.8 and 3.2 weed m-2, respectively, than sole cereal crops by 2.6, 1.9 and 1.8 weed m-2, respectively.The mass of *Galium aparine* decreased (22.9–96.1%) in all crops, except for intercropped wheat. Significantly lower mass of these weeds was in the oat intercrop, and oat and barley sole crop compared to pea sole crop.

At Dotnuva, the number of *Fallopia convolvulus* during the growing season decreased in the majority of crops, except for tricticale sole crop and intercropped with pea. At Joniskelis, the number of these weeds increased during the whole growing season compared to the respective number of weeds in spring. The number of weeds markedly increased in intercropped barley and pea and wheat and barley sole crops. The number of *Fallopia convolvulus* decreased only in intercropped oat compared to the findings in spring. The

decreased in the majority of crops compared to pea sole crop.

intercrops with pea.


Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale

Table 8. The variation of weed numbers (weed m-2) of the most important species in sole crops and intercrops during growing season, data averaged over 2007-2010

At Joniskelis, the highest total mass of weeds was determined in pea sole crop and intercropped with triticale. At Dotnuva, the total dry matter (DM) of weeds in pea sole crop was 38.4% higher compared to the pea sole crop at Joniskelis (Table 9). At Dotnuva, *Chenopodium album*, *Cirsium arvense,* and *Sonchus arvensis* mass accounted for the largest share in the total weed mass. All cereal sole crops and intercrops significantly reduced weeds and the weed mass decreased by 72.0-90.7% compared to pea sole crop. At Joniskelis site, *Cirsium arvense* was spread unevenly in the experimental area; therefore, the total weed mass was substantially higher where this weed was present. The lowest total mass of weeds was determined in oat sole crop and pea intercropped with oat and barley.

The variation of weed total numbers and weight was determined by the response of different weed species to crop suppression. Weed species and their numbers at cereal maturity stage during maturity stage are presented in Tables 7, 8 and 9. The species *Viola arvensis, Veronica arvensis, Thlaspi arvense, Polygonum persicaria* and *Polygonum aviculare* are

*Viola arvensis* Dotnuva -1.0 -0.3 -1.2 -1.9 -0.9 -2.0 -1.2 -1.6 -1.0

*Veronica arvensis* Dotnuva -0.9 -0.3 -0.6 -0.2 -1.2 -0.4 -0.2 -0.4 -0.7

*Thlaspi arvense* Dotnuva -2.6 -1.5 -0.9 -0.9 -0.8 -1.0 -0.7 -1.6 -0.4

*Galium aparine* Dotnuva - - - - - - - - -

*Stellaria media* Dotnuva -0.4 -0.2 -0.4 -1.8 -0.6 -0.4 -1.4 -1.4 -1.2

*Sonchus arvensis* Dotnuva +0.4 -0.4 +0.6 0.0 -0.3 0.0 -0.6 0.0 -0.2

*Cirsium arvense* Dotnuva -0.1 -0.1 -0.3 +0.1 +0.6 0.0 +0.6 +0.3 +0.4

Note. Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi –

Table 8. The variation of weed numbers (weed m-2) of the most important species in sole

At Joniskelis, the highest total mass of weeds was determined in pea sole crop and intercropped with triticale. At Dotnuva, the total dry matter (DM) of weeds in pea sole crop was 38.4% higher compared to the pea sole crop at Joniskelis (Table 9). At Dotnuva, *Chenopodium album*, *Cirsium arvense,* and *Sonchus arvensis* mass accounted for the largest share in the total weed mass. All cereal sole crops and intercrops significantly reduced weeds and the weed mass decreased by 72.0-90.7% compared to pea sole crop. At Joniskelis site, *Cirsium arvense* was spread unevenly in the experimental area; therefore, the total weed mass was substantially higher where this weed was present. The lowest total mass of weeds

The variation of weed total numbers and weight was determined by the response of different weed species to crop suppression. Weed species and their numbers at cereal maturity stage during maturity stage are presented in Tables 7, 8 and 9. The species *Viola arvensis, Veronica arvensis, Thlaspi arvense, Polygonum persicaria* and *Polygonum aviculare* are

crops and intercrops during growing season, data averaged over 2007-2010

was determined in oat sole crop and pea intercropped with oat and barley.

Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs

Joniskelis -0.2 +0.5 -0.4 -1.3 -1.0 +0.6 +0.7 -1.3 -1.3

Joniskelis +6.6 +1.4 +3.1 -2.6 -1.8 +2.4 +2.3 -1.8 +1.2

Joniskelis -4.3 -2.5 -2.7 -4.2 -3.8 -2.9 -3.2 -2.5 -2.8

Dotnuva +0.5 +0.3 -0.4 +0.4 +0.5 +0.5 +0.2 -0.4 -0.2 Joniskelis - - - - - - - - -

Dotnuva +1.0 +0.6 +0.8 -0.2 +0.3 -0.5 +0.1 -0.4 -1.0 Joniskelis - - - - - - - - -

Joniskelis -4.4 -3.8 -2.3 -5.3 -3.2 -1.9 -2.7 -2.6 -1.8

Dotnuva -0.7 -0.3 -0.7 -0.7 +0.5 -0.4 -1.1 -0.2 +0.1 Joniskelis +2.5 +1.0 +2.5 -1.3 +0.3 +1.8 +2.9 +0.4 +0.4

Joniskelis +0.7 -3.8 -1.6 -9.3 -3.4 -4.5 -6.4 -6.5 -11.5

Dotnuva +1.9 -8.6 -12.3 -11.8 -8.5 -9.9 -13.0 -20.0 -9.7 Joniskelis -1.3 -0.8 +0.7 -0.8 +4.6 +1.9 +2.3 -2.8 +1.4

Joniskelis +5.4 +3.1 +4.8 -0.1 +1.4 +0.9 +3.6 0.0 +3.7

Joniskelis +0.7 +0.7 +0.8 +0.2 +0.2 +3.7 +1.3 +0.2 +0.7

Dotnuva -3.1 -12.4 -18.2 -21.0 -12.9 -17.2 -18.2 -28.8 -16.7 Joniskelis +4.9 -9.8 +1.0 -30.1 -12.7 -2.3 -4.8 -24.4 -14.0

Species Place Sole crops and intercrops (BBCH 73)

*Polygonum persicaria*

*Polygonum aviculare*

*Fallopia convolvulus*

*Chenopodium album*

Total number of weeds

pea and triticale

considered less harmful for agricultural crops (Špokienė & Povilionienė, 2003). The suppressing effect for *Viola arvensis, Veronica arvensis* was more pronounced in the lower density crop in Joniskelis. The number of these weed species slightly increased in the cereal sole crop compared to the pea intercropped with cereal. The number of these weeds significantly decreased in oat sole crop and intercropped with pea, and *Veronica arvensis* decreased even in triticale sole crop and intercropped with pea. Although the presence of *Thlaspi arvense* was low, the number of weeds was significantly reduced in all cereal sole crops and intercrops regardless of the site. At Dotnuva site, where *Polygonum persicaria* and *Polygonum aviculare* were found, the cereal sole crop suppressed these weeds slightly sronger compared to the pea intercrop. There were significantly fewer *Polygonum aviculare* plants in wheat, oat sole crops and intercropped with pea compared to the pea sole crop, whereas, *Polygonum persicaria* was suppressed by all cereal sole crops and some of their intercrops with pea.

The number of *Viola arvensis* was low at Dotnuva during cereal maturity stage compared to the findings in spring, (BBCH 32-36); the weed numbers were reduced. However, at Joniskelis, where the incidence of these weed species was higher, the number of weeds was reduced only in oat and triticale sole crops and intercropped with pea (by 1.3, 1.3 and 1.3, 1.0 weed m-2, respectively). The number of *Viola arvensis* increased in pea/wheat and wheat, barley sole crop. At Dotnuva, the number of *Veronica arvensis* during the gropwing season was reduced in all crops 0.2-1.2 weed m-2; however, at Joniskelis, the weed number increased in the majority of the crops. The number of *Viola arvensis* was reduced in oat sole crop (1.8 weed m-2) and pea intercropped with oat (2.8 weeds m-2), triticale (1.8 weed m-2). The number of *Thlaspi arvense* was reduced in all crops (0.4–1.6 weed m-2) at Dotnuva and at Joniskelis (2.5–4.3 weed m-2). The number of *Polygonum persicaria* and *Polygonum aviculare*  increased in the majority of crops. The number of weeds most consistently decreased in oat and triticale sole crops. *Viola arvensis, Veronica arvensis, Thlaspi arvense, Polygonum persicaria, Polygonum aviculare* are less harmful, the mass of the weeds was low and significantly decreased in the majority of crops compared to pea sole crop.

*Galium aparine* and *Fallopia convolvulus* are common in crops. Their numbers significantly decreased in oat sole crop and intercropped with pea; *Galium aparine* was also decreased in pea intercropped with wheat compared to pea sole crop. Cleare advantages of intercrops compared to sole crops were not detected against these two harmful species of weeds.

A strong suppressive effect of crops on *Galium aparine* was identified during cereal maturity stage when the number of weeds declined by 1.9–5.3 weed m-2 compared to the findings in spring. The advantages of intercrops were clear with oat, wheat and triticale intercrops reducing the number of *Galium aparine* by 5.3, 3.8 and 3.2 weed m-2, respectively, than sole cereal crops by 2.6, 1.9 and 1.8 weed m-2, respectively.The mass of *Galium aparine* decreased (22.9–96.1%) in all crops, except for intercropped wheat. Significantly lower mass of these weeds was in the oat intercrop, and oat and barley sole crop compared to pea sole crop.

At Dotnuva, the number of *Fallopia convolvulus* during the growing season decreased in the majority of crops, except for tricticale sole crop and intercropped with pea. At Joniskelis, the number of these weeds increased during the whole growing season compared to the respective number of weeds in spring. The number of weeds markedly increased in intercropped barley and pea and wheat and barley sole crops. The number of *Fallopia convolvulus* decreased only in intercropped oat compared to the findings in spring. The

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 17

pea at Joniskelis. The influence of investigated crops on perennial weeds mass was not as marked as on annual weeds. At Joniskelis, the mass of *Sonchus arvensis* was significantly reduced in several crops including wheat and oat sole crops, and wheat and barley

*Viola arvensis* Dotnuva 0.16 0.16 0.05 0.06 0.14 0.03\* 0.05 0.00\* 0.08

*Thlaspi arvense* Dotnuva 0.72 0.01\* 0.00\* 0.00\* 0.00\* 0.00\* 0.00\* 0.00\* 0.00\*

*Galium aparine* Dotnuva 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

*Stellaria media* Dotnuva 0.71 0.47 1.04 0.21 0.52 0.54 0.20 0.06\* 0.12

*Sonchus arvensis* Dotnuva 4.51 0.26 0.18 4.00 0.49 0.41 0.50 1.82 0.18

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and

Table 9. The weed dry matter mass (DM g m-2) of the most important species in sole crops

At Joniskelis, the number of *Cirsium arvense* increased during the growing season in all investigated crops compared with its number in spring. At Dotnuva, an increase in the number of this weed was not consistent. The number of these weeds was slightly reduced by pea sole crop and intercropped with wheat and barley. More weeds germinated in cereal sole crop compared to intercrops at both expermental sites. The variation of *Cirsium arvense*

Ps P+SWi P+SBi P+Oi P+STi SWs SBs Os STs

Joniskelis 0.75 0.20\*\* 0.27\*\* 0.10\*\* 0.52 0.18\*\* 0.32\* 0.02\*\* 0.44

Dotnuva 0.02 0.02 0.00 0.01 0.00 0.02 0.01 0.00 0.00 Joniskelis 2.02 0.88\*\* 0.63\*\* 0.12\*\* 0.72\*\* 0.44\*\* 0.66\*\* 0.10\*\* 0.88\*\*

Joniskelis 1.11 0.31\* 0.0\*\* 0.00\*\* 0.42 0.02\*\* 0.01\*\* 0.00\*\* 0.01\*\*

Dotnuva 1.00 0.39\*\* 0.02\*\* 0.33\*\* 0.06\*\* 0.11\*\* 0.06\*\* 0.03\*\* 0.04\*\* Joniskelis 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Dotnuva 2.16 0.44\*\* 0.50\*\* 0.14\*\* 0.14\*\* 0.03\*\* 0.10\*\* 0.11\*\* 0.14\*\* Joniskelis 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Joniskelis 3.36 3.87 1.47 0.22\*\* 2.59 2.33 1.04\* 0.13\*\* 1.39

Dotnuva 1.53 0.51 0.21\* 0.09\*\* 0.46\* 0.86 0.02\*\* 0.20\* 0.37\* Joniskelis 9.07 2.08\*\* 3.19\*\* 2.46\*\* 6.55 1.27\*\* 2.87\*\* 0.65\*\* 2.36\*\*

Joniskelis 8.64 5.38\* 3.56\*\* 0.96\*\* 2.85\*\* 1.62\*\* 3.79\*\* 0.45\*\* 2.02\*\*

Dotnuva 37.15 10.54\*\* 5.59\*\* 6.25\*\* 7.68\*\* 5.03\*\* 3.15\*\* 2.44\*\* 4.15\*\* Joniskelis 7.94 2.41\* 1.22\*\* 1.08\*\* 7.27 0.88\*\* 1.63\*\* 0.17\*\* 3.63\*

Joniskelis 2.97 0.19\* 0.24\* 1.58 3.33 0.40\* 0.67 0.05\*\* 0.82

Dotnuva 8.01 1.54 2.48 1.59 6.32 0.06\* 0.92 0.25\* 4.78 Joniskelis 0.66 19.37\* 4.55 8.03 32.53\* 30.30\* 18.77 1.77 19.04\*

Dotnuva 56.89 14.61\*\* 10.27\*\* 12.72\*\* 15.96\*\* 7.34\*\* 5.36\*\* 5.27\*\* 10.02\*\* Joniskelis 41.10 35.64 17.18\* 15.28\*\* 60.44 38.84\* 30.60 4.09\*\* 33.55

Species Place Sole crops and intercrops (BBCH 73)

intercrops when compared to pea sole crop.

*Veronica arvensis*

*Polygonum persicaria*

*Polygonum aviculare*

*Fallopia convolvulus*

*Chenopodium album*

*Cirsium arvense*

Total mass of weeds

and intercrops, data averaged over 2007-2010

triticale

experimental crops had greater influence on *Fallopia convolvulus* mass rather than number. At both experiment sites, intercrops and cereal sole crops significantly decreased the mass of this weed (by 70.6-98.7% and 64.8-92.8%, respectively) except for intercrop and sole crop of wheat (Dotnuva) and intercropped triticale (Joniskelis) compared to pea sole crop.

The incidence of *Stellaria media* was high at Joniskelis. Significantly fewer *Stellaria media* plants were recorded in cereal sole crop (except for spring barley) compared to pea sole crop. The number of this weed was significantly reduced by the intercrops of oat and triticale. The reduction in *Stellaria media* numbers during the cereal maturity stage was marked (regardless of their abundance) compared to the number of these weeds in spring, except for pea sole crop at Joniskelis. At both sites, this weed species was more suppressed by cereal sole crop than the intercrop. The highest reduction of *Stellaria media* was determined in these crops: pea / oats, oats, triticale (Dotnuva and Joniskelis) and barley sole crop (Dotnuva) compared to the respective weed numbers in spring.At Dotnova, the mass of *Stellaria media* was low and the influence of the crops was not significant, except for oat sole crop. The incidence of this weed was high at Joniskelis where the influence of crops on the reduction of weed mass was significant (37.7–94.8%) compared to pea sole crop. The mass of *Stellaria media* was reduced by intercropped or sole oat. Also, this weed was suppressed by wheat, triticale sole crop and their intercrops with pea.

The incidence of *Chenopodium album* was high at Dotnuva (21.9-37.6 weeds m-2); all crops significantly reduced the number of this weed species compared to pea sole crop. At Joniskelis, the number of *Chenopodium album* was reduced only by oat sole crop. The variation of *Chenopodium album* numbers during the growing season showed that these weeds were not as intensively suppressed as other weed species at Dotnuva. The number of *Chenopodium album* reduced in intercrop and sole crop was 8.5–20.0 weeds m-2 compared to the respective weed numbers in spring. The number of this weed species slightly increased in pea sole crop. However, at Joniskelis, the number of *Chenopodium album* incresed in the majority of sole crops and intercrops, where 4-9 times fewer weeds emerged in spring. The number of weeds slightly decreased in wheat and oat intercrops and pea sole crop, but the weed incidence decreased most in oat sole crop (2.8 weeds m-2). The investigated crops at both experimental sites reduced the mass of *Chenopodium album* by 71.6-93.4% at Dotnuva and by 54.3-97.9% at Joniskelis compared to pea sole crop. The mass of this weed was lower in many cereal sole crops compared to intercrops.

Perennial weeds *Sonchus arvensis* and *Cirsium arvense* are more frequent on a clay loam soil, found at Joniskelis, compared to a loamy soil found at Dotnuva. At Joniskelis, *Sonchus arvensis* was more frequent in pea sole crop. The number of this weed significantly reduced in wheat, oat sole crops and intercropped with pea compared to pea sole crop. At Joniskelis, the number of *Cirsium arvense* decreased in all investigated crops, except for oat sole crop. Significantly higher numbers of this weed were found in spring wheat and triticale sole crop compared to pea sole crop. The trends of variation of this weed number were similar in Dotnuva. The crops were less suppressivefor perennial weeds than annual weed species observed in the experiment.

In spring, the number of *Sonchus arvensis* did not differ at either experimental site; however, variation of the weed numbers was noted. Consistent patterns of *Sonchus arvensis* variation were not determined in the higher density crops at Dotnuva. However, the number of weeds increased in all experimental crops, except for oat sole crop and intercropped with

experimental crops had greater influence on *Fallopia convolvulus* mass rather than number. At both experiment sites, intercrops and cereal sole crops significantly decreased the mass of this weed (by 70.6-98.7% and 64.8-92.8%, respectively) except for intercrop and sole crop of

The incidence of *Stellaria media* was high at Joniskelis. Significantly fewer *Stellaria media* plants were recorded in cereal sole crop (except for spring barley) compared to pea sole crop. The number of this weed was significantly reduced by the intercrops of oat and triticale. The reduction in *Stellaria media* numbers during the cereal maturity stage was marked (regardless of their abundance) compared to the number of these weeds in spring, except for pea sole crop at Joniskelis. At both sites, this weed species was more suppressed by cereal sole crop than the intercrop. The highest reduction of *Stellaria media* was determined in these crops: pea / oats, oats, triticale (Dotnuva and Joniskelis) and barley sole crop (Dotnuva) compared to the respective weed numbers in spring.At Dotnova, the mass of *Stellaria media* was low and the influence of the crops was not significant, except for oat sole crop. The incidence of this weed was high at Joniskelis where the influence of crops on the reduction of weed mass was significant (37.7–94.8%) compared to pea sole crop. The mass of *Stellaria media* was reduced by intercropped or sole oat. Also, this weed was

The incidence of *Chenopodium album* was high at Dotnuva (21.9-37.6 weeds m-2); all crops significantly reduced the number of this weed species compared to pea sole crop. At Joniskelis, the number of *Chenopodium album* was reduced only by oat sole crop. The variation of *Chenopodium album* numbers during the growing season showed that these weeds were not as intensively suppressed as other weed species at Dotnuva. The number of *Chenopodium album* reduced in intercrop and sole crop was 8.5–20.0 weeds m-2 compared to the respective weed numbers in spring. The number of this weed species slightly increased in pea sole crop. However, at Joniskelis, the number of *Chenopodium album* incresed in the majority of sole crops and intercrops, where 4-9 times fewer weeds emerged in spring. The number of weeds slightly decreased in wheat and oat intercrops and pea sole crop, but the weed incidence decreased most in oat sole crop (2.8 weeds m-2). The investigated crops at both experimental sites reduced the mass of *Chenopodium album* by 71.6-93.4% at Dotnuva and by 54.3-97.9% at Joniskelis compared to pea sole crop. The mass of this weed was lower

Perennial weeds *Sonchus arvensis* and *Cirsium arvense* are more frequent on a clay loam soil, found at Joniskelis, compared to a loamy soil found at Dotnuva. At Joniskelis, *Sonchus arvensis* was more frequent in pea sole crop. The number of this weed significantly reduced in wheat, oat sole crops and intercropped with pea compared to pea sole crop. At Joniskelis, the number of *Cirsium arvense* decreased in all investigated crops, except for oat sole crop. Significantly higher numbers of this weed were found in spring wheat and triticale sole crop compared to pea sole crop. The trends of variation of this weed number were similar in Dotnuva. The crops were less suppressivefor perennial weeds than annual weed species

In spring, the number of *Sonchus arvensis* did not differ at either experimental site; however, variation of the weed numbers was noted. Consistent patterns of *Sonchus arvensis* variation were not determined in the higher density crops at Dotnuva. However, the number of weeds increased in all experimental crops, except for oat sole crop and intercropped with

wheat (Dotnuva) and intercropped triticale (Joniskelis) compared to pea sole crop.

suppressed by wheat, triticale sole crop and their intercrops with pea.

in many cereal sole crops compared to intercrops.

observed in the experiment.

pea at Joniskelis. The influence of investigated crops on perennial weeds mass was not as marked as on annual weeds. At Joniskelis, the mass of *Sonchus arvensis* was significantly reduced in several crops including wheat and oat sole crops, and wheat and barley intercrops when compared to pea sole crop.


Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat, P+STi – pea and triticale

Table 9. The weed dry matter mass (DM g m-2) of the most important species in sole crops and intercrops, data averaged over 2007-2010

At Joniskelis, the number of *Cirsium arvense* increased during the growing season in all investigated crops compared with its number in spring. At Dotnuva, an increase in the number of this weed was not consistent. The number of these weeds was slightly reduced by pea sole crop and intercropped with wheat and barley. More weeds germinated in cereal sole crop compared to intercrops at both expermental sites. The variation of *Cirsium arvense*

crops.

respectively).

in intercrops compared to pea sole crop.

are presented in Figure 1.

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 19

therefore, the relationship was determined only between the number of this weed and productive density of crops (r= – 0.670, *P*<0.05). At Dotnuva, a strong relationship (r= – 0.856, *P*<0.01) was established between *Cirsium arvense* numbers and productive density of

The relationship established between the total number and mass of weeds and intercrop competitive ability indicators (aggressivity - Ac; competition rate - CRc) showed that, with increasing competition rate of intercrops, weed incidence declined. This relationship was determined only at Joniskelis where the productive density was lower, the variation rate of CRc was higher, and weed species diversity and numbers were increased. With changing competition rates (0.54–15.63), weed number and mass declined by a linear inverse relationship. The correlation was medium (r= – 0.551, *P*<0.05; r= – 0.5031, *P*<0.05,

Researchers from five countries: Denmark, United Kingdom, France, Germany and Italy investigated the influence of pea and barley intercrop sown at different ratios – 45 peas and 150 barley plants m-2, and 90 peas and 150 barley plants m-2 - on dry matter of weeds. The control of weeds was similar in sole barley and in intercrops, and no difference was established between the substitutive and the additive intercrops (Dibet et al., 2006). Researchers report the advantages of various intercropping managements such as pea with wheat against weeds (Szumigalski & van Acker, 2005), pea with barley (Hauggaard-Nielsen et al., 2006), and pea with oats (Rauber et al., 2001). Diversity of weeds was decreased in intercrops in comparison with sole crop (Gharineh & Moradi Telavat, 2009). Like cultivated plants, weeds obtain nutrients through root uptake from the soil solution. As a result, weeds and crops compete for space, nutrients*,* water and light. Both weeds and crop plants are similar in chemical composition; therefore, weeds can accumulate similar or even higher amounts of nutrients than crops *(*Lazauskas, 1990). Nitrogen increases weed and crop biomass (Kristensen et al., 2008). Peas use little nitrogen from the soil since they can fix atmospheric nitrogen for use. As a result, peas provide good conditions for weed growth, especially for high nitrogen demanding weed species. Dibet *et al*. (2006) reported a lower nitrogen concentration, 0.8 g m-2, in weed mass due to competition between weed and cereal

Statistical analyses of sole crop and intercrop grain yields and weed numbers and variation

Strong, inversely proportional relationships were established between grain yield and total weed number, and between grain yield and weed number variation during the growing season. This means that the number of weeds and their variation conversely affected crop yield. These relationships were determined only in lower density crops in clay loam soil (Joniskelis) when weed incidence markedly increased. The analysis of the statistical data suggested that an increase in the total weed number by one weed (regardless of the species) resulted in a grain yield reduction by 27.3 kg ha-1 (Figure 1a). The investigated crop competition characteristics describe the relationship between crop yields and weed number variation during the growing period. The grain yield changed 18.8 kg ha-1 by an inverse trend when changing one weed (Figure 1b). The relationship between grain yield and total weed mass was not significant (r= – 0.564, *P*>0.05). This relationship could determine perennial weed mass, which was especially high and the weeds were spread unevenly. It can be maintained that, in clay loam soil (Joniskelis), the majority of the investigated crops

mass differed between the experimental sites. At Dotnuva, *Cirsium arvense* mass was reduced in the wheat and oat sole crops of higher density but in other investigated crops, we established only a trend towards weed reduction. At Joniskelis, the mass of this perennial weed increased in all investigated crops, particularly in the wheat and triticale sole crops and their intercrops compared to pea sole crop.

Statistical data analysis showed that productive stem density had the greatest effect on weed suppression, while the effect of crop height and mass had a lesser affecting both soil conditions. In loamy soil (Dotnuva), the total number and mass of weeds were significantly related to the productive density (r= – 0.922, *P*<0.01, r= – 0.909, *P*<0.01) within the range 109–478 stems m-2. In clay loam soil (Joniskelis), where productive crop density was lower (81–355 stems m-2), the total number of weeds were significantly reduced by the height of crop (r= – 0.830, *P*<0.01).

Annual weed species had variable responses to the crop density, height and mass. At Joniskelis, the number of *Stellaria media* was significantly reduced with increasing productive density, height and mass of crops (r= – 0.685, *P*<0.05; r= – 0.952, *P*<0.01; r= – 0.816, *P* < 0.01, respectively) and the mass of this weed (respectively r= – 0.820, *P*<0.01; r= – 0.834, *P*<0.01; r= – 0.720, *P*<0.05). At Dotnuva, in the treatments with a lower *Stellaria media* incidence the weed mass was most markedly reduced by crop height (r= – 0.701, *P*<0.05). The investigated crops gave a good suppression of the following annual weeds as well: *Veronica arvensis, Thlaspi arvense, Polygonum aviculare, Fallopia convolvulus.* At Joniskelis, the number and mass of *Fallopia convolvulus* was significantly reduced as productive density of crops increased (r= – 0.759, *P*<0,05; r= – 0.930, *P*<0.01, respectively). The number of these climbing weeds was also significantly reduced by the height and mass of crops (r= – 0.818, *P*<0.01; r= – 0.799, *P*<0.01, respectively).

All crops competed well with *Veronica arvensis.* The number of this weed was significantly reduced by the height and mass of the crop (r= – 0.862, *P*<0.01; r= – 0.681, *P*<0.05, respectively), but weed mass was reduced by the productive density and height of crops (r= – 0.789, *P*<0.05; r= – 0.695, *P* < 0.05, respectively). At Dotnuva, a consistent pattern was not determined due to lower incidence of *Fallopia convolvulus* and *Veronica arvensis.* For *Thlaspi arvense,* the findings at the Dotnuva site were similar to those at Joniskelis. The number and mass of *Thlaspi arvense* were significantly reduced by the productive density of crops (r= – 0.823, *P*<0.01; r= – 0.821, *P*<0.01, Dotnuva and r= – 0.821, *P*<0.01; r= –0.889, *P*<0.01, respectively at Joniskelis). The data of suppression are less consistent for *Galium aparine* and *Chenopodium album* which are harmful weed species in this region. At Dotnuva, in denser crops, *Chenopodium album* numbers and mass were significantly reduced by the productive density of crops (r= – 0.867, *P*<0.01; r= – 0.873, *P*<0.01, respectively). At Joniskelis, in thinner crops, productive density of crops significantly reduced only weed mass (r= – 0.783, *P*<0.05).

*Galium aparine* is a climbing weed; therefore, the spread of this weed was negatively influenced by increasing productive stem numbers and crop height (r= – 0.671, *P*<0.05; r= – 0.670, *P*<0.05, respectively). The data of perennial weeds showed that *Sonchus arvensis* was suppressed more than *Cirsium arvense.* At Joniskelis, the spread of *Sonchus arvensis* depended on the density (r= – 0.719, *P* < 0.05), height (r= – 0.814, *P*<0.01) and mass (r= – 0.754, *P*<0.01) of crops. Also, the mass of *Sonchus arvensis* decreased due to increasing productive density of crops (r= – 0.731, *P*<0.05). At Dotnuva, *Sonchus arvensis* spread less;

mass differed between the experimental sites. At Dotnuva, *Cirsium arvense* mass was reduced in the wheat and oat sole crops of higher density but in other investigated crops, we established only a trend towards weed reduction. At Joniskelis, the mass of this perennial weed increased in all investigated crops, particularly in the wheat and triticale sole crops

Statistical data analysis showed that productive stem density had the greatest effect on weed suppression, while the effect of crop height and mass had a lesser affecting both soil conditions. In loamy soil (Dotnuva), the total number and mass of weeds were significantly related to the productive density (r= – 0.922, *P*<0.01, r= – 0.909, *P*<0.01) within the range 109–478 stems m-2. In clay loam soil (Joniskelis), where productive crop density was lower (81–355 stems m-2), the total number of weeds were significantly reduced by the height of

Annual weed species had variable responses to the crop density, height and mass. At Joniskelis, the number of *Stellaria media* was significantly reduced with increasing productive density, height and mass of crops (r= – 0.685, *P*<0.05; r= – 0.952, *P*<0.01; r= – 0.816, *P* < 0.01, respectively) and the mass of this weed (respectively r= – 0.820, *P*<0.01; r= – 0.834, *P*<0.01; r= – 0.720, *P*<0.05). At Dotnuva, in the treatments with a lower *Stellaria media* incidence the weed mass was most markedly reduced by crop height (r= – 0.701, *P*<0.05). The investigated crops gave a good suppression of the following annual weeds as well: *Veronica arvensis, Thlaspi arvense, Polygonum aviculare, Fallopia convolvulus.* At Joniskelis, the number and mass of *Fallopia convolvulus* was significantly reduced as productive density of crops increased (r= – 0.759, *P*<0,05; r= – 0.930, *P*<0.01, respectively). The number of these climbing weeds was also significantly reduced by the height and mass of crops (r= – 0.818,

All crops competed well with *Veronica arvensis.* The number of this weed was significantly reduced by the height and mass of the crop (r= – 0.862, *P*<0.01; r= – 0.681, *P*<0.05, respectively), but weed mass was reduced by the productive density and height of crops (r= – 0.789, *P*<0.05; r= – 0.695, *P* < 0.05, respectively). At Dotnuva, a consistent pattern was not determined due to lower incidence of *Fallopia convolvulus* and *Veronica arvensis.* For *Thlaspi arvense,* the findings at the Dotnuva site were similar to those at Joniskelis. The number and mass of *Thlaspi arvense* were significantly reduced by the productive density of crops (r= – 0.823, *P*<0.01; r= – 0.821, *P*<0.01, Dotnuva and r= – 0.821, *P*<0.01; r= –0.889, *P*<0.01, respectively at Joniskelis). The data of suppression are less consistent for *Galium aparine* and *Chenopodium album* which are harmful weed species in this region. At Dotnuva, in denser crops, *Chenopodium album* numbers and mass were significantly reduced by the productive density of crops (r= – 0.867, *P*<0.01; r= – 0.873, *P*<0.01, respectively). At Joniskelis, in thinner crops, productive density of crops significantly reduced only weed mass (r= – 0.783, *P*<0.05). *Galium aparine* is a climbing weed; therefore, the spread of this weed was negatively influenced by increasing productive stem numbers and crop height (r= – 0.671, *P*<0.05; r= – 0.670, *P*<0.05, respectively). The data of perennial weeds showed that *Sonchus arvensis* was suppressed more than *Cirsium arvense.* At Joniskelis, the spread of *Sonchus arvensis* depended on the density (r= – 0.719, *P* < 0.05), height (r= – 0.814, *P*<0.01) and mass (r= – 0.754, *P*<0.01) of crops. Also, the mass of *Sonchus arvensis* decreased due to increasing productive density of crops (r= – 0.731, *P*<0.05). At Dotnuva, *Sonchus arvensis* spread less;

and their intercrops compared to pea sole crop.

crop (r= – 0.830, *P*<0.01).

*P*<0.01; r= – 0.799, *P*<0.01, respectively).

therefore, the relationship was determined only between the number of this weed and productive density of crops (r= – 0.670, *P*<0.05). At Dotnuva, a strong relationship (r= – 0.856, *P*<0.01) was established between *Cirsium arvense* numbers and productive density of crops.

The relationship established between the total number and mass of weeds and intercrop competitive ability indicators (aggressivity - Ac; competition rate - CRc) showed that, with increasing competition rate of intercrops, weed incidence declined. This relationship was determined only at Joniskelis where the productive density was lower, the variation rate of CRc was higher, and weed species diversity and numbers were increased. With changing competition rates (0.54–15.63), weed number and mass declined by a linear inverse relationship. The correlation was medium (r= – 0.551, *P*<0.05; r= – 0.5031, *P*<0.05, respectively).

Researchers from five countries: Denmark, United Kingdom, France, Germany and Italy investigated the influence of pea and barley intercrop sown at different ratios – 45 peas and 150 barley plants m-2, and 90 peas and 150 barley plants m-2 - on dry matter of weeds. The control of weeds was similar in sole barley and in intercrops, and no difference was established between the substitutive and the additive intercrops (Dibet et al., 2006). Researchers report the advantages of various intercropping managements such as pea with wheat against weeds (Szumigalski & van Acker, 2005), pea with barley (Hauggaard-Nielsen et al., 2006), and pea with oats (Rauber et al., 2001). Diversity of weeds was decreased in intercrops in comparison with sole crop (Gharineh & Moradi Telavat, 2009). Like cultivated plants, weeds obtain nutrients through root uptake from the soil solution. As a result, weeds and crops compete for space, nutrients*,* water and light. Both weeds and crop plants are similar in chemical composition; therefore, weeds can accumulate similar or even higher amounts of nutrients than crops *(*Lazauskas, 1990). Nitrogen increases weed and crop biomass (Kristensen et al., 2008). Peas use little nitrogen from the soil since they can fix atmospheric nitrogen for use. As a result, peas provide good conditions for weed growth, especially for high nitrogen demanding weed species. Dibet *et al*. (2006) reported a lower nitrogen concentration, 0.8 g m-2, in weed mass due to competition between weed and cereal in intercrops compared to pea sole crop.

Statistical analyses of sole crop and intercrop grain yields and weed numbers and variation are presented in Figure 1.

Strong, inversely proportional relationships were established between grain yield and total weed number, and between grain yield and weed number variation during the growing season. This means that the number of weeds and their variation conversely affected crop yield. These relationships were determined only in lower density crops in clay loam soil (Joniskelis) when weed incidence markedly increased. The analysis of the statistical data suggested that an increase in the total weed number by one weed (regardless of the species) resulted in a grain yield reduction by 27.3 kg ha-1 (Figure 1a). The investigated crop competition characteristics describe the relationship between crop yields and weed number variation during the growing period. The grain yield changed 18.8 kg ha-1 by an inverse trend when changing one weed (Figure 1b). The relationship between grain yield and total weed mass was not significant (r= – 0.564, *P*>0.05). This relationship could determine perennial weed mass, which was especially high and the weeds were spread unevenly. It can be maintained that, in clay loam soil (Joniskelis), the majority of the investigated crops

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 21

pre-crops. Weed density decreased by 23.3 m-2 in wheat grown after pea sole crop, but weed numbers increased on average by 3.3 and 8.6 m-2, respectively, after intercrops and cereal sole crops compared to the 2009 spring period. Different pre-crops did not have any

> Intercrop and sole crop 2009

Ps 35.3 19.3 Ps 70.0 46.7 171.3 42.8 P+SWi 26.0 20.0 P+Oi 51.3\* 62.0 159.3 39.8 P+SBi 22.7 32.7 P+STi 58.0 58.0 171.3 42.8 P+Oi 34.7 24.7 P+SWi 54.0 66.0 179.3 44.8 P+STi 26.0 34.7 P+SBi 54.7 45.3 160.7 40.2 SWs 27.3 27.3 Os 35.3\*\* 47.3 137.3\*\* 34.3 SBs 34.7 28.7 STs 52.7\* 52.7 168.7 42.2 Os 24.0 28.0 SWs 32.7\*\* 47.3 132.0\*\* 33.0\* STs 27.3 32.7 SBs 46.0\*\* 54.0 160.0 40.0 Mean 28.7 27.6 Mean 50.5 53.3 160.0 40.0 intercrop 27.4 28.0 intercrop 54.5 57.8 167.7 41.9 sole crop 28.3 29.2 sole crop 41.7 50.3 149.5 37.4

crop weed m-2 crop weed m-2

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop:

Table 10. The dynamics of weed germination in the crop rotation with cereal sole crop and

Over a four-year period, the total number of weeds ranged from 132.0 to 179.3 m-2. In the crop rotation with intercrops, the weed germination was 12.2% or 18.2 weeds m-2, higher compared to the crop rotation with cereal sole crops. Significantly lower weed germination was seen in the four-course crop rotation, including oat sole crop: Os – SWs –SWs – WWs; SWs – SWs – Os – WWs, compared to the rotation including pea sole crop (Ps – SWs – Ps –

The number of weeds and their variation during cereal maturity stage (BBCH 73) are presented in Table 11. During the first experimental year (2007), the intercrops and sole crops were competitive and gave a good suppression of weeds. The number of weeds decreased by 12-28 m-2 compared to that in spring. According to the averaged data, the intercrops and sole crops did not differ markedly in their ability to suppress weeds. The number of weeds decreased by 20.9-79.1% in cereal sole crops and intercrops compared to pea sole crop. Significantly lower numbers of weeds were found in wheat and oat intercrops and oat sole crops during the cereal maturity stage. At the cereal maturity stage, the number of weeds was on average 4.3 times higher in spring wheat (2008) grown after various cereal

P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

Rotation Total

Winter wheat (WWs) 2010

number over crop rotation

Averaged number over the year

significant influence on weed density in winter wheat.

Spring wheat (SWs) 2008

Intercrop and sole crop 2007

intercrop, 2007-2010, Joniskelis

WWs).

were not strongly competitive and, during maturity stage, there remained 71.3-49.2 weeds m-2 in crops which had a negative impact on grain yield. Oat sole crop and intercropped with pea markedly differed from other investigated crops in that 19.6 and 25.8 weeds m-2 remained during the maturity stage. At Dotnuva, this relationship was not determined. The number of weeds decreased by 54.7-27.0 m-2 and such weed incidence had no significant negative effect on the crop productivity. This shows that sustainable plant communities are formed under organic farming conditions.

Fig. 1. The correlation between grain yield of sole crop, intercrop and total number of weeds (a), and their variation during grain-filling stage (BBCH 73) (b),Joniskelis 2007-2010

#### **2.2.4 The effect of intercrops on subsequent crops in a crop rotation**

During 2007-2010, studies were conducted at Joniskelis to assess the incidence of weeds in the intercropping system. The studies were set up during the transitional period from conventional to organic farming. The dynamics of weed germination (at crop growth stage BBCH 32-36) in the crop rotation is summarized in Table 10. In the spring of 2007, the highest number of weeds was in the pea sole crop. In various spring cereals and their intercrops with pea, weed density decreased by 1.7-35.7% compared to pea sole crop.

The averaged data suggested that the number of weeds in intercrops was slightly lower compared to cereal sole crops. After various sole cereal crops and intercrops with pea, as pre-crop to spring wheat (2008), the number of weeds in spring wheat was similar to that in the pre-crop (2007); however, in spring wheat grown after pea, weed numbers were the lowest. In spring wheat grown after various cereal sole crops and their intercrops with pea, the number of weeds increased by 3.6-69.4% compared with spring wheat grown after pea sole crop. In the third year (2009), under an organic cropping system, weed density significantly increased in sole cereal crops and their intercrops with pea. Weed germination was 1.8 times higher than that in 2007 or 2008. The number of weeds in pea sole crops increased by 50.7 m-2 on average, with an increase of 26.5 m-2 in intercrops and 12.5 m-2 in cereal sole crop compared to the 2008 spring period. The number of weeds was significantly (P<0.01) influenced by crops. Like in 2007, the highest number of weeds was in pea sole crop. The number of weeds decreased by 17.1-53.3% in all other crops tested. Weed germination was significantly lower in oat intercrop and cereal sole crops. The variation of total weed numbers in 2010 was low in winter wheat grown after pea, cereal sole crops and intercrops compared to that in 2009. However, weed germination depended on different

were not strongly competitive and, during maturity stage, there remained 71.3-49.2 weeds m-2 in crops which had a negative impact on grain yield. Oat sole crop and intercropped with pea markedly differed from other investigated crops in that 19.6 and 25.8 weeds m-2 remained during the maturity stage. At Dotnuva, this relationship was not determined. The number of weeds decreased by 54.7-27.0 m-2 and such weed incidence had no significant negative effect on the crop productivity. This shows that sustainable plant communities are

Fig. 1. The correlation between grain yield of sole crop, intercrop and total number of weeds

During 2007-2010, studies were conducted at Joniskelis to assess the incidence of weeds in the intercropping system. The studies were set up during the transitional period from conventional to organic farming. The dynamics of weed germination (at crop growth stage BBCH 32-36) in the crop rotation is summarized in Table 10. In the spring of 2007, the highest number of weeds was in the pea sole crop. In various spring cereals and their intercrops with pea, weed density decreased by 1.7-35.7% compared to pea sole crop.

The averaged data suggested that the number of weeds in intercrops was slightly lower compared to cereal sole crops. After various sole cereal crops and intercrops with pea, as pre-crop to spring wheat (2008), the number of weeds in spring wheat was similar to that in the pre-crop (2007); however, in spring wheat grown after pea, weed numbers were the lowest. In spring wheat grown after various cereal sole crops and their intercrops with pea, the number of weeds increased by 3.6-69.4% compared with spring wheat grown after pea sole crop. In the third year (2009), under an organic cropping system, weed density significantly increased in sole cereal crops and their intercrops with pea. Weed germination was 1.8 times higher than that in 2007 or 2008. The number of weeds in pea sole crops increased by 50.7 m-2 on average, with an increase of 26.5 m-2 in intercrops and 12.5 m-2 in cereal sole crop compared to the 2008 spring period. The number of weeds was significantly (P<0.01) influenced by crops. Like in 2007, the highest number of weeds was in pea sole crop. The number of weeds decreased by 17.1-53.3% in all other crops tested. Weed germination was significantly lower in oat intercrop and cereal sole crops. The variation of total weed numbers in 2010 was low in winter wheat grown after pea, cereal sole crops and intercrops compared to that in 2009. However, weed germination depended on different

(a), and their variation during grain-filling stage (BBCH 73) (b),Joniskelis 2007-2010

y = 3007.2-18.8x r=-0.9210; P>0.01

Grain yield, kg ha-1


Reduction of weed number m-2

formed under organic farming conditions.

0 10 20 30 40 50 60 70 80

Number of weed m-2

**2.2.4 The effect of intercrops on subsequent crops in a crop rotation** 

y = 4658.4-27.3x r=-0.931; P<0.01

Grain yield, kg ha-1

a)

pre-crops. Weed density decreased by 23.3 m-2 in wheat grown after pea sole crop, but weed numbers increased on average by 3.3 and 8.6 m-2, respectively, after intercrops and cereal sole crops compared to the 2009 spring period. Different pre-crops did not have any significant influence on weed density in winter wheat.


Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

Table 10. The dynamics of weed germination in the crop rotation with cereal sole crop and intercrop, 2007-2010, Joniskelis

Over a four-year period, the total number of weeds ranged from 132.0 to 179.3 m-2. In the crop rotation with intercrops, the weed germination was 12.2% or 18.2 weeds m-2, higher compared to the crop rotation with cereal sole crops. Significantly lower weed germination was seen in the four-course crop rotation, including oat sole crop: Os – SWs –SWs – WWs; SWs – SWs – Os – WWs, compared to the rotation including pea sole crop (Ps – SWs – Ps – WWs).

The number of weeds and their variation during cereal maturity stage (BBCH 73) are presented in Table 11. During the first experimental year (2007), the intercrops and sole crops were competitive and gave a good suppression of weeds. The number of weeds decreased by 12-28 m-2 compared to that in spring. According to the averaged data, the intercrops and sole crops did not differ markedly in their ability to suppress weeds. The number of weeds decreased by 20.9-79.1% in cereal sole crops and intercrops compared to pea sole crop. Significantly lower numbers of weeds were found in wheat and oat intercrops and oat sole crops during the cereal maturity stage. At the cereal maturity stage, the number of weeds was on average 4.3 times higher in spring wheat (2008) grown after various cereal

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 23

In 2010, at the winter wheat maturity stage, weed numbers differed little from that in 2009. However, compared with the spring period, weed numbers increased an average of 1.7 times. Averaged data indicate that the greatest increase in weed numbers occurred in winter wheat grown after pea; a smaller increase occurred after intercrops and sole crops. Compared with pea, significantly lower weed numbers were determined in winter wheat

Various crop rotations had significant effects (*P*<0.01) on the total weed number during cereal maturation stage. The highest number of weeds over a four-year period was established in the crop rotation with pea (Ps – SWs – Ps – WWs). Inclusion of semi-leafless pea in the crop rotation tended to increase crop weed incidence. In all other crop rotations with sole cereal crops or pea/cereal intercrops, the total weed incidence significantly declined by 10.7-43.7% (except for the crop rotation: P+SBi – SWs – P+STi - WWs), compared with the crop rotation with pea (Ps – SWs – Ps – WWs). Averaged over one year, significantly lower weed incidence was in the following crop rotations: P+SWi – SWs – P+Oi – WWs; SWs – SWs – Os – WWs and Os – SWs – SWs – WWs, compared with the crop

Weed mass variation in different crops at cereal maturation stage (BBCH 73) is presented in Table 12. For the first year (2007) in intercrops and sole cereal crops, the weed incidence was low; consequently, their mass was low. Compared with pea crop, in all intercrops and sole cereal crops weed mass was significantly lower (61.1-97.3%). The lowest weed mass was recorded in oat and its intercrops with pea. The mass per weed varied in a similar way (except for pea/triticale crop). Averaged data suggest that higher total weed mass and mass

In the next year (2008), in spring wheat crop grown after different pre-crops, weed mass increased by 1.9 times. Different pre-crops exerted varying effects. When spring wheat had been grown after pea, the total weed mass declined by 2.0 times; after intercrops, it increased by 3.0 times and after sole cereals it increasedby 3.7 times, compared with respective weed mass in 2007. Pea/barley and pea/triticale intercrops tended to increase weed mass in spring wheat, compared with pea pre-crop. Other crops, as pre-crops, reduced weed mass. Averaged data indicate that the highest mass per weed was in spring wheat grown after pea sole crop; weed mass was lower after intercrops and it was the lowest after sole cereal crops. Pea/oat intercrop and sole spring wheat crop as pre-crops significantly reduced mass per weed compared with pea as pre-crop. Reduction of mass per weed decreased viability and number of mature seeds (Lazauskas, 1990; Liebman & Davis, 2000). In the third year of the crop rotation, when growing various species of cereals and their intercrops with pea, the total weed number increased by an average of 3 times, compared with the average total weed mass in 2008, or by 5.6 times, compared with 2007. Many of the tested crops significantly reduced weed mass by 70.4-96.3% (except for pea/triticale and triticale crops) compared with sole pea crop. Significantly lower mass per weed was determined in sole cereal crops (except for triticale) and pea/barley crops, compared with

In the final year of the experiment (2010), in the winter wheat crop, total weed mass increased by an average of 29.6% compared with that in 2009. After different pre-crops, total weed mass was variable. In winter wheat grown after pea, total weed mass declined by

crop grown after pea/oat intercrop.

per weed was in intercrops, compared with sole crops.

rotation including pea.

pea crop.

sole crops and intercrops compared to the corresponding period in 2007. Although in 2008 weed emergence was similar to that in 2007, the number of weeds in spring wheat increased by an average 1.7 times compared to that during the spring period.

Averaged data showed that in the wheat crop grown after intercrops, the number of weeds increased by 20 m-2, by 17.9 m-2 after sole crops and by 24.7 m-2after pea, compared with the respective data in spring. Comparison of the effects of various pre-crops on weed incidence in spring wheat showed that the number of weeds declined in pea/oat, sole barely and oat crops (by 6.1, 19.8 and 15.2%, respectively), compared with pea sole pre-crop. However, these differences were not significant.

In 2009, the number of weeds further increased. In sole cereal crops and intercrops, during the cereal maturity stage, the number of weeds was 97.1 m-2, which was 2 times higherthan during the same period in 2008, and 1.9 times more than in spring (2009). Compared with the spring period, weed numbers in pea crop increased by 76 m-2, in intercrops by 49.3-54.0 m-2 (except for pea/oat crop) and in sole cereal crops by 53.3-77.3 m-2 (except for oat crop). Weed numbers declined in pea/oat and sole oat crops by 2.0 and 6.6 m-2 or 3.9 and 18.7% respectively, compared with the respective data in spring. All intercrops and cereal sole crops significantly decreased weed numbers by 15.5-80.3%, except for the sole triticale crop, compared with pea crop. Averaged data suggest that sole cereal crops suppressed weeds slightly more than intercrops. The lowest weed incidence was recorded in pea/oat and sole oat crops.


Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

Table 11. The variation of weed numbers in the crop rotation with intercrops and sole crops at cereal maturity stage, 2007-2010, Joniskelis

sole crops and intercrops compared to the corresponding period in 2007. Although in 2008 weed emergence was similar to that in 2007, the number of weeds in spring wheat increased

Averaged data showed that in the wheat crop grown after intercrops, the number of weeds increased by 20 m-2, by 17.9 m-2 after sole crops and by 24.7 m-2after pea, compared with the respective data in spring. Comparison of the effects of various pre-crops on weed incidence in spring wheat showed that the number of weeds declined in pea/oat, sole barely and oat crops (by 6.1, 19.8 and 15.2%, respectively), compared with pea sole pre-crop. However,

In 2009, the number of weeds further increased. In sole cereal crops and intercrops, during the cereal maturity stage, the number of weeds was 97.1 m-2, which was 2 times higherthan during the same period in 2008, and 1.9 times more than in spring (2009). Compared with the spring period, weed numbers in pea crop increased by 76 m-2, in intercrops by 49.3-54.0 m-2 (except for pea/oat crop) and in sole cereal crops by 53.3-77.3 m-2 (except for oat crop). Weed numbers declined in pea/oat and sole oat crops by 2.0 and 6.6 m-2 or 3.9 and 18.7% respectively, compared with the respective data in spring. All intercrops and cereal sole crops significantly decreased weed numbers by 15.5-80.3%, except for the sole triticale crop, compared with pea crop. Averaged data suggest that sole cereal crops suppressed weeds slightly more than

> Intercrop and sole crop 2009

Ps 17.7 44.0 Ps 146.0 91.3 299.0 74.8 P+SWi 8.7\* 51.3 P+Oi 49.3\*\* 66.7\*\* 176.0\*\* 44.0\*\* P+SBi 10.7 53.7 P+STi 112.0\* 110.0 286.3 71.6 P+Oi 6.7\*\* 41.3 P+SWi 103.3\* 112.7 264.0\* 66.0 P+STi 13.3 45.7 P+SBi 108.7\* 88.7 256.3\*\* 64.1 SWs 11.0 58.7 Os 28.7\*\* 70.0 168.3\*\* 42.1\*\* SBs 14.0 35.3 STs 106.0\* 102.0 257.3\*\* 64.3 Os 3.7\*\* 37.3 SWs 96.7\*\* 80.7 218.3\*\* 54.6\* STs 10.0 45.0 SBs 123.3 88.7 267.0\* 66.8 Mean 10.6 45.8 Mean 91.7 90.1 243.6 60.9 Intercrop 9.9 48 intercrop 93.3 94.5 245.7 61.4 sole crop 9.7 44.1 sole crop 88.7 85.4 227.8 56.9

Rotation Total weed

Winter wheat (WWs) 2010

number during crop rotation Averaged number over the year

intercrops. The lowest weed incidence was recorded in pea/oat and sole oat crops.

Crop weed m-2 crop weed m-2

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop:

Table 11. The variation of weed numbers in the crop rotation with intercrops and sole crops

P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

at cereal maturity stage, 2007-2010, Joniskelis

Spring wheat (SWs) 2008

by an average 1.7 times compared to that during the spring period.

these differences were not significant.

Intercrop and sole crop 2007

In 2010, at the winter wheat maturity stage, weed numbers differed little from that in 2009. However, compared with the spring period, weed numbers increased an average of 1.7 times. Averaged data indicate that the greatest increase in weed numbers occurred in winter wheat grown after pea; a smaller increase occurred after intercrops and sole crops. Compared with pea, significantly lower weed numbers were determined in winter wheat crop grown after pea/oat intercrop.

Various crop rotations had significant effects (*P*<0.01) on the total weed number during cereal maturation stage. The highest number of weeds over a four-year period was established in the crop rotation with pea (Ps – SWs – Ps – WWs). Inclusion of semi-leafless pea in the crop rotation tended to increase crop weed incidence. In all other crop rotations with sole cereal crops or pea/cereal intercrops, the total weed incidence significantly declined by 10.7-43.7% (except for the crop rotation: P+SBi – SWs – P+STi - WWs), compared with the crop rotation with pea (Ps – SWs – Ps – WWs). Averaged over one year, significantly lower weed incidence was in the following crop rotations: P+SWi – SWs – P+Oi – WWs; SWs – SWs – Os – WWs and Os – SWs – SWs – WWs, compared with the crop rotation including pea.

Weed mass variation in different crops at cereal maturation stage (BBCH 73) is presented in Table 12. For the first year (2007) in intercrops and sole cereal crops, the weed incidence was low; consequently, their mass was low. Compared with pea crop, in all intercrops and sole cereal crops weed mass was significantly lower (61.1-97.3%). The lowest weed mass was recorded in oat and its intercrops with pea. The mass per weed varied in a similar way (except for pea/triticale crop). Averaged data suggest that higher total weed mass and mass per weed was in intercrops, compared with sole crops.

In the next year (2008), in spring wheat crop grown after different pre-crops, weed mass increased by 1.9 times. Different pre-crops exerted varying effects. When spring wheat had been grown after pea, the total weed mass declined by 2.0 times; after intercrops, it increased by 3.0 times and after sole cereals it increasedby 3.7 times, compared with respective weed mass in 2007. Pea/barley and pea/triticale intercrops tended to increase weed mass in spring wheat, compared with pea pre-crop. Other crops, as pre-crops, reduced weed mass. Averaged data indicate that the highest mass per weed was in spring wheat grown after pea sole crop; weed mass was lower after intercrops and it was the lowest after sole cereal crops. Pea/oat intercrop and sole spring wheat crop as pre-crops significantly reduced mass per weed compared with pea as pre-crop. Reduction of mass per weed decreased viability and number of mature seeds (Lazauskas, 1990; Liebman & Davis, 2000).

In the third year of the crop rotation, when growing various species of cereals and their intercrops with pea, the total weed number increased by an average of 3 times, compared with the average total weed mass in 2008, or by 5.6 times, compared with 2007. Many of the tested crops significantly reduced weed mass by 70.4-96.3% (except for pea/triticale and triticale crops) compared with sole pea crop. Significantly lower mass per weed was determined in sole cereal crops (except for triticale) and pea/barley crops, compared with pea crop.

In the final year of the experiment (2010), in the winter wheat crop, total weed mass increased by an average of 29.6% compared with that in 2009. After different pre-crops, total weed mass was variable. In winter wheat grown after pea, total weed mass declined by

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 25

and the total weed mass by 16.51 g m-2 higher and 63.31 g m-2 lower, compared with the crop rotations with sole cereals or pea. Averaged data indicate that for any one year, significantly fewer weeds were in the crop rotation including oat or its intercrop with pea. Literature provides data on allelopathy effects against weeds due to direct or indirect release of chemicals from live or dead plants (including microorganisms) (Bhadoria, 2011). The effect of sole oat crop against weeds was longer-lasting than that of pea/oat intercrop. This finding is consistent with other researchers' evidence suggesting that the sequence of oat/pea intercrop harvested for forage followed by winter wheat will suppress warm-

The weed suppression effect of intercrops verses sole crops markedly differed during the plant growing period. Competitive abilities of crops were determined by plant productive density, height, mass, index of aggressiveness of cereals (Ac), and competition rate (CRc). More stable productive densities of intercrops were obtained in a loam soil (286-346 stems m-2) compared to a clay loam soil (211-275 stems m-2). Productive stem density of pea in crop structure was similar. According to plant height, spring cereals ranked as follows: oat > triticale > wheat > barley. Pea plants were the shortest and their height and mass tended to decline in intercrops. In intercrops, cereals had greater influence on weed suppression than

During the crop growing season, sole cereals and pea/cereal intercrops provided better weed suppression than pea (semileafless pea cultivars). At Dotnuva, in denser crop densities, the total weed numbers during the maturity stage declined by 12.4-28.8 weeds m-2 compared with that in spring;in pea crops, the reduction amounted to 3.1 weeds m-2. At Joniskelis, in the crops with a lower population density, the effect on weeds was lower. At lower crop population densities, weed suppression differences between sole crops and intercrops were inappreciable. At both experimental sites, the best weed suppression was exhibited by sole oat crop and its intercrop with pea; total weed numbers during the maturation stage declined by 24.4–28.8 weeds m-2 and 21.0–30.1 weeds m-2, respectively, compared with the spring period. At Dotnuva, all crops significantly reduced weed mass by 72.0-90.7%, compared with pea crop. At Joniskelis, due to higher and uneven incidence of *Cirsium arvense,* the variation of weed mass was less consistent. According to increasing total weed mass, the crops ranked as follows: cereals < intercrops < pea. The lowest weed mass

The variation of total weed numbers and mass was influenced by weed species. With low incidence of *Viola arvensis*, *Veronica arvensis, Thlaspi arvense* and more abundant counts of *Galium aparine,* all crops tended to reduce weed numbers compared with the spring period. With higher incidence of *Viola arvensis*, *Veronica arvensis* and *Galium aparine* (Joniskelis), their number (for *Galium aparine* also mass) decreased most in pea / oat and oat crops, compared with pea crop. With higher incidence of *Fallopia convolvulus*, its numbers were reduced only by pea/oat crop, while other crops increased its number compared with the spring period. However, *Fallopia convolvulus*, *Viola arvensis*, and *Veronica arvensis* mass significantly declined compared with that in pea crop. Sole cereal crops gave a better suppression of *Stellaria media* compared with intercrops. When the incidence of this weed was high, all crops significantly reduced its mass, compared with pea crop. In spring, when the incidence

season weeds during the 2-year interval (Anderson, 2010).

**3. Conclusions** 

was identified in sole oat crop.

pea.


34.3%, after intercrops and sole crops it increased by 23.7 and 94.3%, respectively, compared with the respective data in 2009. Significantly lower total weed mass and mass per weed in winter wheat was recorded when it was grown after pea/oat and sole oat crops.

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop: P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

Table 12. The variation of weed mass in the crop rotation with intercrops and sole crops during cereal maturity stage, 2007-2010, Joniskelis

Summarised data show that in cultivated heavy loam *Cambisol*, during the transition period from an input-intensive to an organic cropping system, weeds emerged more intensively in the third and fourth years of the crop rotation. Averaged data evidence that, in the crop rotations with sole cereal crops and intercrops, weed numbers annually increased. In the crop rotation with pea, the pea promoted weed emergence; pea as pre-crop effect on wheat reduced weed emergence. During the four year, significantly fewer weeds emerged in the crop rotation with sole oat crop.

During the cereal maturity stage, weed numbers and mass were more markedly influenced by sole cereal crops and their intercrops with pea compared with their effect as pre-crops. In the first year, compared with the spring period, weed numbers during the growing season markedly declined; over the following years, weeds were suppressed less and their numbers increased. Cereal sole crops and intercrops had a greater suppression of weeds during the growing season; therefore, their numbers per rotation (except for the crop rotation P+SBi – SWs – P+STi + WWs) and mass (also except for the crop rotation P+SBi – SWs – P+STi – WWs and SBi – SWs –STi – WWs) were significantly lower compared with the crop rotation with pea. Over the four years, during the cereal maturity stage in the crop rotations with intercrops, the total number of weeds was an average of 17.9 m-2 higher and 55.3 m-2 lower, and the total weed mass by 16.51 g m-2 higher and 63.31 g m-2 lower, compared with the crop rotations with sole cereals or pea. Averaged data indicate that for any one year, significantly fewer weeds were in the crop rotation including oat or its intercrop with pea. Literature provides data on allelopathy effects against weeds due to direct or indirect release of chemicals from live or dead plants (including microorganisms) (Bhadoria, 2011). The effect of sole oat crop against weeds was longer-lasting than that of pea/oat intercrop. This finding is consistent with other researchers' evidence suggesting that the sequence of oat/pea intercrop harvested for forage followed by winter wheat will suppress warmseason weeds during the 2-year interval (Anderson, 2010).

#### **3. Conclusions**

24 Weed Control

34.3%, after intercrops and sole crops it increased by 23.7 and 94.3%, respectively, compared with the respective data in 2009. Significantly lower total weed mass and mass per weed in

crop

Ps 23.65 1.338 12.11 0.275 Ps 67.27 0.461 44.17 0.484 147.2 P+SWi 1.77\*\* 0.204\*\* 10.02 0.195 P+Oi 12.44\*\* 0.252 16.51\*\* 0.248\* 40.74\*\* P+SBi 3.21\*\* 0.301\*\* 13.58 0.253 P+STi 71.80 0.641 41.10 0.374 129.69 P+Oi 1.23\*\* 0.184\*\* 7.12\* 0.172\* P+SWi 19.93\*\* 0.193 60.33 0.535 88.61\* P+STi 9.19\*\* 0.689 14.73 0.322 P+SBi 18.61\*\* 0.171\* 33.97 0.383 76.5\*\* SWs 2.71\*\* 0.247\*\* 6.66\* 0.113\* Os 2.50\*\* 0.087\*\* 13.72\*\* 0.196\*\* 25.59\*\* SBs 2.57\*\* 0.184\*\* 7.63 0.216 STs 51.03 0.481 52.25 0.512 113.48 Os 0.63\*\* 0.171\*\* 8.80 0.236 SWs 9.05\*\* 0.094\* 37.06 0.459 55.54\*\* STs 2.91\*\* 0.291\*\* 9.22 0.205 SBs 15.01\*\* 0.122\* 47.78 0.539 74.92\*\* Mean 5.32 0.401 9.99 0.221 Mean 29.74 0.278 38.54 0.414 83.59 intercrop 3.85 0.345 11.36 0.236 intercrop 30.70 0.314 37.98 0.385 83.89 sole crop 2.21 0.223 8.08 0.193 sole crop 19.40 0.196 37.70 0.427 67.38

Rotation Total

single weed (g)

weed mass

total (g m-2)

Winter wheat (WWs), 2010

> single weed (g)

Intercrop and sole crop 2009

> Total (g m-2)

weed mass over crop rotation (g )

winter wheat was recorded when it was grown after pea/oat and sole oat crops.

single weed (g)

Note. \*differences are statistically significant as compared to the control at *P*<0.05, \*\*-at *P*<0.01 Sole crop: Ps –pea, SWs – spring wheat, SBs – spring barley, Os –oat, STs – spring triticale; intercrop:

Table 12. The variation of weed mass in the crop rotation with intercrops and sole crops

Summarised data show that in cultivated heavy loam *Cambisol*, during the transition period from an input-intensive to an organic cropping system, weeds emerged more intensively in the third and fourth years of the crop rotation. Averaged data evidence that, in the crop rotations with sole cereal crops and intercrops, weed numbers annually increased. In the crop rotation with pea, the pea promoted weed emergence; pea as pre-crop effect on wheat reduced weed emergence. During the four year, significantly fewer weeds emerged in the

During the cereal maturity stage, weed numbers and mass were more markedly influenced by sole cereal crops and their intercrops with pea compared with their effect as pre-crops. In the first year, compared with the spring period, weed numbers during the growing season markedly declined; over the following years, weeds were suppressed less and their numbers increased. Cereal sole crops and intercrops had a greater suppression of weeds during the growing season; therefore, their numbers per rotation (except for the crop rotation P+SBi – SWs – P+STi + WWs) and mass (also except for the crop rotation P+SBi – SWs – P+STi – WWs and SBi – SWs –STi – WWs) were significantly lower compared with the crop rotation with pea. Over the four years, during the cereal maturity stage in the crop rotations with intercrops, the total number of weeds was an average of 17.9 m-2 higher and 55.3 m-2 lower,

P+SWi – pea and spring wheat, P+SBi – pea and spring barley; P+Oi – pea and oat

Spring wheat (SWs) 2008

weed mass

during cereal maturity stage, 2007-2010, Joniskelis

crop rotation with sole oat crop.

total (g m-2)

single weed (g)

Intercrop and sole crop 2007

> total (g m-2)

crop

The weed suppression effect of intercrops verses sole crops markedly differed during the plant growing period. Competitive abilities of crops were determined by plant productive density, height, mass, index of aggressiveness of cereals (Ac), and competition rate (CRc). More stable productive densities of intercrops were obtained in a loam soil (286-346 stems m-2) compared to a clay loam soil (211-275 stems m-2). Productive stem density of pea in crop structure was similar. According to plant height, spring cereals ranked as follows: oat > triticale > wheat > barley. Pea plants were the shortest and their height and mass tended to decline in intercrops. In intercrops, cereals had greater influence on weed suppression than pea.

During the crop growing season, sole cereals and pea/cereal intercrops provided better weed suppression than pea (semileafless pea cultivars). At Dotnuva, in denser crop densities, the total weed numbers during the maturity stage declined by 12.4-28.8 weeds m-2 compared with that in spring;in pea crops, the reduction amounted to 3.1 weeds m-2. At Joniskelis, in the crops with a lower population density, the effect on weeds was lower. At lower crop population densities, weed suppression differences between sole crops and intercrops were inappreciable. At both experimental sites, the best weed suppression was exhibited by sole oat crop and its intercrop with pea; total weed numbers during the maturation stage declined by 24.4–28.8 weeds m-2 and 21.0–30.1 weeds m-2, respectively, compared with the spring period. At Dotnuva, all crops significantly reduced weed mass by 72.0-90.7%, compared with pea crop. At Joniskelis, due to higher and uneven incidence of *Cirsium arvense,* the variation of weed mass was less consistent. According to increasing total weed mass, the crops ranked as follows: cereals < intercrops < pea. The lowest weed mass was identified in sole oat crop.

The variation of total weed numbers and mass was influenced by weed species. With low incidence of *Viola arvensis*, *Veronica arvensis, Thlaspi arvense* and more abundant counts of *Galium aparine,* all crops tended to reduce weed numbers compared with the spring period. With higher incidence of *Viola arvensis*, *Veronica arvensis* and *Galium aparine* (Joniskelis), their number (for *Galium aparine* also mass) decreased most in pea / oat and oat crops, compared with pea crop. With higher incidence of *Fallopia convolvulus*, its numbers were reduced only by pea/oat crop, while other crops increased its number compared with the spring period. However, *Fallopia convolvulus*, *Viola arvensis*, and *Veronica arvensis* mass significantly declined compared with that in pea crop. Sole cereal crops gave a better suppression of *Stellaria media* compared with intercrops. When the incidence of this weed was high, all crops significantly reduced its mass, compared with pea crop. In spring, when the incidence

Intercropping of Pea and Spring Cereals for Weed Control in an Organic Farming System 27

Andersen, M. K.; Hauggaard-Nielsen, H.; Weiner, J. & Jensen, E. S. (2007) Competitive

Anderson, R.L.A. (2010) Rotation design to reduce weed density in organic farming. *Renewable Agriculture and Food Systems,* Vol. 25, No. 3, pp. 189–195, ISSN 1742-1705 Anil, L.; Park, R.; Phipps, R.H. & Miller, F.A. (1998) Temperate intercropping of cereals for

to the UK. *Grass and Forage Science,* Vol. 53, pp. 301-317, ISSN 1365-2494 Auskalniene, O. & Auskalnis, A. (2008) The influence of spring wheat plant density on weed

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of *Chenopodium album* was very high (32.8-41.9 m-2) at the Dotnuva site, the number and mass of this weed significantly declined in intercrops and cereal crops compared with pea crop. When the incidence of this weed was lower (4.2-8.5 m-2), only its mass declined more markedly compared with pea crop. At both experimental sites, sole cereal crops, particularly especially oat, reduced weed mass more appreciably than intercrops. Crops had the lowest suppressive effect on perennial weeds, *Sonchus arvensis and Cirsium arvense.* In many crops, the number and mass of these weeds increased. Slightly less sensitive to crop suppression, especially to oat and pea / oat intercrop, was *Sonchus arvensis.* An increase in crop productive density had a significant negative effect on the number and/or mass of many weed species. For many climbing weed species, *Galium aparine* and *Fallopia convolvulus*, an increase in crop height significantly reduced their density. Short-growing weeds *Veronica arvensis Stellaria media* responded negatively to many competitive properties of crops.

The greatest negative effect on crop grain yield (2896.6–4270.4 kg ha-1) in a clay loam soil (Joniskelis) was exerted by weed numbers during crop maturation stage and its variation during the crop growing season. With a simultaneous increase in the number of these weeds (19.6-71.3 m-2 range), the yield of the crops tested statistically declined by 27.3 kg ha-1. It was calculated that during the crop growing season, with one suppressed weed, grain yield increased by 18.8 kg ha-1. In loamy soil (Dotnuva), the remaining number of weeds (27.0-54.7 weed m-2) during cereal maturity stage did not have any significant effect on crop yield (2555.4-3002.9 kg ha-1).

On a cultivated, heavy loam *Cambisol*, during the transition period from an intensive to an organic cropping system, the highest number of weeds emerged and persisted through the growing season in the third and fourth years of crop rotation. During the cereal maturity stage, sole cereal crops and their intercrops with peas had the greatest impact on weed numbers and mass, compared with their effect as pre-crops. Sole cereal crops and intercrops suppressed weeds during the growing season; therefore, in many crop rotations weed numbers and mass were significantly lower compared with a crop rotation with pea. Over a four-year period, during the maturity stage of cereals, in the crop rotations with intercrops, the total number of weeds was an average of 17.9 m-2 higher and 55.3 m-2 lower, and the total mass by 16.51 g m-2 higher and 63.31 g m-2 lower, compared with the respective crop rotations with sole cereal crops or pea. Averaged data showed, that during one year, significantly lower numbers of weeds were in the crop rotation with oat or its intercrop with pea.

#### **4. Acknowledgment**

The study has been supported by the Lithuanian Ministry of Agriculture and the Lithuanian Academy of Science.

#### **5. References**

Andersen, M. K.; Hauggaard-Nielsen, H.; Ambus, P. & Jensen, E. S. (2005) Biomass production, symbiotic nitrogen fixation and inorganic N use in dual and tricomponent annual intercrops, *Plant and Soil*, Vol. 266, pp. 273–287, ISSN 0032-079X

of *Chenopodium album* was very high (32.8-41.9 m-2) at the Dotnuva site, the number and mass of this weed significantly declined in intercrops and cereal crops compared with pea crop. When the incidence of this weed was lower (4.2-8.5 m-2), only its mass declined more markedly compared with pea crop. At both experimental sites, sole cereal crops, particularly especially oat, reduced weed mass more appreciably than intercrops. Crops had the lowest suppressive effect on perennial weeds, *Sonchus arvensis and Cirsium arvense.* In many crops, the number and mass of these weeds increased. Slightly less sensitive to crop suppression, especially to oat and pea / oat intercrop, was *Sonchus arvensis.* An increase in crop productive density had a significant negative effect on the number and/or mass of many weed species. For many climbing weed species, *Galium aparine* and *Fallopia convolvulus*, an increase in crop height significantly reduced their density. Short-growing weeds *Veronica arvensis Stellaria media* responded negatively to many competitive properties of crops.

The greatest negative effect on crop grain yield (2896.6–4270.4 kg ha-1) in a clay loam soil (Joniskelis) was exerted by weed numbers during crop maturation stage and its variation during the crop growing season. With a simultaneous increase in the number of these weeds (19.6-71.3 m-2 range), the yield of the crops tested statistically declined by 27.3 kg ha-1. It was calculated that during the crop growing season, with one suppressed weed, grain yield increased by 18.8 kg ha-1. In loamy soil (Dotnuva), the remaining number of weeds (27.0-54.7 weed m-2) during cereal maturity stage did not have any significant effect on crop yield

On a cultivated, heavy loam *Cambisol*, during the transition period from an intensive to an organic cropping system, the highest number of weeds emerged and persisted through the growing season in the third and fourth years of crop rotation. During the cereal maturity stage, sole cereal crops and their intercrops with peas had the greatest impact on weed numbers and mass, compared with their effect as pre-crops. Sole cereal crops and intercrops suppressed weeds during the growing season; therefore, in many crop rotations weed numbers and mass were significantly lower compared with a crop rotation with pea. Over a four-year period, during the maturity stage of cereals, in the crop rotations with intercrops, the total number of weeds was an average of 17.9 m-2 higher and 55.3 m-2 lower, and the total mass by 16.51 g m-2 higher and 63.31 g m-2 lower, compared with the respective crop rotations with sole cereal crops or pea. Averaged data showed, that during one year, significantly lower numbers of weeds were in the crop rotation with oat or its intercrop with

The study has been supported by the Lithuanian Ministry of Agriculture and the Lithuanian

Andersen, M. K.; Hauggaard-Nielsen, H.; Ambus, P. & Jensen, E. S. (2005) Biomass

production, symbiotic nitrogen fixation and inorganic N use in dual and tricomponent annual intercrops, *Plant and Soil*, Vol. 266, pp. 273–287, ISSN 0032-079X

(2555.4-3002.9 kg ha-1).

**4. Acknowledgment** 

Academy of Science.

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**2** 

*India* 

Manoj Kumar Yadav et al.\*

**Ridge Planted Pigeonpea and Furrow** 

*Department of Geophysics, Banaras Hindu University, Varanasi,* 

**Planted Rice in an Intercropping System as** 

**Affected by Nitrogen and Weed Management** 

Producing more food to feed the burgeoning population from shrinking agricultural land and water resources will be a challenge. Recently, intercropping has received more attention

Intercropping is a crop management system involving the growing of two or more dissimilar crops in distinct row combinations simultaneously on the same land area. In intercropping, the component crop species are usually sown in parallel lines enabling mechanical crop production, maintenance, and harvest. Intercropping involves crop intensification in respect to both time and space dimensions (Ahlawat and Sharma, 2002). Conceptually, an intercropping system helps for risk avoidance from epidemic of insect-pest and diseases and overcome adverse environmental conditions in agro-climatologically unstable regions along with increasing solar radiation utilization and inputs including fertilizer and water utilization compared to monoculture crops. Intercropping not only reduces the risk associated with input costs but also increases profit potential (Rathi and Verma, 1979). Moreover, it provides several major advantages namely, diversification reduces risk associated with crop failure, increased productivity per unit area and time, offers greater yield stability and utilizes the available growth resources more efficiently and sustainably. Furthering rationales of this practice, it caters to the multiple needs of the farmer, is a self-provisioning device, is a mechanism to spread labour peaks, and keeps weeds under check (Singh and Jha, 1984). A number of researchers (Enyi, 1973; Sengupta et al., 1985) reported greater land use efficiency utilizing intercropping and reductions of weed growth through competition. The yield advantage obtained through intercropping has been reported mainly due to efficient utilization and optimization of available natural growth

as a means to increase productivity of crops in per unit area and per unit time.

\* Manoj Kumar Yadav1, R.S. Singh2, Gaurav Mahajan3, Subhash Babu4, Sanjay Kumar Yadav5,

Rakesh Kumar6, Mahesh Kumar Singh7, Amitesh Kumar Singh8 and Amalesh Yadav9

*2,6,7,8 Department of Agronomy, Banaras Hindu University, Varanasi, India 3Department of Agronomy, Jawaharlal Nehru Krishi Vishwavidyalaya, Rewa, India 4Division of Agronomy, Indian Agricultural Research Institute, New Delhi, India* 

*1Department of Geophysics, Banaras Hindu University, Varanasi, India* 

*5Central Potato Research Station (ICAR), Shillong, India 9Department of Botany, University of Lucknow, Lucknow, India* 

**1. Introduction** 


## **Ridge Planted Pigeonpea and Furrow Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management**

Manoj Kumar Yadav et al.\* *Department of Geophysics, Banaras Hindu University, Varanasi, India* 

#### **1. Introduction**

30 Weed Control

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Producing more food to feed the burgeoning population from shrinking agricultural land and water resources will be a challenge. Recently, intercropping has received more attention as a means to increase productivity of crops in per unit area and per unit time.

Intercropping is a crop management system involving the growing of two or more dissimilar crops in distinct row combinations simultaneously on the same land area. In intercropping, the component crop species are usually sown in parallel lines enabling mechanical crop production, maintenance, and harvest. Intercropping involves crop intensification in respect to both time and space dimensions (Ahlawat and Sharma, 2002). Conceptually, an intercropping system helps for risk avoidance from epidemic of insect-pest and diseases and overcome adverse environmental conditions in agro-climatologically unstable regions along with increasing solar radiation utilization and inputs including fertilizer and water utilization compared to monoculture crops. Intercropping not only reduces the risk associated with input costs but also increases profit potential (Rathi and Verma, 1979). Moreover, it provides several major advantages namely, diversification reduces risk associated with crop failure, increased productivity per unit area and time, offers greater yield stability and utilizes the available growth resources more efficiently and sustainably. Furthering rationales of this practice, it caters to the multiple needs of the farmer, is a self-provisioning device, is a mechanism to spread labour peaks, and keeps weeds under check (Singh and Jha, 1984). A number of researchers (Enyi, 1973; Sengupta et al., 1985) reported greater land use efficiency utilizing intercropping and reductions of weed growth through competition. The yield advantage obtained through intercropping has been reported mainly due to efficient utilization and optimization of available natural growth

*2,6,7,8 Department of Agronomy, Banaras Hindu University, Varanasi, India 3Department of Agronomy, Jawaharlal Nehru Krishi Vishwavidyalaya, Rewa, India 4Division of Agronomy, Indian Agricultural Research Institute, New Delhi, India* 

<sup>\*</sup> Manoj Kumar Yadav1, R.S. Singh2, Gaurav Mahajan3, Subhash Babu4, Sanjay Kumar Yadav5,

Rakesh Kumar6, Mahesh Kumar Singh7, Amitesh Kumar Singh8 and Amalesh Yadav9

*<sup>1</sup>Department of Geophysics, Banaras Hindu University, Varanasi, India* 

*<sup>5</sup>Central Potato Research Station (ICAR), Shillong, India* 

*<sup>9</sup>Department of Botany, University of Lucknow, Lucknow, India* 

Ridge Planted Pigeonpea and Furrow

crop failure is halved.

and Shetty, 1981).

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 33

rice both receive their favourable micro-climate at field level. The major advantage of rice intercropping in furrow with ridge planted pigeonpea is that it can give greater yield stability compared to other intercropping choices because they are either adversely affected due to higher soil moisture or waterlogging at initial growth stage, thus the risk of a total

Weed infestation reduces grain yield directly and indirectly. Many crop and weeds have evolved with similar requirements for growth and development (Pujari et al., 1989; Yadav and Singh, 2009). Competition occurs when one of the resources (nutrients, light, moisture and space) fall short of total requirement of the crop and/or weeds. Weeds, by virtue of their high adoptability and faster growth, usually dominate the crop habitat and reduce the yield potential. Due to slow initial growth, wide crop row spacing is inefficient in fully utilizing light and moisture resources at initial growth stages and subsequently yield is reduced through competition with weeds. The inclusion of additional intercrop species can overcome this limitation. The presence of weeds is one of the major constraints to increase the seed yield in grain crops. Weeds are also an important factor responsible for low fertilizer use efficiency. Effective weed control measures are one of the several ways of increasing fertilizer use efficiency in crops in monoculture as well intercropping systems.

The nature and magnitude of crop-weed competition differs considerably between monoculture and intercropping systems. The crop species, population density, sowing geometry, duration and growth rhythm of the component crops, the moisture and fertility status of soil and tillage practices all influence weed flora in intercropping system (Moody

Since weeds are the main concern in many cultivated crops they should be controlled at the proper time. The most critical stage of crop-weed competition was observed between 15 to 45 days after sowing for the pigeonpea based intercropping system (Singh and Singh, 1995). Hand weeding, which is common practice, is very effective if repeated, though it is tedious, time consuming and costly. Moreover, present labour availability for such operations has decreased due to rapid industrialization, increased literacy and migration of labour to urban areas. Further, manual weed control methods are usually initiated after weeds have attained size and thus already competed for some time with the crop. Continuous rains in the rainy season make weed control by hand more difficult due to improper field conditions. In such situations, herbicide use likely will control weeds from the beginning of crop growth and can increase the crop yields. Herbicides not only control weeds and reduce labour cost, but also allow coverage of more area in a relatively shorter time period thus protecting yield potential (Ampong-Nyarko and De Datta, 1993). Many herbicides are crop specific; a herbicide that does not harm both the component crops, usually does not control a broad spectrum of weed species. The herbicides used in intercropping are selective in action for both component crops, but likely have narrow spectrum of weed control, leaving the other weeds to develop and compete with the crop. In addition, herbicidal soil activity expires before the critical period of crop-weed competition. A long duration crop of pigeonpea responded positively to two manual weeding and a pre-emergence herbicide likely substitute first one out of these two (Maheswarappa and Nanjappa, 1994). Higher yield attributes and yield of pigeonpea were also observed in different intercropping system under two sequential hand weedings or by integrated use of herbicides and hand weeding

(Dwivedi et al., 1991; Rafey and Prasad, 1995; Rana et al., 1999).

resources including water (Donald, 1963; Singh and Gupta, 1994), nutrients (Donald, 1963; Dalal, 1974); light (Donald, 1963; Nelliet et al., 1974; Singh and Gupta, 1994) as well as air and space (Singh and Gupta, 1994). In addition, intercropped species can be selected that produce allelopathic effect (Risser, 1969; Rice, 1974). Similarly, Willey (1979) made critical analysis of the yield advantages accrued from the intercropping. He explained that yield advantage occurs because the component crops differ in their use of growth resources in such a way that when they are grown in combination they are able to complement each other to make better overall use of resources than when grown separately. Annidation is the complementary use of resources by exploiting the environment in different ways by the components of a community. Maximizing intercropping advantage is a matter of maximizing the degree of complementarity between the components and minimizing intercrop competition.

Pigeonpea (*Cajanus cajan* L. Millsp.) is seldom or never grown in monoculture except on a very small scale, and mixed cropping is standard on field scale (Aiyer, 1949). Pigeonpea is commonly intercropped with cereals such as sorghum (*Sorghum bicolor* L. Moench), maize (*Zea mays* L.), pearl millet (*Pennisetum typhoides* L.), finger millet (*Eleusine coracana* Gaertn) and rice (*Oryza sativa* L.); grain legumes like black gram (*Vigna mungo* L. Hepper), green gram (*Vigna radiata* L. Wilczek), soybean (*Glycine max* L. Merrill) and oilseed such as sesamum (*Sesamum indicum* L.), groundnut (*Arachis hypogaea* L.) (Jena and Misra, 1988; Parida et al., 1988; Gouranga Kar, 2005; Behera et al., 2009; Ashok et al., 2010).

Practice of intercropping of pigeonpea with different short duration companion crops in India is very common. Being deep rooted, pigeonpea is very well suited for intercropping with the shallow rooted ones. Intercropping besides offering an insurance against failure of the crop due to disease, pests and frost, enables the farmers to obtain a variety of crops of their needs from the same piece of land. Pigeonpea is generally grown with wide row spacing of about 75-80 cm. However, the initial growth is quite slow and the grand growth period starts after 60-70 DAS. A lot of inter-row spaces, therefore, remain vacant during the early stages and get infested by weeds. The space between the rows could be profitably utilised by growing short duration crops such as black gram, green gram, cowpea, rice etc. The row arrangement that utilises a high proportion of the early crop to maximise its yield and allows the late maturing component to fully cover the ground should normally give the highest productivity. Based on the per cent of plant population used for each crop in intercropping system, it is divided into two types *viz.* additive and replacement series. In additive series, one crop is sown with 100% of its recommended population in pure stand which is known as the base crop. Another crop known as intercrop is introduced into base crop by adjusting or changing crop geometry. The population of intercrop is less than its recommended population in pure stand. In replacement series, both the crops called component crops. By scarifying certain proportion of population of one component, another component is introduced. Soybean+pigeonpea (4:2) is one of the example of intercropping in replacement series (Kasbe et al., 2010) and pigeonpea+greengram (1:2) is in additive series (Arjun Sharma et al., 2010).

A new concept of pigeonpea +rice intercropping system under ridge-furrow method of planting has been developed for rice ecosystem of Varanasi in India in additive series (Singh, 2006a). Since both upland rice and pigeonpea are sensitive to moisture regime (rice to drought and pigeonpea to excess soil moisture); however in this system, pigeonpea and

resources including water (Donald, 1963; Singh and Gupta, 1994), nutrients (Donald, 1963; Dalal, 1974); light (Donald, 1963; Nelliet et al., 1974; Singh and Gupta, 1994) as well as air and space (Singh and Gupta, 1994). In addition, intercropped species can be selected that produce allelopathic effect (Risser, 1969; Rice, 1974). Similarly, Willey (1979) made critical analysis of the yield advantages accrued from the intercropping. He explained that yield advantage occurs because the component crops differ in their use of growth resources in such a way that when they are grown in combination they are able to complement each other to make better overall use of resources than when grown separately. Annidation is the complementary use of resources by exploiting the environment in different ways by the components of a community. Maximizing intercropping advantage is a matter of maximizing the degree of complementarity between the components and minimizing

Pigeonpea (*Cajanus cajan* L. Millsp.) is seldom or never grown in monoculture except on a very small scale, and mixed cropping is standard on field scale (Aiyer, 1949). Pigeonpea is commonly intercropped with cereals such as sorghum (*Sorghum bicolor* L. Moench), maize (*Zea mays* L.), pearl millet (*Pennisetum typhoides* L.), finger millet (*Eleusine coracana* Gaertn) and rice (*Oryza sativa* L.); grain legumes like black gram (*Vigna mungo* L. Hepper), green gram (*Vigna radiata* L. Wilczek), soybean (*Glycine max* L. Merrill) and oilseed such as sesamum (*Sesamum indicum* L.), groundnut (*Arachis hypogaea* L.) (Jena and Misra, 1988;

Practice of intercropping of pigeonpea with different short duration companion crops in India is very common. Being deep rooted, pigeonpea is very well suited for intercropping with the shallow rooted ones. Intercropping besides offering an insurance against failure of the crop due to disease, pests and frost, enables the farmers to obtain a variety of crops of their needs from the same piece of land. Pigeonpea is generally grown with wide row spacing of about 75-80 cm. However, the initial growth is quite slow and the grand growth period starts after 60-70 DAS. A lot of inter-row spaces, therefore, remain vacant during the early stages and get infested by weeds. The space between the rows could be profitably utilised by growing short duration crops such as black gram, green gram, cowpea, rice etc. The row arrangement that utilises a high proportion of the early crop to maximise its yield and allows the late maturing component to fully cover the ground should normally give the highest productivity. Based on the per cent of plant population used for each crop in intercropping system, it is divided into two types *viz.* additive and replacement series. In additive series, one crop is sown with 100% of its recommended population in pure stand which is known as the base crop. Another crop known as intercrop is introduced into base crop by adjusting or changing crop geometry. The population of intercrop is less than its recommended population in pure stand. In replacement series, both the crops called component crops. By scarifying certain proportion of population of one component, another component is introduced. Soybean+pigeonpea (4:2) is one of the example of intercropping in replacement series (Kasbe et al., 2010) and pigeonpea+greengram (1:2) is in additive series

A new concept of pigeonpea +rice intercropping system under ridge-furrow method of planting has been developed for rice ecosystem of Varanasi in India in additive series (Singh, 2006a). Since both upland rice and pigeonpea are sensitive to moisture regime (rice to drought and pigeonpea to excess soil moisture); however in this system, pigeonpea and

Parida et al., 1988; Gouranga Kar, 2005; Behera et al., 2009; Ashok et al., 2010).

intercrop competition.

(Arjun Sharma et al., 2010).

rice both receive their favourable micro-climate at field level. The major advantage of rice intercropping in furrow with ridge planted pigeonpea is that it can give greater yield stability compared to other intercropping choices because they are either adversely affected due to higher soil moisture or waterlogging at initial growth stage, thus the risk of a total crop failure is halved.

Weed infestation reduces grain yield directly and indirectly. Many crop and weeds have evolved with similar requirements for growth and development (Pujari et al., 1989; Yadav and Singh, 2009). Competition occurs when one of the resources (nutrients, light, moisture and space) fall short of total requirement of the crop and/or weeds. Weeds, by virtue of their high adoptability and faster growth, usually dominate the crop habitat and reduce the yield potential. Due to slow initial growth, wide crop row spacing is inefficient in fully utilizing light and moisture resources at initial growth stages and subsequently yield is reduced through competition with weeds. The inclusion of additional intercrop species can overcome this limitation. The presence of weeds is one of the major constraints to increase the seed yield in grain crops. Weeds are also an important factor responsible for low fertilizer use efficiency. Effective weed control measures are one of the several ways of increasing fertilizer use efficiency in crops in monoculture as well intercropping systems.

The nature and magnitude of crop-weed competition differs considerably between monoculture and intercropping systems. The crop species, population density, sowing geometry, duration and growth rhythm of the component crops, the moisture and fertility status of soil and tillage practices all influence weed flora in intercropping system (Moody and Shetty, 1981).

Since weeds are the main concern in many cultivated crops they should be controlled at the proper time. The most critical stage of crop-weed competition was observed between 15 to 45 days after sowing for the pigeonpea based intercropping system (Singh and Singh, 1995). Hand weeding, which is common practice, is very effective if repeated, though it is tedious, time consuming and costly. Moreover, present labour availability for such operations has decreased due to rapid industrialization, increased literacy and migration of labour to urban areas. Further, manual weed control methods are usually initiated after weeds have attained size and thus already competed for some time with the crop. Continuous rains in the rainy season make weed control by hand more difficult due to improper field conditions. In such situations, herbicide use likely will control weeds from the beginning of crop growth and can increase the crop yields. Herbicides not only control weeds and reduce labour cost, but also allow coverage of more area in a relatively shorter time period thus protecting yield potential (Ampong-Nyarko and De Datta, 1993). Many herbicides are crop specific; a herbicide that does not harm both the component crops, usually does not control a broad spectrum of weed species. The herbicides used in intercropping are selective in action for both component crops, but likely have narrow spectrum of weed control, leaving the other weeds to develop and compete with the crop. In addition, herbicidal soil activity expires before the critical period of crop-weed competition. A long duration crop of pigeonpea responded positively to two manual weeding and a pre-emergence herbicide likely substitute first one out of these two (Maheswarappa and Nanjappa, 1994). Higher yield attributes and yield of pigeonpea were also observed in different intercropping system under two sequential hand weedings or by integrated use of herbicides and hand weeding (Dwivedi et al., 1991; Rafey and Prasad, 1995; Rana et al., 1999).

Ridge Planted Pigeonpea and Furrow

**2.2.3 Relative humidity** 

as compared to second year.

Soil Physical and Chemical

Soil separates 0 – 15 cm (%)

(1:2.5 soil water ratio)

Electrical conductivity (d S/m at 250C)

**2.2.4 Sunshine duration** 

during 2005-06.

Properties

Sand Silt Clay

*p*H

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 35

The weekly mean maximum relative humidity varied from 62 to 95% with an average of 84% during 2004-05 and it varied from 37 to 92% with an average of 82% during 2005-06. The weekly mean minimum relative humidity varied from 18 to 81%-with an average of 55% during 2004-05 and it varied from 18 to 83% with an average of 52% during 2005-06. The relative humidity indicated considerable variation throughout the growing season during both the years. Data also indicated that the first year was comparatively more humid

> 43.68 30.66 25.66

Textural class Sandy clay loam Textural triangle

Value Analysis Method Employed

(Black, 1967)

0.29 Systronics electrical conductivity

(Jackson, 1973)

(Walkley and Black, 1934)

(Subbiah and Asija, 1973)

(Ammonium acetate extract)

colorimetric method. (Olsen et al., 1954)

(Jackson, 1973)

7.3 Glass electrode *p*H meter (Jackson, 1973)

meter

Organic carbon (%) 0.35 Chromic acid rapid titration method

Available nitrogen (kg N/ha) 208.5 Alkaline permanganate method

Available potassium (kg K /ha) 185.02 Flame photometric method

Table 1. Physio-chemical properties of the experimental field

Available phosphorus (kg P/ha) 18.21 0.5 M NaHCO3 extractable Olsen's

The average duration of bright sunshine day was 6.9 and 7.2 hours in first and second year, respectively. The range of maximum and minimum mean weekly bright sunshine duration was ranged from 2.9 to 10.2 hours during 2004-05 and it ranged from 1.9 to 10.0 hours

Hydrometer method (Bouyoucos, 1962)

The objectives of our present investigation entitled "Ridge Planted Pigeonpea and Furrow Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management" were to study the growth pattern and yields and nutrient uptake as affected by nitrogen and weed management in pigeonpea+ rice intercropping system.

#### **2. Materials and methods**

#### **2.1 Physiographic situation**

The Agricultural Research Farm is situated in the South eastern part of Varanasi city, India at an altitude of 125.93 meter above the MSL, 200 18' north latitude and 800 36' eastern longitude. The experiment was established at the Agricultural Research Farm, Institute of Agricultural Sciences, Banaras Hindu University. The area accurately reflects the agro-climatic conditions of North Gangetic Alluvial Plains with annual rainfall of about 1100 mm.

#### **2.2 Climatic condition**

Varanasi's climate is sub-tropical and is subjected to extremes of weather conditions i.e. heat of summer (33.4-41.4 0C) and cold in winter (9.3-11.8 0C). The temperature increases from mid-February and reaches its maximum by May/June but has a tendency to decrease from July onwards reaching the minimum in December/January. The normal period for the onset of monsoon in the region is third week of June which lasts up to the end of September or sometimes into the first week of October. The area occasionally experiences some winter cyclonic rains during December/ February. The period between March and May is generally dry. The normal annual rainfall of the region is about 1081.4 mm. In terms of percentage of total rainfall, 88 per cent is received from June to September, 5.7 per cent from October to December, 3.3 per cent from January to February and 3 per cent from March to May as per monsoon rains. The mean relative humidity is 62 per cent which rises up to 82 per cent during July to September and fall down to 28 per cent during the end of April and early June.

#### **2.2.1 Rainfall**

The cumulative rainfall received during the period of investigation was 683.0 mm and 783.3 mm in the year 2004-05 and 2005-06, respectively. The distribution of rainfall was more uniform during second year as compared to first year during crop production. The monthwise distribution of the rainfall indicated that July and August of second year received more rain than the corresponding period of the first year.

#### **2.2.2 Temperature**

The weekly mean maximum temperature ranged from 20.0 to 38.6 0C with an average of 30.0 0C during 2004-05 and 18.8 to 44.1 0C with an average of 30.8 0C during 2005-06. The weekly mean minimum temperature ranged from 8.3 0C to 27.4 0C with an average of 19.1 0C during 2004-05 and 7.4 to 30.4 0C with an average of 18.7 0C during 2005-06. The mean fluctuation in maximum and minimum temperature was almost normal during both the years.

#### **2.2.3 Relative humidity**

34 Weed Control

The objectives of our present investigation entitled "Ridge Planted Pigeonpea and Furrow Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management" were to study the growth pattern and yields and nutrient uptake as affected by nitrogen and

The Agricultural Research Farm is situated in the South eastern part of Varanasi city, India at an altitude of 125.93 meter above the MSL, 200 18' north latitude and 800 36' eastern longitude. The experiment was established at the Agricultural Research Farm, Institute of Agricultural Sciences, Banaras Hindu University. The area accurately reflects the agro-climatic conditions of North Gangetic Alluvial Plains with annual rainfall of

Varanasi's climate is sub-tropical and is subjected to extremes of weather conditions i.e. heat of summer (33.4-41.4 0C) and cold in winter (9.3-11.8 0C). The temperature increases from mid-February and reaches its maximum by May/June but has a tendency to decrease from July onwards reaching the minimum in December/January. The normal period for the onset of monsoon in the region is third week of June which lasts up to the end of September or sometimes into the first week of October. The area occasionally experiences some winter cyclonic rains during December/ February. The period between March and May is generally dry. The normal annual rainfall of the region is about 1081.4 mm. In terms of percentage of total rainfall, 88 per cent is received from June to September, 5.7 per cent from October to December, 3.3 per cent from January to February and 3 per cent from March to May as per monsoon rains. The mean relative humidity is 62 per cent which rises up to 82 per cent during July to September and fall down to 28 per cent during the end of April and early

The cumulative rainfall received during the period of investigation was 683.0 mm and 783.3 mm in the year 2004-05 and 2005-06, respectively. The distribution of rainfall was more uniform during second year as compared to first year during crop production. The monthwise distribution of the rainfall indicated that July and August of second year received more

The weekly mean maximum temperature ranged from 20.0 to 38.6 0C with an average of 30.0 0C during 2004-05 and 18.8 to 44.1 0C with an average of 30.8 0C during 2005-06. The weekly mean minimum temperature ranged from 8.3 0C to 27.4 0C with an average of 19.1 0C during 2004-05 and 7.4 to 30.4 0C with an average of 18.7 0C during 2005-06. The mean fluctuation in maximum and minimum temperature was almost normal during both the

weed management in pigeonpea+ rice intercropping system.

**2. Materials and methods 2.1 Physiographic situation** 

about 1100 mm.

June.

years.

**2.2.1 Rainfall** 

**2.2.2 Temperature** 

rain than the corresponding period of the first year.

**2.2 Climatic condition** 

The weekly mean maximum relative humidity varied from 62 to 95% with an average of 84% during 2004-05 and it varied from 37 to 92% with an average of 82% during 2005-06. The weekly mean minimum relative humidity varied from 18 to 81%-with an average of 55% during 2004-05 and it varied from 18 to 83% with an average of 52% during 2005-06. The relative humidity indicated considerable variation throughout the growing season during both the years. Data also indicated that the first year was comparatively more humid as compared to second year.


Table 1. Physio-chemical properties of the experimental field

#### **2.2.4 Sunshine duration**

The average duration of bright sunshine day was 6.9 and 7.2 hours in first and second year, respectively. The range of maximum and minimum mean weekly bright sunshine duration was ranged from 2.9 to 10.2 hours during 2004-05 and it ranged from 1.9 to 10.0 hours during 2005-06.

Ridge Planted Pigeonpea and Furrow

**2.4.4 Seed and sowing** 

**2.4.5 Herbicide application** 

spacing of 20 cm for pigeonpea.

weed infested condition until crop maturity.

DAS and at pod formation growth stage.

**2.4.10 Threshing, cleaning and weighing** 

weight and was converted to t/ha based on net plot size harvest.

**2.4.6 Thinning** 

**2.4.7 Hand weeding** 

**2.4.8 Plant protection** 

**2.4.9 Harvesting** 

Pendimethalin was applied pre-emergence (1 DAS).

cm.

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 37

Seed rate for pigeonpea and rice were 20 and 60 kg/ha, respectively. Pigeonpea seeds were sown on top of the ridges and rice seeds were sown in two rows in each furrow at the same date. The crops were sown on 8th July in 2004 and 12th July in 2005 using full season pigeonpea variety 'Bahar' and early rice variety 'NDR 97'. Row to row spacing of pigeonpea was 75 cm and plant to plant spacing of pigeonpea and row to row spacing of rice were 20

The required quantity (1.0 kg/ha) of pendimethalin was mixed in water and sprayed with a backpack sprayer using the spray volume of 600 litres of water/ha as per treatment.

The extra plants were thinned out at 30 days after sowing to maintain the plant to plant

Hand weeding was accomplished as per the treatment in the experiment. The weeds were removed from hand weeded plots twice at 15 and 45 DAS or once at 45 DAS in integration with herbicidal treatment as per treatment 3, respectively. Weedy check plots were kept

There was no serious incidence of any major pest or disease during period of crop growth. However, as a preventive measure against leaf folder, pod borer attack, two applications of Endosulfan 35 EC at the rate of 2 litres/ha dissolved in 800 litres of water were applied at 65

The crops were harvested at physiological maturity growth stage. Rice was harvested on 10th and 21st October in 2004 and 2005 respectively and pigeonpea on 20th and 28th March in 2005 and 2006 respectively. Firstly, the border rows were harvested and separated. Following border row harvest, crop from net plot was harvested and sun dried. The

The individual net plot's harvested crop bundles were weighed after drying prior to threshing. The grain yield was recorded separately after threshing, winnowing and cleaning. The straw/stalk yield was calculated by subtracting grain yield from the bundle

harvested material from each net plot was bundled, tagged and threshed separately.

#### **2.2.5 Evaporation**

The evaporation data recorded from a United States Weather Bureau class A pan evaporimeter revealed that the weekly average evaporation per day varied from 6.4 to 1.4 mm/day in 2004-05 and 9.5 to 1.5 mm/day in 2005-06. The total evaporation during crop growing period was 942.2 mm in 2004-05 and 1061.9 mm in 2005-06.

#### **2.3 Soil and soil analysis**

In order to know the initial fertility status of the experimental plot, soil sample from 0-15 cm were collected and analysed for mechanical composition and chemical constituents. Data obtained are reported in Table 1. The experimental plot area soil was classified as sandy clay loam in texture, low in nitrogen, and medium in available phosphorus and potassium.

#### **2.4 Technical programme**

Considering the nature of factors evaluated and the convenience of agricultural operation, the experiment was laid out in split plot design with three replications. Six main plot treatments (consisting of all possible combinations of two nitrogen levels in pigeonpea including one control and another 25 kg N/ha as starter application with three nitrogen levels in rice i.e. 50, 75 and 100 kg N/ha) and four sub plot treatments( weed management) were established. Weed management treatments included: 1) a weedy check, 2) pendimethalin at 1.0 kg/ha, 3) pendimethalin at 1.0 kg/ha followed by one hand weeding at 45 days after seeding (DAS) or 4) two sequential hand weeding at 15 and 45 DAS. The whole field was divided into three blocks, each representing a replication. Each block was further divided in six main plots where main plots treatments were randomly allocated within them. Then each main plot was again divided into four equal sub plots and the sub plot treatments were again allocated randomly.

#### **2.4.1 Field preparation**

Proper field preparation is essential for a healthy pigeonpea and rice crop in intercropping. The experimental area was ploughed with tractor drawn mould board plough followed by two passes with a disc. Finally the field was levelled.

#### **2.4.2 Ridge and furrow establishment**

Ridges and furrows were established manually by spade.

#### **2.4.3 Fertilizer application**

The recommended doses of P2O5 and K2O for pigeonpea were 40 and 30, and for rice were 40 and 40 kg/ha, respectively. Quantity of P2O5 and K2O/ha were applied on row basis to each crop separately in the form of single super phosphate and muriate of potash, respectively. Full doses of phosphorus and potassium were applied to pigeonpea and rice as basal applications. Nitrogen was applied as per treatment through urea. Full nitrogen dose of pigeonpea and 75% nitrogen dose of rice were applied as basal and remaining nitrogen dose of rice was top dressed at it's tillering growth stage.

#### **2.4.4 Seed and sowing**

36 Weed Control

The evaporation data recorded from a United States Weather Bureau class A pan evaporimeter revealed that the weekly average evaporation per day varied from 6.4 to 1.4 mm/day in 2004-05 and 9.5 to 1.5 mm/day in 2005-06. The total evaporation during crop

In order to know the initial fertility status of the experimental plot, soil sample from 0-15 cm were collected and analysed for mechanical composition and chemical constituents. Data obtained are reported in Table 1. The experimental plot area soil was classified as sandy clay loam in texture, low in nitrogen, and medium in available phosphorus and potassium.

Considering the nature of factors evaluated and the convenience of agricultural operation, the experiment was laid out in split plot design with three replications. Six main plot treatments (consisting of all possible combinations of two nitrogen levels in pigeonpea including one control and another 25 kg N/ha as starter application with three nitrogen levels in rice i.e. 50, 75 and 100 kg N/ha) and four sub plot treatments( weed management) were established. Weed management treatments included: 1) a weedy check, 2) pendimethalin at 1.0 kg/ha, 3) pendimethalin at 1.0 kg/ha followed by one hand weeding at 45 days after seeding (DAS) or 4) two sequential hand weeding at 15 and 45 DAS. The whole field was divided into three blocks, each representing a replication. Each block was further divided in six main plots where main plots treatments were randomly allocated within them. Then each main plot was again divided into four equal sub plots and the sub

Proper field preparation is essential for a healthy pigeonpea and rice crop in intercropping. The experimental area was ploughed with tractor drawn mould board plough followed by

The recommended doses of P2O5 and K2O for pigeonpea were 40 and 30, and for rice were 40 and 40 kg/ha, respectively. Quantity of P2O5 and K2O/ha were applied on row basis to each crop separately in the form of single super phosphate and muriate of potash, respectively. Full doses of phosphorus and potassium were applied to pigeonpea and rice as basal applications. Nitrogen was applied as per treatment through urea. Full nitrogen dose of pigeonpea and 75% nitrogen dose of rice were applied as basal and remaining nitrogen

growing period was 942.2 mm in 2004-05 and 1061.9 mm in 2005-06.

**2.2.5 Evaporation** 

**2.3 Soil and soil analysis** 

**2.4 Technical programme** 

**2.4.1 Field preparation** 

**2.4.3 Fertilizer application** 

plot treatments were again allocated randomly.

two passes with a disc. Finally the field was levelled.

Ridges and furrows were established manually by spade.

dose of rice was top dressed at it's tillering growth stage.

**2.4.2 Ridge and furrow establishment** 

Seed rate for pigeonpea and rice were 20 and 60 kg/ha, respectively. Pigeonpea seeds were sown on top of the ridges and rice seeds were sown in two rows in each furrow at the same date. The crops were sown on 8th July in 2004 and 12th July in 2005 using full season pigeonpea variety 'Bahar' and early rice variety 'NDR 97'. Row to row spacing of pigeonpea was 75 cm and plant to plant spacing of pigeonpea and row to row spacing of rice were 20 cm.

#### **2.4.5 Herbicide application**

The required quantity (1.0 kg/ha) of pendimethalin was mixed in water and sprayed with a backpack sprayer using the spray volume of 600 litres of water/ha as per treatment. Pendimethalin was applied pre-emergence (1 DAS).

#### **2.4.6 Thinning**

The extra plants were thinned out at 30 days after sowing to maintain the plant to plant spacing of 20 cm for pigeonpea.

#### **2.4.7 Hand weeding**

Hand weeding was accomplished as per the treatment in the experiment. The weeds were removed from hand weeded plots twice at 15 and 45 DAS or once at 45 DAS in integration with herbicidal treatment as per treatment 3, respectively. Weedy check plots were kept weed infested condition until crop maturity.

#### **2.4.8 Plant protection**

There was no serious incidence of any major pest or disease during period of crop growth. However, as a preventive measure against leaf folder, pod borer attack, two applications of Endosulfan 35 EC at the rate of 2 litres/ha dissolved in 800 litres of water were applied at 65 DAS and at pod formation growth stage.

#### **2.4.9 Harvesting**

The crops were harvested at physiological maturity growth stage. Rice was harvested on 10th and 21st October in 2004 and 2005 respectively and pigeonpea on 20th and 28th March in 2005 and 2006 respectively. Firstly, the border rows were harvested and separated. Following border row harvest, crop from net plot was harvested and sun dried. The harvested material from each net plot was bundled, tagged and threshed separately.

#### **2.4.10 Threshing, cleaning and weighing**

The individual net plot's harvested crop bundles were weighed after drying prior to threshing. The grain yield was recorded separately after threshing, winnowing and cleaning. The straw/stalk yield was calculated by subtracting grain yield from the bundle weight and was converted to t/ha based on net plot size harvest.

Ridge Planted Pigeonpea and Furrow

WCE has been expressed in percentage.

Element Method employed

**2.6.1 Nutrient uptake (kg/ha)** 

Yi= Grain yield *i*th component *ei*= equivalent price of *i*th component PGEY has been expressed in tonne/hectare

**2.8 Statistical analysis** 

Where,

**2.6 Chemical analysis of crops and weeds** 

DMC= Dry matter production of weeds/m2 in weedy check.

Total nitrogen Modified Kjeldahl method (Jackson,1973)

Total potassium Flame photometric method (Jackson,1973)

corresponding dry matter and nutrient content (Black, 1967).

Pigeonpea grain equivalent yield (PGEY) was calculated as follows:

applying the procedure as described by Gomez and Gomez (1984).

**2.7 Pigeonpea grain equivalent yield (kg/ha)** 

DMT= Dry matter production of weeds/m2 in the treatment to be compared.

for nitrogen, phosphorus and potassium as per the methods described in Table 2.

Where,

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 39

DMC-DMT WCE <sup>100</sup> DMC

The plant samples from crops (pigeonpea and rice) and weed flora collected within each treatment at crop harvest and thus the maximum growth stage of weeds (60 DAS), respectively were washed with tap water followed by 0.1N HCl, distilled water and then with double distilled water. Plants were first dried under shade then in hot air oven at 60 0C for 48 hours. After recording oven dry weight, plant samples were individually grinded in Willey' Mill and stored in butter paper covers. The powder of plant samples was analysed

Total phosphorus Vanadomolybdo phosphoric yellow colorimetric method (Jackson,1973)

Nutrient content (N,P and K) in grain and stalk/straw of each crop and in entire weed complex were analysed separately using procedure given in Table 2. Nutrient uptake by grain and straw of crops and that of by weeds were calculated in kg/ha by multiplying the

> 1 PGEY ( . ) *n*

*Yi ei*

*i*

The data pertaining to each of the treatments and interactions were analyzed statistically by

 

Table 2. Methods used for determination of chemical composition of crops and weeds

#### **2.5 Observation**

The following observations were taken during the study periods which are described below:

#### **2.5.1 Studies on pigeonpea and rice**

#### **2.5.1.1 Shoot dry matter/ plant or/meter row(g)**

The five pigeonpea plants randomly selected from the sample row were cut carefully at the ground surface and then sun dried. After sun drying, plant samples were collected in paper bags after being cut into smaller pieces and placed in an electric oven at 70 0C for drying to obtain a constant dry weight. The dry weight of the samples then obtained was expressed in g/plant. For dry matter production by rice, all plant samples from 0.50 meter running row length were selected from the sampling rows (leaving aside one border row from the each side) at harvest. The plants were cut at the collar region. The collected sample tillers were oven dried at 60 0C for 48 hours and weighted. The weight of sample tillers thus obtained was converted into g/ running meter by multiplying with conversion factor.

#### **2.5.1.2 Grain yield (t/ha)**

The harvested crop from each net plot was threshed separately. After proper cleaning and drying, grain yield was recorded in kg/plot and finally converted into t/ha by multiplying with conversion factor.

#### **2.5.1.3 Stalk/straw yield (t/ha)**

Stalk/straw yield for each net plot was calculated by subtracting the grain yield from total biological yield and finally expressed in terms of t/ha.

#### **2.5.2 Weed assessment**

Weeds were collected from each individual plot during each year of the investigation for identification. Weed samples were collected by placing a quadrate (0.50 m x 0.50 m) randomly at two places in each plot at 60 DAS.

#### **2.5.2.1 Weed population**

Species wise weed counts were recorded at 60 DAS of crops from the two randomly place quadrates of 0.50 m x 0.50 m (0.25 m2) in each net plot. Thus, weed population/m2 was calculated from total number of all weed species of two quadrates multiplied with conversion factor.

#### **2.5.2.2 Weed dry matter production/m2 (g)**

Weed enclosed in a quadrate of 0.25 m2 (0.50 m x 0.50 m) were removed from the sampling rows at 60 DAS. After sun drying the samples were placed in an oven at 60 0C for 48 hours. The dry weight was multiplied with conversion factor to express in g/m2.

#### **2.5.2.3 Weed control efficiency (%)**

Weed control efficiency (WCE) was calculated at 60 DAS using the formula USDA/ICAR (AICRPWC, 1994).

$$\text{WCE} = \frac{\text{DMC-DMT}}{\text{DMC}} \ge 100^{\circ}$$

Where,

38 Weed Control

The following observations were taken during the study periods which are described below:

The five pigeonpea plants randomly selected from the sample row were cut carefully at the ground surface and then sun dried. After sun drying, plant samples were collected in paper bags after being cut into smaller pieces and placed in an electric oven at 70 0C for drying to obtain a constant dry weight. The dry weight of the samples then obtained was expressed in g/plant. For dry matter production by rice, all plant samples from 0.50 meter running row length were selected from the sampling rows (leaving aside one border row from the each side) at harvest. The plants were cut at the collar region. The collected sample tillers were oven dried at 60 0C for 48 hours and weighted. The weight of sample tillers thus obtained

The harvested crop from each net plot was threshed separately. After proper cleaning and drying, grain yield was recorded in kg/plot and finally converted into t/ha by multiplying

Stalk/straw yield for each net plot was calculated by subtracting the grain yield from total

Weeds were collected from each individual plot during each year of the investigation for identification. Weed samples were collected by placing a quadrate (0.50 m x 0.50 m)

Species wise weed counts were recorded at 60 DAS of crops from the two randomly place quadrates of 0.50 m x 0.50 m (0.25 m2) in each net plot. Thus, weed population/m2 was calculated from total number of all weed species of two quadrates multiplied with

Weed enclosed in a quadrate of 0.25 m2 (0.50 m x 0.50 m) were removed from the sampling rows at 60 DAS. After sun drying the samples were placed in an oven at 60 0C for 48 hours.

Weed control efficiency (WCE) was calculated at 60 DAS using the formula USDA/ICAR

The dry weight was multiplied with conversion factor to express in g/m2.

was converted into g/ running meter by multiplying with conversion factor.

**2.5 Observation** 

**2.5.1.2 Grain yield (t/ha)** 

with conversion factor.

**2.5.2 Weed assessment** 

**2.5.2.1 Weed population** 

conversion factor.

(AICRPWC, 1994).

**2.5.1.3 Stalk/straw yield (t/ha)** 

biological yield and finally expressed in terms of t/ha.

randomly at two places in each plot at 60 DAS.

**2.5.2.2 Weed dry matter production/m2 (g)** 

**2.5.2.3 Weed control efficiency (%)** 

**2.5.1 Studies on pigeonpea and rice** 

**2.5.1.1 Shoot dry matter/ plant or/meter row(g)** 

DMC= Dry matter production of weeds/m2 in weedy check. DMT= Dry matter production of weeds/m2 in the treatment to be compared. WCE has been expressed in percentage.

#### **2.6 Chemical analysis of crops and weeds**

The plant samples from crops (pigeonpea and rice) and weed flora collected within each treatment at crop harvest and thus the maximum growth stage of weeds (60 DAS), respectively were washed with tap water followed by 0.1N HCl, distilled water and then with double distilled water. Plants were first dried under shade then in hot air oven at 60 0C for 48 hours. After recording oven dry weight, plant samples were individually grinded in Willey' Mill and stored in butter paper covers. The powder of plant samples was analysed for nitrogen, phosphorus and potassium as per the methods described in Table 2.


Table 2. Methods used for determination of chemical composition of crops and weeds

#### **2.6.1 Nutrient uptake (kg/ha)**

Nutrient content (N,P and K) in grain and stalk/straw of each crop and in entire weed complex were analysed separately using procedure given in Table 2. Nutrient uptake by grain and straw of crops and that of by weeds were calculated in kg/ha by multiplying the corresponding dry matter and nutrient content (Black, 1967).

#### **2.7 Pigeonpea grain equivalent yield (kg/ha)**

Pigeonpea grain equivalent yield (PGEY) was calculated as follows:

$$\text{PGEY} = \sum\_{i=1}^{n} \text{(Yi.ei)}$$

Where, Yi= Grain yield *i*th component *ei*= equivalent price of *i*th component PGEY has been expressed in tonne/hectare

#### **2.8 Statistical analysis**

The data pertaining to each of the treatments and interactions were analyzed statistically by applying the procedure as described by Gomez and Gomez (1984).

Ridge Planted Pigeonpea and Furrow

Treatment Pigeonpea

N level in Pigeonpea(kg/ha)

N level in Rice(kg/ha)

Weed Management

Pendimethalin@ 1kg / ha +one hand weeding at 45 DAS

Two hand weedings at 15 and 45 DAS

years)

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 41

(1992), Mazid et al. (1998), Panda et al. (1999) and Bindra et al. (2000). Many researchers also reported that cereal component in legumes based intercropping yielded more at higher levels of nitrogen application (Reddy et al. 1980; Ramesh and Surve 1984; Ofori and Stern 1986; Ezumah et al. 1987; Rao et al. 1987; Kaushik and Gautam 1987; Chowdhury and Rosario 1992; Rafey and Prasad 1992; Bhagat and Dhar 1995; Kushwaha and Chandel 1997; Mandal et al. 2000; Sarwagi and Tripathi 1999; Shivay et al. 1999; Shivay and Singh 2000; Singh, 2006b). Whereas, same were failed to show its effect on pigeonpea (Table 3 and 4). This might be due to the fact that localized placement of nitrogen made was first available to that crop for which it was applied. Forage area of pigeonpea at initial growth stage (50 DAS) was slow due their slow growth habit. Contrary to this, forage area of short duration rice was higher due to faster initial growth rate and planting in furrow between ridges, likely taking most of applied nitrogen easily by themselves in comparison to pigeonpea. Mahapatra et al. (1990) and Singh (2006b) were also find the similar result.

> dry matter/ plant at harvest (g)

0 151.4 187.3 256.9 193.9 76.4 25 168.0 209.9 286.7 225.0 77.9 CD(*P*=0.05) 7.6 11.9 13.9 10.9 0.5

50 156.1 175.2 256.3 194.7 76.6 75 160.5 204.0 274.4 211.7 77.5 100 162.6 216.7 284.7 222.1 77.5 CD(*P*=0.05) NS 14.6 17.0 13.4 0.6

Weedy check 136.1 60.4 545.0 498.8 - Pendimethalin@ 1kg / ha 159.0 178.6 266.1 205.2 58.7

CD(*P*=0.05) 3.2 6.5 8.0 7.7 0.3

Table 3. Effect of nitrogen levels and weed management practices on crop growth and weed and weed control efficiency under pigeon pea + rice intercropping system (mean of two

Rice dry matter /m row at harvest (g)

Weed number /m2

171.0 255.1 161.7 79.7 83.9

172.9 300.4 114.3 54.3 89.0

Weed dry weight /m2(g) Weed control efficiency (%)

#### **3. Results and discussion**

#### **3.1 Weeds**

The weed flora of the experimental field included: Jungle rice [*Echinochloa colona* (L.) Link.], barnyard grass [ *Echinochloa crusgalli* (L.) Beauv.], bermuda grass [*Cynodon dactylon* (L.) Pers], goose grass [*Eleusine indica* (L.) Gaerth.], crab grass [*Digitaria sanguinalis*(L.) Scop.], crowfoot grass [*Dactyloctenium aegypticum* (L.) P. Beauv.], purple nutsedge *(Cyperus rotundus* Linn.), variable flatsedge (*Cyperus difformis* Linn.), ricefield flatsedge (*Cyperus iria* Linn.), grass-like fimbry [*Fimbristylis miliacea* (L.) Vahl.], goat weed (*Ageratum conyzoides* L.), dayflower (*Commelina benghalensis* Linn.), climbing dayflower (*Commelina diffusa* L.), hairy spurge (*Euphorbia hirta* Linn.), asian spiderflower (*Cleome viscosa* L.), wild carrot weed (*Parthenium hysterophorus* L.), pink node flower (*Caesulia axillaris* Roxb.), silver cock's comb (*Celosia argentea* L.), gale of the wind (*Phyllanthus niruri* Linn.), false daisy [*Eclipta alba*(L.) Hassk.] and wild jute (*Corchorus acutengulus* lamk).

Application of 25 kg N/ha in pigeonpea increased weed population and their dry weight/m2 as compared to control (Table 3). Analysis further reveals that weed density and dry weight/m2 increased with increasing levels of nitrogen to rice up to 100 kg N/ha (Table 3). This likely due to weeds utilizing a greater quantity of applied and available nutrients. Thus, higher dose of nitrogen accelerated weed emergence and growth. Weed control efficiency increased with increasing nitrogen levels for pigeonpea and rice. Similar findings were also reported by Pujari et al. (1989) and Yadav and Singh (2009).

Data presented in Table 3 also indicates that two hand weeding at 15 and 45 DAS resulted in the lowest density and dry weight of weeds/m2 followed by pendimethalin + one hand weeding at 45 DAS; both treatments were superior over other weed management treatments. This result was likely owing to better indiscriminate control of all types of weeds by hand weeding. These findings were in close agreement with those of Shetty and Krantz (1976), Ampong-Nyarko and De Datta(1993) and Reddy et al. (2007). Reflecting minimum density and dry weight results, maximum weed control efficiency was obtained with two hand weeding at 15 and 45 DAS. This finding is in agreement with finding of Sinha et al. (1989a and b), Goyal et al. (1991), Parthi et al. (1991), Mahapatra (1991), Prasad and Srivastava (1991), Maheswarappa and Nanjappa (1994), Rafey and Prasad (1995), Patil and Pandey (1996), Mishra et al. (1998), Singh et al. (1998c), Singh et al. (1999), Rana and Pal (1999), Rana et al. (1999), Manickam et al. (2000), Reddy et al. (2007) and Singh (2007).

#### **3.2 Growth, yields and pigeonpea grain equivalent yield**

Dry matter production, grain and straw yield of pigeonpea and rice, and pigeonpea grain equivalent yield were increased with application of 25 kg N/ha to pigeonpea over control (Table 3 and 4). Similar findings have been reported earlier by Singh et al. (1978), Bhandhari et al. (1989), Chittapur et al. (1994), Patel and Patel (1994), Singh et al.(1998a and b), Mandal et al. (1999) and Singh (2006b). Dry matter production, grain and straw yield of rice were increased significantly up to 75 kg N/ha applied to rice (Table 3 and 4). The improvement in the dry matter production and yields of rice might be attributed to the adequate supply of photosynthate to sink under sufficient supply of nitrogen. These results were supported by the findings of Samui et al. (1979), Reddy et al. (1986), Abdulsalam and Subramaniam (1988), Purushotham et al. (1988), Raju et al. (1990), Dubey et al. (1991), Bhattacharya and Singh

The weed flora of the experimental field included: Jungle rice [*Echinochloa colona* (L.) Link.], barnyard grass [ *Echinochloa crusgalli* (L.) Beauv.], bermuda grass [*Cynodon dactylon* (L.) Pers], goose grass [*Eleusine indica* (L.) Gaerth.], crab grass [*Digitaria sanguinalis*(L.) Scop.], crowfoot grass [*Dactyloctenium aegypticum* (L.) P. Beauv.], purple nutsedge *(Cyperus rotundus* Linn.), variable flatsedge (*Cyperus difformis* Linn.), ricefield flatsedge (*Cyperus iria* Linn.), grass-like fimbry [*Fimbristylis miliacea* (L.) Vahl.], goat weed (*Ageratum conyzoides* L.), dayflower (*Commelina benghalensis* Linn.), climbing dayflower (*Commelina diffusa* L.), hairy spurge (*Euphorbia hirta* Linn.), asian spiderflower (*Cleome viscosa* L.), wild carrot weed (*Parthenium hysterophorus* L.), pink node flower (*Caesulia axillaris* Roxb.), silver cock's comb (*Celosia argentea* L.), gale of the wind (*Phyllanthus niruri* Linn.), false daisy [*Eclipta alba*(L.)

Application of 25 kg N/ha in pigeonpea increased weed population and their dry weight/m2 as compared to control (Table 3). Analysis further reveals that weed density and dry weight/m2 increased with increasing levels of nitrogen to rice up to 100 kg N/ha (Table 3). This likely due to weeds utilizing a greater quantity of applied and available nutrients. Thus, higher dose of nitrogen accelerated weed emergence and growth. Weed control efficiency increased with increasing nitrogen levels for pigeonpea and rice. Similar findings

Data presented in Table 3 also indicates that two hand weeding at 15 and 45 DAS resulted in the lowest density and dry weight of weeds/m2 followed by pendimethalin + one hand weeding at 45 DAS; both treatments were superior over other weed management treatments. This result was likely owing to better indiscriminate control of all types of weeds by hand weeding. These findings were in close agreement with those of Shetty and Krantz (1976), Ampong-Nyarko and De Datta(1993) and Reddy et al. (2007). Reflecting minimum density and dry weight results, maximum weed control efficiency was obtained with two hand weeding at 15 and 45 DAS. This finding is in agreement with finding of Sinha et al. (1989a and b), Goyal et al. (1991), Parthi et al. (1991), Mahapatra (1991), Prasad and Srivastava (1991), Maheswarappa and Nanjappa (1994), Rafey and Prasad (1995), Patil and Pandey (1996), Mishra et al. (1998), Singh et al. (1998c), Singh et al. (1999), Rana and Pal (1999), Rana et al. (1999), Manickam et al. (2000), Reddy et al. (2007) and Singh (2007).

Dry matter production, grain and straw yield of pigeonpea and rice, and pigeonpea grain equivalent yield were increased with application of 25 kg N/ha to pigeonpea over control (Table 3 and 4). Similar findings have been reported earlier by Singh et al. (1978), Bhandhari et al. (1989), Chittapur et al. (1994), Patel and Patel (1994), Singh et al.(1998a and b), Mandal et al. (1999) and Singh (2006b). Dry matter production, grain and straw yield of rice were increased significantly up to 75 kg N/ha applied to rice (Table 3 and 4). The improvement in the dry matter production and yields of rice might be attributed to the adequate supply of photosynthate to sink under sufficient supply of nitrogen. These results were supported by the findings of Samui et al. (1979), Reddy et al. (1986), Abdulsalam and Subramaniam (1988), Purushotham et al. (1988), Raju et al. (1990), Dubey et al. (1991), Bhattacharya and Singh

**3. Results and discussion** 

Hassk.] and wild jute (*Corchorus acutengulus* lamk).

were also reported by Pujari et al. (1989) and Yadav and Singh (2009).

**3.2 Growth, yields and pigeonpea grain equivalent yield** 

**3.1 Weeds** 

(1992), Mazid et al. (1998), Panda et al. (1999) and Bindra et al. (2000). Many researchers also reported that cereal component in legumes based intercropping yielded more at higher levels of nitrogen application (Reddy et al. 1980; Ramesh and Surve 1984; Ofori and Stern 1986; Ezumah et al. 1987; Rao et al. 1987; Kaushik and Gautam 1987; Chowdhury and Rosario 1992; Rafey and Prasad 1992; Bhagat and Dhar 1995; Kushwaha and Chandel 1997; Mandal et al. 2000; Sarwagi and Tripathi 1999; Shivay et al. 1999; Shivay and Singh 2000; Singh, 2006b). Whereas, same were failed to show its effect on pigeonpea (Table 3 and 4). This might be due to the fact that localized placement of nitrogen made was first available to that crop for which it was applied. Forage area of pigeonpea at initial growth stage (50 DAS) was slow due their slow growth habit. Contrary to this, forage area of short duration rice was higher due to faster initial growth rate and planting in furrow between ridges, likely taking most of applied nitrogen easily by themselves in comparison to pigeonpea. Mahapatra et al. (1990) and Singh (2006b) were also find the similar result.


Table 3. Effect of nitrogen levels and weed management practices on crop growth and weed and weed control efficiency under pigeon pea + rice intercropping system (mean of two years)

Ridge Planted Pigeonpea and Furrow

(kg/ha)

(kg/ha)

(kg/ha)

(kg/ha)

**3.3 Nutrient uptake** 

also observed similar results.

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 43

Pigeonpea grain yield Rice grain yield Weed dry weight

Pigeonpea grain N uptake Rice grain N uptake Weed N uptake

Pigeonpea grain P uptake Rice grain P uptake Weed P uptake

Pigeonpea grain K uptake Rice grain K uptake Weed K uptake

> Two HW at 15 and 45DAS

Fig. 1. Effect of weed management practices on weed dry weight, crop yield and nutrient

Pendimethalin + one HW at 45DAS

Application of 25 kg N/ha to pigeonpea increased NPK uptake by grain as well by stalk/straw of pigeonpea and rice over the control. Weed NPK uptake were also higher with 25 kg N/ha applied to pigeonpea. NPK uptake by rice grain and straw increased significantly with each successive increase in nitrogen level applied to them up to 75 kg N/ha (Table 5). Nitrogen levels applied to rice also increased weed NPK uptake up to 100 kg/ha. This is likely due to the optimum nitrogen application and ultimately resulted in subsequent uptake of other nutrients (phosphorus and potassium) due to increased growth. The maximum nutrient uptake under higher nitrogen dose might be due to better root establishment and thus enhanced translocation of absorbed nutrients from soil to plant ultimately resulting in higher growth and yield. Singh (2006b) and Yadav and Singh (2009)

uptake under pigeon pea + rice intercropping system (mean of two years).

kg/ha

Weedy check Pendimethalin 1.0


Table 4. Effect of nitrogen levels and weed management practices on yields under pigeon pea +rice intercropping system (mean of two years). DAS: days after sowing

Among weed management practices, two sequential hand weeding recorded maximum dry matter accumulation and yields of pigeonpea and rice and minimum weed density and dry weight which was followed by pendimethalin + one hand weeding at 45 DAS (Table 3 and 4). This was likely owing to minimum weed competition for water, nutrient and space etc. (Fig. 1). Similar observations were seen by Dwivedi et al. (1991), Mahapatra (1991), Parthi et al. (1991), Dahama et al. (1992), Varshney (1993), Rafey and Prasad (1995), Mahalle (1996), Patil and Pandey (1996), Mishra et al. (1998), Rana and Pal (1999), Rana et al. (1999) and Reddy et al. (2007). The minimum yields were attained in the weedy check. This was again likely owing to higher weed competition for water, nutrient and space etc. (Fig. 1). Similar results were also reported by Ghobrial (1981), Dwivedi et al. (1991), Mahapatra (1991), Rafey and Prasad (1995) and Chandra Pal et al. (2000).

Fig. 1. Effect of weed management practices on weed dry weight, crop yield and nutrient uptake under pigeon pea + rice intercropping system (mean of two years).

#### **3.3 Nutrient uptake**

42 Weed Control

Grain yield (t/ha)

0 1.9 6.3 0.7 1.2 2.1

25 2.3 7.0 0.8 1.4 2.5

CD(*P*=0.05) 0.2 0.5 0.1 0.1 0.17

50 2.0 6.4 0.7 1.2 2.2

75 2.1 6.7 0.8 1.3 2.3

100 2.1 6.9 0.8 1.4 2.4

CD(*P*=0.05) NS NS 0.1 0.1 0.21

Weedy check 1.5 6.0 0.2 0.4 1.6

Pendimethalin@ 1kg / ha 2.0 6.6 0.7 1.2 2.2

Two hand weeding at 15 and 45 DAS 2.4 7.1 1.2 1.9 2.7

CD(*P*=0.05) 0.1 0.2 0.1 0.1 0.07

Among weed management practices, two sequential hand weeding recorded maximum dry matter accumulation and yields of pigeonpea and rice and minimum weed density and dry weight which was followed by pendimethalin + one hand weeding at 45 DAS (Table 3 and 4). This was likely owing to minimum weed competition for water, nutrient and space etc. (Fig. 1). Similar observations were seen by Dwivedi et al. (1991), Mahapatra (1991), Parthi et al. (1991), Dahama et al. (1992), Varshney (1993), Rafey and Prasad (1995), Mahalle (1996), Patil and Pandey (1996), Mishra et al. (1998), Rana and Pal (1999), Rana et al. (1999) and Reddy et al. (2007). The minimum yields were attained in the weedy check. This was again likely owing to higher weed competition for water, nutrient and space etc. (Fig. 1). Similar results were also reported by Ghobrial (1981), Dwivedi et al. (1991), Mahapatra (1991), Rafey

Table 4. Effect of nitrogen levels and weed management practices on yields under pigeon pea +rice intercropping system (mean of two years). DAS: days after sowing

Stalk yield (t/ha)

Pigeonpea Rice Pigeonpea

Straw yield (t/ha)

Grain yield (t/ha)

2.3 7.0 1.0 1.6 2.6

grain equivalent yield (t/ha)

Treatment

N level in Pigeonpea(kg/ha)

N level in Rice(kg/ha)

Weed Management

Pendimethalin@ 1kg / ha +one hand weeding at 45 DAS

and Prasad (1995) and Chandra Pal et al. (2000).

Application of 25 kg N/ha to pigeonpea increased NPK uptake by grain as well by stalk/straw of pigeonpea and rice over the control. Weed NPK uptake were also higher with 25 kg N/ha applied to pigeonpea. NPK uptake by rice grain and straw increased significantly with each successive increase in nitrogen level applied to them up to 75 kg N/ha (Table 5). Nitrogen levels applied to rice also increased weed NPK uptake up to 100 kg/ha. This is likely due to the optimum nitrogen application and ultimately resulted in subsequent uptake of other nutrients (phosphorus and potassium) due to increased growth. The maximum nutrient uptake under higher nitrogen dose might be due to better root establishment and thus enhanced translocation of absorbed nutrients from soil to plant ultimately resulting in higher growth and yield. Singh (2006b) and Yadav and Singh (2009) also observed similar results.

Ridge Planted Pigeonpea and Furrow

N level in

N level in Rice(kg/ha)

/ ha

/ ha

at 45 DAS

Pigeonpea(kg/ha)

Weed Management

Pendimethalin@ 1kg

Pendimethalin@ 1kg

+one hand weeding

Two hand weeding at 15 and 45 DAS

applied nitrogen over crop.

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 45

uptake(kg/ha)

Rice

W

ea

Potassium uptake(kg/ha)

Pigeonpea Rice

Nitrogen uptake(kg/ha) Phosphorus

W Treatment G S G S G S G S G S G S

0 58.4 43.3 8.1 5.2 21.1 17.4 7.0 1.3 1.4 12.2 12.7 57.6 2.6 18.1 25.6 25 73.1 49.8 9.5 5.8 25.2 21.5 8.0 1.5 1.6 15.4 15.8 65.2 3.1 20.4 30.7 C.D.(*P*=0.05) 6.2 5.0 0.5 0.3 2.0 1.7 1.0 0.1 0.2 1.1 1.5 6.0 0.3 1.9 2.3

50 63.0 44.1 7.8 5.0 21.2 18.7 7.0 1.2 1.4 12.5 13.7 58.1 2.5 17.6 25.8 75 66.3 46.9 9.0 5.6 23.3 19.6 7.6 1.4 1.6 13.9 14.4 62.0 2.9 19.6 28.5 100 67.9 48.7 9.7 5.9 24.9 20.0 7.9 1.5 1.7 15.1 14.7 64.0 3.1 20.7 30.1 C.D.(*P*=0.05) NS NS 0.6 0.4 2.4 NS NS 0.2 0.2 1.4 NS NS 0.4 2.4 2.8

Weedy check 46.7 39.6 2.3 1.7 56.7 13.9 5.8 0.4 0.4 33.2 10.1 53.1 0.8 6.1 67.8

C.D.(*P*=0.05) 2.6 2.1 0.3 0.2 1.5 0.7 0.4 0.1 0.1 0.8 0.6 2.6 0.2 1.0 1.7

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to grain yield and grain nutrient uptake by pigeonpea (Fig. 3). Application of 25 kg N/ha to pigeonpea under two hand weeded plots resulted in maximum yield and nutrient uptake by pigeonpea and this treatment was similar to application of 25 kg N/ha to pigeonpea with pendimethalin + one hand weeding at 45 DAS. Further, there was minimum removal of NPK by weeds (Fig. 6) in this treatment which was ultimately utilized by the crop and promoted its growth and yield. All weed management practices along with no nitrogen application in pigeonpea gave higher pigeonpea grain yield and nutrient uptake over the weedy check along with application of 25 kg N/ha in pigeonpea. This might be due pigeonpea growing better even without nitrogen addition where weeds were controlled. In case of weedy check, weeds were dominant competitor to

Table 5. Effect of nitrogen levels and weed management practices on nutrient uptake by component crops and weed under pigeon pea +rice intercropping system (mean of two

years).G: Grain, S: Straw/Stalk, W: Weed, DAS: days after sowing

64.8 46.1 7.6 5.0 22.0 19.2 7.4 1.1 1.4 13.4 14.0 60.8 2.5 18.0 27.2

74.6 49.8 11.2 6.9 8.3 22.1 8.3 1.7 1.9 5.2 16.2 65.2 3.6 23.9 10.5

76.9 50.8 14.2 8.5 5.5 22.7 8.4 2.2 2.4 3.5 16.7 66.4 4.6 29.1 7.0

Pigeonpea Rice W Pigeonp

Among the weed management practices, two sequential hand weeding recorded higher NPK uptake by grain and straw of pigeonpea and rice and this treatment was followed by with pendimethalin + one hand weeding at 45 DAS (Table 5). Contrary to this, minimum NPK removals by weed were associated with these treatment and maximum with weedy check (Table 5). This might be due to applied inputs assimilated efficiently by weeds under weedy condition and by crops under weed free condition (Table 5 and Fig. 1). These results are in agreement with findings of Singh et al. (1980), Singh and Singh (1985), Sinha et al.(1989a and b), Goyal et al. (1991), Maheswarappa and Nanjappa (1994), Singh et al. (1998c) and Singh (2007).

Fig. 2. Interaction effect of nitrogen levels and weed management practices on rice grain yield underpigeon pea + rice intercropping system (mean of two years).

#### **3.4 Interaction effect**

Interaction effect of nitrogen levels in pigeonpea and rice and weed management practices was significant in respect to grain yield of rice (Fig. 2). Grain yield of rice increased with increasing level of nitrogen applied to rice up to 100 kg N/ha with or without 25 kg N/ha applied to pigeonpea in combination with all weed management treatments except weedy check where all nitrogen level failed to show any significant increase in grain yield of rice. This might be due to fact that rice grew better even with lower nitrogen addition where weeds were controlled than weedy check. In case of weedy check, weeds were dominant competitor to applied nitrogen over crop. Soundara and Mahapatra (1978) found that the maximum grain yield of direct seeded rice with application of 100 kg N/ha along with two sequential hand weedings where weeds effectively controlled. Sharma (1997) observed that grain yield of rice increased significantly with N application up to 60 kg/ha when weeds were controlled. He also observed that grain yield of rice remained unaffected with N application under weedy conditions due to severe competition.

#### Ridge Planted Pigeonpea and Furrow Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 45

44 Weed Control

Among the weed management practices, two sequential hand weeding recorded higher NPK uptake by grain and straw of pigeonpea and rice and this treatment was followed by with pendimethalin + one hand weeding at 45 DAS (Table 5). Contrary to this, minimum NPK removals by weed were associated with these treatment and maximum with weedy check (Table 5). This might be due to applied inputs assimilated efficiently by weeds under weedy condition and by crops under weed free condition (Table 5 and Fig. 1). These results are in agreement with findings of Singh et al. (1980), Singh and Singh (1985), Sinha et al.(1989a and b), Goyal et al. (1991), Maheswarappa and Nanjappa (1994), Singh et al. (1998c)

Fig. 2. Interaction effect of nitrogen levels and weed management practices on rice grain

100 kg N/ha in rice

50 kg N/ha in rice

75 kg N/ha in rice

Weedy check

Pendimethalin 1.0 kg/ha

25 kg N/ha in pigeonpea

100 kg N/ha in rice

Interaction effect of nitrogen levels in pigeonpea and rice and weed management practices was significant in respect to grain yield of rice (Fig. 2). Grain yield of rice increased with increasing level of nitrogen applied to rice up to 100 kg N/ha with or without 25 kg N/ha applied to pigeonpea in combination with all weed management treatments except weedy check where all nitrogen level failed to show any significant increase in grain yield of rice. This might be due to fact that rice grew better even with lower nitrogen addition where weeds were controlled than weedy check. In case of weedy check, weeds were dominant competitor to applied nitrogen over crop. Soundara and Mahapatra (1978) found that the maximum grain yield of direct seeded rice with application of 100 kg N/ha along with two sequential hand weedings where weeds effectively controlled. Sharma (1997) observed that grain yield of rice increased significantly with N application up to 60 kg/ha when weeds were controlled. He also observed that grain yield of rice remained unaffected with N

yield underpigeon pea + rice intercropping system (mean of two years).

75 kg N/ha in rice

Pendimethalin + one HW at 45DAS

Two HW at 15 and 45DAS

0 kg N/ha in pigeonpea

application under weedy conditions due to severe competition.

and Singh (2007).

Rice grain yield(t/ha)

**3.4 Interaction effect** 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

> 50 kg N/ha in rice


Table 5. Effect of nitrogen levels and weed management practices on nutrient uptake by component crops and weed under pigeon pea +rice intercropping system (mean of two years).G: Grain, S: Straw/Stalk, W: Weed, DAS: days after sowing

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to grain yield and grain nutrient uptake by pigeonpea (Fig. 3). Application of 25 kg N/ha to pigeonpea under two hand weeded plots resulted in maximum yield and nutrient uptake by pigeonpea and this treatment was similar to application of 25 kg N/ha to pigeonpea with pendimethalin + one hand weeding at 45 DAS. Further, there was minimum removal of NPK by weeds (Fig. 6) in this treatment which was ultimately utilized by the crop and promoted its growth and yield. All weed management practices along with no nitrogen application in pigeonpea gave higher pigeonpea grain yield and nutrient uptake over the weedy check along with application of 25 kg N/ha in pigeonpea. This might be due pigeonpea growing better even without nitrogen addition where weeds were controlled. In case of weedy check, weeds were dominant competitor to applied nitrogen over crop.

Ridge Planted Pigeonpea and Furrow

results were also reported by Sharma (1997).

0.0 1.0 2.0 3.0 4.0

0.0 0.5 1.0 1.5 2.0

0.0 1.0 2.0 3.0 4.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

(t/ha)

(t/ha)

(kg/ha)

(kg/ha)

(kg/ha)

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 47

N/ha to rice under two hand weeded plots resulted in maximum grain yield and grain nutrient uptake by rice which was similar to the application of 75 kg N/ha under two sequential hand weeded plots and superior than rest of the other treatment combination (Fig. 5). All weed management practices in combination with 50 kg N/ha applied in rice produced higher grain yield and NPK uptake by rice over weedy check in combination with 100 kg N applied in rice. This might be due to crop plants utilizing nitrogen more efficiently even at lower level of nitrogen (50 kg/ha) in absence of weeds than higher level of nitrogen (100 kg/ha) in presence of weeds because most of which was utilized by weeds. Similar

0 kg N/ha 25 kg N/ha

Pigeonpea grain equivalent yield

Rice grain yield

Rice grain N uptake

Rice grain P uptake

Rice grain K uptake

Pendimethalin + one HW at 45DAS

Two HW at 15 and 45DAS

Fig. 4. Interaction effect of nitrogen levels in pigeonpea and weed management practices on pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake under

kg/ha

pigeon pea + rice intercropping system (mean of two years).

Weedy check Pendimethalin 1.0

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake (Fig. 4). Application of 25 kg N/ha to pigeonpea following two sequential hand weedings gave maximum pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake and minimum yield and nutrient uptake resulting from no nitrogen application under weedy condition. These results agree with findings of Soundara and Mahapatra (1978) and Sharma (1997). The significant increase in pigeonpea grain equivalent yield, rice grain yield and rice grain N, P and K uptake with N application were observed only in weed controlled plots (Fig. 4). This might be due to rice compete strongly with pigeonpea for nitrogen in absence of weeds when first at its log phase and second at its lag phase of growth.

Fig. 3. Interaction effect of nitrogen levels in pigeonpea and weed management practices on pigeopea grain yield and pigeopea grain nutrient uptake under pigeon pea + rice intercropping system (mean of two years).

Interaction effect of nitrogen levels in rice and weed management practices was significant in respect to grain yield and grain nutrient uptake by rice (Fig. 5). Application of 100 kg

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake (Fig. 4). Application of 25 kg N/ha to pigeonpea following two sequential hand weedings gave maximum pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake and minimum yield and nutrient uptake resulting from no nitrogen application under weedy condition. These results agree with findings of Soundara and Mahapatra (1978) and Sharma (1997). The significant increase in pigeonpea grain equivalent yield, rice grain yield and rice grain N, P and K uptake with N application were observed only in weed controlled plots (Fig. 4). This might be due to rice compete strongly with pigeonpea for nitrogen in absence of weeds when first at its log phase and second at its lag

0 kg N/ha 25 kg N/ha

Pigeonpea grain yield

Pigeonpea grain N uptake

Pigeonpea grain P uptake

Pigeonpea grain K uptake

Fig. 3. Interaction effect of nitrogen levels in pigeonpea and weed management practices on

Pendimethalin + one HW at 45DAS Two HW at 15 and 45DAS

Interaction effect of nitrogen levels in rice and weed management practices was significant in respect to grain yield and grain nutrient uptake by rice (Fig. 5). Application of 100 kg

pigeopea grain yield and pigeopea grain nutrient uptake under pigeon pea + rice

kg/ha

Weedy check Pendimethalin 1.0

intercropping system (mean of two years).

phase of growth.

0.0 0.5 1.0 1.5 2.0 2.5 3.0

(t/ha)

(kg/ha)

(kg/ha)

(kg/ha)

N/ha to rice under two hand weeded plots resulted in maximum grain yield and grain nutrient uptake by rice which was similar to the application of 75 kg N/ha under two sequential hand weeded plots and superior than rest of the other treatment combination (Fig. 5). All weed management practices in combination with 50 kg N/ha applied in rice produced higher grain yield and NPK uptake by rice over weedy check in combination with 100 kg N applied in rice. This might be due to crop plants utilizing nitrogen more efficiently even at lower level of nitrogen (50 kg/ha) in absence of weeds than higher level of nitrogen (100 kg/ha) in presence of weeds because most of which was utilized by weeds. Similar results were also reported by Sharma (1997).

Fig. 4. Interaction effect of nitrogen levels in pigeonpea and weed management practices on pigeonpea grain equivalent yield, rice grain yield and rice grain nutrient uptake under pigeon pea + rice intercropping system (mean of two years).

Ridge Planted Pigeonpea and Furrow

under severe competition.

(kg/ha)

(g/m2)

(kg/ha)

(kg/ha)

(mean of two years).

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 49

two hand weeding at 15 and 45 DAS or pendimethalin + one hand weeding at 45 DAS did not cause any significant variation in NPK removal by weeds. Maximum weed dry weight and weed nutrient uptake were recorded with application of 100 kg N/ha to rice under the weedy check (Fig. 7). This might be due to weeds utilizing more inputs than crop plant

0 kg N/ha 25 kg N/ha

Weed dry weight

Weed N uptake

Weed P Uptake

Weed K uptake

Pendimethalin + one HW at 45DAS Two HW at 15 and 45DAS

Fig. 6. Interaction effect of nitrogen levels in pigeonpea and weed management practices on weed dry weight and weed nutrient uptake under pigeon pea + rice intercropping system

kg/ha

Weedy check Pendimethalin 1.0

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to weed dry weight and weed nutrient uptake (Fig. 6). The weed dry weight and weed nutrient uptake were recorded lower with or without application of 25 kg N/ha to pigeonpea under weed controlled plots than with or without application of 25 kg N/ha to pigeonpea under weedy check. Whereas, application of 25 kg N/ha to pigeonpea increased the weed dry weight and weed nutrient uptake only under weedy check. This due to one would exert severe competition on another under their dominance.

Fig. 5. Interaction effect of nitrogen levels in rice and weed management practices on rice grain yield and rice grain nutrient uptake under pigeon pea + rice intercropping system (mean of two years).

Interaction effect of nitrogen levels in rice and weed management practices was significant in respect to weed dry weight and weed nutrient uptake (Fig. 7). All weed management practices recorded lower nutrient removal by weeds irrespective of nitrogen levels applied in rice over weedy check which had maximum nutrient removal by weeds. This might be due to lower weed density and their dry weight in weed free condition ultimately resulting in lower NPK removal by weeds. Varying nitrogen levels applied in rice either along with

Interaction effect of nitrogen levels in pigeonpea and weed management practices was significant in respect to weed dry weight and weed nutrient uptake (Fig. 6). The weed dry weight and weed nutrient uptake were recorded lower with or without application of 25 kg N/ha to pigeonpea under weed controlled plots than with or without application of 25 kg N/ha to pigeonpea under weedy check. Whereas, application of 25 kg N/ha to pigeonpea increased the weed dry weight and weed nutrient uptake only under weedy check. This due

> 50 kg N/ha 75 kg N/ha 100 kg N/ha Rice grain yield

> > Rice grain N uptake

Rice grain P uptake

Rice grain K uptake

Pendimethalin + one HW at 45DAS Two HW at 15 and 45DAS

Fig. 5. Interaction effect of nitrogen levels in rice and weed management practices on rice grain yield and rice grain nutrient uptake under pigeon pea + rice intercropping system

kg/ha

Weedy check Pendimethalin 1.0

Interaction effect of nitrogen levels in rice and weed management practices was significant in respect to weed dry weight and weed nutrient uptake (Fig. 7). All weed management practices recorded lower nutrient removal by weeds irrespective of nitrogen levels applied in rice over weedy check which had maximum nutrient removal by weeds. This might be due to lower weed density and their dry weight in weed free condition ultimately resulting in lower NPK removal by weeds. Varying nitrogen levels applied in rice either along with

(mean of two years).

0.0 0.5 1.0 1.5 2.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 1.0 2.0 3.0 4.0 5.0

(t/ha)

(kg/ha)

(kg/ha)

(kg/ha)

to one would exert severe competition on another under their dominance.

two hand weeding at 15 and 45 DAS or pendimethalin + one hand weeding at 45 DAS did not cause any significant variation in NPK removal by weeds. Maximum weed dry weight and weed nutrient uptake were recorded with application of 100 kg N/ha to rice under the weedy check (Fig. 7). This might be due to weeds utilizing more inputs than crop plant under severe competition.

Fig. 6. Interaction effect of nitrogen levels in pigeonpea and weed management practices on weed dry weight and weed nutrient uptake under pigeon pea + rice intercropping system (mean of two years).

Ridge Planted Pigeonpea and Furrow

**6. Acknowledgment** 

Delhi, India

439-443

**7. References** 

Planted Rice in an Intercropping System as Affected by Nitrogen and Weed Management 51

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#### **4. Conclusion**

Pigeonpea and rice could be fertilized with 25 kg N/ha and 75 kg N/ha, respectively, in an intercropping system integrated with two sequential hand weeding at 15 and 45 DAS for higher growth, yield and nutrient uptake by the crops. The next most effective treatment was application of 25 kg N/ha to pigeonpea and 75 kg N/ha to rice in the intercropping system integrated with a pre emergence application of pendimethalin at the rate of 1.0 kg/ha followed by one hand weeding at 45 DAS.

#### **5. Future research**

Studies are required to investigate the effect of rice cultivars, nitrogen levels under weeded and weedy condition in a pigeonpea+ rice intercropping system.

#### **6. Acknowledgment**

The senior author would like to express his gratitude to University Grants Commission, New Delhi for providing research fellowship during his Ph.D. programme.

#### **7. References**

50 Weed Control

50 kg N/ha 75 kg N/ha 100 kg N/ha Weed dry weight

Weed N uptake

Weed P uptake

Weed K Uptake

Pendimethalin + one HW at 45DAS

Two HW at 15 and 45DAS

Fig. 7. Interaction effect of nitrogen levels in pigeonpea and weed management practices on weed nutrient uptake under pigeon pea + rice intercropping system (mean of two years)

1.0 kg/ha

Pigeonpea and rice could be fertilized with 25 kg N/ha and 75 kg N/ha, respectively, in an intercropping system integrated with two sequential hand weeding at 15 and 45 DAS for higher growth, yield and nutrient uptake by the crops. The next most effective treatment was application of 25 kg N/ha to pigeonpea and 75 kg N/ha to rice in the intercropping system integrated with a pre emergence application of pendimethalin at the rate of 1.0

Studies are required to investigate the effect of rice cultivars, nitrogen levels under weeded

**4. Conclusion** 

(g/m2)

((kg/ha)

(kg/ha)

(kg/ha)

**5. Future research** 

kg/ha followed by one hand weeding at 45 DAS.

and weedy condition in a pigeonpea+ rice intercropping system.

Weedy check Pendimethalin


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rainfed conditions. *Indian Journal of Agronomy* 29(1): 101-106

rainfed lowland situation. *Indian Journal of Agronomy* 44(4): 722-727.

rice(*Oryza sativa*) and soybean (*Glycine max*) intercropping. *Indian Journal of* 

performance of rice under flood prone lowland conditions. *Journal of Agricultural* 

Maize (*Zea mays)* as influenced by cropping systems and nitrogen levels. *Annals of* 

competition on yield and nutrient uptake by direct-seeded rice (*Oryza sativa*) in

and phosphorus on growth and yield of pigeonpea (*Cajanus cajan*). *Indian Journal of* 

weed control in pigeonpea under humid-subtropical conditions of Pantnagar.

pigeonpea+urdbean intercropping system. *Indian Journal of Pulses Research* 8(1): 29-

planting. 8th Indian Agricultural Scientists and Farmer's Congress, February 21-22,

systems under adequate water supply conditions. *Indian Journal of Agricultural* 


Patil, B.M. and Pandey, J. (1996). Chemical weed control in pigeonpea (*Cajanus cajan*)

Prasad, K. and Srivastava, V.C. (1991). Weed management in pure and mixed crops of

Pujari, B.T.; Hosamani, M.M; Sharma, K.M.S.; Goudreddy, B.S. and Patel, V.C. (1989).

Rafey, A. and Prasad, N.K. (1992). Biological potential and economic feasibility of maize(*Zea* 

Rafey, A. and Prasad, N.K. (1995). Influence of weed management practices in pigeonpea

Raju, U.R.; Jaganathan, A. and Rao, R.S. (1990). Performance of scented rice varieties under

Ramesh, D.G. and Surve, D.N. (1984). Intercropping of legumes in sorghum with different levels of nitrogen. *Journal of Maharashtra Agricultural Universities* 9(3): 313-315 Rana, K.S. and Pal, M. (1999). Effect of intercropping systems and weed control on crop

Rana, K.S.; Mahendra Pal and Rana, D.S. (1999). Nutrient depletion by pigeonpea ( *Cajanus* 

Rao, M.R.; Rego, T.J. and Willey, R.W. (1987). Response of cereals to nitrogen in sole cropping and intercropping with different legumes. *Plant and Soil* 101(2): 167-177 Rathi, K.S. and Verma, V.S. (1979). Potato and mustard- A new companionship. *Indian* 

Reddy, G.R.S.; Reddy, G.B.; Ramaiah, N.V. and Reddy, G.V. (1986). Effect of rates and

Reddy, K.C.S.; Hussain, M.M. and Krantz, B.A. (1980). Effect of nitrogen levels and spacing

Reddy, M.M; Madhavilatha, A. and Rao, L.J. (2007). Integrated weed management in

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Risser, P.G. (1969). Competitive relationship among herbaceous grassland plants. *Biological* 

under rainfed conditions. *Indian Journal of Agronomy* 44(2): 267-270

Karnataka. *Journal of Maharashtra Agricultural Universities* 14(2): 189-192 Purushotham, S.; Kulakarmi, K.P. and Sharma, K.M.S. (1988).Comparative performance of

529-535

143-146

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*Farming* 28(11): 13-14

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182

416-418

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*Agricultural Sciences* 62(2): 110-113

*Agricultural Sciences* 65(4): 281-282

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pigeonpea(*Cajanus cajan*)+soybean(*Glycine max*) intercropping system in vertisols


**1. Introduction** 

profitability for the grower.

**1.1 History of mulching in vegetable systems** 

**3** 

*USA* 

**Mulches for Weed Management** 

The practice of applying mulches for the production of vegetables is thousands of years old (Lightfoot, 1994; Rowe-Dutton, 1957). Typically mulching involves placing a layer of material on the soil around the crop of interest to modify the growing environment to improve crop productivity. The primary purpose for using mulches is for weed suppression in the crop to be grown. Mulches typically function by blocking light or creating environmental conditions which can prevent germination or suppress weed growth shortly after germination. However, numerous other benefits are often obtained including: increased earliness, moisture conservation, temperature regulation of the root zone and above-ground growing environment, reduced nutrient leaching, altered insect and disease pressures, and, in some instances, reduced soil compaction or improved soil organic matter (Lamont, 2005; Lamont, 1993; Ngouajio and McGiffen, 2004; Rowe-Dutton, 1957). The use of mulches typically results in higher yields and quality in vegetable crops enhancing

A wide variety of mulches have been utilized throughout history. Lithic-mulches, which include pebbles and gravel as well as volcanic ash, may be some of the earliest documented mulches used in vegetable production. Depending on the site and crop grown, lithicmulches could take the form of mounds around individual plants, long rows or ridges of larger stones, or vast areas where an entire production site is covered in pebbles or volcanic ash (Lightfoot, 1994). Although primarily used in areas with scarce moisture, lithic-mulches also modulate fluctuations in soil temperatures as well as reduce weeds (Lightfoot, 1994). Some of the earliest documented sites where lithic-mulches were used date to 200 B.C. and are found in the Negev desert of Israel (Kedar, 1957). These mulches may have been used with grapevines or olive trees; though, it is unclear if they were used for vegetable production (Mayerson, 1959 as cited in Lightfoot, 1994). The Maori people of New Zealand used gravel mulches in fields between the years of 1200 and 1800 AD to grow sweetpotatoes [*Ipomoea batatas* (L.) Lam.] and maize (*Zea mays* L.) (Lightfoot, 1994; Rigg and Bruce, 1923). In a practice dating back several hundred years, growers in the Lanzhou area of China have used river pebbles at a depth of 7-10 cm for the production of melons (*Cucumis* sp.*)*

**in Vegetable Production** 

Timothy Coolong

*Department of Horticulture, University of Kentucky,* 


## **Mulches for Weed Management in Vegetable Production**

Timothy Coolong *Department of Horticulture, University of Kentucky, USA* 

#### **1. Introduction**

56 Weed Control

Singh, S.P.; Singh, S.P. and Misra, P.K. (1998b). Response of short duration

Singh, V.K.; Singh, N.P.; Sharma, B.B. and Sahu, J.P. (1998c). Effect of planting method and

(*Cajanus cajan*) in foothills region. *Indian Journal of Agronomy* 43(4): 685-688 Sinha, A.C.; Mandal, B.B. and Jana, P.K. (1989a). Effect of time of sowing, row spacing and

Sinha, A.C.; Mandal, B.B. and Jana, P.K. (1989b). Effect of time of sowing, row spacing and

Soundara, M.S.R. and Mahapatra, I.C. (1978). Relative efficiency of slow release and split

Subbiah, B.V. and Asija,G.L. (1973). A rapid procedure for estimation of available nitrogen

Varshney, J.G. (1993). Weed management in pigeonpea(*Cajanus cajan*) and greengram

Walkley, A. and Black, I.A. (1934). An examination of the Digtijareff method for determining

Willey, R.W. (1979). Intercropping - its importance and research needs - 1. Competition and

Yadav, M.K. and Singh, R.S. (2009). Effect of nitrogen levels and weed management

*Journal of Agronomy* 43(4): 681-684

34(3): 283-285

4-7

272

*Indian Journal of Agricultural Sciences* 59(6): 353-358

seeded upland rice. *Oryza* 15(2): 117-123

yield advantage. *Field Crop Abstracts* 32(1): 1-10

in soils. *Current Science* 28(8): 259-260

method. *Soil Science* 37: 29-33

pigeonpea(*Cajanus cajan*) to nitrogen, *Rhizobium* inoculation and phosphorus. *Indian* 

weed control practice on weed management and productivity of pigeonpea

weed control treatments on weeds and grain yield of pigeonpea(*Cajanus cajan*).

weed control practices on production of pigeonpea. *Indian Journal of Agronomy*

application of nitrogenous fertilizers and weed control methods on yield of direct-

(*Phaseolus radiatus*) intercropping system. *Indian journal of Agricultural Sciences* 63(1):

soil organic matter and a proposed modification of the chromic acid titration

practices on pigeonpea(*Cajanus cajan*) and rice(*Oryza sativa*) intercropping system under ridge-furrow planting system. *Indian Journal of Agricultural Sciences* 79(4):268The practice of applying mulches for the production of vegetables is thousands of years old (Lightfoot, 1994; Rowe-Dutton, 1957). Typically mulching involves placing a layer of material on the soil around the crop of interest to modify the growing environment to improve crop productivity. The primary purpose for using mulches is for weed suppression in the crop to be grown. Mulches typically function by blocking light or creating environmental conditions which can prevent germination or suppress weed growth shortly after germination. However, numerous other benefits are often obtained including: increased earliness, moisture conservation, temperature regulation of the root zone and above-ground growing environment, reduced nutrient leaching, altered insect and disease pressures, and, in some instances, reduced soil compaction or improved soil organic matter (Lamont, 2005; Lamont, 1993; Ngouajio and McGiffen, 2004; Rowe-Dutton, 1957). The use of mulches typically results in higher yields and quality in vegetable crops enhancing profitability for the grower.

#### **1.1 History of mulching in vegetable systems**

A wide variety of mulches have been utilized throughout history. Lithic-mulches, which include pebbles and gravel as well as volcanic ash, may be some of the earliest documented mulches used in vegetable production. Depending on the site and crop grown, lithicmulches could take the form of mounds around individual plants, long rows or ridges of larger stones, or vast areas where an entire production site is covered in pebbles or volcanic ash (Lightfoot, 1994). Although primarily used in areas with scarce moisture, lithic-mulches also modulate fluctuations in soil temperatures as well as reduce weeds (Lightfoot, 1994). Some of the earliest documented sites where lithic-mulches were used date to 200 B.C. and are found in the Negev desert of Israel (Kedar, 1957). These mulches may have been used with grapevines or olive trees; though, it is unclear if they were used for vegetable production (Mayerson, 1959 as cited in Lightfoot, 1994). The Maori people of New Zealand used gravel mulches in fields between the years of 1200 and 1800 AD to grow sweetpotatoes [*Ipomoea batatas* (L.) Lam.] and maize (*Zea mays* L.) (Lightfoot, 1994; Rigg and Bruce, 1923). In a practice dating back several hundred years, growers in the Lanzhou area of China have used river pebbles at a depth of 7-10 cm for the production of melons (*Cucumis* sp.*)*

Mulches for Weed Management in Vegetable Production 59

under mulches compared to bare ground leading to enhanced pineapple production

Paper-based mulches were utilized through much of the early 20th century with positive results. Thompson and Platenius (1931) reported positive results with paper mulches in several vegetable crops including pepper (*Capsicum* sp.), tomato, and muskmelon (*Cucumis melo* L.). Paper mulches controlled weeds, increased soil temperatures and moisture levels, resulting in greater yields (Thompson and Platenius, 1931). By altering the growing environment, black-paper mulches affected root distribution of several vegetable crops when compared to bare-ground production (Knavel and Mohr, 1967). The use of paper mulches, particularly those impregnated with asphalt or tar, suppressed weeds, conserved moisture, and warmed the soil, increasing yields in most warm-season crops. However,

In addition to paper and organic mulches several other substances were evaluated through the early and mid 20th century. Aluminum foil was shown to be an effective mulch, increasing yields (Burgis, 1950). The reflective nature of aluminum foil mulch actually cooled soil, while affecting insect predation and the spread of some insect-transmitted diseases (Adlerz and Everett, 1968; Burgis, 1950; Wolfenbarger and Moore, 1968). However, while effective and more durable than paper, aluminum foil mulches were never

Petroleum-based spray mulches were also evaluated as an in-row band for cucurbits grown in Florida (Nettles, 1963). These spray mulches functioned as an effective pre-emergent herbicide and warmed the soil. Early and total season yields were significantly greater for petroleum mulches compared to a bare-ground control (Nettles, 1963; Takatori et al., 1963). Despite success with cucurbits, petroleum spray mulches were found to be no more effective

Invented in its solid form in 1935 by British chemists Fawcett and Gibson, and first made into a sheet form in 1938, polyethylene has changed vegetable production around the world (Lamont et al., 1993; Lamont, 1996; Partington, 1970; Wright, 1968). Much of the pioneering research using low density polyethylene (LDPE) mulch was conducted by Dr. Emery Emmert in the 1950s at the University of Kentucky. In his earliest research, Emmert utilized 0.0015 gauge (1.5 mil-thick) black and black-aluminum pigmented plastic sheets. Transplanted tomatoes and direct-seeded pole beans (*Phaseolus* sp.) were some of the first crops tested with plastic mulches (Emmert, 1956; Emmert, 1957). Irrigation was achieved by cutting furrows in the ground next to the crop, covering with plastic, and cutting holes in the plastic for the water to penetrate the plant bed. In the earliest trials with plastic mulches, Emmert found similar results as previous researchers observed with paper mulches. Weed control and yields, particularly early in the season, were significantly better in treatments grown using the plastic mulch compared to a non-mulched control. In some treatments, Emmert reported an increase in yield of more than 200 bushels/acre (5000 kg/ha) for pole beans grown on plastic compared to a bare-ground control (Emmert, 1957). Although

issues with cost and durability led to the development of alternative mulches.

(Stewart et al., 1926).

**1.3 Other mulches** 

**2. Polyethylene mulch** 

implemented on a large scale due to high costs.

than non-mulched controls for potato production (Hensel, 1968).

(Lightfoot, 1994; Rowe-Dutton, 1957). Pieces of slate were also used as mulches under melons in England nearly 200 years ago; however, this was likely a way to keep fruit dry as well as warm the plants rather than for weed control (Williams 1824 as cited in Rowe-Dutton, 1957).

Dust mulching is another practice that persists to this day; although, it is not a true mulching technique since no materials are applied to the soil. Dust mulching is the practice of repeatedly and shallowly cultivating the soil surrounding the crop to create a pulverized (dust) layer of soil (James, 1945). A theory, though proven to be incorrect, is that by creating a finely textured layer of soil at the surface, capillarity in the soil is "broken" and the movement of water out of the soil via evaporation is reduced (James, 1945; Ladewig, 1951). It is generally accepted that the primary benefit from dust mulching comes from the destruction of weeds around the crop and not a reduction in evaporation at the soil surface (Rowe-Dutton, 1957).

Organic-based mulches such as plant waste, straw, sawdust, and manure have also been used to a great extent for vegetable production. Traditionally, organic mulches have consisted of materials which are locally plentiful. Organic-based mulches can be as diverse as the region in which they are used. For instance, banana (*Musa* sp.) leaves and water hyacinth [*Eichhornia crassipes* (Mart.) Solms] have been used for mulching tomato (*Solanum lycopersicum* L.) in Bangladesh (Kayum et al., 2008), while cane (*Saccharum officinarum* L.) bagasse (sugarcane stalks) have been used in Hawaii (Gilbert, 1956) and sawdust in Pennsylvania (Isenberg and Odland, 1950). When applied as a thick layer, organic-based mulches can effectively suppress weeds and increase soil moisture levels (Diaz-Perez et al., 2004). However, research dating to the late 19th century has shown variable results of organic mulches on yield. In areas with warm temperatures and limitations on water for plant growth, straw mulches have been found to positively affect growth and yields of several cucurbits (Emerson, 1903). However, when used in cool climates, the addition of straw mulch, while beneficial for controlling weeds, has been shown to retard the growth of warm season vegetables and decrease yields (Rowe-Dutton, 1957). Nonetheless, organic mulches remain popular due to their low cost and ready availability.

#### **1.2 Paper-based mulches**

Paper-based mulches represent some of the earliest mulching systems developed for fruit and vegetable production. Paper was an ideal mulch because it could be transported long distances and easily applied from a roll in the field. Paper-based mulches were extensively used in Hawaii in sugarcane production in the early 20th century. In sugarcane production, lightweight-tar or asphalt-impregnated paper mulches were placed over rows of seed cane and held to the ground with soil. Newly emerged cane shoots were sharp enough to pierce the mulch and continue to grow, while weeds were unable to penetrate the paper and died shortly after germination (Stewart et al., 1926). Using a system remarkably similar to modern plastic mulch, Stewart et al. (1926) evaluated asphalt-impregnated paper for in-row weed control for pineapple [*Ananas comosus* (L.) Merr]. In that trial, the mulches were unrolled over beds using a tractor mounted mulch layer, much like those used today. In addition to reducing weed pressure on the crop, paper mulches generally increased the soil temperature by several degrees Fahrenheit on sunny days, with little effect on cloudy or rainy days (Stewart et al., 1926). Soil moisture and nitrate levels were also generally greater under mulches compared to bare ground leading to enhanced pineapple production (Stewart et al., 1926).

Paper-based mulches were utilized through much of the early 20th century with positive results. Thompson and Platenius (1931) reported positive results with paper mulches in several vegetable crops including pepper (*Capsicum* sp.), tomato, and muskmelon (*Cucumis melo* L.). Paper mulches controlled weeds, increased soil temperatures and moisture levels, resulting in greater yields (Thompson and Platenius, 1931). By altering the growing environment, black-paper mulches affected root distribution of several vegetable crops when compared to bare-ground production (Knavel and Mohr, 1967). The use of paper mulches, particularly those impregnated with asphalt or tar, suppressed weeds, conserved moisture, and warmed the soil, increasing yields in most warm-season crops. However, issues with cost and durability led to the development of alternative mulches.

#### **1.3 Other mulches**

58 Weed Control

(Lightfoot, 1994; Rowe-Dutton, 1957). Pieces of slate were also used as mulches under melons in England nearly 200 years ago; however, this was likely a way to keep fruit dry as well as warm the plants rather than for weed control (Williams 1824 as cited in Rowe-

Dust mulching is another practice that persists to this day; although, it is not a true mulching technique since no materials are applied to the soil. Dust mulching is the practice of repeatedly and shallowly cultivating the soil surrounding the crop to create a pulverized (dust) layer of soil (James, 1945). A theory, though proven to be incorrect, is that by creating a finely textured layer of soil at the surface, capillarity in the soil is "broken" and the movement of water out of the soil via evaporation is reduced (James, 1945; Ladewig, 1951). It is generally accepted that the primary benefit from dust mulching comes from the destruction of weeds around the crop and not a reduction in evaporation at the soil surface

Organic-based mulches such as plant waste, straw, sawdust, and manure have also been used to a great extent for vegetable production. Traditionally, organic mulches have consisted of materials which are locally plentiful. Organic-based mulches can be as diverse as the region in which they are used. For instance, banana (*Musa* sp.) leaves and water hyacinth [*Eichhornia crassipes* (Mart.) Solms] have been used for mulching tomato (*Solanum lycopersicum* L.) in Bangladesh (Kayum et al., 2008), while cane (*Saccharum officinarum* L.) bagasse (sugarcane stalks) have been used in Hawaii (Gilbert, 1956) and sawdust in Pennsylvania (Isenberg and Odland, 1950). When applied as a thick layer, organic-based mulches can effectively suppress weeds and increase soil moisture levels (Diaz-Perez et al., 2004). However, research dating to the late 19th century has shown variable results of organic mulches on yield. In areas with warm temperatures and limitations on water for plant growth, straw mulches have been found to positively affect growth and yields of several cucurbits (Emerson, 1903). However, when used in cool climates, the addition of straw mulch, while beneficial for controlling weeds, has been shown to retard the growth of warm season vegetables and decrease yields (Rowe-Dutton, 1957). Nonetheless, organic

Paper-based mulches represent some of the earliest mulching systems developed for fruit and vegetable production. Paper was an ideal mulch because it could be transported long distances and easily applied from a roll in the field. Paper-based mulches were extensively used in Hawaii in sugarcane production in the early 20th century. In sugarcane production, lightweight-tar or asphalt-impregnated paper mulches were placed over rows of seed cane and held to the ground with soil. Newly emerged cane shoots were sharp enough to pierce the mulch and continue to grow, while weeds were unable to penetrate the paper and died shortly after germination (Stewart et al., 1926). Using a system remarkably similar to modern plastic mulch, Stewart et al. (1926) evaluated asphalt-impregnated paper for in-row weed control for pineapple [*Ananas comosus* (L.) Merr]. In that trial, the mulches were unrolled over beds using a tractor mounted mulch layer, much like those used today. In addition to reducing weed pressure on the crop, paper mulches generally increased the soil temperature by several degrees Fahrenheit on sunny days, with little effect on cloudy or rainy days (Stewart et al., 1926). Soil moisture and nitrate levels were also generally greater

mulches remain popular due to their low cost and ready availability.

Dutton, 1957).

(Rowe-Dutton, 1957).

**1.2 Paper-based mulches** 

In addition to paper and organic mulches several other substances were evaluated through the early and mid 20th century. Aluminum foil was shown to be an effective mulch, increasing yields (Burgis, 1950). The reflective nature of aluminum foil mulch actually cooled soil, while affecting insect predation and the spread of some insect-transmitted diseases (Adlerz and Everett, 1968; Burgis, 1950; Wolfenbarger and Moore, 1968). However, while effective and more durable than paper, aluminum foil mulches were never implemented on a large scale due to high costs.

Petroleum-based spray mulches were also evaluated as an in-row band for cucurbits grown in Florida (Nettles, 1963). These spray mulches functioned as an effective pre-emergent herbicide and warmed the soil. Early and total season yields were significantly greater for petroleum mulches compared to a bare-ground control (Nettles, 1963; Takatori et al., 1963). Despite success with cucurbits, petroleum spray mulches were found to be no more effective than non-mulched controls for potato production (Hensel, 1968).

#### **2. Polyethylene mulch**

Invented in its solid form in 1935 by British chemists Fawcett and Gibson, and first made into a sheet form in 1938, polyethylene has changed vegetable production around the world (Lamont et al., 1993; Lamont, 1996; Partington, 1970; Wright, 1968). Much of the pioneering research using low density polyethylene (LDPE) mulch was conducted by Dr. Emery Emmert in the 1950s at the University of Kentucky. In his earliest research, Emmert utilized 0.0015 gauge (1.5 mil-thick) black and black-aluminum pigmented plastic sheets. Transplanted tomatoes and direct-seeded pole beans (*Phaseolus* sp.) were some of the first crops tested with plastic mulches (Emmert, 1956; Emmert, 1957). Irrigation was achieved by cutting furrows in the ground next to the crop, covering with plastic, and cutting holes in the plastic for the water to penetrate the plant bed. In the earliest trials with plastic mulches, Emmert found similar results as previous researchers observed with paper mulches. Weed control and yields, particularly early in the season, were significantly better in treatments grown using the plastic mulch compared to a non-mulched control. In some treatments, Emmert reported an increase in yield of more than 200 bushels/acre (5000 kg/ha) for pole beans grown on plastic compared to a bare-ground control (Emmert, 1957). Although

Mulches for Weed Management in Vegetable Production 61

At the end of the growing season, plastic mulches must be removed from the field; though in warmer climates mulches are often double or triple cropped (Hanna and Adams, 1989). Double cropping plastic mulch decreases input costs for growers; however, weed pressures are often increased during the second crop as pre-emergent herbicides have dissipated. Although additional herbicides may be applied to spaces between rows; in-row weeds, growing through the planting holes of the previous crop, can be difficult to control (Waterer et al., 2008). In regions with shorter growing seasons, most plastic mulch is removed after one crop, though double-cropping mulches that have been left in fields over a winter have been evaluated (Waterer et al., 2008). To remove plastic mulch from fields, a specialized piece of equipment (mulch lifter) is required. A mulch lifter is a device which undercuts and

The earliest plastic mulches evaluated were 1.5 mil-thick and black (Emmert, 1957). There are now arrays of mulches available. The most common mulches are 1.0 or 1.25 mil-thick and are sold on a 1.2 m-wide roll, though widths of 0.9 – 1.5 m are also produced. Mulches that are thinner than 1.0 mil are easily punctured by weeds. Most degradable plastic mulches are 0.5-0.75 mil-thick, which allows for quicker decomposition. Rolls of mulch commonly range from 730 – 1830 m in length. Mulches may be smooth or embossed. Mulches that are embossed tend to resist excessive expansion and contraction which can

The most popular plastic mulch world-wide is black, though white-on-black and clear mulches are also used (Schales, 1990). Other colors that that have been evaluated include: blue, green, red, yellow, brown, white, and silver (Brault et al., 2002; Gough, 2001; Hanna, 2000; Ngouajio and Ernest, 2004). Different colored mulches have multiple effects on the crops being grown. The optical properties of various colored mulches can influence soil and air temperatures around the crop as well as impact weed growth under the mulch. Moreover, in some cases, colored mulches can alter insect behaviour, which can directly (insect feeding) and indirectly (vectoring diseases) affect crop growth. Colored mulches can be separated into those that do not discriminate between different wavelengths of light transmitted and those that selectively prevent transmission of photosynthetically active radiation (PAR) (400-700 nm) (Ngouajio and Ernest, 2004; Tarara, 2000). Mulches that selectively filter out light in the PAR range are called infrared transmitting (IRT) mulches. In addition to restricting light of the PAR range, IRT mulches tend to transmit high percentages of light at longer wavelengths (>900 nm). By selectively filtering light in the PAR range and transmitting longer wavelength light energy, IRT mulches allow for greater

lifts plastic mulch out of the soil at which time it can be collected and disposed.

cause mulches to become loose from raised beds (Lamont, 1993).

soil warming while reducing light available for weed growth.

**2.3.1 Non-IRT colored mulches effects on light, temperature, and weed growth** 

The most common non-IRT mulches are black, clear, white-on-black, and reflective silver. A myriad of other colors exist including: yellow, blue, red, and green (Figure 1). These colored mulches comprise a very small portion of the total mulch utilized. Although benefits have

**2.2 Characteristics of plastic mulches** 

**2.3 Colored mulches** 

expensive, Emmert estimated that if the plastic material lasted four years in a field, the annual cost would be approximately \$12-\$16 per acre per year (Emmert, 1957).

Much early research evaluated the effect of mulches on yields and microclimate. Soil temperatures were generally higher under black and clear plastic mulches than non-mulched controls (Army and Hudspeth, 1960; Clarkson and Frazier, 1957; Harris, 1965; Nettles, 1963; Oebker and Hopen, 1974; Takatori et al., 1964). Moisture and nitrate levels were generally greater under plastic mulches (Clarkson, 1960; Harris, 1965). This led to earlier (7-14 day) and greater yields in most crops tested (Clarkson and Frazier, 1957). Interestingly, much of the earliest research with plastic mulches indicated that they altered the soil-root zone microclimate in a similar manner as previously reported for asphalt-impregnated paper mulches in the 1920s and 1930s. However, unlike early paper mulches, the plastic-mulch production system has become the dominant mulching tactic for vegetable production.

#### **2.1 Equipment for the plastic-mulch production system**

Early plastic mulches were placed in the field by hand; however, to increase efficiency, specialized equipment was developed. Initial land preparation is similar for bare-ground and plastic-mulch production systems. Soil is ploughed and disked until a fine tilth is achieved. A piece of equipment which can form a raised bed and lay plastic mulch in a single operation is pulled through the field to form the planting bed. When using a raised-bed plastic-mulch system, rows must be spaced further apart in order to accommodate the bed shaping equipment than would be necessary in a flat-bed system. Therefore, raised-bed plastic-mulch rows are typically spaced on 1.7 to 2.2 m centers. Raised beds are often preferred with plastic mulches because they warm quicker than flat beds and offer superior drainage (Lamont, 1996; Tarara, 2000). Herbicides which must be incorporated with tillage may be applied to the soil prior to bed formation or under the mulch while it is being laid in the field. Chemical fumigants are often knifed into the soil under plastic mulches during this process as well (Hartz et al., 1993). Fumigation is an important component of many plastic-mulch production systems. Fumigants have the ability to kill weed seeds, which may potentially germinate, as well as control soil pathogenic fungi, bacteria, and nematodes (Goring, 1962; Wilhelm and Paulus, 1980). Drip irrigation tubing is placed under plastic mulch during the same process. Early research with plastic mulches was conducted using overhead irrigation or furrow irrigation (Emmert, 1957); however, with the introduction of drip irrigation in the 1970s, the vast majority of plastic-mulch production now utilizes this method (Hartz, 1996). The combination of drip irrigation with plastic mulch has significantly increased irrigation water use efficiency in vegetable production (Howell, 2001).

After the plastic is laid in the field, transplants can be placed by hand or using a mechanized transplanter. Plastic mulches must fit tightly against the soil; not only to obtain the maximum benefit of heat transfer from mulch to soil; but also because warm air, when trapped under the mulch, can escape through the holes where transplants are placed, desiccating and damaging the crop (Lamont, 2005). Due to the increased productivity of plastic mulches, in-row spacing of plants is often less compared to bare-ground production systems. Crops which may normally be planted in a single row fashion when grown without mulches are often planted in double rows with plastic mulches (Lamont, 1991). Plant populations per unit area may also be increased in plastic-mulch production systems.

expensive, Emmert estimated that if the plastic material lasted four years in a field, the

Much early research evaluated the effect of mulches on yields and microclimate. Soil temperatures were generally higher under black and clear plastic mulches than non-mulched controls (Army and Hudspeth, 1960; Clarkson and Frazier, 1957; Harris, 1965; Nettles, 1963; Oebker and Hopen, 1974; Takatori et al., 1964). Moisture and nitrate levels were generally greater under plastic mulches (Clarkson, 1960; Harris, 1965). This led to earlier (7-14 day) and greater yields in most crops tested (Clarkson and Frazier, 1957). Interestingly, much of the earliest research with plastic mulches indicated that they altered the soil-root zone microclimate in a similar manner as previously reported for asphalt-impregnated paper mulches in the 1920s and 1930s. However, unlike early paper mulches, the plastic-mulch

production system has become the dominant mulching tactic for vegetable production.

Early plastic mulches were placed in the field by hand; however, to increase efficiency, specialized equipment was developed. Initial land preparation is similar for bare-ground and plastic-mulch production systems. Soil is ploughed and disked until a fine tilth is achieved. A piece of equipment which can form a raised bed and lay plastic mulch in a single operation is pulled through the field to form the planting bed. When using a raised-bed plastic-mulch system, rows must be spaced further apart in order to accommodate the bed shaping equipment than would be necessary in a flat-bed system. Therefore, raised-bed plastic-mulch rows are typically spaced on 1.7 to 2.2 m centers. Raised beds are often preferred with plastic mulches because they warm quicker than flat beds and offer superior drainage (Lamont, 1996; Tarara, 2000). Herbicides which must be incorporated with tillage may be applied to the soil prior to bed formation or under the mulch while it is being laid in the field. Chemical fumigants are often knifed into the soil under plastic mulches during this process as well (Hartz et al., 1993). Fumigation is an important component of many plastic-mulch production systems. Fumigants have the ability to kill weed seeds, which may potentially germinate, as well as control soil pathogenic fungi, bacteria, and nematodes (Goring, 1962; Wilhelm and Paulus, 1980). Drip irrigation tubing is placed under plastic mulch during the same process. Early research with plastic mulches was conducted using overhead irrigation or furrow irrigation (Emmert, 1957); however, with the introduction of drip irrigation in the 1970s, the vast majority of plastic-mulch production now utilizes this method (Hartz, 1996). The combination of drip irrigation with plastic mulch has significantly increased irrigation water

After the plastic is laid in the field, transplants can be placed by hand or using a mechanized transplanter. Plastic mulches must fit tightly against the soil; not only to obtain the maximum benefit of heat transfer from mulch to soil; but also because warm air, when trapped under the mulch, can escape through the holes where transplants are placed, desiccating and damaging the crop (Lamont, 2005). Due to the increased productivity of plastic mulches, in-row spacing of plants is often less compared to bare-ground production systems. Crops which may normally be planted in a single row fashion when grown without mulches are often planted in double rows with plastic mulches (Lamont, 1991). Plant populations per unit area may also be increased in plastic-mulch production systems.

**2.1 Equipment for the plastic-mulch production system** 

use efficiency in vegetable production (Howell, 2001).

annual cost would be approximately \$12-\$16 per acre per year (Emmert, 1957).

At the end of the growing season, plastic mulches must be removed from the field; though in warmer climates mulches are often double or triple cropped (Hanna and Adams, 1989). Double cropping plastic mulch decreases input costs for growers; however, weed pressures are often increased during the second crop as pre-emergent herbicides have dissipated. Although additional herbicides may be applied to spaces between rows; in-row weeds, growing through the planting holes of the previous crop, can be difficult to control (Waterer et al., 2008). In regions with shorter growing seasons, most plastic mulch is removed after one crop, though double-cropping mulches that have been left in fields over a winter have been evaluated (Waterer et al., 2008). To remove plastic mulch from fields, a specialized piece of equipment (mulch lifter) is required. A mulch lifter is a device which undercuts and lifts plastic mulch out of the soil at which time it can be collected and disposed.

#### **2.2 Characteristics of plastic mulches**

The earliest plastic mulches evaluated were 1.5 mil-thick and black (Emmert, 1957). There are now arrays of mulches available. The most common mulches are 1.0 or 1.25 mil-thick and are sold on a 1.2 m-wide roll, though widths of 0.9 – 1.5 m are also produced. Mulches that are thinner than 1.0 mil are easily punctured by weeds. Most degradable plastic mulches are 0.5-0.75 mil-thick, which allows for quicker decomposition. Rolls of mulch commonly range from 730 – 1830 m in length. Mulches may be smooth or embossed. Mulches that are embossed tend to resist excessive expansion and contraction which can cause mulches to become loose from raised beds (Lamont, 1993).

#### **2.3 Colored mulches**

The most popular plastic mulch world-wide is black, though white-on-black and clear mulches are also used (Schales, 1990). Other colors that that have been evaluated include: blue, green, red, yellow, brown, white, and silver (Brault et al., 2002; Gough, 2001; Hanna, 2000; Ngouajio and Ernest, 2004). Different colored mulches have multiple effects on the crops being grown. The optical properties of various colored mulches can influence soil and air temperatures around the crop as well as impact weed growth under the mulch. Moreover, in some cases, colored mulches can alter insect behaviour, which can directly (insect feeding) and indirectly (vectoring diseases) affect crop growth. Colored mulches can be separated into those that do not discriminate between different wavelengths of light transmitted and those that selectively prevent transmission of photosynthetically active radiation (PAR) (400-700 nm) (Ngouajio and Ernest, 2004; Tarara, 2000). Mulches that selectively filter out light in the PAR range are called infrared transmitting (IRT) mulches. In addition to restricting light of the PAR range, IRT mulches tend to transmit high percentages of light at longer wavelengths (>900 nm). By selectively filtering light in the PAR range and transmitting longer wavelength light energy, IRT mulches allow for greater soil warming while reducing light available for weed growth.

#### **2.3.1 Non-IRT colored mulches effects on light, temperature, and weed growth**

The most common non-IRT mulches are black, clear, white-on-black, and reflective silver. A myriad of other colors exist including: yellow, blue, red, and green (Figure 1). These colored mulches comprise a very small portion of the total mulch utilized. Although benefits have

Mulches for Weed Management in Vegetable Production 63

al.; Standifer et al., 1984). However, to properly solarize soil, clear plastic must be exposed to high light and temperatures for a fairly long period of time; therefore, its use is limited in

Clear plastic functions well for soil solarization, but its use as a mulch is limited. Higher yields have been reported for crops such as strawberries (*Fragaria* sp.) when using clear plastic in combination with soil fumigation with methyl bromide and chloropicrin (Johnson and Fennimore, 2005). However, due to the methyl bromide phase-out and the absence of suitable replacements (Locascio et al., 1997), the ability to control weeds under clear-plastic mulches has limited their use. In non-fumigated soils, clear mulches only controlled 64% of weeds compared to black mulches (Johnson and Fennimore, 2005). Clear plastic is generally unsuitable as a mulch unless supplemental herbicides or fumigants are applied to control

Black plastic is the predominate mulch utilized in vegetable production today. Much of this popularity is due to a lower cost per acre compared to other mulches. However, blackplastic mulch also effectively warms the soil, improving early crop production and eliminates most in-row weed growth. Unlike clear mulches, black plastic absorbs nearly all shortwave radiation to heat the soil (Ham et al., 1993). By absorbing radiation, black-plastic mulch heats the soil through conduction. A tightly formed plant bed where the mulch makes consistent contact with the soil is necessary for optimal soil warming (Lamont, 1993; Tarara, 2000). By absorbing nearly all shortwave radiation, the surface temperatures of black plastic mulches can reach 55 oC (Tarara, 2000). Soil temperatures 10 cm under the mulch may increase 3-5 oC (Ham et al., 1993). Once crop canopies develop, shading of the mulches increases, and soil temperatures under mulches often decrease compared to bare-ground treatments. Though weed seeds may germinate under black-plastic mulch, subsequent weed growth is limited, with the notable exception of yellow and purple nutsedges (*Cyperus* spp.) (Patterson, 1998). Therefore, black plastic is the mulch of choice for early season vegetable

White-on-black and silver-reflective plastic mulches are less popular than black plastic, but still serve an important role in vegetable production and weed management. During periods when soil temperatures are elevated, warming the soil with black-plastic mulch can actually harm plants and reduce yields. To avoid damaging the crop, but still provide in-row weed control, white and silver reflective mulches were developed. White mulches were largely ineffective for weed control, without the use of fumigants or herbicides, because they transmitted too much light. Ngouajio and Ernest (2004) reported that white mulches transmitted 48% of solar radiation. This level of light transmission led to substantial weed growth under white mulch. Trials where black mulches were painted white demonstrated benefits of a reflective mulch where weeds could be controlled (Decoteau et al., 1988). White and black-colored mulches are now coextruded forming white-on-black mulch. This mulch is popular because it combines the weed control properties of black mulches (Johnson and Fennimore, 2005) with the soil cooling properties of white-reflective mulch. Ham et al. (1993) reported that white-on-black and silver mulches reflect 48%and 39% of shortwave radiation, respectively. The reflection of shortwave radiation can result in slightly lower root-zone temperatures in reflective mulches compared to bare soil (Diaz-Perez, 2010; Diaz-

cooler climates (Katan and DeVay, 1991).

weeds (Lamont, 2005).

production.

Perez et al., 2005; Ham et al., 1993; Tarara, 2000).

been obtained from colored mulches, particularly red in tomatoes (Decoteau et al., 1989), some allow excessive light transmittance, resulting in unacceptable weed growth. The potential for weed growth and higher costs associated with colored plastic mulches has limited their use.

Fig. 1. Muskmelons being grown on blue, brown, red, and white-on-black mulches1.

As would be expected, clear mulches transmit the most shortwave radiation (84%) and absorb the least (5%) (Ham et al., 1993). Clear mulches also reflect a high percentage (88%) of long-wave radiation. Clear-plastic mulches increase soil temperatures from 4.4 – 7.8 oC when measured at a depth of 5 cm below the soil surface (Lamont et al., 1993). However, the ability of clear mulch to heat the soil also depends on how it is applied. As noted, clear mulches largely transmit shortwave radiation and reflect long-wave radiation. When clear mulches are loosely applied, long-wave radiation emitted from the soil becomes trapped under the plastic creating a greenhouse-type environment (Ham et al., 1993; Lamont, 1993; Liakatas et al., 1986). However, if the clear mulch is placed tightly on the soil surface, then less convective heating occurs and soil temperature increases may not be as large as expected (Ham and Kluitenberg, 1994; Ham et al., 1993). Diurnal temperature fluctuations are also greater in clear plastic that has not been held tightly to the soil compared to those that have (Tarara, 2000). It has also been reported that the warming effects of clear mulches compared to other colors are substantially reduced in overcast or cloudy environments with less solar radiation (Johnson and Fennimore, 2005).

Clear plastics are utilized for soil solarization. This is the process by which light energy from the sun is trapped, heating the soil enough to cause thermal degradation of bacterial, nematode, fungal, or weed pests (Katan, 1981b; Katan and DeVay, 1991). Soil is prepared for the crop of interest and then solarized for a period of time prior to planting. Disturbing the soil after solarization reduces weed control. When soils are disturbed after solarization, weed seeds that were deep in the soil and unaffected by the treatment, can be brought to the surface to germinate. Clear plastic is the best choice for solarization due to superior heating ability. Reports from California show soil temperatures, measured at a depth of 5 cm from the surface, reaching 60 oC under clear plastic (Katan, 1981a; Katan, 1981b). Plastics are applied more loosely for solarization than they are when mulching plant beds. This may explain the higher temperatures observed in solarization trials than when using clear plastic as a mulch. Solarization has been documented to control a variety of weed pests in many crops (Basavaraju and Nanjappa, 1999; Katan and DeVay, 1991; Law et al., 2008; Megueni et

<sup>1</sup> Photos courtesy of Dr. John Strang, University of Kentucky, Department of Horticulture.

been obtained from colored mulches, particularly red in tomatoes (Decoteau et al., 1989), some allow excessive light transmittance, resulting in unacceptable weed growth. The potential for weed growth and higher costs associated with colored plastic mulches has

Fig. 1. Muskmelons being grown on blue, brown, red, and white-on-black mulches1.

less solar radiation (Johnson and Fennimore, 2005).

As would be expected, clear mulches transmit the most shortwave radiation (84%) and absorb the least (5%) (Ham et al., 1993). Clear mulches also reflect a high percentage (88%) of long-wave radiation. Clear-plastic mulches increase soil temperatures from 4.4 – 7.8 oC when measured at a depth of 5 cm below the soil surface (Lamont et al., 1993). However, the ability of clear mulch to heat the soil also depends on how it is applied. As noted, clear mulches largely transmit shortwave radiation and reflect long-wave radiation. When clear mulches are loosely applied, long-wave radiation emitted from the soil becomes trapped under the plastic creating a greenhouse-type environment (Ham et al., 1993; Lamont, 1993; Liakatas et al., 1986). However, if the clear mulch is placed tightly on the soil surface, then less convective heating occurs and soil temperature increases may not be as large as expected (Ham and Kluitenberg, 1994; Ham et al., 1993). Diurnal temperature fluctuations are also greater in clear plastic that has not been held tightly to the soil compared to those that have (Tarara, 2000). It has also been reported that the warming effects of clear mulches compared to other colors are substantially reduced in overcast or cloudy environments with

Clear plastics are utilized for soil solarization. This is the process by which light energy from the sun is trapped, heating the soil enough to cause thermal degradation of bacterial, nematode, fungal, or weed pests (Katan, 1981b; Katan and DeVay, 1991). Soil is prepared for the crop of interest and then solarized for a period of time prior to planting. Disturbing the soil after solarization reduces weed control. When soils are disturbed after solarization, weed seeds that were deep in the soil and unaffected by the treatment, can be brought to the surface to germinate. Clear plastic is the best choice for solarization due to superior heating ability. Reports from California show soil temperatures, measured at a depth of 5 cm from the surface, reaching 60 oC under clear plastic (Katan, 1981a; Katan, 1981b). Plastics are applied more loosely for solarization than they are when mulching plant beds. This may explain the higher temperatures observed in solarization trials than when using clear plastic as a mulch. Solarization has been documented to control a variety of weed pests in many crops (Basavaraju and Nanjappa, 1999; Katan and DeVay, 1991; Law et al., 2008; Megueni et

1 Photos courtesy of Dr. John Strang, University of Kentucky, Department of Horticulture.

limited their use.

al.; Standifer et al., 1984). However, to properly solarize soil, clear plastic must be exposed to high light and temperatures for a fairly long period of time; therefore, its use is limited in cooler climates (Katan and DeVay, 1991).

Clear plastic functions well for soil solarization, but its use as a mulch is limited. Higher yields have been reported for crops such as strawberries (*Fragaria* sp.) when using clear plastic in combination with soil fumigation with methyl bromide and chloropicrin (Johnson and Fennimore, 2005). However, due to the methyl bromide phase-out and the absence of suitable replacements (Locascio et al., 1997), the ability to control weeds under clear-plastic mulches has limited their use. In non-fumigated soils, clear mulches only controlled 64% of weeds compared to black mulches (Johnson and Fennimore, 2005). Clear plastic is generally unsuitable as a mulch unless supplemental herbicides or fumigants are applied to control weeds (Lamont, 2005).

Black plastic is the predominate mulch utilized in vegetable production today. Much of this popularity is due to a lower cost per acre compared to other mulches. However, blackplastic mulch also effectively warms the soil, improving early crop production and eliminates most in-row weed growth. Unlike clear mulches, black plastic absorbs nearly all shortwave radiation to heat the soil (Ham et al., 1993). By absorbing radiation, black-plastic mulch heats the soil through conduction. A tightly formed plant bed where the mulch makes consistent contact with the soil is necessary for optimal soil warming (Lamont, 1993; Tarara, 2000). By absorbing nearly all shortwave radiation, the surface temperatures of black plastic mulches can reach 55 oC (Tarara, 2000). Soil temperatures 10 cm under the mulch may increase 3-5 oC (Ham et al., 1993). Once crop canopies develop, shading of the mulches increases, and soil temperatures under mulches often decrease compared to bare-ground treatments. Though weed seeds may germinate under black-plastic mulch, subsequent weed growth is limited, with the notable exception of yellow and purple nutsedges (*Cyperus* spp.) (Patterson, 1998). Therefore, black plastic is the mulch of choice for early season vegetable production.

White-on-black and silver-reflective plastic mulches are less popular than black plastic, but still serve an important role in vegetable production and weed management. During periods when soil temperatures are elevated, warming the soil with black-plastic mulch can actually harm plants and reduce yields. To avoid damaging the crop, but still provide in-row weed control, white and silver reflective mulches were developed. White mulches were largely ineffective for weed control, without the use of fumigants or herbicides, because they transmitted too much light. Ngouajio and Ernest (2004) reported that white mulches transmitted 48% of solar radiation. This level of light transmission led to substantial weed growth under white mulch. Trials where black mulches were painted white demonstrated benefits of a reflective mulch where weeds could be controlled (Decoteau et al., 1988). White and black-colored mulches are now coextruded forming white-on-black mulch. This mulch is popular because it combines the weed control properties of black mulches (Johnson and Fennimore, 2005) with the soil cooling properties of white-reflective mulch. Ham et al. (1993) reported that white-on-black and silver mulches reflect 48%and 39% of shortwave radiation, respectively. The reflection of shortwave radiation can result in slightly lower root-zone temperatures in reflective mulches compared to bare soil (Diaz-Perez, 2010; Diaz-Perez et al., 2005; Ham et al., 1993; Tarara, 2000).

Mulches for Weed Management in Vegetable Production 65

Numerous studies show that vegetables grown with plastic mulches typically out yield those grown on bare ground, even with complete weed control for the bare-ground plots. (Table 1). It has been well documented that plastic mulches reduce evaporation, nutrient leaching, and soil compaction in the plant bed (Lamont, 2005). However, the impact of plastic mulch on root architecture and root-zone temperatures are particularly notable; especially as the yield benefits of black plastic mulch are often greater in the spring than in

[mean ± s.e (kg·ha-1)]

**2.4 Polyethylene mulches influence the root zone affecting weeds and crops** 

Treatmemt Total Yield

treatments grown in summer and fall [adapted from (Coolong, 2010)].

compared to bare-ground production (Gough, 2001).

Spring Fall

Black Plastic 37905 ± 1492 az 27214 ± 953 a Bare ground hand-weeded 19693 ± 1352 b 21843 ± 1214 b Bare ground non-weeded 10524 ± 722 c 17330 ± 1866 b z Treatments within a column not followed by the same letter are different by Duncan's Multiple Range

Table 1. Yields of summer squash (*Cucurbita* sp.) under black plastic mulch and bare-ground

Knavel and Mohr (1967) reported summer squash, tomato, and pepper plants had significantly more and longer roots when grown with plastic mulches compared to unmulched controls. However in graphic representations, roots under plastic mulches were also significantly shallower and spread out over the surface of the bed compared to bareground plots (Knavel and Mohr, 1967). Other trials have reported that plastic mulches influenced adventitious root development, but overall root architecture remained similar

Significant research has been conducted evaluating the impact of mulch type and color on root-zone temperature and subsequent yield impacts (Diaz-Perez, 2009; Diaz-Perez, 2010; Diaz-Perez and Batal, 2002; Diaz-Perez et al., 2005). Generally, black plastic mulch is preferred for spring plantings as a method to warm the root zone and increase yields (Diaz-Perez, 2009; Diaz-Perez, 2010; Diaz-Perez et al., 2005). However, during summer, soil temperatures under black plastic mulches may be greater than 30 oC (Ham et al., 1993; Tarara, 2000; Tindall et al., 1991). Vegetable growth and yield has been shown to respond quadratically to root-zone temperature, increasing up to a point then rapidly decreasing (Coolong and Randle, 2006; Diaz-Perez, 2010; Tindall et al., 1990). Depending on the crop grown, the critical root-zone temperature for maximum yield and growth may be several degrees cooler than is present under the black plastic mulch. Diaz-Perez and Batal (2002) reported an increase of 5-fruit per plant for tomatoes grown in black plastic mulch compared to bare-ground. However, in the same trial, plants grown on reflective gray and silver mulches, which reduced root-zone temperatures compared to black plastic, had an additional 6-7-fruit per plant compared to the black-plastic mulch treatment. Because of high root-zone temperatures, reflective mulches are encouraged for summer-planted crops. To double-crop black plastic mulches during the summer, a system was developed which utilized a photodegradable black mulch placed over a white non-degradable mulch

the fall after soil has warmed (Table 1).

Test *P*<0.05

White-on-black and silver mulches reflect significantly more light into the plant canopy than black mulches, though this decreases as the canopy expands. The upwardly reflected light from white or silver mulches decreases the ratio of red to far-red light compared to black mulches (Decotcau, 2007; Decoteau et al., 1988). The alteration of the light microenvironment is thought to lead to greater leaf areas, shorter internodes, and greater branching in plants grown on reflective mulches compared to black plastic (Decotcau, 2007; Decoteau et al., 1988; Diaz-Perez, 2010). However, the impact of the optical characteristics of the reflective mulches is limited at certain heights above the bed, and wanes as the plant canopy forms (Lamont, 2005). It is also difficult to isolate differences in light effects on plant growth from root-zone temperatures when comparing different colored mulches (Diaz-Perez and Batal, 2002). Light reflective mulches have also been suggested to influence insect predation on vegetable crops as well (Brown and Brown, 1992; Caldwell and Clarke, 1999; Csizinszky et al., 1995; Funderburk, 2009; Lu, 1990).

#### **2.3.2 The effect of IRT mulches on temperature, light, and weed growth**

IRT mulches allow transmission of light outside of the PAR spectrum. By transmitting infrared radiation, but excluding PAR, IRT mulches combine the soil-warming benefits of clear plastic mulches with the weed control of black plastic mulch. IRT mulches are most commonly manufactured in green and brown colors. Ngouajio and Ernest (2004) reported that IRT-green and IRT-brown mulches transmitted 42% and 26% of light, respectively, between the wavelengths of 400 and 1100 nm. This was compared to just 1% in black and 2% in white-on-black mulches, respectively. However, the green and brown-IRT mulches transmitted 16% and 6% of PAR (400-700 nm), respectively. Ham et al. (1993) reported 37% of total short-wave light (300-1100 nm) transmitted for an IRT mulch, while Johnson and Fennimore (2005) reported 10.6% and 10.9% transmittance of PAR (400-700 nm) for green and brown IRT mulches, respectively. This selective transmittance of light allows IRT mulches to provide similar weed control as black-plastic mulches (Johnson and Fennimore, 2005; Ngouajio and Ernest, 2004).

The soil warming properties of IRT mulches are reported to be more similar to clear plastic mulches (Lamont, 1993). However, effects of IRT mulches on soil temperatures may vary. Ngouajio and Ernest (2004) reported heat accumulation in growing degree days (base 10 oC) in IRT mulches was similar to black plastic mulch and better than white and white-on-black mulches. In that trial, clear mulches were not included for comparison. Johnson and Fennimore (2005), using a degree-hour model for heat accumulation, reported that IRT brown and green mulches accumulated 5200 and 6300 degree hours, respectively, while clear and black-plastic mulches accumulated 11000 and 4400 degree hours, respectively. This suggests that IRT mulches provide soil warming abilities between clear and blackplastic mulches. However, in the same trial, the authors reported that black-plastic mulch accumulated more degree hours than clear and IRT mulches in a cooler, cloudier location. Ham et al. (1993) trialled several mulches and reported that IRT mulch had similar soil warming characteristics as clear-plastic mulch. However, both IRT and clear-plastic mulches failed to warm the soil as much as a black plastic mulch (Ham et al., 1993). Therefore, while IRT mulches may control weeds as well as black-plastic mulch, the relative soil warming abilities of IRT mulches compared to black plastic may vary based on local climate.

White-on-black and silver mulches reflect significantly more light into the plant canopy than black mulches, though this decreases as the canopy expands. The upwardly reflected light from white or silver mulches decreases the ratio of red to far-red light compared to black mulches (Decotcau, 2007; Decoteau et al., 1988). The alteration of the light microenvironment is thought to lead to greater leaf areas, shorter internodes, and greater branching in plants grown on reflective mulches compared to black plastic (Decotcau, 2007; Decoteau et al., 1988; Diaz-Perez, 2010). However, the impact of the optical characteristics of the reflective mulches is limited at certain heights above the bed, and wanes as the plant canopy forms (Lamont, 2005). It is also difficult to isolate differences in light effects on plant growth from root-zone temperatures when comparing different colored mulches (Diaz-Perez and Batal, 2002). Light reflective mulches have also been suggested to influence insect predation on vegetable crops as well (Brown and Brown, 1992; Caldwell and Clarke, 1999;

Csizinszky et al., 1995; Funderburk, 2009; Lu, 1990).

2005; Ngouajio and Ernest, 2004).

**2.3.2 The effect of IRT mulches on temperature, light, and weed growth** 

IRT mulches allow transmission of light outside of the PAR spectrum. By transmitting infrared radiation, but excluding PAR, IRT mulches combine the soil-warming benefits of clear plastic mulches with the weed control of black plastic mulch. IRT mulches are most commonly manufactured in green and brown colors. Ngouajio and Ernest (2004) reported that IRT-green and IRT-brown mulches transmitted 42% and 26% of light, respectively, between the wavelengths of 400 and 1100 nm. This was compared to just 1% in black and 2% in white-on-black mulches, respectively. However, the green and brown-IRT mulches transmitted 16% and 6% of PAR (400-700 nm), respectively. Ham et al. (1993) reported 37% of total short-wave light (300-1100 nm) transmitted for an IRT mulch, while Johnson and Fennimore (2005) reported 10.6% and 10.9% transmittance of PAR (400-700 nm) for green and brown IRT mulches, respectively. This selective transmittance of light allows IRT mulches to provide similar weed control as black-plastic mulches (Johnson and Fennimore,

The soil warming properties of IRT mulches are reported to be more similar to clear plastic mulches (Lamont, 1993). However, effects of IRT mulches on soil temperatures may vary. Ngouajio and Ernest (2004) reported heat accumulation in growing degree days (base 10 oC) in IRT mulches was similar to black plastic mulch and better than white and white-on-black mulches. In that trial, clear mulches were not included for comparison. Johnson and Fennimore (2005), using a degree-hour model for heat accumulation, reported that IRT brown and green mulches accumulated 5200 and 6300 degree hours, respectively, while clear and black-plastic mulches accumulated 11000 and 4400 degree hours, respectively. This suggests that IRT mulches provide soil warming abilities between clear and blackplastic mulches. However, in the same trial, the authors reported that black-plastic mulch accumulated more degree hours than clear and IRT mulches in a cooler, cloudier location. Ham et al. (1993) trialled several mulches and reported that IRT mulch had similar soil warming characteristics as clear-plastic mulch. However, both IRT and clear-plastic mulches failed to warm the soil as much as a black plastic mulch (Ham et al., 1993). Therefore, while IRT mulches may control weeds as well as black-plastic mulch, the relative soil warming

abilities of IRT mulches compared to black plastic may vary based on local climate.

#### **2.4 Polyethylene mulches influence the root zone affecting weeds and crops**

Numerous studies show that vegetables grown with plastic mulches typically out yield those grown on bare ground, even with complete weed control for the bare-ground plots. (Table 1). It has been well documented that plastic mulches reduce evaporation, nutrient leaching, and soil compaction in the plant bed (Lamont, 2005). However, the impact of plastic mulch on root architecture and root-zone temperatures are particularly notable; especially as the yield benefits of black plastic mulch are often greater in the spring than in the fall after soil has warmed (Table 1).


z Treatments within a column not followed by the same letter are different by Duncan's Multiple Range Test *P*<0.05

Table 1. Yields of summer squash (*Cucurbita* sp.) under black plastic mulch and bare-ground treatments grown in summer and fall [adapted from (Coolong, 2010)].

Knavel and Mohr (1967) reported summer squash, tomato, and pepper plants had significantly more and longer roots when grown with plastic mulches compared to unmulched controls. However in graphic representations, roots under plastic mulches were also significantly shallower and spread out over the surface of the bed compared to bareground plots (Knavel and Mohr, 1967). Other trials have reported that plastic mulches influenced adventitious root development, but overall root architecture remained similar compared to bare-ground production (Gough, 2001).

Significant research has been conducted evaluating the impact of mulch type and color on root-zone temperature and subsequent yield impacts (Diaz-Perez, 2009; Diaz-Perez, 2010; Diaz-Perez and Batal, 2002; Diaz-Perez et al., 2005). Generally, black plastic mulch is preferred for spring plantings as a method to warm the root zone and increase yields (Diaz-Perez, 2009; Diaz-Perez, 2010; Diaz-Perez et al., 2005). However, during summer, soil temperatures under black plastic mulches may be greater than 30 oC (Ham et al., 1993; Tarara, 2000; Tindall et al., 1991). Vegetable growth and yield has been shown to respond quadratically to root-zone temperature, increasing up to a point then rapidly decreasing (Coolong and Randle, 2006; Diaz-Perez, 2010; Tindall et al., 1990). Depending on the crop grown, the critical root-zone temperature for maximum yield and growth may be several degrees cooler than is present under the black plastic mulch. Diaz-Perez and Batal (2002) reported an increase of 5-fruit per plant for tomatoes grown in black plastic mulch compared to bare-ground. However, in the same trial, plants grown on reflective gray and silver mulches, which reduced root-zone temperatures compared to black plastic, had an additional 6-7-fruit per plant compared to the black-plastic mulch treatment. Because of high root-zone temperatures, reflective mulches are encouraged for summer-planted crops. To double-crop black plastic mulches during the summer, a system was developed which utilized a photodegradable black mulch placed over a white non-degradable mulch

Mulches for Weed Management in Vegetable Production 67

Although plastic mulches provide excellent in-row weed control and enhance productivity of many vegetable crops, waste is a significant issue. It is estimated that world-wide plastic film use is 700,000 tons per year (Espi et al., 2006) with more than 140,000 tons used annually in the U.S. (Shogren, 2001). Most of these mulches end up in landfills or are burned (Hemphill Jr, 1993; Kyrikou and Briassoulis, 2007). As landfill space becomes limited and concerns rise about discarding plastic, which may potentially contain pesticide residues, disposal of mulch films has become a significant issue for farmers. Recycling is not typically an option as used mulches contain dirt and debris from production fields that must first be removed prior to the recycling process. At this time, processes to remove dirt from mulches are too expensive. An alternative to recycling that has been pilot-tested was to compress used-plastic mulches into dense pellets and using them as a fuel source. These pellets have been effectively co-fired with coal in trials conducted at Pennsylvania State University (Lawrence et al., 2010). Plastic mulches are petroleum-based products and contain roughly the same energy content as fuel oil on a weight basis (Hemphill Jr, 1993). In addition to environmental concerns, the costs for removal and disposal of plastic mulches are

Economic and environmental concerns have spurred interest in degradable mulch films. Designing degradable mulches with properties similar to LDPE is challenging. The degradable mulch must be flexible, lightweight, prevent light transmittance, and degrade in a timely manner after harvest. Exposure to light, temperature, and moisture can influence degradation (Kyrikou and Briassoulis, 2007). Normalizing degradation rates between growing regions with vastly different climates is a challenge as well. Some crops will quickly form a canopy shading mulches and thus delaying degradation; while others do not. Developing a mulch that will degrade on-demand at a competitive cost is a challenge.

Degradable plastic mulches are often labeled as biodegradable. However, to be considered biodegradable, a polymer must be completely converted by microorganisms to water, minerals, carbon dioxide and biomass (Kyrikou and Briassoulis, 2007). Some mulches that have been marketed as biopolymers do not biodegrade, but fragment, leaving synthetic polymers in the environment in microscopic fragments. Starch-polymer blends fragment as the starch co-polymers degrade, with the synthetic co-polymer remaining in the field (Halley et al., 2001). It is debated whether the synthetic polymers which are left in the field biodegrade. Nonetheless, a variety of mulches have been developed that are reported to

Halley et al. (2001) developed a mulch film using modified-starch polymers that performed as well as a conventional polyethylene mulch for pepper production. This mulch withstood 14 weeks of water exposure and remained largely stable during crop production. However, just two weeks after the mulch was plowed into the soil, it was visually undetectable, with composting trials indicating that the mulch completely degraded to carbon dioxide and water after 45 days (Halley et al., 2001). Waterer (2010) tested clear, black, and wavelengthselective starch-based mulches. In this trial, clear and wavelength-selective starch-based mulches degraded quickly in the field. The clear starch-based mulch broke down completely

**3. Waste issues and mulches** 

approximatley \$250/ha (Waterer, 2010).

**3.1 Degradable mulch films** 

completely degrade.

(Graham et al., 1995). The black mulch warmed the soil in the spring and then degraded, exposing the white mulch used for a second planting. This system was effective in reducing soil temperatures late in the summer; however, a co-extrusion process has not been commercialized for developing such a system.

#### **2.4.1 Mulch type influences weed morphology**

Two common weeds that are not controlled by black plastic mulches are purple and yellow nutsedge. These are two of the most problematic weeds for vegetable production in the Southern U.S. (Webster and MacDonald, 2001). Unlike most weeds, both yellow and purple nutsedge have the ability to pierce plastic mulches (Figure 2) and successfully compete with crops (William, 1976; William and Warren, 1975).

Fig. 2. Yellow nutsedge penetrating white-on-black mulch.

Traditionally, growers have relied on fumigation with methyl bromide to control nutsedge when using plastic mulches. However, as methyl bromide use has been phased out with the exception of some critical-use exemptions, the management of yellow and purple nutsedge under plastic mulches has become a pressing issue (Webster, 2005). Interestingly, some research has demonstrated that controlling yellow and purple nutsedge may depend on the light transmittance of mulches used. Purple nutsedge shoot and tuber growth was shown to be greater under white-on-black mulch compared to IRT mulch when grown under sunlight in a greenhouse (Patterson, 1998). However, in the same trial, all mulches failed to prevent nutsedge shoot emergence when treatments were conducted in total darkness in growth chambers. This suggests that the transmission of light may alter the ability of purple nutsedge to penetrate mulches. Chase et al. (1998) reported similar results when evaluating yellow and purple nutsedge. In that trial, yellow and purple nutsedges penetrated black mulches to a greater extent than clear and IRT mulches. All mulches controlled yellow nutsedge to a greater degree than purple nutsedge. The authors theorized that nutsedge rhizomes have a sharp tip that will penetrate opaque mulches. However, upon exposure to light, photomorphogenic initiation of leaf expansion occurs and the leaves do not have the ability to penetrate the plastic mulches as well as the rhizome (Chase et al., 1998). Although nutsedges will sprout under clear or IRT mulches, they rarely penetrate through the film. Webster (2005) reported similar results, also noting the greater relative ability of purple compared to yellow nutsedge to overcome any plastic mulch. Over time, this may result in a shift in the weed population from yellow to purple nutsedge in mulched vegetable cropping systems (Webster, 2005).

#### **3. Waste issues and mulches**

66 Weed Control

(Graham et al., 1995). The black mulch warmed the soil in the spring and then degraded, exposing the white mulch used for a second planting. This system was effective in reducing soil temperatures late in the summer; however, a co-extrusion process has not been

Two common weeds that are not controlled by black plastic mulches are purple and yellow nutsedge. These are two of the most problematic weeds for vegetable production in the Southern U.S. (Webster and MacDonald, 2001). Unlike most weeds, both yellow and purple nutsedge have the ability to pierce plastic mulches (Figure 2) and successfully compete with

Traditionally, growers have relied on fumigation with methyl bromide to control nutsedge when using plastic mulches. However, as methyl bromide use has been phased out with the exception of some critical-use exemptions, the management of yellow and purple nutsedge under plastic mulches has become a pressing issue (Webster, 2005). Interestingly, some research has demonstrated that controlling yellow and purple nutsedge may depend on the light transmittance of mulches used. Purple nutsedge shoot and tuber growth was shown to be greater under white-on-black mulch compared to IRT mulch when grown under sunlight in a greenhouse (Patterson, 1998). However, in the same trial, all mulches failed to prevent nutsedge shoot emergence when treatments were conducted in total darkness in growth chambers. This suggests that the transmission of light may alter the ability of purple nutsedge to penetrate mulches. Chase et al. (1998) reported similar results when evaluating yellow and purple nutsedge. In that trial, yellow and purple nutsedges penetrated black mulches to a greater extent than clear and IRT mulches. All mulches controlled yellow nutsedge to a greater degree than purple nutsedge. The authors theorized that nutsedge rhizomes have a sharp tip that will penetrate opaque mulches. However, upon exposure to light, photomorphogenic initiation of leaf expansion occurs and the leaves do not have the ability to penetrate the plastic mulches as well as the rhizome (Chase et al., 1998). Although nutsedges will sprout under clear or IRT mulches, they rarely penetrate through the film. Webster (2005) reported similar results, also noting the greater relative ability of purple compared to yellow nutsedge to overcome any plastic mulch. Over time, this may result in a shift in the weed population from yellow to purple nutsedge in mulched vegetable cropping

commercialized for developing such a system.

**2.4.1 Mulch type influences weed morphology** 

crops (William, 1976; William and Warren, 1975).

Fig. 2. Yellow nutsedge penetrating white-on-black mulch.

systems (Webster, 2005).

Although plastic mulches provide excellent in-row weed control and enhance productivity of many vegetable crops, waste is a significant issue. It is estimated that world-wide plastic film use is 700,000 tons per year (Espi et al., 2006) with more than 140,000 tons used annually in the U.S. (Shogren, 2001). Most of these mulches end up in landfills or are burned (Hemphill Jr, 1993; Kyrikou and Briassoulis, 2007). As landfill space becomes limited and concerns rise about discarding plastic, which may potentially contain pesticide residues, disposal of mulch films has become a significant issue for farmers. Recycling is not typically an option as used mulches contain dirt and debris from production fields that must first be removed prior to the recycling process. At this time, processes to remove dirt from mulches are too expensive. An alternative to recycling that has been pilot-tested was to compress used-plastic mulches into dense pellets and using them as a fuel source. These pellets have been effectively co-fired with coal in trials conducted at Pennsylvania State University (Lawrence et al., 2010). Plastic mulches are petroleum-based products and contain roughly the same energy content as fuel oil on a weight basis (Hemphill Jr, 1993). In addition to environmental concerns, the costs for removal and disposal of plastic mulches are approximatley \$250/ha (Waterer, 2010).

#### **3.1 Degradable mulch films**

Economic and environmental concerns have spurred interest in degradable mulch films. Designing degradable mulches with properties similar to LDPE is challenging. The degradable mulch must be flexible, lightweight, prevent light transmittance, and degrade in a timely manner after harvest. Exposure to light, temperature, and moisture can influence degradation (Kyrikou and Briassoulis, 2007). Normalizing degradation rates between growing regions with vastly different climates is a challenge as well. Some crops will quickly form a canopy shading mulches and thus delaying degradation; while others do not. Developing a mulch that will degrade on-demand at a competitive cost is a challenge.

Degradable plastic mulches are often labeled as biodegradable. However, to be considered biodegradable, a polymer must be completely converted by microorganisms to water, minerals, carbon dioxide and biomass (Kyrikou and Briassoulis, 2007). Some mulches that have been marketed as biopolymers do not biodegrade, but fragment, leaving synthetic polymers in the environment in microscopic fragments. Starch-polymer blends fragment as the starch co-polymers degrade, with the synthetic co-polymer remaining in the field (Halley et al., 2001). It is debated whether the synthetic polymers which are left in the field biodegrade. Nonetheless, a variety of mulches have been developed that are reported to completely degrade.

Halley et al. (2001) developed a mulch film using modified-starch polymers that performed as well as a conventional polyethylene mulch for pepper production. This mulch withstood 14 weeks of water exposure and remained largely stable during crop production. However, just two weeks after the mulch was plowed into the soil, it was visually undetectable, with composting trials indicating that the mulch completely degraded to carbon dioxide and water after 45 days (Halley et al., 2001). Waterer (2010) tested clear, black, and wavelengthselective starch-based mulches. In this trial, clear and wavelength-selective starch-based mulches degraded quickly in the field. The clear starch-based mulch broke down completely

Mulches for Weed Management in Vegetable Production 69

Shogren and Hochmuth, 2004; Vandenberg and Tiessen, 1972). Coating 30-40 lb (14-18 kg) kraft paper with vegetable oils will retard degradation by repelling water and also by filling voids in the cellulose fibers of paper, preventing microorganism infiltration (Shogren, 1999). When oils are applied to the kraft-paper mulches, field-life can be increased to 14 weeks; giving adequate weed control and yields comparable to black-plastic mulch (Shogren, 1999; Shogren and David, 2006). Coolong (2010) reported adequate weed control and yields comparable to black plastic mulches for 40-lb kraft paper coated with a thin layer of clear polyethylene. Mating a thin degradable coating to paper mulch may be a potential solution to the premature degradation of paper mulches; however, the weight and subsequent shipping costs for paper-based mulches at the present time precludes them from

Mulches have been used for centuries for weed control in vegetable crops. Despite the development of a range of herbicides available, mulches still continue to play a significant role in the production of vegetable crops. The introduction of polyethylene mulches in the 1950s significantly altered the way mulches were utilized. When combined with tillage techniques and herbicides, plastic mulches allow vegetable growers to maintain nearly weed-free fields. The ability of plastic mulches to alter crop microclimate can also lead to improved earliness, quality, and yields (Lamont, 2005). Plastic mulches are now an indispensible part of the modern vegetable production system. However, as concerns regarding the environmental impact of the disposal of mulches increase, alternatives are being sought. Paper-based mulches are degradable and made of a renewable resource, but are bulky and costly to produce. Organic mulches such as straw improve soil health by increasing organic matter and improving soil structure. However, they do not provide the same soil warming benefits as polyethylene mulches. This may limit their use in certain crops or cooler climates. As technologies improve, a completely degradable mulch film made from natural polymers may replace traditional polyethylene mulches. However, until that time polyethylene plastic will remain the most widely used mulch for the production of

Adlerz, W. and Everett, P. 1968. Aluminum foil and white polyethylene mulches to repel

Army, T. and Hudspeth, E. 1960. Alteration of the microclimate of the seed zone. *Agronomy* 

Basavaraju, H.K. and Nanjappa, H.V. 1999. Weed dynamics in chilli-maize cropping

Brault, D., Stewart, K.A., and Jenni, S. 2002. Optical properties of paper and polyethylene

Brown, S.L. and Brown, J.E. 1992. Effect of plastic mulch color and insecticides on thrips

mulches used for weed control in lettuce. *HortScience.* 37:87-91.

populations and damage to tomato. *HortTechnology.* 2:208-210.

aphids and control watermelon mosaic. *Journal of Economic Entomology.* 61:1276-

sequence as influenced by soil solarization. *Indian Journal of Weed Science.* 31:183-

widespread use.

**4. Summary and conclusions** 

warm-season vegetable crops.

*Journal.* 52:17-22.

**5. References** 

1279.

186.

within 8 weeks of application. Although weed growth occurred in the clear and wavelength selective-starch mulches, the yields of the crops trialled (zucchini, cantaloupe, pepper, eggplant, and corn) were not significantly different between mulch types (starch-based polyethylene) of a given color (clear, wavelength-selective, black). The black-colored starch mulch remained intact for the entire growing season (Waterer, 2010). This trial was conducted at a northern latitude (Saskatoon, Saskatchewan, CA) with a short cool growing season. Different results may be expected in warmer environments.

Another commonly utilized polymer for degradable mulches is polybutylene adipate-coterephthalate (PBAT). PBAT is reportedly a fully biodegradable polymer that has similar physical characteristics as traditional LDPE mulches, although, PBAT mulches are typically slightly thinner and tear easier than common LDPE mulches (Kijchavengkul et al., 2008a; Witt et al., 2001). During the time PBAT mulches are set out in the field for crop production they begin photodegrading with a period of intensive biodegradation after crop removal and subsequent plowing into the ground. However, the absolute biodegradability of the PBAT mulches has been questioned due to cross-linking that can occur between benzene rings contained in the PBAT polymer (Kijchavengkul et al., 2008a; Kijchavengkul et al., 2008b). Typically, white or green PBAT mulches have been found to degrade quicker than black mulches, often breaking apart while the crop is in the field (Moreno and Moreno, 2008; Ngouajio et al., 2008). White-colored PBAT films can contain titanium oxide, which may catalyze photodegradation leading to premature breakdown (Gesenhues, 2000; Kijchavengkul et al., 2008a). Black-colored PBAT mulches typically last longer and it is proposed that the carbon black added to the PBAT film absorbs light energy, reducing photodegradation (Kijchavengkul et al., 2008a; Schnabel, 1981).

Trials of PBAT mulches indicate that white-colored PBAT mulches have lower yields compared to black-polyethylene mulches, but black-PBAT mulches usually perform as well as traditional polyethylene mulches (Miles et al., 2006; Moreno and Moreno, 2008; Ngouajio et al., 2008). Usually white-PBAT mulches break down prematurely allowing weeds to grow, affecting crop yields. Interestingly, soil temperatures under black-PBAT mulches are often lower than under black-polyethylene mulches (Moreno and Moreno, 2008; Ngouajio et al., 2008). Although promising, PBAT and starch-based mulches are not used on a large scale at this time.

#### **3.2 Degradable paper mulches**

Nearly 100 years after the use of paper mulches was documented in Hawaii, they are again being evaluated for use in vegetable production (Stewart et al., 1926). Paper is a renewable resource that readily biodegrades. Newspaper-based mulches represent an available and cost effective resource and have been frequently trialled; though they often deteriorate rapidly under field conditions, reducing effectiveness (Shogren, 2001). Shredded newspapers have been successfully used as a weed suppressing mulch in organic hightunnel cucumber production (Sanchez et al., 2008). A high-tunnel environment (no wind or rain) is conducive for using newspaper mulches. Traditionally, paper mulches degrade quickly under field conditions and may tear when using traditional mulch-laying and planting equipment (Coolong, 2010). To improve the durability of paper-based mulches, several trials have utilized mulches with polyethylene, wax, or vegetable oil coatings used to slow degradation of paper mulches in the field (Shogren, 1999; Shogren and David, 2006;

within 8 weeks of application. Although weed growth occurred in the clear and wavelength selective-starch mulches, the yields of the crops trialled (zucchini, cantaloupe, pepper, eggplant, and corn) were not significantly different between mulch types (starch-based polyethylene) of a given color (clear, wavelength-selective, black). The black-colored starch mulch remained intact for the entire growing season (Waterer, 2010). This trial was conducted at a northern latitude (Saskatoon, Saskatchewan, CA) with a short cool growing

Another commonly utilized polymer for degradable mulches is polybutylene adipate-coterephthalate (PBAT). PBAT is reportedly a fully biodegradable polymer that has similar physical characteristics as traditional LDPE mulches, although, PBAT mulches are typically slightly thinner and tear easier than common LDPE mulches (Kijchavengkul et al., 2008a; Witt et al., 2001). During the time PBAT mulches are set out in the field for crop production they begin photodegrading with a period of intensive biodegradation after crop removal and subsequent plowing into the ground. However, the absolute biodegradability of the PBAT mulches has been questioned due to cross-linking that can occur between benzene rings contained in the PBAT polymer (Kijchavengkul et al., 2008a; Kijchavengkul et al., 2008b). Typically, white or green PBAT mulches have been found to degrade quicker than black mulches, often breaking apart while the crop is in the field (Moreno and Moreno, 2008; Ngouajio et al., 2008). White-colored PBAT films can contain titanium oxide, which may catalyze photodegradation leading to premature breakdown (Gesenhues, 2000; Kijchavengkul et al., 2008a). Black-colored PBAT mulches typically last longer and it is proposed that the carbon black added to the PBAT film absorbs light energy, reducing

Trials of PBAT mulches indicate that white-colored PBAT mulches have lower yields compared to black-polyethylene mulches, but black-PBAT mulches usually perform as well as traditional polyethylene mulches (Miles et al., 2006; Moreno and Moreno, 2008; Ngouajio et al., 2008). Usually white-PBAT mulches break down prematurely allowing weeds to grow, affecting crop yields. Interestingly, soil temperatures under black-PBAT mulches are often lower than under black-polyethylene mulches (Moreno and Moreno, 2008; Ngouajio et al., 2008). Although promising, PBAT and starch-based mulches are not used on a large

Nearly 100 years after the use of paper mulches was documented in Hawaii, they are again being evaluated for use in vegetable production (Stewart et al., 1926). Paper is a renewable resource that readily biodegrades. Newspaper-based mulches represent an available and cost effective resource and have been frequently trialled; though they often deteriorate rapidly under field conditions, reducing effectiveness (Shogren, 2001). Shredded newspapers have been successfully used as a weed suppressing mulch in organic hightunnel cucumber production (Sanchez et al., 2008). A high-tunnel environment (no wind or rain) is conducive for using newspaper mulches. Traditionally, paper mulches degrade quickly under field conditions and may tear when using traditional mulch-laying and planting equipment (Coolong, 2010). To improve the durability of paper-based mulches, several trials have utilized mulches with polyethylene, wax, or vegetable oil coatings used to slow degradation of paper mulches in the field (Shogren, 1999; Shogren and David, 2006;

season. Different results may be expected in warmer environments.

photodegradation (Kijchavengkul et al., 2008a; Schnabel, 1981).

scale at this time.

**3.2 Degradable paper mulches** 

Shogren and Hochmuth, 2004; Vandenberg and Tiessen, 1972). Coating 30-40 lb (14-18 kg) kraft paper with vegetable oils will retard degradation by repelling water and also by filling voids in the cellulose fibers of paper, preventing microorganism infiltration (Shogren, 1999). When oils are applied to the kraft-paper mulches, field-life can be increased to 14 weeks; giving adequate weed control and yields comparable to black-plastic mulch (Shogren, 1999; Shogren and David, 2006). Coolong (2010) reported adequate weed control and yields comparable to black plastic mulches for 40-lb kraft paper coated with a thin layer of clear polyethylene. Mating a thin degradable coating to paper mulch may be a potential solution to the premature degradation of paper mulches; however, the weight and subsequent shipping costs for paper-based mulches at the present time precludes them from widespread use.

#### **4. Summary and conclusions**

Mulches have been used for centuries for weed control in vegetable crops. Despite the development of a range of herbicides available, mulches still continue to play a significant role in the production of vegetable crops. The introduction of polyethylene mulches in the 1950s significantly altered the way mulches were utilized. When combined with tillage techniques and herbicides, plastic mulches allow vegetable growers to maintain nearly weed-free fields. The ability of plastic mulches to alter crop microclimate can also lead to improved earliness, quality, and yields (Lamont, 2005). Plastic mulches are now an indispensible part of the modern vegetable production system. However, as concerns regarding the environmental impact of the disposal of mulches increase, alternatives are being sought. Paper-based mulches are degradable and made of a renewable resource, but are bulky and costly to produce. Organic mulches such as straw improve soil health by increasing organic matter and improving soil structure. However, they do not provide the same soil warming benefits as polyethylene mulches. This may limit their use in certain crops or cooler climates. As technologies improve, a completely degradable mulch film made from natural polymers may replace traditional polyethylene mulches. However, until that time polyethylene plastic will remain the most widely used mulch for the production of warm-season vegetable crops.

#### **5. References**


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http://vegetables.wsu.edu/MulchReport06.pdf


**4** 

*Iran* 

G.R. Mohammadi

**Living Mulch as a Tool to Control Weeds** 

*Faculty of Agriculture and Natural Resources, Razi University, Kermanshah,* 

Weeds are a serious constraint to increased production in crops due to reduced yield and economic returns. Weed problems are particularly problematic in row crops as a result of widely spaced crop rows. Weed control in most agroecosystems is highly dependent on conventional cultivation and herbicide applications. Conventional interrow cultivation represents an additional cost for the producer due to the consumption of fossil fuels (Lybecker et al. 1988) and is also associated with increased soil erosion as soil particles are more susceptible to displacement after tillage (Dabney et al. 1993; Fuller et al. 1995). Moreover, ground and surface water pollution by pesticides are causes for concern (Hallberg 1989), and herbicides used in crops have been among the pesticides most frequently detected in these waters (National Research Council 1989). Improving water quality and decreasing herbicide carry over is one of the more important environmental issues for farmers and agriculture researchers (Stoller et al. 1993). Herbicide-resistant weed ecotypes are being discovered more frequently, due to increased herbicide applications and subsequent selection, is also posing a serious threat to agricultural production (Holt and

Increased interest in sustainable agricultural systems has led to significant developments in cropping practices over the past decade (Thiessen-Martenes et al., 2001). Interest in alternative and sustainable agricultural production systems that require fewer production inputs is growing (Calkins and Swanson 1995). The current emphasis on reduced pesticide use has led to increased interest in alternative weed management methods (Bellinder et al. 1994). In sustainable agriculture, an alternative method to chemical and mechanical weed control in crops is the use of living mulches. Living mulches are cover crops that are planted between the rows of a main crop such as corn (*Zea mays* L.), soybean (*Glycine max* L.), etc., and are maintained as a living ground cover during the growing season of the main crop. Although living mulches are sometimes referred to as cover crops, they grow at least part of

In addition to providing adequate cover to reduce soil erosion (Wall et al. 1991) and increase soil water infiltration (Bruce et al. 1992), legume living mulches improve soil nutrient status through addition of organic nitrogen (N) (Holderbaum et al. 1990; Brown et al., 1993) via

**1. Introduction** 

LeBaron 1990).

the time simultaneously with the crop.

**in Agroecosystems: A Review** 

*Department of Crop Production and Breeding,* 


## **Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review**

G.R. Mohammadi

*Department of Crop Production and Breeding, Faculty of Agriculture and Natural Resources, Razi University, Kermanshah, Iran* 

#### **1. Introduction**

74 Weed Control

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Waterer, D., Hrycan, W., and Simms, T. 2008. Potential to double-crop plastic mulch.

Webster, T.M. 2005. Mulch type affects growth and tuber production of yellow nutsedge

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Wolfenbarger, D. and Moore, W. 1968. Insect abundances on tomatoes and squash mulched with aluminum and plastic sheetings1. *Journal of Economic Entomology.* 61:34-36. Wright, J. 1968. Production of polyethylene film. P*roceedings of the National Agricultural* 

Biodegradation of aliphatic-aromatic copolyesters: Evaluation of the final biodegradability and ecotoxicological impact of degradation intermediates.

(*Cyperus esculentus*) and purple nutsedge (*Cyperus rotundus*). *Weed Science.* 53:834-

Tarara, J.M. 2000. Microclimate modification with plastic mulch. *HortScience.* 35:169-180. Thompson, H. and Platenius, H. 1931. Results of paper mulch experiments with vegetable crops. *Proceedings of the American Society for Horticultural Science.* 28:305-309. Tindall, J.A., Mills, H., and Radcliffe, D. 1990. The effect of root zone temperature on

nutrient uptake of tomato. *Journal of Plant Nutrition.* 13:939-956.

mulch on growth and flowering of tomato. *HortScience.* 7:464-465.

vegetable crops. *Canadian Journal of Plant Science.* 90:737-743.

William, R. 1976. Purple nutsedge: Tropical scourge. *HortScience.* 11:357-364.

*Canadian Journal of Plant Science.* 88:187-193.

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*Weed Science.*317-323.

*Chemosphere.* 44:289-299.

*Plastics Congress* 8:72-79.

industry. *Plant Disease.* 64:264-270.

stand establishment in preliminary vegetable crop studies. *California Agriculture.*

polyethylene films on soil temperature and plant growth. *Proceedings of the* 

polymerized vegetable oil mulches. *HortScience.* 39:1588-1591.

seed populations. *Weed Science.* 32:569-573.

*American Society for Horticultural Science.* 85:532-540.

soils. *Soil Science.* 22:35-39.

17:2-3.

838.

Weeds are a serious constraint to increased production in crops due to reduced yield and economic returns. Weed problems are particularly problematic in row crops as a result of widely spaced crop rows. Weed control in most agroecosystems is highly dependent on conventional cultivation and herbicide applications. Conventional interrow cultivation represents an additional cost for the producer due to the consumption of fossil fuels (Lybecker et al. 1988) and is also associated with increased soil erosion as soil particles are more susceptible to displacement after tillage (Dabney et al. 1993; Fuller et al. 1995). Moreover, ground and surface water pollution by pesticides are causes for concern (Hallberg 1989), and herbicides used in crops have been among the pesticides most frequently detected in these waters (National Research Council 1989). Improving water quality and decreasing herbicide carry over is one of the more important environmental issues for farmers and agriculture researchers (Stoller et al. 1993). Herbicide-resistant weed ecotypes are being discovered more frequently, due to increased herbicide applications and subsequent selection, is also posing a serious threat to agricultural production (Holt and LeBaron 1990).

Increased interest in sustainable agricultural systems has led to significant developments in cropping practices over the past decade (Thiessen-Martenes et al., 2001). Interest in alternative and sustainable agricultural production systems that require fewer production inputs is growing (Calkins and Swanson 1995). The current emphasis on reduced pesticide use has led to increased interest in alternative weed management methods (Bellinder et al. 1994). In sustainable agriculture, an alternative method to chemical and mechanical weed control in crops is the use of living mulches. Living mulches are cover crops that are planted between the rows of a main crop such as corn (*Zea mays* L.), soybean (*Glycine max* L.), etc., and are maintained as a living ground cover during the growing season of the main crop. Although living mulches are sometimes referred to as cover crops, they grow at least part of the time simultaneously with the crop.

In addition to providing adequate cover to reduce soil erosion (Wall et al. 1991) and increase soil water infiltration (Bruce et al. 1992), legume living mulches improve soil nutrient status through addition of organic nitrogen (N) (Holderbaum et al. 1990; Brown et al., 1993) via

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 77

The symbiotic relationship of legume living mulches with rhizobia bacteria allows them to use N from the atmosphere. Of major interest is whether some of the fixed N will be available to a cereal grown simultaneously with the legume. If this is the case, living mulches of legumes could reduce the need for fertilizer N. The primary mechanism of N transfer from a legume to a nonlegume is decomposition of leaves, roots, and stems of the legume (Fujita et al. 1992). However, observations in cereal–legume intercrops have confirmed that N is also excreted from legume roots and leached from leaves, thus

More efficient use of environmental resources is another important benefit of a living mulch system. For example, a legume such as crownvetch (*Coronilla varia* L.) with different root architecture than corn might absorb relatively immobile potassium (K) from deep zones in the soil that would not be accessed by corn (Vandermeer, 1990). Corn requires less soil nutrients (Richie et al. 1993) and light as it matures late in the growing season. In the presence of kura clover (*Trifolium ambiguum* M. Bieb.) as a living mulch, nutrients and light penetrating through the maturing corn canopy may be utilized by the living mulch rather

Living mulches are crops grown simultaneously with the main crop that can suppress weed growth significantly without reducing main crop yield through an ability to grow fast or because they are planted at a high density (De Haan et al., 1994). Living mulches can suppress weed growth by competing for light (Teasdale 1993), water and nutrients (Mayer and Hartwig 1986), and through the production of allelopathic compounds (White et al. 1989) which may ultimately result in reduced herbicide applications. Many studies have confirmed the weed suppressing ability of living mulches in different cropping systems.

There is wide agreement in the literature that a vigorous living mulch will suppress weeds growing at the same time as the living mulch (Stivers-Young 1998; Akobundu et al. 2000; Creamer and Baldwin 2000; Blackshaw et al. 2001; Favero et al. 2001; Grimmer and Masiunas 2004; Peachey et al. 2004; Brennan and Smith 2005). In one study (Echtenkamp and Moomaw 1989), chewing fescue or red fescue *(Festuca rubra* L.*)* and ladino clover *(Trifolium repens* L.*)* were effective living mulches for controlling weed growth. Reductions in weed infestations have been reported with sunn hemp (*Crotalaria juncea* L.) as a living mulch in citrus and avocado (*Persea americana* Mill.) (Linares et al. 2008; Severino and Christoffoleti 2004). According to Mohammadi (2010) weed dry weight was reduced by 34 and 50.9% when hairy vetch (*Vicia villosa* Roth) was interseeded in corn at 25 and 50 kg ha-1,

Moynihan et al. (1996) also reported a 65% reduction in fall weed biomass compared with non-living mulch control following a grain barley (*Hordeum vulgare* L.) and medic (*Medicago*  sp.) intercrop. Velvetbean (*Mucuna pruriens* L.) suppressed the radical growth of the local weeds alegria (*Amaranthus hypochondriacus* L.) by 66% and barnyardgrass (*Echinochloa crusgalli* L.) by 26.5% (Caamal-Maldonado et al. 2001). In another research, a subterranean clover (*Trifolium subterraneum* L.) living mulch reduced weed biomass and increased soybean yield by 91 % relative to weedy control plots (Ilnicki and Enache 1992). The list of weed

suppression intensity by different living mulch species are shown in Table 1.

becoming available to the cereal immediately (Fujita et al. 1992).

than fostering weed growth (Zemenchik et al. 2000).

**3.1 Weed suppression** 

respectively.

fixed atmospheric nitrogen which improves soil physical properties (McVay et al. 1989; Latif et al. 1992). Incorporating legume living mulches can also increase the yield of the succeeding crop (Bollero and Bullock 1994; Decker et al. 1994). Leguminous living mulches have the potential to reduce dependence on fossil fuels and reduce negative environmental effects of crop production systems. Some functions these living mulches can perform are (1) fixing atmospheric N that is made available to main crop, (2) protecting soil from erosion during the main crop growing season, (3) improving soil quality, (4) reducing evaporation and increasing infiltration during the main crop growing season and (5) suppressing weeds (SAN, 1998). Improvement of soil organic matter and production of forage for animal feed are other potential uses of living mulches.

#### **2. Necessity to develop alternative weed control methods in agroecosystems**

Weeds are one of the major problems in crop production around the world, and we are trending toward controlling these weeds with herbicides, which comes with an increased environmental impact. At present, in most agroecosystems, weed control highly depends on chemical and mechanical practices that are very expensive, hazardous for the environment and, consequently, unsustainable. For example, currently about 95 % of the soybean acreage in the state of Minnesota in the United States (U.S.) is treated with herbicides with about 6 million kg of herbicides applied annually (Minnesota Agricultural Statistics 2001). Herbicide costs account for 35% of the variable cost of production. Overall, in the U. S. alone, 75% of crop production is based on herbicide input (Duke 1999).

However, herbicide-based control has failed to achieve long-term weed seedbank management (Mortensen et al. 2000; Weber and Gut 2005). Even with herbicides, weeds remain prominent in croplands and producers still lose considerable crop yield due to weeds (Bridges 1994). Furthermore, herbicide resistance is forcing producers to use more expensive management tactics, thereby increasing production costs. Public concern over safety has also caused a reassessment of toxicological and environmental impacts of synthetic herbicides. Therefore, because synthetic herbicides represent a significant expense and environmental concern and cannot be used by those wishing to be certified as organic producers, many producers seek alternative weed control strategies.

#### **3. Benefits of living mulch systems**

Living mulches have the potential to form an important component in agroecosystems and can be a useful tool for weed suppression in sustainable agricultural systems (Teasdale 1996; Bond and Grundy 2001; Kruidhof et al. 2008) including many useful advantages such as: improvement of soil structure (Harris et al., 1966), regulation of soil water content (Hoyt and Hargrove 1986), enhancement of soil organic matter, carbon dynamics and microbiological function (Steenwerth and Belina 2008), reducing soil erosion (Malik et al., 2000), soil enrichment by nitrogen fixation (Sainju et al. 2001), insectarium for many beneficial arthropod species (Grafton-Cardwell et al. 1999), and enhancement of populations of soil macrofauna (Blanchart et al. 2006). Living mulches also have the potential to suppress weed growth (De Haan et al. 1994), increase soil water infiltration (Bruce et al. 1992), decrease soil erosion (Cripps and Bates 1993), contribute N to the main crop (Corak et al. 1991) and reduce economic risk (Hanson et al. 1993).

The symbiotic relationship of legume living mulches with rhizobia bacteria allows them to use N from the atmosphere. Of major interest is whether some of the fixed N will be available to a cereal grown simultaneously with the legume. If this is the case, living mulches of legumes could reduce the need for fertilizer N. The primary mechanism of N transfer from a legume to a nonlegume is decomposition of leaves, roots, and stems of the legume (Fujita et al. 1992). However, observations in cereal–legume intercrops have confirmed that N is also excreted from legume roots and leached from leaves, thus becoming available to the cereal immediately (Fujita et al. 1992).

More efficient use of environmental resources is another important benefit of a living mulch system. For example, a legume such as crownvetch (*Coronilla varia* L.) with different root architecture than corn might absorb relatively immobile potassium (K) from deep zones in the soil that would not be accessed by corn (Vandermeer, 1990). Corn requires less soil nutrients (Richie et al. 1993) and light as it matures late in the growing season. In the presence of kura clover (*Trifolium ambiguum* M. Bieb.) as a living mulch, nutrients and light penetrating through the maturing corn canopy may be utilized by the living mulch rather than fostering weed growth (Zemenchik et al. 2000).

#### **3.1 Weed suppression**

76 Weed Control

fixed atmospheric nitrogen which improves soil physical properties (McVay et al. 1989; Latif et al. 1992). Incorporating legume living mulches can also increase the yield of the succeeding crop (Bollero and Bullock 1994; Decker et al. 1994). Leguminous living mulches have the potential to reduce dependence on fossil fuels and reduce negative environmental effects of crop production systems. Some functions these living mulches can perform are (1) fixing atmospheric N that is made available to main crop, (2) protecting soil from erosion during the main crop growing season, (3) improving soil quality, (4) reducing evaporation and increasing infiltration during the main crop growing season and (5) suppressing weeds (SAN, 1998). Improvement of soil organic matter and production of forage for animal feed

**2. Necessity to develop alternative weed control methods in agroecosystems**  Weeds are one of the major problems in crop production around the world, and we are trending toward controlling these weeds with herbicides, which comes with an increased environmental impact. At present, in most agroecosystems, weed control highly depends on chemical and mechanical practices that are very expensive, hazardous for the environment and, consequently, unsustainable. For example, currently about 95 % of the soybean acreage in the state of Minnesota in the United States (U.S.) is treated with herbicides with about 6 million kg of herbicides applied annually (Minnesota Agricultural Statistics 2001). Herbicide costs account for 35% of the variable cost of production. Overall, in the U. S. alone, 75% of

However, herbicide-based control has failed to achieve long-term weed seedbank management (Mortensen et al. 2000; Weber and Gut 2005). Even with herbicides, weeds remain prominent in croplands and producers still lose considerable crop yield due to weeds (Bridges 1994). Furthermore, herbicide resistance is forcing producers to use more expensive management tactics, thereby increasing production costs. Public concern over safety has also caused a reassessment of toxicological and environmental impacts of synthetic herbicides. Therefore, because synthetic herbicides represent a significant expense and environmental concern and cannot be used by those wishing to be certified as organic

Living mulches have the potential to form an important component in agroecosystems and can be a useful tool for weed suppression in sustainable agricultural systems (Teasdale 1996; Bond and Grundy 2001; Kruidhof et al. 2008) including many useful advantages such as: improvement of soil structure (Harris et al., 1966), regulation of soil water content (Hoyt and Hargrove 1986), enhancement of soil organic matter, carbon dynamics and microbiological function (Steenwerth and Belina 2008), reducing soil erosion (Malik et al., 2000), soil enrichment by nitrogen fixation (Sainju et al. 2001), insectarium for many beneficial arthropod species (Grafton-Cardwell et al. 1999), and enhancement of populations of soil macrofauna (Blanchart et al. 2006). Living mulches also have the potential to suppress weed growth (De Haan et al. 1994), increase soil water infiltration (Bruce et al. 1992), decrease soil erosion (Cripps and Bates 1993), contribute N to the main crop (Corak et al. 1991) and

are other potential uses of living mulches.

**3. Benefits of living mulch systems** 

reduce economic risk (Hanson et al. 1993).

crop production is based on herbicide input (Duke 1999).

producers, many producers seek alternative weed control strategies.

Living mulches are crops grown simultaneously with the main crop that can suppress weed growth significantly without reducing main crop yield through an ability to grow fast or because they are planted at a high density (De Haan et al., 1994). Living mulches can suppress weed growth by competing for light (Teasdale 1993), water and nutrients (Mayer and Hartwig 1986), and through the production of allelopathic compounds (White et al. 1989) which may ultimately result in reduced herbicide applications. Many studies have confirmed the weed suppressing ability of living mulches in different cropping systems.

There is wide agreement in the literature that a vigorous living mulch will suppress weeds growing at the same time as the living mulch (Stivers-Young 1998; Akobundu et al. 2000; Creamer and Baldwin 2000; Blackshaw et al. 2001; Favero et al. 2001; Grimmer and Masiunas 2004; Peachey et al. 2004; Brennan and Smith 2005). In one study (Echtenkamp and Moomaw 1989), chewing fescue or red fescue *(Festuca rubra* L.*)* and ladino clover *(Trifolium repens* L.*)* were effective living mulches for controlling weed growth. Reductions in weed infestations have been reported with sunn hemp (*Crotalaria juncea* L.) as a living mulch in citrus and avocado (*Persea americana* Mill.) (Linares et al. 2008; Severino and Christoffoleti 2004). According to Mohammadi (2010) weed dry weight was reduced by 34 and 50.9% when hairy vetch (*Vicia villosa* Roth) was interseeded in corn at 25 and 50 kg ha-1, respectively.

Moynihan et al. (1996) also reported a 65% reduction in fall weed biomass compared with non-living mulch control following a grain barley (*Hordeum vulgare* L.) and medic (*Medicago*  sp.) intercrop. Velvetbean (*Mucuna pruriens* L.) suppressed the radical growth of the local weeds alegria (*Amaranthus hypochondriacus* L.) by 66% and barnyardgrass (*Echinochloa crusgalli* L.) by 26.5% (Caamal-Maldonado et al. 2001). In another research, a subterranean clover (*Trifolium subterraneum* L.) living mulch reduced weed biomass and increased soybean yield by 91 % relative to weedy control plots (Ilnicki and Enache 1992). The list of weed suppression intensity by different living mulch species are shown in Table 1.

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 79

mulch species (Table 2). Corn yield was increased 79% and weed dry weight was reduced 80.5% when the plots were interseeded with hairy vetch as compared with full season

> **100-seed weight (g)**

Hairy vetch 1188.78 ab 1.10 b 725.30 ab 30.94 ab 254.83 ab 4.82 ab 2.88 a 32.13 d

Persian clover 1063.37 c 1.05 bc 683.60 bc 26.91 c 247.50 b 4.44 abc 2.74 ab 53.65 bc

White clover 999.52 cd 1.00 cd 671.10 bc 26.84 c 248.00 b 4.60 abc 2.23 de 46.33 cd

Black alfalfa 945.16 d 1.03 cd 653.50 c 26.78 c 251.50 b 3.96 c 2.17 e 65.73 b

Alfalfa 938.08 d 1.02 cd 645.05 c 27.16 c 251.75 b 4.19 bc 2.41 cd 62.15 bc

Weedy 664.03 e 0.98 d 519.40 d 23.49 d 205.42 c 3.13 d 1.89 f 164.78 a

LSD (0.05) 109.32 0.07 63.71 2.27 20.41 0.74 0.24 17.21

Living mulches should be species that establish more rapidly than weeds and whose peak period of growth coincides with that of early weed emergence but does not coincide with that of the crop. Ideally the living mulch should suppress weed growth during the critical period for weed establishment, i.e., the period when emerging weeds will cause a loss in crop yield (Buhler et al. 2001). Beard (1973) recommended chewing fescue as a good living mulch because it adapts to the shady conditions under corn and soybean. This grass is also

Total biomass production and nitrogen fixation are the main factors determining the suitability of leguminous species for improvement of soil fertility, but if used as a component crop in intercropping systems, competitive ability is another obvious criterion. Morphological growth characteristics, such as early relative growth rate of leaf area and earliness of height development, have been identified to determine competition in

Similar letters at each column indicate the non significant difference at the 0.05 level of probability. Table 2. Means comparison of corn plant traits and weed dry weight under different living

**Height (cm)** 

1282.89 a 1.20 a 760.10 a 32.34 a 273.41 a 5.10 a 2.73 ab 0.00 e

1084.31 bc 1.05 bc 689.08 bc 29.86 b 252.99 b 4.52 abc 2.57 bc 49.23 bcd

**Leaf area index** 

**Leaf nitrogen content (%)**  **Weed dry weight (g m-2)** 

weedy conditions.

Weed free control

Berseem clover

**Treatment Yield** 

**(g m-2)** 

mulch treatments (from Mohammadi 2009).

well adapted to dry and poor soils.

intercropping systems (Kropff and van Laar 1993).

**Ear per plant** 

**Seed per ear**


\* Percentage suppression relative to a control without living mulch.

Table 1. Suppression of weeds by different living mulch species.

#### **4. Factors determining the success of a living mulch system to suppress weeds**

#### **4.1 Living mulch species**

Interseeding of a crop and living mulches have not always resulted in a positive gain (Nordquist and Wicks 1974; De Haan et al. 1997). Consequently, the success of these kinds of living mulch-crop systems is largely determined by the selection of the most appropriate species and, additionally, by the design of an optimal management strategy for the intercrop. Living mulches differ in their ability to establish well in an interseeding situation. For example, Exner and Cruse (1993) found that alfalfa (*Medicago sativa* L.) and sweet clover (*Melilotus officinalis* L.) usually established better and produced more cover than either red clover (*Trifolium pratense* L.) or alsike clover (*T. hybridum* L.) when interseeded under corn. The competitive ability against weeds is also another important characteristic determining the suitability of a plant species as a living mulch. In a study on six leguminous species (Persian clover, *Trifolium resupinatum* L.; white clover, *T. repens* L.; berseem clover, *T. alexandrinum* L.; hairy vetch; alfalfa; and black alfalfa, *M. lupulina* L.), Mohammadi (2009) found that the highest corn (as the main crop) plant traits including yield, yield components, height, leaf area index and leaf nitrogen content and the lowest weed dry weight were obtained from the plots interseeded with hairy vetch as compared with the other living

Red clover, hairy vetch 75 Palada et al. (1982) Subterranean clover 53-94 Enache and Ilnicki (1990) Hairy vetch 70-90 Oliver et al. (1992) Subterranean clover 91 Ilnicki and Enache (1992)

**suppression\*** 

95-99

71-90

29-48

Hairy vetch 96 Hoffman et al. (1993)

Velvetbean 68 Caamal-Maldonado et al. (2001) Hairy vetch 79 Reddy and Koger (2004) Alfalfa 34.2-56.9 Ghosheh et al. (2004) Rye 37-76 Brainard and Bellinder (2004)

Hairy vetch 34-50.9 Mohammadi (2010)

**4. Factors determining the success of a living mulch system to suppress** 

Interseeding of a crop and living mulches have not always resulted in a positive gain (Nordquist and Wicks 1974; De Haan et al. 1997). Consequently, the success of these kinds of living mulch-crop systems is largely determined by the selection of the most appropriate species and, additionally, by the design of an optimal management strategy for the intercrop. Living mulches differ in their ability to establish well in an interseeding situation. For example, Exner and Cruse (1993) found that alfalfa (*Medicago sativa* L.) and sweet clover (*Melilotus officinalis* L.) usually established better and produced more cover than either red clover (*Trifolium pratense* L.) or alsike clover (*T. hybridum* L.) when interseeded under corn. The competitive ability against weeds is also another important characteristic determining the suitability of a plant species as a living mulch. In a study on six leguminous species (Persian clover, *Trifolium resupinatum* L.; white clover, *T. repens* L.; berseem clover, *T. alexandrinum* L.; hairy vetch; alfalfa; and black alfalfa, *M. lupulina* L.), Mohammadi (2009) found that the highest corn (as the main crop) plant traits including yield, yield components, height, leaf area index and leaf nitrogen content and the lowest weed dry weight were obtained from the plots interseeded with hairy vetch as compared with the other living

60.1-80.5 Mohammadi (2009)

Yellow mustard (*Sinapis alba* L.) 80 De Haan et al. (1994) Annual medics (*Medicago* spp.) 65 Moynihan et al. (1996) Annual medics (*Medicago* spp.) 41-69 De Haan et al. (1997) Subterranean clover, white clover 45-51 Brandsaeter et al. (1998)

**Reference** 

Skora Neto (1993)

**Living mulch species Percentage weed** 

Black mucuna (*Mucuna pruriens* L.), smooth rattlebox (*Crotalaria pallida* L.) Jack bean (*Canaualia ensiformis* L.), pigeon pea (*Cajanus cajan* L.) Cowpea (*Vigna unguiculata* L.)

Persian clover, white clover, berseem clover, hairy vetch, alfalfa, and black alfalfa

Percentage suppression relative to a control without living mulch. Table 1. Suppression of weeds by different living mulch species.

\*

**weeds** 

**4.1 Living mulch species** 



Similar letters at each column indicate the non significant difference at the 0.05 level of probability.

Table 2. Means comparison of corn plant traits and weed dry weight under different living mulch treatments (from Mohammadi 2009).

Living mulches should be species that establish more rapidly than weeds and whose peak period of growth coincides with that of early weed emergence but does not coincide with that of the crop. Ideally the living mulch should suppress weed growth during the critical period for weed establishment, i.e., the period when emerging weeds will cause a loss in crop yield (Buhler et al. 2001). Beard (1973) recommended chewing fescue as a good living mulch because it adapts to the shady conditions under corn and soybean. This grass is also well adapted to dry and poor soils.

Total biomass production and nitrogen fixation are the main factors determining the suitability of leguminous species for improvement of soil fertility, but if used as a component crop in intercropping systems, competitive ability is another obvious criterion. Morphological growth characteristics, such as early relative growth rate of leaf area and earliness of height development, have been identified to determine competition in intercropping systems (Kropff and van Laar 1993).

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 81

optimum living mulch density is achieved, beyond which, no further decrease in weed biomass could be obtained. Generally, the biomass produced by a living mulch highly depends on its planting rate. Moreover, there is often a negative correlation between living mulch and weed biomass (Akemo et al. 2000; Ross et al. 2001; Sheaffer et al. 2002). Meschede et al. (2007) expressed that the biomass accumulation by the living mulches was inversely proportional to the weed biomass. Mohammadi (2010) also reported that increasing the hairy vetch dry weight led to the reduction of weed dry weight produced. As for every 1.18 g m-2 hairy vetch dry weight produced, 1 g m-2 weed dry weight was reduced (Fig. 1).

> 0 50 100 150 200 250 300 350 **Hairy vetch dry weight (g m-2)**

Fig. 1. Relationship between hairy vetch dry weight and weed dry weight loss in field corn as obtained using linear regression model, y = 442.81 - 0.8485x, R2 = 0.52. The points indicate

However, Akobundu et al. (2000) found that development of early ground cover was more important than the quantity of dry matter produced for suppression of cogongrass by

Sowing time of a living mulch is also a very important factor for controlling the weed flora. Some weed species will germinate faster than the living mulch, while some of them will germinate simultaneously with mulch species and others will germinate after the living mulch. Species that germinate after the living mulch cannot grow well since the mulch species shades and mechanically blocks growth of these weed species (Kitis et al. 2011). This

There are a number of mechanisms by which living mulches can suppress weeds such as: their competition for light (Teasdale 1993; Teasdale and Mohler 1993), moisture and nutrient

the individual weed-infested plot values (n = 24) (from Mohammadi 2010).

can lead to the alteration of weed flora in cropping systems.

**5. Mechanisms by which living mulches can suppress weeds** 

0

velvetbean as a living mulch.

100

200

300

**Weed dry weight (g m-2)**

400

500

600

Different phenological characteristics and growth patterns were observed among living mulches species ranging from the short-lived species *Mucuna pruriens*, which germinated quickly and covered the ground surface rapidly (LAI=1 at GDD=476°Cd), to the long-lived species *Aeschynomene histrix*, which is slow to establish and only reached a canopy LAI of 1 at around 800°Cd. These characteristics make *M. pruriens* a relatively strong competitor, which may explain its use against the perennial grass *Imperata cylindrica* in maize-based systems in Africa and North Honduras (Versteeg and Koudopon 1990; Akobundo 1993; Triomphe 1996). Based on early growth characteristics, *Crotalaria juncea*, *Cajanus cajan* and *M. pruriens* can be considered as species with a higher competitive ability than *Calopogonium mucunoides*, *Stylosanthes hamata* and *A. histrix*. This can be explained by the combination of high initial growth rates for height and leaf area development. Additionally, the high final height of *C. juncea* and *C. cajan* may confer higher competitiveness throughout the growing season (Akanvou et al. 2001).

According to De Haan et al. (1994) medics used as living mulches in row crops should be small, prostrate, and early maturing. Because of their prostrate growth habit, short life span, and good seedling vigour, medics have potential as living mulches. A living mulch should control weeds, have a relatively short growing season, provide a constant N supply, and give minimal competition to the main crop for water, light, and nutrients (De Haan et al. 1994).

In general, ideal living mulches for weed suppression should have the following characteristics:


Usually, living mulches that establish an early leaf canopy cover are most competitive with weeds.

#### **4.2 Living mulch planting rate and time**

The success of a living mulch system also depends on appropriate management. Both time and rate of living mulch interseeding can be important factors determining the success of a crop-living mulch system. These factors are critical to reduce living mulch competition with the main crop for environmental resources while allowing the mulch to grow and cover the soil surface sufficiently to reap potential benefits such as weed suppression.

For example, interseeding rye (*Secale* sp.) or small-grain living mulches tended to provide higher levels of weed suppression when interseeded at or near planting of the main crop (Rajalahti et al. 1999; Brainard and Bellinder 2004). In another study, Mohammadi (2010) observed that the plant traits of corn and weed dry weight were not significantly influenced by hairy vetch (as a living mulch) planting times (simultaneous with corn planting or 10 days after corn emergence), but increased hairy vetch planting rate from 0 to 50 kg ha-1 improved corn yield (by 11%) and reduced weed dry weight (by 50.9%). It was hypothesized that as living mulch density is increased, canopy closure would occur more rapidly, decreasing the amount of photosynthetically active radiation (PAR) available beneath the canopy. This would result in a concomitant decrease in weed biomass until an

Different phenological characteristics and growth patterns were observed among living mulches species ranging from the short-lived species *Mucuna pruriens*, which germinated quickly and covered the ground surface rapidly (LAI=1 at GDD=476°Cd), to the long-lived species *Aeschynomene histrix*, which is slow to establish and only reached a canopy LAI of 1 at around 800°Cd. These characteristics make *M. pruriens* a relatively strong competitor, which may explain its use against the perennial grass *Imperata cylindrica* in maize-based systems in Africa and North Honduras (Versteeg and Koudopon 1990; Akobundo 1993; Triomphe 1996). Based on early growth characteristics, *Crotalaria juncea*, *Cajanus cajan* and *M. pruriens* can be considered as species with a higher competitive ability than *Calopogonium mucunoides*, *Stylosanthes hamata* and *A. histrix*. This can be explained by the combination of high initial growth rates for height and leaf area development. Additionally, the high final height of *C. juncea* and *C. cajan* may confer higher competitiveness throughout the growing

According to De Haan et al. (1994) medics used as living mulches in row crops should be small, prostrate, and early maturing. Because of their prostrate growth habit, short life span, and good seedling vigour, medics have potential as living mulches. A living mulch should control weeds, have a relatively short growing season, provide a constant N supply, and give minimal competition to the main crop for water, light, and nutrients (De Haan et al.

In general, ideal living mulches for weed suppression should have the following

Usually, living mulches that establish an early leaf canopy cover are most competitive with

The success of a living mulch system also depends on appropriate management. Both time and rate of living mulch interseeding can be important factors determining the success of a crop-living mulch system. These factors are critical to reduce living mulch competition with the main crop for environmental resources while allowing the mulch to grow and cover the

For example, interseeding rye (*Secale* sp.) or small-grain living mulches tended to provide higher levels of weed suppression when interseeded at or near planting of the main crop (Rajalahti et al. 1999; Brainard and Bellinder 2004). In another study, Mohammadi (2010) observed that the plant traits of corn and weed dry weight were not significantly influenced by hairy vetch (as a living mulch) planting times (simultaneous with corn planting or 10 days after corn emergence), but increased hairy vetch planting rate from 0 to 50 kg ha-1 improved corn yield (by 11%) and reduced weed dry weight (by 50.9%). It was hypothesized that as living mulch density is increased, canopy closure would occur more rapidly, decreasing the amount of photosynthetically active radiation (PAR) available beneath the canopy. This would result in a concomitant decrease in weed biomass until an

1. Ability to provide a complete ground cover of dense vegetation.

2. Rapid establishment and growth that develops a canopy faster than weeds. 3. Selectivity between suppression of weeds and the associated crop (Teasdale 2003).

soil surface sufficiently to reap potential benefits such as weed suppression.

season (Akanvou et al. 2001).

**4.2 Living mulch planting rate and time** 

1994).

weeds.

characteristics:

optimum living mulch density is achieved, beyond which, no further decrease in weed biomass could be obtained. Generally, the biomass produced by a living mulch highly depends on its planting rate. Moreover, there is often a negative correlation between living mulch and weed biomass (Akemo et al. 2000; Ross et al. 2001; Sheaffer et al. 2002). Meschede et al. (2007) expressed that the biomass accumulation by the living mulches was inversely proportional to the weed biomass. Mohammadi (2010) also reported that increasing the hairy vetch dry weight led to the reduction of weed dry weight produced. As for every 1.18 g m-2 hairy vetch dry weight produced, 1 g m-2 weed dry weight was reduced (Fig. 1).

Fig. 1. Relationship between hairy vetch dry weight and weed dry weight loss in field corn as obtained using linear regression model, y = 442.81 - 0.8485x, R2 = 0.52. The points indicate the individual weed-infested plot values (n = 24) (from Mohammadi 2010).

However, Akobundu et al. (2000) found that development of early ground cover was more important than the quantity of dry matter produced for suppression of cogongrass by velvetbean as a living mulch.

Sowing time of a living mulch is also a very important factor for controlling the weed flora. Some weed species will germinate faster than the living mulch, while some of them will germinate simultaneously with mulch species and others will germinate after the living mulch. Species that germinate after the living mulch cannot grow well since the mulch species shades and mechanically blocks growth of these weed species (Kitis et al. 2011). This can lead to the alteration of weed flora in cropping systems.

#### **5. Mechanisms by which living mulches can suppress weeds**

There are a number of mechanisms by which living mulches can suppress weeds such as: their competition for light (Teasdale 1993; Teasdale and Mohler 1993), moisture and nutrient

**5.1 Light** 

velvetbean living mulch.

growing in this condition (Davis and Simmons 1994).

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 83

Plants grown together frequently compete primarily for solar radiation (Redfearn et al. 1999). Two components of light affect the outcome of competition: quantity and quality. The quantitative component of light (i.e., intensity and amount intercepted by a plant) determines canopy photosynthesis, whereas light quality is a driving variable of plant morphology. Both aspects of light are changed in a crop–weed competition situation when compared to the sole crop or weed canopy. Most crops and weeds attain their maximum photosynthetic rates at high levels of irradiance. In a mixed crop–weed community, mutual shading of leaves causes reduction of available photosynthetic photon flux density (PPFD),

In general, one of the important factors of weed suppression mechanisms of living mulch is light interception. Because plants need light to develop and living mulches are blocking sunlight reaching the weeds, weed species, especially decumbent weeds, cannot get enough light for germination and growth. Kruidhof et al. (2008) reported that weed suppression is positively correlated to early light interception by the living mulch and is sustained by the strong negative correlation between cumulative light interception and weed biomass. Similarly, according to Steinmaus et al. (2008), weed suppression was linked to light interception by the mulch cover for most weed species. Caamal-Maldonado et al. (2001) also found that canopy closure of velvetbean decreased the amount of light reaching the soil and inhibited weed growth. They reported that smooth pigweed (*Amaranthus hybridus* L.) and spiny amaranth (*Amaranthus spinosus* L.), among other weeds, were well controlled by a

Several studies have shown that the presence and nearness of the other vegetation influences the far red/red (FR/R) ratio received by a plant ( Ballare et al. 1987; Kasperbauer 1987; Smith et al. 1990), where the FR/R ratio received by plants in a dense canopy was higher than the FR/R ratio in a sparse canopy. In fact, weeds that grow underneath or within a canopy are not only exposed to a reduced amount of PPFD, but they also receive a different quality of light than the plants grown in full sunlight. Light within the lower canopy is enriched in FR radiation (730–740 nm). This is caused by selective absorption of red light (660–670 nm) by photosynthetic pigments and FR light reflectance from and transmittance by green leaves. Chlorophyll preferentially absorbs R light and reflects FR light, thereby decreasing R/FR as sunlight moves through plant canopies. In turn, the pool of R light-absorbing phytochrome decreases relative to that of the FR light-absorbing pool, creating a signal transduction pathway that leads to an altered growth response (Rajcan and Swanton 2001). This causes the FR/R ratio of the light in the lower portion of the canopy to be higher than the FR/R ratio of the incoming light above the canopy. This may lead one to speculate that weeds growing with living mulches would have a lower root/shoot ratio than the living mulch-free condition, which would be a major disadvantage for a plant later in the season when competition for below-ground resources (such as water) may be more limiting (Rajcan and Swanton 2001). Moreover, decreased tillering may be another morphological change in weed grass species

Living mulches can also change phenological development of weeds. Because FR is a determinant of photoperiod, within a dense crop canopy (FR enriched), long-day weed species may have accelerated phenological development, whereas short-day weeds (i.e.,

which results in reduction of photosynthetic rates (Rajcan and Swanton 2001).

availability (Mayer and Hartwig 1986,); stimulating microorganisms; shading; changes in physical factors of soil such as pH, water holding capacity, temperature and aeration; and the release of allelochemicals (Leather 1983; Liebel and Worsham 1983; Putnam and DeFrank 1983; Weston et al. 1989; Yenish et al. 1995; Liebman and Davis 2000). Overall, weed suppression is thought to be based on alleopathic properties, physical impedance of germination and seedling growth, and competition for light, water, and nutrients (Teasdale, 1993; Teasdale and Mohler, 1993).

Because weed and living mulch plants compete for the same resources, weeds can be suppressed by the introduction of living mulches into cropping systems. In other words, including a living mulch in a cropping system can contribute to weed suppression by occupying the niche that would normally be filled by weeds (Teasdale 1998). Once established, living mulches can rapidly occupy the open space between the rows of the main crop and use the light, water, and nutritional resources that would otherwise be available to weeds. This can result in the inhibition of weed seed germination and reduction in the growth and development of weed seedlings. Therefore, weeds attempting to establish along with a living mulch would be in competition for resources and may not develop sufficiently. Moreover, physical impediments to weed seedlings is another mechanism by which living mulches suppress weeds (Facelli and Pickett 1991; Teasdale 1996; Teasdale and Mohler 1993).

If a living mulch becomes established before the emergence of weeds, then the presence of green vegetation covering the soil creates a radiation environment that is unfavorable for weed germination, emergence, and growth. Moreover, a more diverse biological and physical environment at the surface of soils such as that associated with living mulches offers opportunities for regulating and minimizing weed populations (Teasdale 2003).

Weed seed germination can be negatively affected by quality and quantity of light and the smaller amplitude of soil temperature fluctuation that result from the presence of living mulches (Gallagher et al. 1999; Teasdale 1998). Germination of weed seeds may be inhibited by complete light interception (Phatak 1992) by the living mulch or by secretion of allelochemicals (White et al. 1989; Overland 1966). A delay in emergence of weeds because of the presence of living mulches can also adversely affect weed seed production. Moreover, the presence of living mulches leads to greater seed mortality of weeds by favoring predators (Cromar et al. 1999). Teasdale (1998) also suggested that living-mulch suppression of weeds occurs through resource competition, promoting conditions that are unfavorable for germination and establishment, retaining living mulch residues as ground cover, and by means of allelopathy.

Water competition is another mechanism by which living mulches suppress weeds. Plants exposed to water stress for a limited time (i.e., several hours) respond by a reduction in the transpiration rate through a lowering of the leaf water potential and closing of stomata. Stomatal closing will affect the rate of leaf photosynthesis, which influences the growth and yield. However, under prolonged moisture stress (i.e., days to weeks), whole plant photosynthesis is reduced with a possibility of permanent damage to the photosynthetic apparatus (Nissanka et al. 1997). The severity of this damage will affect total dry matter accumulation and allocation among various organs of the plant. However, since most cropliving mulch systems are sufficiently supported by water and nutrients, it seems that light is the most important resource for competition between living mulches and weeds.

#### **5.1 Light**

82 Weed Control

availability (Mayer and Hartwig 1986,); stimulating microorganisms; shading; changes in physical factors of soil such as pH, water holding capacity, temperature and aeration; and the release of allelochemicals (Leather 1983; Liebel and Worsham 1983; Putnam and DeFrank 1983; Weston et al. 1989; Yenish et al. 1995; Liebman and Davis 2000). Overall, weed suppression is thought to be based on alleopathic properties, physical impedance of germination and seedling growth, and competition for light, water, and nutrients (Teasdale,

Because weed and living mulch plants compete for the same resources, weeds can be suppressed by the introduction of living mulches into cropping systems. In other words, including a living mulch in a cropping system can contribute to weed suppression by occupying the niche that would normally be filled by weeds (Teasdale 1998). Once established, living mulches can rapidly occupy the open space between the rows of the main crop and use the light, water, and nutritional resources that would otherwise be available to weeds. This can result in the inhibition of weed seed germination and reduction in the growth and development of weed seedlings. Therefore, weeds attempting to establish along with a living mulch would be in competition for resources and may not develop sufficiently. Moreover, physical impediments to weed seedlings is another mechanism by which living mulches

suppress weeds (Facelli and Pickett 1991; Teasdale 1996; Teasdale and Mohler 1993).

If a living mulch becomes established before the emergence of weeds, then the presence of green vegetation covering the soil creates a radiation environment that is unfavorable for weed germination, emergence, and growth. Moreover, a more diverse biological and physical environment at the surface of soils such as that associated with living mulches offers opportunities for regulating and minimizing weed populations (Teasdale 2003).

Weed seed germination can be negatively affected by quality and quantity of light and the smaller amplitude of soil temperature fluctuation that result from the presence of living mulches (Gallagher et al. 1999; Teasdale 1998). Germination of weed seeds may be inhibited by complete light interception (Phatak 1992) by the living mulch or by secretion of allelochemicals (White et al. 1989; Overland 1966). A delay in emergence of weeds because of the presence of living mulches can also adversely affect weed seed production. Moreover, the presence of living mulches leads to greater seed mortality of weeds by favoring predators (Cromar et al. 1999). Teasdale (1998) also suggested that living-mulch suppression of weeds occurs through resource competition, promoting conditions that are unfavorable for germination and establishment, retaining living mulch residues as ground cover, and by

Water competition is another mechanism by which living mulches suppress weeds. Plants exposed to water stress for a limited time (i.e., several hours) respond by a reduction in the transpiration rate through a lowering of the leaf water potential and closing of stomata. Stomatal closing will affect the rate of leaf photosynthesis, which influences the growth and yield. However, under prolonged moisture stress (i.e., days to weeks), whole plant photosynthesis is reduced with a possibility of permanent damage to the photosynthetic apparatus (Nissanka et al. 1997). The severity of this damage will affect total dry matter accumulation and allocation among various organs of the plant. However, since most cropliving mulch systems are sufficiently supported by water and nutrients, it seems that light is

the most important resource for competition between living mulches and weeds.

1993; Teasdale and Mohler, 1993).

means of allelopathy.

Plants grown together frequently compete primarily for solar radiation (Redfearn et al. 1999). Two components of light affect the outcome of competition: quantity and quality. The quantitative component of light (i.e., intensity and amount intercepted by a plant) determines canopy photosynthesis, whereas light quality is a driving variable of plant morphology. Both aspects of light are changed in a crop–weed competition situation when compared to the sole crop or weed canopy. Most crops and weeds attain their maximum photosynthetic rates at high levels of irradiance. In a mixed crop–weed community, mutual shading of leaves causes reduction of available photosynthetic photon flux density (PPFD), which results in reduction of photosynthetic rates (Rajcan and Swanton 2001).

In general, one of the important factors of weed suppression mechanisms of living mulch is light interception. Because plants need light to develop and living mulches are blocking sunlight reaching the weeds, weed species, especially decumbent weeds, cannot get enough light for germination and growth. Kruidhof et al. (2008) reported that weed suppression is positively correlated to early light interception by the living mulch and is sustained by the strong negative correlation between cumulative light interception and weed biomass. Similarly, according to Steinmaus et al. (2008), weed suppression was linked to light interception by the mulch cover for most weed species. Caamal-Maldonado et al. (2001) also found that canopy closure of velvetbean decreased the amount of light reaching the soil and inhibited weed growth. They reported that smooth pigweed (*Amaranthus hybridus* L.) and spiny amaranth (*Amaranthus spinosus* L.), among other weeds, were well controlled by a velvetbean living mulch.

Several studies have shown that the presence and nearness of the other vegetation influences the far red/red (FR/R) ratio received by a plant ( Ballare et al. 1987; Kasperbauer 1987; Smith et al. 1990), where the FR/R ratio received by plants in a dense canopy was higher than the FR/R ratio in a sparse canopy. In fact, weeds that grow underneath or within a canopy are not only exposed to a reduced amount of PPFD, but they also receive a different quality of light than the plants grown in full sunlight. Light within the lower canopy is enriched in FR radiation (730–740 nm). This is caused by selective absorption of red light (660–670 nm) by photosynthetic pigments and FR light reflectance from and transmittance by green leaves. Chlorophyll preferentially absorbs R light and reflects FR light, thereby decreasing R/FR as sunlight moves through plant canopies. In turn, the pool of R light-absorbing phytochrome decreases relative to that of the FR light-absorbing pool, creating a signal transduction pathway that leads to an altered growth response (Rajcan and Swanton 2001). This causes the FR/R ratio of the light in the lower portion of the canopy to be higher than the FR/R ratio of the incoming light above the canopy. This may lead one to speculate that weeds growing with living mulches would have a lower root/shoot ratio than the living mulch-free condition, which would be a major disadvantage for a plant later in the season when competition for below-ground resources (such as water) may be more limiting (Rajcan and Swanton 2001). Moreover, decreased tillering may be another morphological change in weed grass species growing in this condition (Davis and Simmons 1994).

Living mulches can also change phenological development of weeds. Because FR is a determinant of photoperiod, within a dense crop canopy (FR enriched), long-day weed species may have accelerated phenological development, whereas short-day weeds (i.e.,

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 85

weed suppression, this could reduce dependency on synthetic herbicides that are potentially

Hairy vetch is a well-known living mulch in the U.S. and Europe. It provides a number of advantages in agroecosystems. Benefits include: nitrogen fixation, quick addition of biomass, prevention of soil erosion and promotion of soil porosity, amelioration of microclimate, and, primarily, weed suppression owing to its allelopathic effects (Fujii 2001). It is extensively used to suppress weeds in different cropping systems. Johnson et al. (1993) observed that hairy vetch mulch completely inhibits the weeds under a no-tillage system. According to Fujii (2001), complete weed control can be achieved by direct application of the hairy vetch to the rice (*Oryza sativa* L.) paddy fields. He suggested that hairy vetch is a promising legume in abandoned paddy fields, grasslands and orchards in the central and southern parts of Japan and its inhibitory effect toward weeds was similar to that of

Oliver et al. (1992) also reported that a hairy vetch (*Vicia villosa* Roth ssp. villosa) living mulch established into soybean reduced morningglory (*Ipomoea lacunosa* L.) and spotted spurge (*Euphorbia maculata* L.) biomass by about 90% and large crabgrass (*Digitaria ischaemum* Schreb) biomass by about 70 % compared to weedy controls. When grown with the hairy vetch living mulch, soybean had yields that were comparable to a conventional production system using herbicides. In another study, Mohammadi (2010) suggested that interseeding of hairy vetch as a living mulch can be used as a beneficial method to control weeds in corn fields without causing any reduction in corn yield. In other research, best results were obtained from vetch species such as hairy vetch among living mulch species for weed control because of competitive ability, high biomass, densely growing habit, and allelopathic features of these species (Moonen and Barberi 2002; Batool and Hamid 2006;

Overall, hairy vetch is a legume living mulch that suppresses weed emergence and supplies nitrogen for sustainable cropping systems (Ngouajio and Mennan 2005; Choi and Daimon 2008) and it can be proposed as a promising candidate for an integrated weed management

Living mulches are generally considered to be more competitive with weeds than cover crop residue because they are actively growing and can compete efficiently for water, nutrients, and light. Dead cover crop residue does not suppress weeds as consistently as living mulches (Teasdale and Daughtry 1993; Reddy and Koger 2004). Once weed seedlings become established, cover crop residue will usually have a negligible impact on weed growth and seed production or may even stimulate these processes through conservation of soil moisture and release of nutrients (Teasdale and Daughtry 1993; Haramoto and Gallandt 2005). A living mulch competes with emerging and growing weeds for essential resources and inhibits emergence and growth more than cover crop residue does (Teasdale and

hazardous to our environment.

herbicide applications.

program.

**6. Hairy vetch as a good living mulch** 

Nakatsubo et al. 2008; Mohammadi 2009).

**7. Living mulch vs. cover crop residue** 

Daughtry 1993; Reddy and Koger 2004).

pigweed) will take longer to complete their life cycle (Huang et al., 2000). Branching and tillering are also influenced by FR light ( Begonia et al. 1988; Davis and Simmons 1994; Ghersa et al. 1994; McLachlan et al. 1993). Thus, the competitive ability of weed species would be also affected by light quality. Weed seed germination is also influenced by living mulches. It is known that light can break weed seed dormancy and stimulate germination (Hartmann and Nezadal, 1990). Therefore, including living mulches in cropping systems can prevent weed seed germination by shading the soil surface.

In general, the common important traits that determine competition for light between plants are inherent to the species. Amongst these traits are growth rate and architecture of the canopy (Davis and Garcia 1983; Kropff and van Laar 1993).

#### **5.2 Allelopathy**

The term allelopathy was first introduced by Hans Molisch in 1937 and refers to chemical interactions among plants, including those mediated by microorganisms. Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plantproduced secondary products to the soil rhizosphere (Weston 2005).

In certain cropping situations, allelopathy may have the potential to be integrated into a weed management plan in order to reduce the use of synthetic herbicides as well as provide other added benefits from the allelopathic crop. Allelopathy could potentially be used for weed control by producing and releasing allelochemicals from leaves, flowers, seeds, stems, and roots of living or decomposing plant materials (Weston 1996). Allelopathic compounds can be released into the soil by a variety of mechanisms that include decomposition of residues, root exudation, and volatilization (Weston 2005). They can be broadly classified into plant phenolics and terpenoids, which show great chemical diversity and are involved in a number of metabolic and ecological processes (Sung et al. 2010). These naturally produced secondary compounds can have chemical structures as complex as synthetic herbicides; they can also have the same wide range of selectivity and control for weeds (Westra 2010).

Allelopathy is another mechanism by which living mulches may suppress weeds (Fujii 1999). However, this is difficult to separate experimentally from mechanisms relating to competition for growth resources. In some situations, the allelopathic properties of living mulches can be used to control weeds. For example, the allelopathic properties of winter rye (*Secale cereale* L.), ryegrasses (*Lolium spp*), and subterranean clover (*Trifolium subterraneum* L.) can be used to control weeds in sweet corn (*Zea mays* var "rugosa") and snap beans (*Phaseolus vulgaris* L.) (De Gregorio and Ashley 1986). Root exudation produces allelopathic compounds that are actively secreted directly into the soil rhizosphere by living root systems. The allelochemicals then move through the soil by diffusion and come into contact neighboring plants. This creates a radius effect, where proximity to the allelopathic species results in greater concentrations of the allelochemical, which, in turn, typically decreases the growth of neighboring plants (Westra 2010).

Usually, using allelopathic species as a living mulch can provide normal weed suppression traits seen for mulch, as well as slowly releasing allelochemicals from their biomass which provide further weed suppression especially for weed seedling control. Therefore, if allelopathic living mulches could be incorporated in certain cropping systems to provide weed suppression, this could reduce dependency on synthetic herbicides that are potentially hazardous to our environment.

#### **6. Hairy vetch as a good living mulch**

84 Weed Control

pigweed) will take longer to complete their life cycle (Huang et al., 2000). Branching and tillering are also influenced by FR light ( Begonia et al. 1988; Davis and Simmons 1994; Ghersa et al. 1994; McLachlan et al. 1993). Thus, the competitive ability of weed species would be also affected by light quality. Weed seed germination is also influenced by living mulches. It is known that light can break weed seed dormancy and stimulate germination (Hartmann and Nezadal, 1990). Therefore, including living mulches in cropping systems can

In general, the common important traits that determine competition for light between plants are inherent to the species. Amongst these traits are growth rate and architecture of the

The term allelopathy was first introduced by Hans Molisch in 1937 and refers to chemical interactions among plants, including those mediated by microorganisms. Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-

In certain cropping situations, allelopathy may have the potential to be integrated into a weed management plan in order to reduce the use of synthetic herbicides as well as provide other added benefits from the allelopathic crop. Allelopathy could potentially be used for weed control by producing and releasing allelochemicals from leaves, flowers, seeds, stems, and roots of living or decomposing plant materials (Weston 1996). Allelopathic compounds can be released into the soil by a variety of mechanisms that include decomposition of residues, root exudation, and volatilization (Weston 2005). They can be broadly classified into plant phenolics and terpenoids, which show great chemical diversity and are involved in a number of metabolic and ecological processes (Sung et al. 2010). These naturally produced secondary compounds can have chemical structures as complex as synthetic herbicides; they can also

Allelopathy is another mechanism by which living mulches may suppress weeds (Fujii 1999). However, this is difficult to separate experimentally from mechanisms relating to competition for growth resources. In some situations, the allelopathic properties of living mulches can be used to control weeds. For example, the allelopathic properties of winter rye (*Secale cereale* L.), ryegrasses (*Lolium spp*), and subterranean clover (*Trifolium subterraneum* L.) can be used to control weeds in sweet corn (*Zea mays* var "rugosa") and snap beans (*Phaseolus vulgaris* L.) (De Gregorio and Ashley 1986). Root exudation produces allelopathic compounds that are actively secreted directly into the soil rhizosphere by living root systems. The allelochemicals then move through the soil by diffusion and come into contact neighboring plants. This creates a radius effect, where proximity to the allelopathic species results in greater concentrations of the allelochemical, which, in turn, typically decreases the

Usually, using allelopathic species as a living mulch can provide normal weed suppression traits seen for mulch, as well as slowly releasing allelochemicals from their biomass which provide further weed suppression especially for weed seedling control. Therefore, if allelopathic living mulches could be incorporated in certain cropping systems to provide

prevent weed seed germination by shading the soil surface.

canopy (Davis and Garcia 1983; Kropff and van Laar 1993).

produced secondary products to the soil rhizosphere (Weston 2005).

have the same wide range of selectivity and control for weeds (Westra 2010).

growth of neighboring plants (Westra 2010).

**5.2 Allelopathy** 

Hairy vetch is a well-known living mulch in the U.S. and Europe. It provides a number of advantages in agroecosystems. Benefits include: nitrogen fixation, quick addition of biomass, prevention of soil erosion and promotion of soil porosity, amelioration of microclimate, and, primarily, weed suppression owing to its allelopathic effects (Fujii 2001). It is extensively used to suppress weeds in different cropping systems. Johnson et al. (1993) observed that hairy vetch mulch completely inhibits the weeds under a no-tillage system. According to Fujii (2001), complete weed control can be achieved by direct application of the hairy vetch to the rice (*Oryza sativa* L.) paddy fields. He suggested that hairy vetch is a promising legume in abandoned paddy fields, grasslands and orchards in the central and southern parts of Japan and its inhibitory effect toward weeds was similar to that of herbicide applications.

Oliver et al. (1992) also reported that a hairy vetch (*Vicia villosa* Roth ssp. villosa) living mulch established into soybean reduced morningglory (*Ipomoea lacunosa* L.) and spotted spurge (*Euphorbia maculata* L.) biomass by about 90% and large crabgrass (*Digitaria ischaemum* Schreb) biomass by about 70 % compared to weedy controls. When grown with the hairy vetch living mulch, soybean had yields that were comparable to a conventional production system using herbicides. In another study, Mohammadi (2010) suggested that interseeding of hairy vetch as a living mulch can be used as a beneficial method to control weeds in corn fields without causing any reduction in corn yield. In other research, best results were obtained from vetch species such as hairy vetch among living mulch species for weed control because of competitive ability, high biomass, densely growing habit, and allelopathic features of these species (Moonen and Barberi 2002; Batool and Hamid 2006; Nakatsubo et al. 2008; Mohammadi 2009).

Overall, hairy vetch is a legume living mulch that suppresses weed emergence and supplies nitrogen for sustainable cropping systems (Ngouajio and Mennan 2005; Choi and Daimon 2008) and it can be proposed as a promising candidate for an integrated weed management program.

#### **7. Living mulch vs. cover crop residue**

Living mulches are generally considered to be more competitive with weeds than cover crop residue because they are actively growing and can compete efficiently for water, nutrients, and light. Dead cover crop residue does not suppress weeds as consistently as living mulches (Teasdale and Daughtry 1993; Reddy and Koger 2004). Once weed seedlings become established, cover crop residue will usually have a negligible impact on weed growth and seed production or may even stimulate these processes through conservation of soil moisture and release of nutrients (Teasdale and Daughtry 1993; Haramoto and Gallandt 2005). A living mulch competes with emerging and growing weeds for essential resources and inhibits emergence and growth more than cover crop residue does (Teasdale and Daughtry 1993; Reddy and Koger 2004).

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 87

found corn dry matter yield to be reduced by up to 47% and grain yield by up to 31% when alfalfa was interseeded at the time of corn establishment. Hoffman et al. (1993) observed a

**Weed life cycle stage Cover crop residue Living mulch**  Germination Moderate High

Growth Low High Seed production Low Moderate Seed survival None?a Moderate?a Perennial structure survival None?a Low-moderate?a,b

a More research is needed to provide definitive estimates of cover crop effects on these processes. b When living mulches are combined with other practices such as soil disturbance or mowing, perennial

Typically, a living mulch that is competitive enough to suppress weeds will also suppress crop growth and yield. Much of the research with living mulches has focused on documenting and alleviating this problem (Liebman and Staver 2001; Teasdale, 1998). Many studies in the North Central U.S. on legume interseeding in established corn stands report grain yield losses that are attributed to moisture stress (Kurtz et al. 1952; Pendleton et al. 1957), N deficiency (Scott et al. 1987; Triplett 1962), and reduced corn populations associated with wider row spacing (Schaller and Larson 1955; Stringfield and Thatcher 1951). Marks (1993) also suggested that reduced growth of the main crop may be due to competition for water or some other limited resource, or the mulch may be having an allelopathic effect.

De Haan et al. (1997) used burr medic (*Medicago polymorpha* L.) and snail medic [*Medicago scutellata* (L.) Mill.] as living mulches in corn and found that, although both medics suppressed weeds, corn and medics competed strongly for resources. Consequently, medic living mulches significantly reduced corn grain yields. The reduction was due to competition for nutrients or moisture when medic and corn were planted at the same time. Yield loss in transplanted cabbage due to competition with the living mulch for light or

When the growth of a living mulch is not restricted, or when soil moisture is inadequate, even a relatively vigorous crop like potato may suffer competition and loss of yield (Rajalahti and Bellinder 1996). Generally, without irrigation, it becomes more challenging to implement a living mulch system. However, there are successful examples of annual or biennial living mulches established after emergence of the main crop, which gives the main crop a competitive advantage (Scott et al. 1987; Wall et al. 1991). If living mulches are established before or after the main crop is planted, competition of the living mulch for water may reduce crop yields (Echtenkamp and Moomaw 1989; Eberlein et al. 1992; Masiunas et al. 1997; Teasdale et al. 2000). Thus, it can be concluded that living mulches can severely compete with the main crop for water which is particularly problematic during a

Table 3. Potential impact of typical cover crop residue or living mulch on inhibition of

Emergence/establishment Moderate High

corn reduction of over 76% in corn grown with untreated hairy vetch.

structure survival may be more effectively reduced.

weeds at various life cycle stages (from Teasdale et al. 2007).

moisture was also recorded by Bottenberg et al. (1997).

In one study, a chemically stunted stand of crownvetch gave better weed control than dead rye mulch (Hartwig 1989). Teasdale and Daughtry (1993) found that weed suppression by live hairy vetch was more than that by paraquat desiccated cover crop residues. Therefore, weed control can be maximized by keeping hairy vetch live for a longer period rather than killing / desiccating. Living plant tissue of wheat (*Triticum* sp.), crimson clover (*Trifolium incarnatum* L.), subterranean clover and rye inhibited the emergence of weeds like ivyleaf morning glory (*Ipomoea hederacea* L.) and redroot pigweed (*Amaranthus retroflexus* L.) (Lehman and Blum 1997). However, if these were used after desiccation with glyphosate, only wheat and crimson clover were inhibitory. Likewise, subterranean clover cover crops, when used as living mulch under field conditions, can efficiently control weeds such as fall panicum (*Panicum dichotomiflorusm* Michx) and ivyleaf morning glory without affecting the yield of corn (Enache and Ilnicki 1990; Ilnicki and Enache 1992).

Several requirements for breaking dormancy and promoting germination of weed seeds in soils (light with a high red-to-far red ratio and high daily soil temperature amplitude) are reduced more by living mulches than by desiccated residue (Teasdale and Daughtry 1993). A living mulch absorbs red light and will reduce the red/far-red ratio sufciently to inhibit phytochrome-mediated seed germination, whereas cover crop residue has a minimal effect on this ratio (Teasdale and Daughtry 1993).

Enache and Ilnicki (1990) reported that weed biomass was reduced 53 to 94 percent by subterranean clover living mulch whereas weed biomass in desiccated rye mulch ranged from an 11 percent decrease to a 76 percent increase compared to a no-mulch control. In another study, a live hairy vetch cover crop was more effective than a desiccated cover crop in suppressing weed emergence during the first four weeks and throughout the season (Teasdale et al. 1991). In addition, if growth suppression is sufcient, a living mulch can inhibit weed seed production (Brainard and Bellinder 2004; Brennan and Smith 2005). Weed seed predation at the soil surface was also higher when living mulch vegetation was present (Davis and Liebman 2003; Gallandt et al. 2005), suggesting a role for living mulches in enhancing weed seed mortality.

Generally, it can be concluded that living mulches will suppress weeds more completely and at more phases of the weed life cycle than will cover crop residue. The inhibitory effect of typical cover crop residue or living mulch on weeds at various life cycle stages has been shown in Table 3.

#### **8. Competition between living mulch and main crop**

Although living mulches can efficiently suppress weeds, they may compete for nutrients and water with the main crop (Echtenkamp and Moomaw 1989) which can reduce yields. For example, Elkins et al. (1983) examined the use of tall fescue *(Festuca arundinacea* Schreb*)*, smooth bromegrass *(Bromus inermis* Leyss*)*, and orchargrass *(Dactylis glomerata* L.*)* as living mulches. They found corn yield was reduced 5% to 10% at the end of the harvest. Regnier and Janke (1990) indicated that the majority of previously conducted studies showed that the species, when selected as living mulches do not suppress weeds selectively, but suppress the crop as well; therefore, living mulches must be managed carefully to reduce their competition with the crop. In that regard, Jeranyama et al. (1998) found a reduction of 13 to 18% in grain yield when corn was intercropped with legumes. Norquidst and Wicks (1974)

In one study, a chemically stunted stand of crownvetch gave better weed control than dead rye mulch (Hartwig 1989). Teasdale and Daughtry (1993) found that weed suppression by live hairy vetch was more than that by paraquat desiccated cover crop residues. Therefore, weed control can be maximized by keeping hairy vetch live for a longer period rather than killing / desiccating. Living plant tissue of wheat (*Triticum* sp.), crimson clover (*Trifolium incarnatum* L.), subterranean clover and rye inhibited the emergence of weeds like ivyleaf morning glory (*Ipomoea hederacea* L.) and redroot pigweed (*Amaranthus retroflexus* L.) (Lehman and Blum 1997). However, if these were used after desiccation with glyphosate, only wheat and crimson clover were inhibitory. Likewise, subterranean clover cover crops, when used as living mulch under field conditions, can efficiently control weeds such as fall panicum (*Panicum dichotomiflorusm* Michx) and ivyleaf morning glory without affecting the

Several requirements for breaking dormancy and promoting germination of weed seeds in soils (light with a high red-to-far red ratio and high daily soil temperature amplitude) are reduced more by living mulches than by desiccated residue (Teasdale and Daughtry 1993). A living mulch absorbs red light and will reduce the red/far-red ratio sufciently to inhibit phytochrome-mediated seed germination, whereas cover crop residue has a minimal effect

Enache and Ilnicki (1990) reported that weed biomass was reduced 53 to 94 percent by subterranean clover living mulch whereas weed biomass in desiccated rye mulch ranged from an 11 percent decrease to a 76 percent increase compared to a no-mulch control. In another study, a live hairy vetch cover crop was more effective than a desiccated cover crop in suppressing weed emergence during the first four weeks and throughout the season (Teasdale et al. 1991). In addition, if growth suppression is sufcient, a living mulch can inhibit weed seed production (Brainard and Bellinder 2004; Brennan and Smith 2005). Weed seed predation at the soil surface was also higher when living mulch vegetation was present (Davis and Liebman 2003; Gallandt et al. 2005), suggesting a role for living mulches in

Generally, it can be concluded that living mulches will suppress weeds more completely and at more phases of the weed life cycle than will cover crop residue. The inhibitory effect of typical cover crop residue or living mulch on weeds at various life cycle stages has been

Although living mulches can efficiently suppress weeds, they may compete for nutrients and water with the main crop (Echtenkamp and Moomaw 1989) which can reduce yields. For example, Elkins et al. (1983) examined the use of tall fescue *(Festuca arundinacea* Schreb*)*, smooth bromegrass *(Bromus inermis* Leyss*)*, and orchargrass *(Dactylis glomerata* L.*)* as living mulches. They found corn yield was reduced 5% to 10% at the end of the harvest. Regnier and Janke (1990) indicated that the majority of previously conducted studies showed that the species, when selected as living mulches do not suppress weeds selectively, but suppress the crop as well; therefore, living mulches must be managed carefully to reduce their competition with the crop. In that regard, Jeranyama et al. (1998) found a reduction of 13 to 18% in grain yield when corn was intercropped with legumes. Norquidst and Wicks (1974)

yield of corn (Enache and Ilnicki 1990; Ilnicki and Enache 1992).

**8. Competition between living mulch and main crop** 

on this ratio (Teasdale and Daughtry 1993).

enhancing weed seed mortality.

shown in Table 3.


found corn dry matter yield to be reduced by up to 47% and grain yield by up to 31% when alfalfa was interseeded at the time of corn establishment. Hoffman et al. (1993) observed a corn reduction of over 76% in corn grown with untreated hairy vetch.

a More research is needed to provide definitive estimates of cover crop effects on these processes. b When living mulches are combined with other practices such as soil disturbance or mowing, perennial structure survival may be more effectively reduced.

Table 3. Potential impact of typical cover crop residue or living mulch on inhibition of weeds at various life cycle stages (from Teasdale et al. 2007).

Typically, a living mulch that is competitive enough to suppress weeds will also suppress crop growth and yield. Much of the research with living mulches has focused on documenting and alleviating this problem (Liebman and Staver 2001; Teasdale, 1998). Many studies in the North Central U.S. on legume interseeding in established corn stands report grain yield losses that are attributed to moisture stress (Kurtz et al. 1952; Pendleton et al. 1957), N deficiency (Scott et al. 1987; Triplett 1962), and reduced corn populations associated with wider row spacing (Schaller and Larson 1955; Stringfield and Thatcher 1951). Marks (1993) also suggested that reduced growth of the main crop may be due to competition for water or some other limited resource, or the mulch may be having an allelopathic effect.

De Haan et al. (1997) used burr medic (*Medicago polymorpha* L.) and snail medic [*Medicago scutellata* (L.) Mill.] as living mulches in corn and found that, although both medics suppressed weeds, corn and medics competed strongly for resources. Consequently, medic living mulches significantly reduced corn grain yields. The reduction was due to competition for nutrients or moisture when medic and corn were planted at the same time. Yield loss in transplanted cabbage due to competition with the living mulch for light or moisture was also recorded by Bottenberg et al. (1997).

When the growth of a living mulch is not restricted, or when soil moisture is inadequate, even a relatively vigorous crop like potato may suffer competition and loss of yield (Rajalahti and Bellinder 1996). Generally, without irrigation, it becomes more challenging to implement a living mulch system. However, there are successful examples of annual or biennial living mulches established after emergence of the main crop, which gives the main crop a competitive advantage (Scott et al. 1987; Wall et al. 1991). If living mulches are established before or after the main crop is planted, competition of the living mulch for water may reduce crop yields (Echtenkamp and Moomaw 1989; Eberlein et al. 1992; Masiunas et al. 1997; Teasdale et al. 2000). Thus, it can be concluded that living mulches can severely compete with the main crop for water which is particularly problematic during a

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 89

free check, whereas all other species caused significant yield reductions. Therefore, it is suited to this system because it does not grow aggressively early in the year when it could

Newenhouse and Dana (1989) also evaluated different grass living mulches for strawberries (*Fragaria* sp.) and found perennial ryegrass (*Lolium perenne* L.) was best because it covered the ground quickly but did not spread into the crop rows. In raspberries (*Rubus* sp.), a white clover (*Trifolium repens* L.) living mulch did not affect the crop but perennial ryegrass

According to Akanvou et al. (2001), slow-growing species with longer duration such as *Stylosanthes hamata* and *Aeschynomene histrix* are expected to be less competitive and therefore appropriate for early establishment in rice-legume intercropping systems. Intercropping research has shown that most legumes do not compete strongly with cereals for light, N, P, and K, whereas they compete equally for water (Ofori and Stern 1987; Vandermeer 1990). The low stature of most legumes and their horizontally positioned leaves reduce competition for light with tall, erect cereals. Since many legumes are C3 crops with low light saturation points and low temperature optima, one might expect these legumes to complement a C4 crop such as corn that has a high light saturation point and high temperature optimum (Ofori and Stern 1987). Instead of competing for N, legumes may instead contribute N to the main crop (Fujita et al. 1992). Because of their different root systems (less fibrous and often having a taproot), competition for the immobile nutrients P and K can be expected to be limited (Ofori and Stern 1987; Vandermeer 1990). Legumes are

Generally, the competitive ability is an obvious characteristic determining the suitability of a plant species as a living mulch. For example, tall and vigorously growing legumes with relatively large leaves and rapid leaf expansion might be detrimental to the associated crop, whereas poorly competing species will be out-competed and will therefore contribute little

Appropriate management is essential to avoid or decrease the interspecific competition between intercropped species. Several approaches have been used to reduce competition between the living mulch and main crop species without eliminating the desirable attributes

The classical attempts to reduce competition in living mulch systems have focused on chemical or mechanical suppression of mulch growth or screening for less competitive living mulches. Reducing interference between a white clover living mulch and sweet corn (*Zea mays* L. var. *saccharata*) by chemical suppression or mechanical suppression has been reported by Vrabel (1983) and by Grubinger and Minotti (1990), respectively. Reduced interference by mechanical suppression of white clover and subterranean clover living mulches in white cabbage (*Brassica oleracea* var. *capitata* L.) is also reported by Brandsæter et

Timely mowing of a clover (*Trifolium spp*.) living mulch prevented the competition in transplanted broccoli (*Brassica oleracea* L.) (Costello and Altieri 1994). Ilnicki and Enache

therefore promising candidates for living mulches in agroecosystems.

to improving soil fertility (Akanvou et al. 2001).

**9.2 Application of appropriate practices** 

and benefits of the living mulch.

al. (1998).

reduce corn yield.

reduced berry yield (Freyman 1989).

dry period. In one study, corn yields were not negatively affected by competition from the crownvetch, birdsfoot trefoil (*Lotus corniculatus* L.), and flatpea (*Lathyrus sylvestris* L.) living mulches in years with adequate precipitation. However, in a year with very low rainfall in July and August, crownvetch and birdsfoot trefoil reduced corn yields (Duiker and Hartwig 2004).

In general, although legume living mulches compete weakly with cereals for light, N, phosphorus (P), and K, they can compete strongly for water. If water stress is eliminated by irrigation, living mulches of legumes rarely reduce and sometimes increase main crop yields (Grubinger and Minotti 1990; Fischer and Burrill 1993; Costello 1994).

#### **9. The ways to prevent or reduce the competition between living mulch and main crop**

A serious problem in living mulch cropping systems is reduced main crop yield because of competition. Management of living mulches becomes critical to reduce competition with the main crop for resources while allowing the mulch to grow sufficiently to reap potential benefits. Different ways have been suggested to overcome this problem in such cropping systems. One of them is the selection of suitable living mulch species and the others have been employed to suppress the living mulch, such as tillage, mowing, and herbicides (Grubinger and Minotti 1990; Fischer and Burrill 1993; Costello 1994; Martin et al. 1999; Zemenchik et al. 2000).

#### **9.1 Selection of suitable species**

It is important to make the correct choice of a living mulch (Ingels et al. 1994). According to Ilnicki and Enache (1992), to avoid competition with the main crop in the subterranean clover cropping system, it is essential to use species and cultivars which have a low canopy height and terminate vegetative growth early in the summer. Greater potential benefits might be expected from living mulches with a very different active growth period than the main crop. For example, kura clover does not produce abundant dry matter during dry periods of the growing season and should therefore compete less than other perennial legumes with corn for limited resources, especially water (Zemenchik et al. 2000). Kura clover and corn in the living mulch systems were more compatible after tasselling because the species differed greatly in stature and corn had sequestered much of the resources necessary to complete its life cycle. This ecological differentiation is a necessary condition for coexistence according to the competitive exclusion principle (Hardin 1960).

Another approach suggested by Ilnicki and Enache (1992) was to use winter annual legumes, e.g., subterranean clover, as a living mulch. Winter annual legumes sown in late summer grow vegetatively during autumn, become dormant in winter, and resume vegetative growth the following spring. Later in the spring or early summer the plant flowers, senesces, and dies. Because of this unique life cycle, a main crop transplanted into the senescencing mulch would be able to use all available water and nutrients.

Moynihan et al. (1996) reported that black medic was found to be the least competitive medic species when it was intercropped with barley as a living mulch. In another study, black medic did not significantly reduce corn yields compared with the medic and weed-

dry period. In one study, corn yields were not negatively affected by competition from the crownvetch, birdsfoot trefoil (*Lotus corniculatus* L.), and flatpea (*Lathyrus sylvestris* L.) living mulches in years with adequate precipitation. However, in a year with very low rainfall in July and August, crownvetch and birdsfoot trefoil reduced corn yields (Duiker and Hartwig

In general, although legume living mulches compete weakly with cereals for light, N, phosphorus (P), and K, they can compete strongly for water. If water stress is eliminated by irrigation, living mulches of legumes rarely reduce and sometimes increase main crop yields

**9. The ways to prevent or reduce the competition between living mulch and** 

A serious problem in living mulch cropping systems is reduced main crop yield because of competition. Management of living mulches becomes critical to reduce competition with the main crop for resources while allowing the mulch to grow sufficiently to reap potential benefits. Different ways have been suggested to overcome this problem in such cropping systems. One of them is the selection of suitable living mulch species and the others have been employed to suppress the living mulch, such as tillage, mowing, and herbicides (Grubinger and Minotti 1990; Fischer and Burrill 1993; Costello 1994; Martin et al. 1999;

It is important to make the correct choice of a living mulch (Ingels et al. 1994). According to Ilnicki and Enache (1992), to avoid competition with the main crop in the subterranean clover cropping system, it is essential to use species and cultivars which have a low canopy height and terminate vegetative growth early in the summer. Greater potential benefits might be expected from living mulches with a very different active growth period than the main crop. For example, kura clover does not produce abundant dry matter during dry periods of the growing season and should therefore compete less than other perennial legumes with corn for limited resources, especially water (Zemenchik et al. 2000). Kura clover and corn in the living mulch systems were more compatible after tasselling because the species differed greatly in stature and corn had sequestered much of the resources necessary to complete its life cycle. This ecological differentiation is a necessary condition

Another approach suggested by Ilnicki and Enache (1992) was to use winter annual legumes, e.g., subterranean clover, as a living mulch. Winter annual legumes sown in late summer grow vegetatively during autumn, become dormant in winter, and resume vegetative growth the following spring. Later in the spring or early summer the plant flowers, senesces, and dies. Because of this unique life cycle, a main crop transplanted into

Moynihan et al. (1996) reported that black medic was found to be the least competitive medic species when it was intercropped with barley as a living mulch. In another study, black medic did not significantly reduce corn yields compared with the medic and weed-

for coexistence according to the competitive exclusion principle (Hardin 1960).

the senescencing mulch would be able to use all available water and nutrients.

(Grubinger and Minotti 1990; Fischer and Burrill 1993; Costello 1994).

2004).

**main crop** 

Zemenchik et al. 2000).

**9.1 Selection of suitable species** 

free check, whereas all other species caused significant yield reductions. Therefore, it is suited to this system because it does not grow aggressively early in the year when it could reduce corn yield.

Newenhouse and Dana (1989) also evaluated different grass living mulches for strawberries (*Fragaria* sp.) and found perennial ryegrass (*Lolium perenne* L.) was best because it covered the ground quickly but did not spread into the crop rows. In raspberries (*Rubus* sp.), a white clover (*Trifolium repens* L.) living mulch did not affect the crop but perennial ryegrass reduced berry yield (Freyman 1989).

According to Akanvou et al. (2001), slow-growing species with longer duration such as *Stylosanthes hamata* and *Aeschynomene histrix* are expected to be less competitive and therefore appropriate for early establishment in rice-legume intercropping systems. Intercropping research has shown that most legumes do not compete strongly with cereals for light, N, P, and K, whereas they compete equally for water (Ofori and Stern 1987; Vandermeer 1990). The low stature of most legumes and their horizontally positioned leaves reduce competition for light with tall, erect cereals. Since many legumes are C3 crops with low light saturation points and low temperature optima, one might expect these legumes to complement a C4 crop such as corn that has a high light saturation point and high temperature optimum (Ofori and Stern 1987). Instead of competing for N, legumes may instead contribute N to the main crop (Fujita et al. 1992). Because of their different root systems (less fibrous and often having a taproot), competition for the immobile nutrients P and K can be expected to be limited (Ofori and Stern 1987; Vandermeer 1990). Legumes are therefore promising candidates for living mulches in agroecosystems.

Generally, the competitive ability is an obvious characteristic determining the suitability of a plant species as a living mulch. For example, tall and vigorously growing legumes with relatively large leaves and rapid leaf expansion might be detrimental to the associated crop, whereas poorly competing species will be out-competed and will therefore contribute little to improving soil fertility (Akanvou et al. 2001).

#### **9.2 Application of appropriate practices**

Appropriate management is essential to avoid or decrease the interspecific competition between intercropped species. Several approaches have been used to reduce competition between the living mulch and main crop species without eliminating the desirable attributes and benefits of the living mulch.

The classical attempts to reduce competition in living mulch systems have focused on chemical or mechanical suppression of mulch growth or screening for less competitive living mulches. Reducing interference between a white clover living mulch and sweet corn (*Zea mays* L. var. *saccharata*) by chemical suppression or mechanical suppression has been reported by Vrabel (1983) and by Grubinger and Minotti (1990), respectively. Reduced interference by mechanical suppression of white clover and subterranean clover living mulches in white cabbage (*Brassica oleracea* var. *capitata* L.) is also reported by Brandsæter et al. (1998).

Timely mowing of a clover (*Trifolium spp*.) living mulch prevented the competition in transplanted broccoli (*Brassica oleracea* L.) (Costello and Altieri 1994). Ilnicki and Enache

Living Mulch as a Tool to Control Weeds in Agroecosystems: A Review 91

5. Suppressing the living mulch so as to reduce its competitiveness with the crop using the

c. Strip tillage to provide suitable planting conditions without competition within the crop

It can be concluded that although living mulches are efficient tools to suppress weeds in cropping systems, but an appropriate management program is very essential to reduce the competition with the main crop for environmental resources and enhance the potential

Akanvou, R., L. Bastiaans, M. J. Kropff, J. Goudriaan and M. Becker. 2001. Characterization

Akemo, M.C., Regnier, E.E. and Bennett, M.A. 2000. Weed suppression in spring-sown rye-

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Akobundu, I.O., Udensi, U.E. and Chikoye, D. 2000. Velvetbean suppresses speargrass and increases maize yield. International Journal of Pest Management 46, 103-108. Ballare, C.L., R.A. Sanchez, A.L. Scopel, J.J. Casal and C.M. Ghersa. 1987. Early detection of

Batool S., Hamid R. 2006. Effect of cover crops mulch on weed control in orchards. Pakistan

Bellinder, R.R., G. Gummeson and C. Karlson. 1994. Percentage-driven government mandates for pesticide reduction, The Swedish model. Weed Technol. 8, 350-359. Blackshaw, R.E., Moyer, J .R., Doram, R.C. and Boswell, A.L. 2001. Yellow sweetclover,

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green manure, and its residues effectively suppress weeds during fallow. Weed

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d. Timely mowing to reduce the height and vigour of the living mulch (Teasdale 2003).

a. A broadcast application of an herbicide at a rate that is suppressive but not lethal. b. A banded application of a herbicide to kill the living mulch in the crop row so as to reduce competition within the row area but permit weed suppression by the living

row but to permit weed suppression by the living mulch between rows.

these cropping systems is very essential.

benefits of living mulch such as weed suppressing ability.

pea cover crop mixes. Weed Technology 14, 545-549.

Society, Oregon State University, Corvallis.

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following methods:

mulch between rows.

**10. References** 

the limiting factor in crop production. Therefore, preparation of sufficient water for

(1992) also found that mowing of a subterranean clover mulch was necessary to reduce early competition when sweet corn, tomato (*Lycopersicon esculentum* Mill.) and cabbage crops were planted into it. Mulongoy and Akobundo (1990) proposed the use of growth retardants to reduce growth of the associated legumes in maize. Werner (1988) investigated the influence of different living mulch species on weed density and diversity. Weed numbers were reduced and maize yield was not affected where growth of the living mulch was reduced by cutting or flaming treatments.

Another way to avoid or decrease the competition in such systems is to intercrop a main crop and a living mulch with a synchronized onset of maximum vegetative growth. This synchronization of living mulch and main crop could be achieved in different ways (Brandsæter and Netland 1999). Muller-Scharer and Potter (1991) concluded that living mulches should be seeded to emerge in the middle of the vegetation period of the main crop. De Haan et al. (1994) have studied the opposite way to avoid interference problems in living mulch systems in the north central region of the U.S.. They tried to develop a springseeded living mulch that had been selected for its ability to suppress weeds without affecting crop yield. This living mulch flowered 3 weeks after emergence and began senescence 5 weeks after emergence.

Shifting the relative sowing dates of the various intercropped components in a crop-living mulch system is an important means to ensure a better use of available resources and to minimize yield loss of the main crop (Midmore 1993). Usually, delaying the sowing time of living mulches might reduce the interaction effects. For example, velvetbean planted as living mulch 20 days after corn reduced weed biomass by 68% with no negative effects on corn yield (Caamal-Maldonado et al. 2001). Corn grain yield was not reduced when living mulch seeding was delayed until the corn was 15 to 30 cm in height (Scott et al. 1987), suggesting that yield can be maintained by delaying the seeding date of the living mulch. In another study, annual medics interseeded several weeks after corn planting did not affect corn yield (De Haan et al. 1997). Moreover, delaying the planting time of the main crop until senescing of living mulch might also decrease the interspecific competition. For example, competition was not a problem when dwarf beans (*Phaseolus vulgaris* L.) were planted into a clover mulch as it began senescing (Ilnicki and Enache 1992).

In general, the efficient management approaches to prevent or reduce the competition between living mulches and a main crop include:


the limiting factor in crop production. Therefore, preparation of sufficient water for these cropping systems is very essential.


It can be concluded that although living mulches are efficient tools to suppress weeds in cropping systems, but an appropriate management program is very essential to reduce the competition with the main crop for environmental resources and enhance the potential benefits of living mulch such as weed suppressing ability.

#### **10. References**

90 Weed Control

(1992) also found that mowing of a subterranean clover mulch was necessary to reduce early competition when sweet corn, tomato (*Lycopersicon esculentum* Mill.) and cabbage crops were planted into it. Mulongoy and Akobundo (1990) proposed the use of growth retardants to reduce growth of the associated legumes in maize. Werner (1988) investigated the influence of different living mulch species on weed density and diversity. Weed numbers were reduced and maize yield was not affected where growth of the living mulch was reduced by

Another way to avoid or decrease the competition in such systems is to intercrop a main crop and a living mulch with a synchronized onset of maximum vegetative growth. This synchronization of living mulch and main crop could be achieved in different ways (Brandsæter and Netland 1999). Muller-Scharer and Potter (1991) concluded that living mulches should be seeded to emerge in the middle of the vegetation period of the main crop. De Haan et al. (1994) have studied the opposite way to avoid interference problems in living mulch systems in the north central region of the U.S.. They tried to develop a springseeded living mulch that had been selected for its ability to suppress weeds without affecting crop yield. This living mulch flowered 3 weeks after emergence and began

Shifting the relative sowing dates of the various intercropped components in a crop-living mulch system is an important means to ensure a better use of available resources and to minimize yield loss of the main crop (Midmore 1993). Usually, delaying the sowing time of living mulches might reduce the interaction effects. For example, velvetbean planted as living mulch 20 days after corn reduced weed biomass by 68% with no negative effects on corn yield (Caamal-Maldonado et al. 2001). Corn grain yield was not reduced when living mulch seeding was delayed until the corn was 15 to 30 cm in height (Scott et al. 1987), suggesting that yield can be maintained by delaying the seeding date of the living mulch. In another study, annual medics interseeded several weeks after corn planting did not affect corn yield (De Haan et al. 1997). Moreover, delaying the planting time of the main crop until senescing of living mulch might also decrease the interspecific competition. For example, competition was not a problem when dwarf beans (*Phaseolus vulgaris* L.) were planted into a

In general, the efficient management approaches to prevent or reduce the competition

1. Using low-growing living mulch that competes primarily for light. In this case, as long as the living mulch becomes established before the weeds, it would maintain weed suppression by excluding light but would not impact taller growing crops and would not compete with the crop excessively for soil resources such as water and nutrients. 2. Timely planting the living mulch so that the time of peak growth of the living mulch does not coincide with the critical period during which competition would have the

3. Reducing crop row spacing and/or increase crop population to enhance the

4. Providing supplemental water and nitrogen to compensate for resources used by living mulch plants. Usually, soil moisture depletion by living mulches will become the primary management consideration in those areas of the world where soil moisture is

competitiveness of the main crop relative to the living mulch.

cutting or flaming treatments.

senescence 5 weeks after emergence.

clover mulch as it began senescing (Ilnicki and Enache 1992).

between living mulches and a main crop include:

greatest impact on main crop yield.


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**5** 

*USA* 

**Utilization of Sunn Hemp for Cover Crops and** 

The use of smother crops or cover crop residue to suppress weed growth in agriculture is not a recent innovation; yet, only recently have smother, or cover crops, received considerable attention. The need to develop increasingly integrated pest management and sustainable food production systems has encouraged a greater interest to thoroughly evaluate effective utilization of cover crops in agricultural systems. In addition to providing a measure of weed control through physical obstruction and/or biochemical suppression, cover crops provide numerous environmental benefits that can promote long-term viability of farm lands (Jordan et al. 1999; Phatak et al. 2002; Yenish et al. 1996). Implementation of cover crops can reduce soil erosion, reduce runoff and improve water availability, improve soil structure, enhance soil organic matter, and increase diversity of soil biota (Bugg and Dutcher 1989; Reeves 1994; Wang et al. 2002a). These soil improvements, along with weed suppression capabilities, have made cover crops ideally suited for use in current and future

Autumn-seeded cover crops include cereal grains, such as oat (*Avena sativa* L.) or rye (*Secale cereale* L.), Brassicas, like mustard (*Brassica* spp.) and radish (*Raphanus sativus* L.), or legumes, like clover (*Trifolium* spp.) or vetch (*Vicia* spp.) (SARE 2007). Each type of cover provides ecological benefits; however, leguminous cover crops are capable of providing biologically fixed nitrogen (N) which is available for uptake by the succeeding cash crop (Balkcom and Reeves 2005; Cherr et al. 2006; Karlen and Doran 1991; Wang et al. 2005). This source of nitrogen can greatly reduce N fertilizer applications necessary for the subsequent crop, and is of particular interest in low-input agriculture systems (Deberkow and Reichelderfer 1988). The drawback when utilizing legume cover crops, in comparison to grain covers, is acceleration of residue decomposition (Cherr et al. 2006; Somda et al. 1991). For weed control purposes, cover crops with plant portions containing relatively high C:N and high residue levels, such as cereal grains or sunn hemp (*Crotalaria juncea* L.), offer increased weed suppression for a relatively lengthier period of time during the growing season compared to cover crops with low C:N ratios (Cherr et al. 2006; Vigil and Kissel 1995). Legume cover crops have low C:N ratios, thus generally decompose more rapidly than cereal grains and require substantial biomass for extended ground cover. To resolve this issue, research has examined the use of tropical legume cover crops in temperate

**1. Introduction** 

sustainable agronomic systems.

**Weed Control in Temperate Climates** 

Andrew J. Price1, Jessica Kelton2 and Jorge Mosjidis2

*1United States Department of Agriculture,* 

*2Auburn University,* 


## **Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates**

Andrew J. Price1, Jessica Kelton2 and Jorge Mosjidis2 *1United States Department of Agriculture, 2Auburn University, USA* 

#### **1. Introduction**

100 Weed Control

Yenish, J. P., Worsham, A. D. and Chilton, W. S. 1995. Disappearance of DIBOA-glucoside, DIBOA, and BOA from rye cover crop residue. Weed Science 43, 18-20. Zemenchik, R. A., K. A. Albrecht, C. M. Boerboom, and J. G. Lauer. 2000. Corn production

with kura clover as a living mulch. Agron. J. 92, 698–705.

The use of smother crops or cover crop residue to suppress weed growth in agriculture is not a recent innovation; yet, only recently have smother, or cover crops, received considerable attention. The need to develop increasingly integrated pest management and sustainable food production systems has encouraged a greater interest to thoroughly evaluate effective utilization of cover crops in agricultural systems. In addition to providing a measure of weed control through physical obstruction and/or biochemical suppression, cover crops provide numerous environmental benefits that can promote long-term viability of farm lands (Jordan et al. 1999; Phatak et al. 2002; Yenish et al. 1996). Implementation of cover crops can reduce soil erosion, reduce runoff and improve water availability, improve soil structure, enhance soil organic matter, and increase diversity of soil biota (Bugg and Dutcher 1989; Reeves 1994; Wang et al. 2002a). These soil improvements, along with weed suppression capabilities, have made cover crops ideally suited for use in current and future sustainable agronomic systems.

Autumn-seeded cover crops include cereal grains, such as oat (*Avena sativa* L.) or rye (*Secale cereale* L.), Brassicas, like mustard (*Brassica* spp.) and radish (*Raphanus sativus* L.), or legumes, like clover (*Trifolium* spp.) or vetch (*Vicia* spp.) (SARE 2007). Each type of cover provides ecological benefits; however, leguminous cover crops are capable of providing biologically fixed nitrogen (N) which is available for uptake by the succeeding cash crop (Balkcom and Reeves 2005; Cherr et al. 2006; Karlen and Doran 1991; Wang et al. 2005). This source of nitrogen can greatly reduce N fertilizer applications necessary for the subsequent crop, and is of particular interest in low-input agriculture systems (Deberkow and Reichelderfer 1988). The drawback when utilizing legume cover crops, in comparison to grain covers, is acceleration of residue decomposition (Cherr et al. 2006; Somda et al. 1991). For weed control purposes, cover crops with plant portions containing relatively high C:N and high residue levels, such as cereal grains or sunn hemp (*Crotalaria juncea* L.), offer increased weed suppression for a relatively lengthier period of time during the growing season compared to cover crops with low C:N ratios (Cherr et al. 2006; Vigil and Kissel 1995). Legume cover crops have low C:N ratios, thus generally decompose more rapidly than cereal grains and require substantial biomass for extended ground cover. To resolve this issue, research has examined the use of tropical legume cover crops in temperate

legumes.

Phillips 1980).

**3.1 Nitrogen fixation in legumes** 

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 103

However, a major constraint to the use of winter legumes covers is the lack of ample growing time between cash crops (Mansoer et al. 1997). Traditional planting windows for cover crops do not allow for maximum growth of cover crop species prior to the onset of cold temperatures; earlier planting of legumes would require a harvest of summer crops before maturity. In addition, planting cash crops often interferes with maturation of cover crops. Current limitations with legume biomass production have warranted research to resolve these issues in order to make use of the nitrogen fixation properties offered by

With the majority of atmospheric nitrogen present in a form unavailable for plant use (N2), biological fixation of N2 to NH3 by bacteria is a critical process for contributing nitrogen to the soil environment (Meyer et al. 1978; Novoa and Loomis 1981). With legumes, bacteria fix atmospheric nitrogen within root nodules while the plant provides needed carbohydrates to facilitate the process (Figure 1). This symbiotic relationship allows many leguminous crops to be grown without the addition of synthetic fertilizers (Lindemann and Glover 2003;

Fig. 1. Nitrogen fixation by bacteria, such as *Rhizobium*, occurs in root nodules of legumes.

practices, has boosted interest in the use of a number of legumes for cover crops.

As cover crops, legumes release nitrogen accumulations through vegetative decomposition, making NH3 available for succeeding crops (Lindemann and Glover 2003). The rising cost of synthetic fertilizer, and the desire for viable alternatives to traditional high-input production

The amount of N biologically fixed by a legume is dependent on a number of environmental conditions and management practices that affect biomass production (Holderbaum et al.

regions to facilitate N fixation while achieving suitable levels of biomass (Balkcom et al. 2011; Gallaher et al. 2001; Marshall et al. 2002; Mosjidis and Wehtje 2011).

Sunn hemp, a tropical legume that most likely originated from the Indo-Pakistani subcontinent, has been identified as a potential alternative to traditional legume cover crops employed in the southern portion of the United States (Cook and White 1996; Mansoer et al. 1997; Montgomery 1954; Mosjidis and Wetje 2011). As a tropical legume, sunn hemp can produce larger quantities of biomass in a shorter time period than winter legumes from temperate zones, while still providing an agronomically important amount of fixed N (Mansoer et al. 1997; Reeves et al. 1996; Wang et al. 2002b). The increased biomass production from sunn hemp would improve and extend weed control compared to other legume covers. Research continues worldwide to evaluate this species to determine its potential for widespread use in sustainable agricultural production, as well as to identify any limitations with the use of sunn hemp.

This chapter briefly explores the biological features of sunn hemp that make it a suitable cover crop and reviews previous research concerning weed suppression by sunn hemp. It also outlines current research projects targeting constraints on extensive adoption which include plant breeding studies and herbicide evaluations to improve sunn hemp production for seed availability. In order to improve weed managment options, it is necessary to continue investigating alternative methods to achieve effective, yet sustainable, weed control.

#### **2. Cover crops**

As stated previously, cover crops provide numerous environmental and weed suppression benefits. Implementation of cover crops into a production system is often in response to the need to reduce soil erosion and water runoff (Hartwig and Ammon 2002). However, with current advances toward sustainable growing practices as well as a need to reduce input costs, growers have begun to integrate cover crops for their weed control capabilities. The use of cover crops is typically found in conservation agriculture settings; cover crop residue left on the soil surface at planting provides a measure of weed control through shading of the soil and/or through allelopathy, chemical inhibition of plant germination, and as physical barrier for weed growth (Creamer et al. 1996; Price et al. 2007; Teasdale 1996).

To maximize weed suppression, high-residue cover crop systems that provide at least 4,500 kg ha-1 of biomass for ground cover are generally utilized (Balkcom et al. 2007). In these instances, winter cereal grain crops such as rye or oat are employed to attain the greatest amounts of residue prior to cash crop planting to maintain ground cover for an extended period into the growing season (Duiker and Curran 2005; Price et al. 2006, 2007; Ruffo and Bollero 2003). Although cereal crops can be established with relatively low costs and offer maximum biomass production for weed suppression, some systems would benefit more from the use of fall or winter legume cover crops.

#### **3. Legume cover crops**

Leguminous cover crops provide many benefits also achieved with other cover crop species including erosion control, improved water filtration, and improved soil organic matter. However, a major constraint to the use of winter legumes covers is the lack of ample growing time between cash crops (Mansoer et al. 1997). Traditional planting windows for cover crops do not allow for maximum growth of cover crop species prior to the onset of cold temperatures; earlier planting of legumes would require a harvest of summer crops before maturity. In addition, planting cash crops often interferes with maturation of cover crops. Current limitations with legume biomass production have warranted research to resolve these issues in order to make use of the nitrogen fixation properties offered by legumes.

#### **3.1 Nitrogen fixation in legumes**

102 Weed Control

regions to facilitate N fixation while achieving suitable levels of biomass (Balkcom et al.

Sunn hemp, a tropical legume that most likely originated from the Indo-Pakistani subcontinent, has been identified as a potential alternative to traditional legume cover crops employed in the southern portion of the United States (Cook and White 1996; Mansoer et al. 1997; Montgomery 1954; Mosjidis and Wetje 2011). As a tropical legume, sunn hemp can produce larger quantities of biomass in a shorter time period than winter legumes from temperate zones, while still providing an agronomically important amount of fixed N (Mansoer et al. 1997; Reeves et al. 1996; Wang et al. 2002b). The increased biomass production from sunn hemp would improve and extend weed control compared to other legume covers. Research continues worldwide to evaluate this species to determine its potential for widespread use in sustainable agricultural production, as well as to identify

This chapter briefly explores the biological features of sunn hemp that make it a suitable cover crop and reviews previous research concerning weed suppression by sunn hemp. It also outlines current research projects targeting constraints on extensive adoption which include plant breeding studies and herbicide evaluations to improve sunn hemp production for seed availability. In order to improve weed managment options, it is necessary to continue investigating alternative methods to achieve effective, yet sustainable, weed

As stated previously, cover crops provide numerous environmental and weed suppression benefits. Implementation of cover crops into a production system is often in response to the need to reduce soil erosion and water runoff (Hartwig and Ammon 2002). However, with current advances toward sustainable growing practices as well as a need to reduce input costs, growers have begun to integrate cover crops for their weed control capabilities. The use of cover crops is typically found in conservation agriculture settings; cover crop residue left on the soil surface at planting provides a measure of weed control through shading of the soil and/or through allelopathy, chemical inhibition of plant germination, and as physical barrier for weed growth (Creamer et al. 1996; Price et al. 2007; Teasdale 1996).

To maximize weed suppression, high-residue cover crop systems that provide at least 4,500 kg ha-1 of biomass for ground cover are generally utilized (Balkcom et al. 2007). In these instances, winter cereal grain crops such as rye or oat are employed to attain the greatest amounts of residue prior to cash crop planting to maintain ground cover for an extended period into the growing season (Duiker and Curran 2005; Price et al. 2006, 2007; Ruffo and Bollero 2003). Although cereal crops can be established with relatively low costs and offer maximum biomass production for weed suppression, some systems would benefit more

Leguminous cover crops provide many benefits also achieved with other cover crop species including erosion control, improved water filtration, and improved soil organic matter.

2011; Gallaher et al. 2001; Marshall et al. 2002; Mosjidis and Wehtje 2011).

any limitations with the use of sunn hemp.

from the use of fall or winter legume cover crops.

**3. Legume cover crops** 

control.

**2. Cover crops** 

With the majority of atmospheric nitrogen present in a form unavailable for plant use (N2), biological fixation of N2 to NH3 by bacteria is a critical process for contributing nitrogen to the soil environment (Meyer et al. 1978; Novoa and Loomis 1981). With legumes, bacteria fix atmospheric nitrogen within root nodules while the plant provides needed carbohydrates to facilitate the process (Figure 1). This symbiotic relationship allows many leguminous crops to be grown without the addition of synthetic fertilizers (Lindemann and Glover 2003; Phillips 1980).

Fig. 1. Nitrogen fixation by bacteria, such as *Rhizobium*, occurs in root nodules of legumes.

As cover crops, legumes release nitrogen accumulations through vegetative decomposition, making NH3 available for succeeding crops (Lindemann and Glover 2003). The rising cost of synthetic fertilizer, and the desire for viable alternatives to traditional high-input production practices, has boosted interest in the use of a number of legumes for cover crops.

The amount of N biologically fixed by a legume is dependent on a number of environmental conditions and management practices that affect biomass production (Holderbaum et al.

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 105

covers such as clover and vetch or between crop harvest and cereal cover crop planting

Sunn hemp is readily used as a rotational crop in tropical regions with cash crops such as rice, cotton, and corn (Purseglove 1974). Its value as a cover crop is due to its biomass production, N accumulation, reduced pests and pathogen infestation, and weed suppression achieved when planted (Wang et al. 2002b). While sunn hemp is not winter hardy, its adaptability to various soil types and precipitation amounts has allowed sunn hemp to be grown in temperate regions as a green manure (Dempsey 1975). In the US, sunn hemp was used in this manner during the 1930's until reduced seed availability caused interest in the crop to diminish (Cook and White 1996). Recently, sunn hemp has again received attention as a potential alternative to winter annual legume cover crops in these temperate climates (Balkcom and Reeves 2005; Mansoer et al. 1997). Sunn hemp's vigorous growth and nitrogen production can provide needed ground cover to control erosion and an N source to succeeding cash crops. To fully utilize sunn hemp nitrogen release, investigations have also been conducted to determine the suitability of sunn hemp grown as a late summer cover crop between harvest and winter planting of cash crop or cereal cover crop (Balkcom et al.

The rapid growth of sunn hemp in a relatively short period of time allows for a relatively high amount of biomass production prior to the onset of cool temperatures in mild climates across the southeastern US. Previous research has reported sunn hemp biomass to average between 1 and 9 Mg ha-1 in 45 to 90 days after planting, respectively (Mansoer et al. 1997; Morris et al. 1986; Schomberg et al. 2007; Reeves et al. 1996; Yadvinder et al. 1992). Although environmental conditions affect potential biomass production, substantial amounts of biomass can be achieved under typical late-summer and autumn conditions to aid in erosion

As a legume cover crop, sunn hemp can fix atmospheric nitrogen that is available over time to succeeding crops as it decomposes. With high fertilizer prices and sustainability concerns with synthetic soil amendments, the potential use of sunn hemp to provide nitrogen to crops such as cotton, corn, and rice has prompted research to determine N availability from a sunn hemp cover crop (Balkcom and Reeves 2005; Chung et al. 2000; Sangakkara et al. 2004; Schomberg et al. 2007). Nitrogen production with sunn hemp varies depending on many factors; however, reported N values in sunn hemp range between 110 and 160 kg ha-1 (Balkcom and Reeves 2005; Mansoer et al. 1997; Marshall et al. 2001). In most investigations, sunn hemp nitrogen content equals or exceeds N content of traditional winter legume cover

With nitrogen release during winter months reported at approximately 75 kg ha-1, remaining N may still be utilized by spring planted crops (Reeves et al. 1996). Due to winter nitrogen loss from leaching and utilization, however, an alternative scenario mentioned previously for sunn hemp use is to employ the crop as a late summer cover prior to grain cover crop or winter cash crop planting in the fall (Balkcom et al. 2011; Schomberg et al. 2007). Released nitrogen from sunn hemp residue would be available to the subsequent crop

control, nematode and weed suppression, and N accumulation before frost occurs.

(Balkcom and Reeves 2005; Mansoer et al. 1997).

2011; Creamer and Baldwin 2000; Schomberg et al. 2007).

while minimizing N losses during winter months.

**4.1 Sunn hemp cover crops** 

crops (Reeves 1994).

1990; Reeves 1994; Wagger 1989). Traditional fall-seeded legume cover crops have limited biomass production prior to cold temperatures, which can limit N accumulation and availability to subsequent crops. Utilization of tropical legumes, such as sunn hemp, may allow for greater biomass production and N accumulation during the available growing season between fall harvest and onset of winter in temperate climates (Mansoer et al. 1997).

#### **3.2 Weed control with legume cover crops**

Weed control obtained through legume cover crops has been well researched for a variety of cash crops (Caamal-Maldonado et al. 2001; DeGregoria and Ashley 1986; Teasdale 1988). Common legume covers have been shown to suppress growth of many species like pigweed (*Amaranthus* spp.), foxtail (*Setaria* spp.), and morningglory (*Ipomoeae* spp.) (Collins et al. 2007; Teasdale 1988; White et al. 1989). Either as a ground cover or through allelopathy, as noted in subterranean clover (*Trifolium subterraneum* L.), weed control achieved through legume cover crops has the potential to reduce early-season herbicide use in agricultural systems (Hartwig and Ammon 2002; Leather 1983; Mosjidis and Wehtje 2011). However, in most climates, the inability to produce high biomass with winter legumes, coupled with rapid decomposition, weed suppression is largely obtained during active cover growth and just after cover crop termination (Reddy 2001; Teasdale 1996). The use of sunn hemp immediately behind an early harvest summer cash crop like corn (*Zea mays* L.) may allow for extended post-harvest weed control through increased biomass production as well as slower decomposition rates in comparison to some other legume choices (Cherr et al. 2006; Cobo et al. 2002; Mansoer et al. 1997).

#### **4. Sunn hemp**

Sunn hemp, or Indian hemp, has become an important crop in regions such as India and Brazil that have climates well suited to the tropical, herbaceous annual (Bhardwaj et al. 2005; Duke 1981) (Figure 2). Typically utilized as a green manure due to its nitrogen accumulation, sunn hemp is also grown as a fiber crop; it can also be grown for forage since this *Crotalaria* species is nontoxic to animals (Rotar and Joy 1983). As a vigorously growing, relatively drought tolerant plant species, sunn hemp has been shown to thrive in a variety of soil types and with variable rainfall, but it is still most successful in tropical or subtropical environments (Wang et al. 2002b).

The utilization of this species in cooler, temperate climates, such as those found in the continental United States (US), began in the early 1930's in response to sunn hemp's potential as a green manure and for suppression of root-knot nematodes (Cook and White 1996; Cook et al. 1998; Dempsey 1975). At the onset of World War II, increased demands for rope fiber drew more attention to sunn hemp as an alternative for imported cordage material (Cook and White 1996; Wilson et al. 1965). During the 1950's and 1960's, US research placed particular emphasis on sunn hemp production as a quickly-renewed source of fiber for paper materials (Nelson et al. 1961). Although most attention for nonwood fiber sources has been concentrated on kenaf (*Hibiscus cannabinus* L.), some research continues to identify sunn hemp as a potential source that can be produced in the US, particularly in Hawaii, southern Texas, and south Florida (Cook and Scott 1998; Webber and Bledsoe 1993). In temperate regions of the US, however, more recent research has evaluated the cover crop potential of sunn hemp as a frost-terminated, late-summer alternative to winter legume covers such as clover and vetch or between crop harvest and cereal cover crop planting (Balkcom and Reeves 2005; Mansoer et al. 1997).

#### **4.1 Sunn hemp cover crops**

104 Weed Control

1990; Reeves 1994; Wagger 1989). Traditional fall-seeded legume cover crops have limited biomass production prior to cold temperatures, which can limit N accumulation and availability to subsequent crops. Utilization of tropical legumes, such as sunn hemp, may allow for greater biomass production and N accumulation during the available growing season between fall harvest and onset of winter in temperate climates (Mansoer et al. 1997).

Weed control obtained through legume cover crops has been well researched for a variety of cash crops (Caamal-Maldonado et al. 2001; DeGregoria and Ashley 1986; Teasdale 1988). Common legume covers have been shown to suppress growth of many species like pigweed (*Amaranthus* spp.), foxtail (*Setaria* spp.), and morningglory (*Ipomoeae* spp.) (Collins et al. 2007; Teasdale 1988; White et al. 1989). Either as a ground cover or through allelopathy, as noted in subterranean clover (*Trifolium subterraneum* L.), weed control achieved through legume cover crops has the potential to reduce early-season herbicide use in agricultural systems (Hartwig and Ammon 2002; Leather 1983; Mosjidis and Wehtje 2011). However, in most climates, the inability to produce high biomass with winter legumes, coupled with rapid decomposition, weed suppression is largely obtained during active cover growth and just after cover crop termination (Reddy 2001; Teasdale 1996). The use of sunn hemp immediately behind an early harvest summer cash crop like corn (*Zea mays* L.) may allow for extended post-harvest weed control through increased biomass production as well as slower decomposition rates in comparison to some other legume choices (Cherr et al. 2006;

Sunn hemp, or Indian hemp, has become an important crop in regions such as India and Brazil that have climates well suited to the tropical, herbaceous annual (Bhardwaj et al. 2005; Duke 1981) (Figure 2). Typically utilized as a green manure due to its nitrogen accumulation, sunn hemp is also grown as a fiber crop; it can also be grown for forage since this *Crotalaria* species is nontoxic to animals (Rotar and Joy 1983). As a vigorously growing, relatively drought tolerant plant species, sunn hemp has been shown to thrive in a variety of soil types and with variable rainfall, but it is still most successful in tropical or subtropical

The utilization of this species in cooler, temperate climates, such as those found in the continental United States (US), began in the early 1930's in response to sunn hemp's potential as a green manure and for suppression of root-knot nematodes (Cook and White 1996; Cook et al. 1998; Dempsey 1975). At the onset of World War II, increased demands for rope fiber drew more attention to sunn hemp as an alternative for imported cordage material (Cook and White 1996; Wilson et al. 1965). During the 1950's and 1960's, US research placed particular emphasis on sunn hemp production as a quickly-renewed source of fiber for paper materials (Nelson et al. 1961). Although most attention for nonwood fiber sources has been concentrated on kenaf (*Hibiscus cannabinus* L.), some research continues to identify sunn hemp as a potential source that can be produced in the US, particularly in Hawaii, southern Texas, and south Florida (Cook and Scott 1998; Webber and Bledsoe 1993). In temperate regions of the US, however, more recent research has evaluated the cover crop potential of sunn hemp as a frost-terminated, late-summer alternative to winter legume

**3.2 Weed control with legume cover crops** 

Cobo et al. 2002; Mansoer et al. 1997).

environments (Wang et al. 2002b).

**4. Sunn hemp** 

Sunn hemp is readily used as a rotational crop in tropical regions with cash crops such as rice, cotton, and corn (Purseglove 1974). Its value as a cover crop is due to its biomass production, N accumulation, reduced pests and pathogen infestation, and weed suppression achieved when planted (Wang et al. 2002b). While sunn hemp is not winter hardy, its adaptability to various soil types and precipitation amounts has allowed sunn hemp to be grown in temperate regions as a green manure (Dempsey 1975). In the US, sunn hemp was used in this manner during the 1930's until reduced seed availability caused interest in the crop to diminish (Cook and White 1996). Recently, sunn hemp has again received attention as a potential alternative to winter annual legume cover crops in these temperate climates (Balkcom and Reeves 2005; Mansoer et al. 1997). Sunn hemp's vigorous growth and nitrogen production can provide needed ground cover to control erosion and an N source to succeeding cash crops. To fully utilize sunn hemp nitrogen release, investigations have also been conducted to determine the suitability of sunn hemp grown as a late summer cover crop between harvest and winter planting of cash crop or cereal cover crop (Balkcom et al. 2011; Creamer and Baldwin 2000; Schomberg et al. 2007).

The rapid growth of sunn hemp in a relatively short period of time allows for a relatively high amount of biomass production prior to the onset of cool temperatures in mild climates across the southeastern US. Previous research has reported sunn hemp biomass to average between 1 and 9 Mg ha-1 in 45 to 90 days after planting, respectively (Mansoer et al. 1997; Morris et al. 1986; Schomberg et al. 2007; Reeves et al. 1996; Yadvinder et al. 1992). Although environmental conditions affect potential biomass production, substantial amounts of biomass can be achieved under typical late-summer and autumn conditions to aid in erosion control, nematode and weed suppression, and N accumulation before frost occurs.

As a legume cover crop, sunn hemp can fix atmospheric nitrogen that is available over time to succeeding crops as it decomposes. With high fertilizer prices and sustainability concerns with synthetic soil amendments, the potential use of sunn hemp to provide nitrogen to crops such as cotton, corn, and rice has prompted research to determine N availability from a sunn hemp cover crop (Balkcom and Reeves 2005; Chung et al. 2000; Sangakkara et al. 2004; Schomberg et al. 2007). Nitrogen production with sunn hemp varies depending on many factors; however, reported N values in sunn hemp range between 110 and 160 kg ha-1 (Balkcom and Reeves 2005; Mansoer et al. 1997; Marshall et al. 2001). In most investigations, sunn hemp nitrogen content equals or exceeds N content of traditional winter legume cover crops (Reeves 1994).

With nitrogen release during winter months reported at approximately 75 kg ha-1, remaining N may still be utilized by spring planted crops (Reeves et al. 1996). Due to winter nitrogen loss from leaching and utilization, however, an alternative scenario mentioned previously for sunn hemp use is to employ the crop as a late summer cover prior to grain cover crop or winter cash crop planting in the fall (Balkcom et al. 2011; Schomberg et al. 2007). Released nitrogen from sunn hemp residue would be available to the subsequent crop while minimizing N losses during winter months.

hemp.

suppression by *C. juncea* will continue.

production in sunn hemp stands.

**4.4 Breeding and seed availability** 

**4.3 Weed control in sunn hemp production** 

will be critical for successful production in the future.

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 107

In contrast, weed suppression specifically by sunn hemp cover crops has been minimally investigated and only recently has it received more attention. General comments concerning the potential of sunn hemp to suppress weed species have been reported in several studies (Reeves et al. 1996; SARE 2007). Weed control by sunn hemp has been mostly attributed to vigorous plant growth and rapid shading of the ground (Duke 1981). In fact, Mosjidis and Wehtje (2011) demonstrated that there was a progressive reduction in weed biomass as a sunn hemp stand increased up to 100 plants/m2. Furthermore, recent research has suggested allelopathic compounds released from sunn hemp also cause weed suppression (Adler and Chase 2007; Collins et al. 2007; Leather and Forrence 1990; Price et al. 2008). More research is necessary to determine the extent of allelochemical functions in sunn

Significant weed control can be achieved under moderate to high levels of sunn hemp (Mosjidis and Wehtje 2011; SARE 2007; Severino and Christoffoleti 2004). However, several weed species, such as nutsedge (*Cyperus* spp.), morningglory (*Ipomoea* spp.), and bermudagrass (*Cynodon dactylon*), are capable of thriving in a sunn hemp stand (Chaudhury et al. 2007; Collins et al. 2007; McKee et al. 1946). It is expected that, as sunn hemp utilization for cover crops and weed control grows, research efforts to fully understand weed

Little research has been conducted to determine weed control strategies in sunn hemp production. *C. juncea* grown as a green manure or cover crop typically does not require extensive weed management but production of sunn hemp for seed production may benefit from additional weed control practices. Due to sunn hemp's rapid growth and possible allelopathic effects, it can be easily established and can out-compete neighboring weed species. In fact, areas that rely on hand removal of weed species in agricultural productions generally do not employ this practice in sunn hemp grown for fiber, since weed competition is minimal (Chaudhury et al. 2007). However, in regions that utilize herbicides for weed control, early season weed suppression with herbicide applications may increase seed

At present, no herbicides are labelled for use in sunn hemp production, but research by Mosjidis and Wehtje (2011) identified a preemergent herbicide, pendimethalin, as a potential treatment that would provide effective weed control during establishment. This research also found that sunn hemp could also tolerate 2,4-DB as a postemergent application (Mosjidis and Wehtje 2011). Imazethapyr was also determined to be safe for use in sunn hemp and effective against yellow nutsedge (*Cyperus esculentus* L.), which is not well suppressed by sunn hemp (Collins et al. 2007; Mosjidis and Wehtje 2011). With advancements in sunn hemp breeding and increased utilization of this species in a broader geographical range, continued research efforts to determine additional herbicide programs

As a native tropical species, sunn hemp use in temperate regions faces some challenges. Although sunn hemp experiences rapid vegetative growth in a short time frame, viable seed

Fig. 2. The vigorous growth of sunn hemp cultivars developed at Auburn University, Alabama, can be seen here, 70 days after planting (Photo by J.A. Mosjidis).

#### **4.2 Weed suppression by sunn hemp**

It has been noted by many researchers that *C. juncea* can suppress populations of pests such as nematodes and vigorous weed species (Collins et al. 2007; Fassuliotis and Skucas 1969; McSorley et al. 1994; Taylor 1985; Wang et al. 2001). Much of the pest control potential of sunn hemp has focused primarily on nematode control of such species as *Meloidogyne* spp., *Rotylenchulus reniformis*, *Radopholus similis,* and *Heterodera glycines* (Birchfield and Bristline 1956; Desaeger and Rao 2000; Good et al. 1965; Marla et al. 2008; Robinson et al. 1998; Wang et al. 2004). Although some questions remain concerning suppression by sunn hemp for specific nematode species, extensive research has provided considerable knowledge as to how *C. juncea* reduces certain nematode populations (Halbrendt 1996; Kloepper et al. 1991; LaMondia 1996; Rodriguez-Kabana 1994; Wang et al. 2002b).

In contrast, weed suppression specifically by sunn hemp cover crops has been minimally investigated and only recently has it received more attention. General comments concerning the potential of sunn hemp to suppress weed species have been reported in several studies (Reeves et al. 1996; SARE 2007). Weed control by sunn hemp has been mostly attributed to vigorous plant growth and rapid shading of the ground (Duke 1981). In fact, Mosjidis and Wehtje (2011) demonstrated that there was a progressive reduction in weed biomass as a sunn hemp stand increased up to 100 plants/m2. Furthermore, recent research has suggested allelopathic compounds released from sunn hemp also cause weed suppression (Adler and Chase 2007; Collins et al. 2007; Leather and Forrence 1990; Price et al. 2008). More research is necessary to determine the extent of allelochemical functions in sunn hemp.

Significant weed control can be achieved under moderate to high levels of sunn hemp (Mosjidis and Wehtje 2011; SARE 2007; Severino and Christoffoleti 2004). However, several weed species, such as nutsedge (*Cyperus* spp.), morningglory (*Ipomoea* spp.), and bermudagrass (*Cynodon dactylon*), are capable of thriving in a sunn hemp stand (Chaudhury et al. 2007; Collins et al. 2007; McKee et al. 1946). It is expected that, as sunn hemp utilization for cover crops and weed control grows, research efforts to fully understand weed suppression by *C. juncea* will continue.

#### **4.3 Weed control in sunn hemp production**

106 Weed Control

Fig. 2. The vigorous growth of sunn hemp cultivars developed at Auburn University,

It has been noted by many researchers that *C. juncea* can suppress populations of pests such as nematodes and vigorous weed species (Collins et al. 2007; Fassuliotis and Skucas 1969; McSorley et al. 1994; Taylor 1985; Wang et al. 2001). Much of the pest control potential of sunn hemp has focused primarily on nematode control of such species as *Meloidogyne* spp., *Rotylenchulus reniformis*, *Radopholus similis,* and *Heterodera glycines* (Birchfield and Bristline 1956; Desaeger and Rao 2000; Good et al. 1965; Marla et al. 2008; Robinson et al. 1998; Wang et al. 2004). Although some questions remain concerning suppression by sunn hemp for specific nematode species, extensive research has provided considerable knowledge as to how *C. juncea* reduces certain nematode populations (Halbrendt 1996; Kloepper et al. 1991;

Alabama, can be seen here, 70 days after planting (Photo by J.A. Mosjidis).

LaMondia 1996; Rodriguez-Kabana 1994; Wang et al. 2002b).

**4.2 Weed suppression by sunn hemp** 

Little research has been conducted to determine weed control strategies in sunn hemp production. *C. juncea* grown as a green manure or cover crop typically does not require extensive weed management but production of sunn hemp for seed production may benefit from additional weed control practices. Due to sunn hemp's rapid growth and possible allelopathic effects, it can be easily established and can out-compete neighboring weed species. In fact, areas that rely on hand removal of weed species in agricultural productions generally do not employ this practice in sunn hemp grown for fiber, since weed competition is minimal (Chaudhury et al. 2007). However, in regions that utilize herbicides for weed control, early season weed suppression with herbicide applications may increase seed production in sunn hemp stands.

At present, no herbicides are labelled for use in sunn hemp production, but research by Mosjidis and Wehtje (2011) identified a preemergent herbicide, pendimethalin, as a potential treatment that would provide effective weed control during establishment. This research also found that sunn hemp could also tolerate 2,4-DB as a postemergent application (Mosjidis and Wehtje 2011). Imazethapyr was also determined to be safe for use in sunn hemp and effective against yellow nutsedge (*Cyperus esculentus* L.), which is not well suppressed by sunn hemp (Collins et al. 2007; Mosjidis and Wehtje 2011). With advancements in sunn hemp breeding and increased utilization of this species in a broader geographical range, continued research efforts to determine additional herbicide programs will be critical for successful production in the future.

#### **4.4 Breeding and seed availability**

As a native tropical species, sunn hemp use in temperate regions faces some challenges. Although sunn hemp experiences rapid vegetative growth in a short time frame, viable seed

Utilization of Sunn Hemp for Cover Crops and Weed Control in Temperate Climates 109

set (Chaudhury et al. 2007). In cooler climates above this latitude, temperatures below optimal usually occur before sunn hemp seed can mature. The recent development of cultivar, 'AU Golden' and 'AU Durbin' has been shown to produce viable seed in these temperate regions; research is on-going to determine best management practices for

The progression of agricultural systems towards more sustainable, yet high yielding production has required researchers to identify numerous alternative weed management practices that can be employed along with traditional weed control tactics. The use of sunn hemp as a cover crop, either as a substitute for winter annual legumes or as a late summer cover between harvest and winter crops, delivers effective weed control while providing ground cover and a nitrogen source for subsequent crops. Continued research with sunn hemp and crops similar to this species may provide even more benefits to weed control

Adler, M.J., and C.A. Chase. 2007. A comparative analysis of the allelopathic potential of

Balkcom, K.S. and D.W. Reeves. 2005. Sunn hemp utilized as a legume cover crop for corn

Balkcom, K.S., H. Schomberg, D.W. Reeves, and A. Clark. 2007. Managing cover crops in

Balkcom, K.S., J.M. Massey, J.A. Mosjidis, A.J. Price, and S.F. Enloe. 2011. Planting date and

Bhardwaj, H.L., C.L. Webber, and G.S. Sakamoto. 2005. Cultivation of kenaf and sunn hemp in the mid-Atlantic United States. Industrial Crops and Products 22: 151-155. Birchfield, W. and F. Bristline. 1956. Cover crop in relation to the burrowing nematode,

Bugg, R.L. and J.D. Dutcher. 1989. Warm-season cover crops for pecan orchards:

Caamal-Maldonado, J.A., J.J. Jimenez-Osornio, A. Torres-Barragan, and A.L. Anaya. 2001.

Chaudhury, J., D.P. Singh, and S.K. Hazra. 2007. Sunn Hemp (*Crotalaria juncea* L.). Central

Cherr, C.M., J.M.S. Scholberg, and R. McSorley. 2006. Green manure as nitrogen source for sweet corn in a warm-temperate environment. Agronomy Journal 98: 1173-1180.

cover crop. International Journal of Agronomy 2011: 1-8.

*Radopholus similis*. Plant Disease Reporter 40: 398-399.

cropping systems. Agronomy Journal 93: 27-36.

http://assamagribusiness.nic.in/Sunnhemp.pdf

leguminous summer cover crops: cowpea, sunn hemp and velvetbean. HortScience

conservation tillage systems. *In* A. Clark (ed.). *Managing Cover Crops Profitably.*

seeding rate effects on sunn hemp biomass and nitrogen production for a winter

horticultural and entomological implications. Biology, Agriculture, and

The use of allelopathic legume cover and mulch species for weed control in

Research Institute for Jute and Allied Fibres (ICAR). available at:

implementing these sunn hemp cultivars (Balkcom et al. 2011; Mosjidis 2007, 2010).

**5. Conclusions** 

efforts in the future.

42: 289-293.

production. Agronomy Journal 97: 26-31.

SARE. College Park, MD: 44-61.

Horticulture 6: 123-148.

**6. References** 

production typically requires a longer season than can be achieved before winter conditions in temperate regions (Li et al. 2009)(sunn hemp seed pictured in Figure 3). The lack of seed production outside of tropical and subtropical climates severely limits seed availability to producers in cooler climates. Moreover, with seed costs ranging from \$90 to \$130 (US) per hectare, implementation of *C. juncea* as a cover crop can be an expensive task for growers (Li et al. 2009; Petcher 2009).

Fig. 3. Sunn hemp seed production can yield 450 to 1000 kg of seed per hectare (Photo by J.A. Mosjidis).

A good deal of breeding research has been conducted in countries throughout the world that grow sunn hemp for fiber production (Kundu 1964; Ram and Singh 2011). In India, particularly, cultivars are developed for high fiber yield and resistance to wilt diseases (Chaudhury et al. 2007). Most commonly used in this region is 'Kharif sunn' or 'K-12' which produce high yield of good quality fiber (Chaudhury et al. 2007). Other important cultivars in India include 'SS-11' and 'T-6' which is a day-neutral variety, while most varieties are short day plants (Chaudhury et al. 2007). In other regions of sunn hemp production, varieties such as 'Somerset' in South Africa and 'KRC-1' in Brazil are commonly used.

Due to elevated seed costs and limited seed production in the US, sunn hemp breeding research has focused on developing a *C. juncea* cultivar that can produce seeds in climatic conditions prevalent in the southeastern US. For successful seed production in traditional sunn hemp regions, characteristics of seed production locations should typically be below 24° N latitude, not fall below 10° C, have ample sunlight and not receive rainfall during fruit set (Chaudhury et al. 2007). In cooler climates above this latitude, temperatures below optimal usually occur before sunn hemp seed can mature. The recent development of cultivar, 'AU Golden' and 'AU Durbin' has been shown to produce viable seed in these temperate regions; research is on-going to determine best management practices for implementing these sunn hemp cultivars (Balkcom et al. 2011; Mosjidis 2007, 2010).

#### **5. Conclusions**

108 Weed Control

production typically requires a longer season than can be achieved before winter conditions in temperate regions (Li et al. 2009)(sunn hemp seed pictured in Figure 3). The lack of seed production outside of tropical and subtropical climates severely limits seed availability to producers in cooler climates. Moreover, with seed costs ranging from \$90 to \$130 (US) per hectare, implementation of *C. juncea* as a cover crop can be an expensive task for growers (Li

Fig. 3. Sunn hemp seed production can yield 450 to 1000 kg of seed per hectare (Photo by

A good deal of breeding research has been conducted in countries throughout the world that grow sunn hemp for fiber production (Kundu 1964; Ram and Singh 2011). In India, particularly, cultivars are developed for high fiber yield and resistance to wilt diseases (Chaudhury et al. 2007). Most commonly used in this region is 'Kharif sunn' or 'K-12' which produce high yield of good quality fiber (Chaudhury et al. 2007). Other important cultivars in India include 'SS-11' and 'T-6' which is a day-neutral variety, while most varieties are short day plants (Chaudhury et al. 2007). In other regions of sunn hemp production, varieties such as 'Somerset' in South Africa and 'KRC-1' in Brazil are commonly used.

Due to elevated seed costs and limited seed production in the US, sunn hemp breeding research has focused on developing a *C. juncea* cultivar that can produce seeds in climatic conditions prevalent in the southeastern US. For successful seed production in traditional sunn hemp regions, characteristics of seed production locations should typically be below 24° N latitude, not fall below 10° C, have ample sunlight and not receive rainfall during fruit

et al. 2009; Petcher 2009).

J.A. Mosjidis).

The progression of agricultural systems towards more sustainable, yet high yielding production has required researchers to identify numerous alternative weed management practices that can be employed along with traditional weed control tactics. The use of sunn hemp as a cover crop, either as a substitute for winter annual legumes or as a late summer cover between harvest and winter crops, delivers effective weed control while providing ground cover and a nitrogen source for subsequent crops. Continued research with sunn hemp and crops similar to this species may provide even more benefits to weed control efforts in the future.

### **6. References**


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Li, Y., Q. Wang, W. Klassen, and E.A. Hanlon. 2009. Sunn hemp-A cover crop in Florida.

Lindemann, W.C. and C.R. Glover. 2003. Nitrogen fixation by legumes. New Mexico State

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Cobo, J.G., E. Barrios, D.C.L, Kass and R.J. Thomas. 2002. Decomposition and nutrient

Cook, C.G. and G.A. White. 1996. *Crotalaria juncea*: A potential multi-purpose fiber crop. *In* J. Janick (ed.) *Progress in New Crops.* ASHS Press. Arlington, VA: 389-394. Cook, C.G. and A.W. Scott. 1998. Plant population effects on stalk growth, yield, and bark fiber content of sunn hemp. Industrial Crops and Products 8: 97-103. Cook, C.G., A.W. Scott, P. Chow. 1998. Planting date and cultivar effects on growth and

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**1. Introduction** 

the absence of resource competition (Einhellig 1994a).

systems, as well as organic production.

**6** 

*USA* 

*1Auburn University* 

*2United States Department of Agriculture* 

**Allelopathic Weed Suppression** 

**Through the Use of Cover Crops** 

Jessica Kelton1, Andrew J. Price2 and Jorge Mosjidis1

There has long been observed an inhibitive response by plant species to certain neighboring plants. The Greek philosopher and botanist, Theophrastus, noted this effect from cabbage as early as 300 BC (Willis 1985). Since that time, others have documented similar plant interactions. In 1937, Austrian botanist, Hans Molisch, described this phenomenon as allelopathy, which he determined to be the result of biochemical interactions between plants (Molisch 1937; Putnam and Duke 1978). When first described, allelopathy referred to both deleterious and beneficial interactions between species; since that time, however, allelopathy has been applied to only adverse plant interactions, rather than to both. First described by a Roman scholar during the first century, black walnut (*Juglans nigra* L.) has long served as the common example of allelopathic effects with its ability to inhibit growth of surrounding plants either through decaying leaves or nuts or from the tree itself (Weir et al. 2004). Researchers have continued to examine allelopathy and the mechanism for biochemical inhibition, which was initially scrutinized by many since differentiation between this effect and plant competition remained uncertain (Weir et al. 2004). Subsequent bioassays involving specific chemical compounds extracted from plants have confirmed that certain species do, in fact, produce biochemicals that can inhibit plant germination and growth in

With confirmation of allelopathy, many investigations have been conducted in order to determine how best to utilize this effect for possible weed control in agricultural settings (Khanh et al. 2005; Olofsdotter 2001; Weston 1996). The ability to inhibit weed growth through the implementation of cover crops into a crop rotation has been a focal point for this research for several reasons. In addition to weed suppression and control through allelopathy, as well as a mulching effect, cover crops provide substantial environmental benefits such as reduced erosion and water runoff (Price et al. 2006; Truman et al. 2003). Moreover, cover crops are readily available and easily adapted to many agricultural situations. Because of these many benefits, including natural weed suppression through allelopathy, the use of cover crops has become a vital component of sustainable agriculture

Ensuring sufficient food and fiber production for future generations can be hampered by limited options for weed control, particularly in developing countries where yields are


## **Allelopathic Weed Suppression Through the Use of Cover Crops**

Jessica Kelton1, Andrew J. Price2 and Jorge Mosjidis1 *1Auburn University 2United States Department of Agriculture USA* 

#### **1. Introduction**

114 Weed Control

Wang, K., B.S. Sipes, and D.P. Schmitt. 2002b. *Crotalaria* as a cover crop for nematode

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and J.E. Simon (eds.). New Crops. Wiley, New York, NY: 416-421.

and seed. Florida Agricultural Experiment Station Circular S168.

with green manures. Advances in Soil Science 20: 237-309.

no-tillage corn (*Zea mays*). Weed Technology 10: 815-821.

changes associated with decomposition of *Crotalaria juncea* amendment in

soil water and nutrients in a subtropical area. Journal of Soil and Water

and 'Everglades 71', two new cultivars of kenaf (*Hibiscus cannabinus* L.) for the fiber

management: a review. Nematropica 32: 35-57.

litterbags. Applied Soil Ecology 27: 31-45.

aqueous extracts. Weed Science 37: 674-679.

Conservation 60: 58-63.

There has long been observed an inhibitive response by plant species to certain neighboring plants. The Greek philosopher and botanist, Theophrastus, noted this effect from cabbage as early as 300 BC (Willis 1985). Since that time, others have documented similar plant interactions. In 1937, Austrian botanist, Hans Molisch, described this phenomenon as allelopathy, which he determined to be the result of biochemical interactions between plants (Molisch 1937; Putnam and Duke 1978). When first described, allelopathy referred to both deleterious and beneficial interactions between species; since that time, however, allelopathy has been applied to only adverse plant interactions, rather than to both. First described by a Roman scholar during the first century, black walnut (*Juglans nigra* L.) has long served as the common example of allelopathic effects with its ability to inhibit growth of surrounding plants either through decaying leaves or nuts or from the tree itself (Weir et al. 2004). Researchers have continued to examine allelopathy and the mechanism for biochemical inhibition, which was initially scrutinized by many since differentiation between this effect and plant competition remained uncertain (Weir et al. 2004). Subsequent bioassays involving specific chemical compounds extracted from plants have confirmed that certain species do, in fact, produce biochemicals that can inhibit plant germination and growth in the absence of resource competition (Einhellig 1994a).

With confirmation of allelopathy, many investigations have been conducted in order to determine how best to utilize this effect for possible weed control in agricultural settings (Khanh et al. 2005; Olofsdotter 2001; Weston 1996). The ability to inhibit weed growth through the implementation of cover crops into a crop rotation has been a focal point for this research for several reasons. In addition to weed suppression and control through allelopathy, as well as a mulching effect, cover crops provide substantial environmental benefits such as reduced erosion and water runoff (Price et al. 2006; Truman et al. 2003). Moreover, cover crops are readily available and easily adapted to many agricultural situations. Because of these many benefits, including natural weed suppression through allelopathy, the use of cover crops has become a vital component of sustainable agriculture systems, as well as organic production.

Ensuring sufficient food and fiber production for future generations can be hampered by limited options for weed control, particularly in developing countries where yields are

Allelopathic Weed Suppression Through the Use of Cover Crops 117

Like most allelochemicals, phenolic acids are secondary plant compounds typified by a hydroxylated aromatic ring structure. To date, a number of phenolic acids have been determined to have allelopathic properties and have been measured in extracts from a variety of plant species (Figure 1). Species which have been noted to produce phenolic acids include: rice (*Oryza sativa* L.), wheat (*Triticum aestivum* L.), mango (*Mangifera indica* L.), and spotted knapweed (*Centaurea stoebe* L.) (Bais et al. 2003; Chung et al. 2002; El-Rokiek et al. 2010; Fitter 2003). Many species, such as rice, contain multiple phenolic compounds along with other allelopathic compounds. In two studies, researchers isolated nine individual phenolic acids from rice hull extracts and 14 different phenolic acids from buffalograss [*Buchloe dactyloides* (Nutt.) Engelm] (Chung et al. 2002; Wu et al. 1998). At this time, however, it is not clear to what degree individual allelochemicals interact to produce plant inhibition. Some reports show a synergistic effect when allelochemicals are in a mixture, while other studies indicate decreased plant inhibition in the presence of a mixture when

compared to individual chemical inhibition (Chung et al. 2002; Einhellig 1996).

Caffeic acid Coumaric acid Vanillic acid

Fig. 1. Phenolic acids identified in many plant species, such as oat (*Avena sativa* L.) and rice

Syringic acid *p*-hydrobenzoic Ferulic acid

Although modes of action for allelopathic chemicals are not readily understood for each identified allelochemical, phenolic acids have been the focus of many studies designed to establish the basis of their allelopathy (Putnam 1985). Early research with phenolic acids indicated that some phenolic acids could function though increasing cell membrane permeability, thus affecting ion transport and metabolism (Glass and Dunlop 1974). More recent studies report disruption of cell division and malformed cellular structures in plants

(*Oryza sativa* L.), have been found to have allelopathic properties.

**3.1 Phenolic acids** 

reduced by up to 25% by weed competition. Identifying and describing sustainable weed control measures that can be implemented to reduce weed pressure in a number of settings can help safeguard the productivity of agriculture. Therefore, the objectives of this chapter are to describe the fundamentals of allelopathy and how to utilize allelopathic compounds for weed control through cover crop use. The chapter also highlights many of the identified biochemicals, their structures, and the respective cover crops in which they are found. Lastly, we describe the degree of allelopathic potential for a number of cover crops, as determined by laboratory testing.

#### **2. Production and release of allelopathic compounds**

Allelochemicals enter the environment from plants in a number of ways, such as plant degradation, volatilization, leaching from plant leaves, and from root exudation (Bertin et al. 2003; Weir et al. 2004). During active plant growth, particularly in early growth stages or during periods of stress, root exudation, either through diffusion, ion channels, or vesicle transport, is the primary method for release of many organic and inorganic compounds into the rhizosphere (Battey and Blackbourn 1993; Uren 2000). These compounds serve a multitude of functions such as improving nutrient uptake, root lubrication, plant growth regulation, microorganism defense, and waste removal (Bertin et al. 2003; Fan et al. 1997; Uren 2000).

A large proportion of identified allelochemicals are noted to be secondary compounds formed during photosynthetic processes (Einhellig 1994b; Swain 1977). Since many allelopathic chemicals appear to perform no primary metabolic functions, although some compounds such as cinnamic acid and salicylic acid do serve other functions within a plant, it is unclear at this point as to what regulates the release of these compounds (Einhellig 1994a). Many environmental plant stressors have been observed to increase allelochemical release but not necessarily chemical production (Bertin et al. 2003; Inderjit and Weston 2003; Sterling et al. 1987). Plant stressors such as elevated temperature, reduced water availability, and herbivory may cause increased allelochemical release; however, a definitive correlation between environmental factors and allelopathic compounds has yet to be made (Bertin et al. 2003; Pramanik et al. 2000). Continued research directed at isolating and identifying individual root exudates while manipulating environmental stress factors may help to increase our understanding of allelochemical release into the rhizosphere.

#### **3. Allelopathic compounds**

Many allelochemicals have been identified since experiments began to isolate and determine allelopathic potentials of plant compounds. Compounds that have been identified thus far include a variety of chemical classes such as phenolic acids, coumarins, benzoquinones, terpenoids, glucosinolates, and tannins (Chung et al. 2002; Putnam and Duke 1978; Seigler 1996; Swain 1977; Vyvyan 2002). These and other allelochemicals are found in many plant species from woody to herbaceous plants, grasses and broadleaves, weeds and crops. There are many details left to be determined such as regulation and production stimuli and mode of action for inhibition. It is also not readily understood to what extent allelopathic compounds interact with each other and other chemical compounds within the rhizosphere to inhibit surrounding plants. The following sections present several of the structural classes of recognized allelochemicals as well as specific compounds within each group.

#### **3.1 Phenolic acids**

116 Weed Control

reduced by up to 25% by weed competition. Identifying and describing sustainable weed control measures that can be implemented to reduce weed pressure in a number of settings can help safeguard the productivity of agriculture. Therefore, the objectives of this chapter are to describe the fundamentals of allelopathy and how to utilize allelopathic compounds for weed control through cover crop use. The chapter also highlights many of the identified biochemicals, their structures, and the respective cover crops in which they are found. Lastly, we describe the degree of allelopathic potential for a number of cover crops, as

Allelochemicals enter the environment from plants in a number of ways, such as plant degradation, volatilization, leaching from plant leaves, and from root exudation (Bertin et al. 2003; Weir et al. 2004). During active plant growth, particularly in early growth stages or during periods of stress, root exudation, either through diffusion, ion channels, or vesicle transport, is the primary method for release of many organic and inorganic compounds into the rhizosphere (Battey and Blackbourn 1993; Uren 2000). These compounds serve a multitude of functions such as improving nutrient uptake, root lubrication, plant growth regulation, microorganism defense, and waste removal (Bertin et al. 2003; Fan et al. 1997; Uren 2000).

A large proportion of identified allelochemicals are noted to be secondary compounds formed during photosynthetic processes (Einhellig 1994b; Swain 1977). Since many allelopathic chemicals appear to perform no primary metabolic functions, although some compounds such as cinnamic acid and salicylic acid do serve other functions within a plant, it is unclear at this point as to what regulates the release of these compounds (Einhellig 1994a). Many environmental plant stressors have been observed to increase allelochemical release but not necessarily chemical production (Bertin et al. 2003; Inderjit and Weston 2003; Sterling et al. 1987). Plant stressors such as elevated temperature, reduced water availability, and herbivory may cause increased allelochemical release; however, a definitive correlation between environmental factors and allelopathic compounds has yet to be made (Bertin et al. 2003; Pramanik et al. 2000). Continued research directed at isolating and identifying individual root exudates while manipulating environmental stress factors may help to

Many allelochemicals have been identified since experiments began to isolate and determine allelopathic potentials of plant compounds. Compounds that have been identified thus far include a variety of chemical classes such as phenolic acids, coumarins, benzoquinones, terpenoids, glucosinolates, and tannins (Chung et al. 2002; Putnam and Duke 1978; Seigler 1996; Swain 1977; Vyvyan 2002). These and other allelochemicals are found in many plant species from woody to herbaceous plants, grasses and broadleaves, weeds and crops. There are many details left to be determined such as regulation and production stimuli and mode of action for inhibition. It is also not readily understood to what extent allelopathic compounds interact with each other and other chemical compounds within the rhizosphere to inhibit surrounding plants. The following sections present several of the structural classes

increase our understanding of allelochemical release into the rhizosphere.

of recognized allelochemicals as well as specific compounds within each group.

determined by laboratory testing.

**3. Allelopathic compounds** 

**2. Production and release of allelopathic compounds** 

Like most allelochemicals, phenolic acids are secondary plant compounds typified by a hydroxylated aromatic ring structure. To date, a number of phenolic acids have been determined to have allelopathic properties and have been measured in extracts from a variety of plant species (Figure 1). Species which have been noted to produce phenolic acids include: rice (*Oryza sativa* L.), wheat (*Triticum aestivum* L.), mango (*Mangifera indica* L.), and spotted knapweed (*Centaurea stoebe* L.) (Bais et al. 2003; Chung et al. 2002; El-Rokiek et al. 2010; Fitter 2003). Many species, such as rice, contain multiple phenolic compounds along with other allelopathic compounds. In two studies, researchers isolated nine individual phenolic acids from rice hull extracts and 14 different phenolic acids from buffalograss [*Buchloe dactyloides* (Nutt.) Engelm] (Chung et al. 2002; Wu et al. 1998). At this time, however, it is not clear to what degree individual allelochemicals interact to produce plant inhibition. Some reports show a synergistic effect when allelochemicals are in a mixture, while other studies indicate decreased plant inhibition in the presence of a mixture when compared to individual chemical inhibition (Chung et al. 2002; Einhellig 1996).

Fig. 1. Phenolic acids identified in many plant species, such as oat (*Avena sativa* L.) and rice (*Oryza sativa* L.), have been found to have allelopathic properties.

Although modes of action for allelopathic chemicals are not readily understood for each identified allelochemical, phenolic acids have been the focus of many studies designed to establish the basis of their allelopathy (Putnam 1985). Early research with phenolic acids indicated that some phenolic acids could function though increasing cell membrane permeability, thus affecting ion transport and metabolism (Glass and Dunlop 1974). More recent studies report disruption of cell division and malformed cellular structures in plants

Allelopathic Weed Suppression Through the Use of Cover Crops 119

and Paya 1996; Maddi et al. 1992; Razavi 2011). The broad activity of these compounds has made pharmaceutical use difficult due to the potential for non-target activity. Although allelopathic research has yet to indicate that the broad spectrum activity of coumarins could limit future use of these compounds for weed control, this may require further investigation

Gluconapin Glucobrassicin Progoitrin

Fig. 2. Glucosinolates, allelopathic compounds known to be produced by plants in the Brassicaceae family as well as other families, are produced in both the root and shoot

Glucoiberin Glucotropaeolin Glucoraphenin

Neoglucobrassicin

Gluconasturtin

as research moves forward.

regions of plants.

Sinigrin

exposed to phenolic acids (Li et al. 2010). Reduced respiration and reduced photosynthetic rates, due to decreased photosynthetic products such as chlorophyll, have also been reported in the presence of phenolic acids (Patterson 1981; Yu et al. 2003). Other studies have cited altered plant enzymatic functions, inhibited protein synthesis, and inactivated plant hormones as inhibitory mechanisms from these allelochemicals (Batish et al. 2008; Li et al. 2010). Each mechanism of plant inhibition can lead to the reduced growth and/or death of an exposed plant; however, it is likely multiple functions within a plant are being affected simultaneously due to the mixture of allelochemicals released from a plant species. Despite the extensive research with phenolic acids, target sites for allelochemical activity within affected plant species remain to be determined for many phenolic compounds.

#### **3.2 Glucosinolates**

Glucosinolates occur in many plant species, but are widely known to be produced by species within the Brassicaceae family (Figure 2) (Haramoto and Gallandt 2005; Malik et al. 2008; Mithen 2001). Members of this family include: wild radish (*R*a*phanus raphanistrum* L.), white mustard (*Sinapis alba* L.), turnip (*Brassica campestris* L.), and rapeseed (*Brassica napus*  L.). Glucosinolates, secondary metabolites containing sulfur and nitrogen, are enzymatically hydrolyzed by myrosinase in the presence of water to form isothiocynates, the active allelochemicals (Haramoto and Gallandt 2005; Norsworthy and Meehan 2005; Petersen et al. 2001; Price et al. 2005). Previous research examining extracts from glucosinolate-producing plant species have shown inhibition of other species through reduced germination, reduced seedling emergence and reduced size, as well as delayed seed germination (Al-Khatib et al. 1997; Brown and Morra 1996; Malik et al. 2008; Norsworthy et al. 2007; Wolf et al. 1984). Although specific modes of action have not been thoroughly investigated for each compound, it is evident that some plant species are able to tolerate these allelochemicals more readily than other species (Norsworthy and Meehan 2005). Some suggest that seed size variability plays a role in determining inhibitory effects of these allelochemicals; however, this may not be the only determinant for tolerance to these compounds (Haramoto and Gallandt 2005; Westoby et al. 1996). Future research with these allelopathic compounds will likely seek to answer this question, along with identifying the mode of action for plant inhibition, in order to utilize these compounds more effectively in agricultural production.

#### **3.3 Coumarins**

Coumarin compounds (Figure 3) are found in a range of plant species, particularly from the Apiaceae, Asteraceae and Fabaceae families (Razavi 2011). Coumarins and their derivatives have been identified in plants such as lettuce (*Lactuca sativa* L.), wild oat (*Avena sativa* L.), sweet vernalgrass (*Anthoxanthum odoratum* L.), and a number of other species (Abenavoli et al. 2004; Razavi 2011). Like many other allelochemicals, coumarins have been found to inhibit plant growth by reduced seedling germination and reduced root and shoot growth, likely with interference in photosynthesis, respiration, nutrient uptake and metabolism (Abenavoli et al. 2001; Abenavoli et al. 2004; Razavi et al 2010; Yamamoto 2008).

In addition to plant inhibition, biological activity of coumarins includes antibacterial, nematicidal, antifungal, and insecticidal activity; moreover, pharmacological activity of coumarins has been commonly noted in a number of instances with specific compounds functioning to reduce edema and inflammation (Casley-Smith and Casley-Smith 1992; Hoult

exposed to phenolic acids (Li et al. 2010). Reduced respiration and reduced photosynthetic rates, due to decreased photosynthetic products such as chlorophyll, have also been reported in the presence of phenolic acids (Patterson 1981; Yu et al. 2003). Other studies have cited altered plant enzymatic functions, inhibited protein synthesis, and inactivated plant hormones as inhibitory mechanisms from these allelochemicals (Batish et al. 2008; Li et al. 2010). Each mechanism of plant inhibition can lead to the reduced growth and/or death of an exposed plant; however, it is likely multiple functions within a plant are being affected simultaneously due to the mixture of allelochemicals released from a plant species. Despite the extensive research with phenolic acids, target sites for allelochemical activity within

Glucosinolates occur in many plant species, but are widely known to be produced by species within the Brassicaceae family (Figure 2) (Haramoto and Gallandt 2005; Malik et al. 2008; Mithen 2001). Members of this family include: wild radish (*R*a*phanus raphanistrum* L.), white mustard (*Sinapis alba* L.), turnip (*Brassica campestris* L.), and rapeseed (*Brassica napus*  L.). Glucosinolates, secondary metabolites containing sulfur and nitrogen, are enzymatically hydrolyzed by myrosinase in the presence of water to form isothiocynates, the active allelochemicals (Haramoto and Gallandt 2005; Norsworthy and Meehan 2005; Petersen et al. 2001; Price et al. 2005). Previous research examining extracts from glucosinolate-producing plant species have shown inhibition of other species through reduced germination, reduced seedling emergence and reduced size, as well as delayed seed germination (Al-Khatib et al. 1997; Brown and Morra 1996; Malik et al. 2008; Norsworthy et al. 2007; Wolf et al. 1984). Although specific modes of action have not been thoroughly investigated for each compound, it is evident that some plant species are able to tolerate these allelochemicals more readily than other species (Norsworthy and Meehan 2005). Some suggest that seed size variability plays a role in determining inhibitory effects of these allelochemicals; however, this may not be the only determinant for tolerance to these compounds (Haramoto and Gallandt 2005; Westoby et al. 1996). Future research with these allelopathic compounds will likely seek to answer this question, along with identifying the mode of action for plant inhibition, in order to utilize these compounds more effectively in agricultural production.

Coumarin compounds (Figure 3) are found in a range of plant species, particularly from the Apiaceae, Asteraceae and Fabaceae families (Razavi 2011). Coumarins and their derivatives have been identified in plants such as lettuce (*Lactuca sativa* L.), wild oat (*Avena sativa* L.), sweet vernalgrass (*Anthoxanthum odoratum* L.), and a number of other species (Abenavoli et al. 2004; Razavi 2011). Like many other allelochemicals, coumarins have been found to inhibit plant growth by reduced seedling germination and reduced root and shoot growth, likely with interference in photosynthesis, respiration, nutrient uptake and metabolism

In addition to plant inhibition, biological activity of coumarins includes antibacterial, nematicidal, antifungal, and insecticidal activity; moreover, pharmacological activity of coumarins has been commonly noted in a number of instances with specific compounds functioning to reduce edema and inflammation (Casley-Smith and Casley-Smith 1992; Hoult

(Abenavoli et al. 2001; Abenavoli et al. 2004; Razavi et al 2010; Yamamoto 2008).

affected plant species remain to be determined for many phenolic compounds.

**3.2 Glucosinolates** 

**3.3 Coumarins** 

and Paya 1996; Maddi et al. 1992; Razavi 2011). The broad activity of these compounds has made pharmaceutical use difficult due to the potential for non-target activity. Although allelopathic research has yet to indicate that the broad spectrum activity of coumarins could limit future use of these compounds for weed control, this may require further investigation as research moves forward.

Fig. 2. Glucosinolates, allelopathic compounds known to be produced by plants in the Brassicaceae family as well as other families, are produced in both the root and shoot regions of plants.

Allelopathic Weed Suppression Through the Use of Cover Crops 121

From the sunflower plant (*Helianthus annuus* L.), several compounds have been identified as being allelopathic (Leather 1983; Vyvyan 2002). The heliannuols are classified as phenolic sesquiterpenes and are noted for allelopathic as well as pharmacological activity (Vyvyan 2002). In addition to having been isolated from the sunflower, similarly structured compounds have been detected in animal species as well (Harrison and Crews 1997). Most notable about heliannuolic compounds is their ability to suppress plant growth at relatively low concentrations. Although they have been shown to inhibit growth of many broadleaf weed species, heliannuols appear to have a stimulating effect upon monocotyleden species (Weidenhamer 1996; Vyvyan 2002). This aspect of heliannuol activity may prove difficult

Fig. 4. Compounds, such as DIMBOA, heliannuol A, and sorgoleone, continue to be studied

Sorgoleone

DIMBOA Heliannuol A

Benzoquinone compounds, primarily sorgoleone, isolated from sorghum [*Sorghum bicolor*  (L.) Moench], have also been determined to be highly allelopathic (Netzly et al. 1998). Research with this compound indicates plant growth inhibition is achieved through disruption of photosynthesis as well as reduced chlorophyll development (Einhellig and Souza 1992). Like some other compounds, sorgoleone exhibits selective activity with inhibition of many germinating seedlings but little activity against certain species such as morningglory (*Ipomoea spp.*) (Nimbal et al. 1996). Research conducted with sorghum root exudates compares sorgoleone activity to that of the herbicide, diuron, but has many target

for their allelopathic properties.

when developing weed control applications of these compounds.

#### **3.4 Other allelopathic compounds**

Many other allelochemicals have been detected in a wide range of species; however, a few compounds have been more widely researched. Classes of allelochemicals under thorough investigation, such as the benzoxazinoids, heliannuols, and benzoquinones, offer potential benefits for weed control in agricultural systems (Figure 4) (Macias et al. 2005; Vyvyan 2002). These classes, described briefly below, represent only a few of the many other compounds that may one day provide substantial weed suppression through allelopathy.

Benzoxazinoid compounds, identified in cereal grains such as wheat and rye, include DIBOA [2,4-dihydroxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] and DIMBOA [2,3-dihydroxy-7 methoxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] (Burgos and Talbert 2000; Macias et al. 2005). These compounds are easily degraded into other allelopathic forms, BOA (2 benzoxazolinone) and MBOA (7-methoxy-2-benzoxazolinone), within the soil and can diminish plant germination and growth (Barnes et al. 1987; Burgos and Talbert 2000). In light of the allelopathic properties of BOA and MBOA, it is now recognized that continued research efforts are needed to understand the role of breakdown products of initial allelochemicals in inhibiting plant growth (Macias et al. 2005).

Umbelliferone Scopoletin Scopolin

Imperatorin Psoralen Bergapten

Fig. 3. Coumarins and their subgroups have been identified as allelopathic compounds in

Many other allelochemicals have been detected in a wide range of species; however, a few compounds have been more widely researched. Classes of allelochemicals under thorough investigation, such as the benzoxazinoids, heliannuols, and benzoquinones, offer potential benefits for weed control in agricultural systems (Figure 4) (Macias et al. 2005; Vyvyan 2002). These classes, described briefly below, represent only a few of the many other compounds that may one day provide substantial weed suppression through

Benzoxazinoid compounds, identified in cereal grains such as wheat and rye, include DIBOA [2,4-dihydroxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] and DIMBOA [2,3-dihydroxy-7 methoxy-(2*H*)-1,4-benzoxazin-3(4*H*)-one] (Burgos and Talbert 2000; Macias et al. 2005). These compounds are easily degraded into other allelopathic forms, BOA (2 benzoxazolinone) and MBOA (7-methoxy-2-benzoxazolinone), within the soil and can diminish plant germination and growth (Barnes et al. 1987; Burgos and Talbert 2000). In light of the allelopathic properties of BOA and MBOA, it is now recognized that continued research efforts are needed to understand the role of breakdown products of initial

several plant families including Apiaceae and Fabaceae.

allelochemicals in inhibiting plant growth (Macias et al. 2005).

**3.4 Other allelopathic compounds** 

allelopathy.

From the sunflower plant (*Helianthus annuus* L.), several compounds have been identified as being allelopathic (Leather 1983; Vyvyan 2002). The heliannuols are classified as phenolic sesquiterpenes and are noted for allelopathic as well as pharmacological activity (Vyvyan 2002). In addition to having been isolated from the sunflower, similarly structured compounds have been detected in animal species as well (Harrison and Crews 1997). Most notable about heliannuolic compounds is their ability to suppress plant growth at relatively low concentrations. Although they have been shown to inhibit growth of many broadleaf weed species, heliannuols appear to have a stimulating effect upon monocotyleden species (Weidenhamer 1996; Vyvyan 2002). This aspect of heliannuol activity may prove difficult when developing weed control applications of these compounds.

Fig. 4. Compounds, such as DIMBOA, heliannuol A, and sorgoleone, continue to be studied for their allelopathic properties.

Benzoquinone compounds, primarily sorgoleone, isolated from sorghum [*Sorghum bicolor*  (L.) Moench], have also been determined to be highly allelopathic (Netzly et al. 1998). Research with this compound indicates plant growth inhibition is achieved through disruption of photosynthesis as well as reduced chlorophyll development (Einhellig and Souza 1992). Like some other compounds, sorgoleone exhibits selective activity with inhibition of many germinating seedlings but little activity against certain species such as morningglory (*Ipomoea spp.*) (Nimbal et al. 1996). Research conducted with sorghum root exudates compares sorgoleone activity to that of the herbicide, diuron, but has many target

Allelopathic Weed Suppression Through the Use of Cover Crops 123

(*Raphanus sativus* L.), an indicator species, and cotton (*Gossypium hirsutum* L.) established levels of inhibition for radicle elongation by extracts from cover crops, primarily legumes

Legume cover crops have the ability to fix atmospheric nitrogen that potentially provides a nitrogen source to the subsequent crop without the need for additional fertilizer applications (Balkcom et al. 2007; Hartwig and Ammon 2002). Legume species such as vetch (*Vicia villosa* Roth), clover (*Trifolium spp.*), black medic (*Medicago lupulina* L.), and winter pea (*Pisum sativum* L.) are typically used as cover crops in agricultural production in the United States (Figure 5) (SARE 2007). Other legume crops beginning to be researched as possible choices for cover crops are sunn hemp (*Crotalaria juncea* L.) and white lupin (*Lupinus albus*  L.); however, their availability and use are not as widespread as the previously mentioned legumes. In addition to being a nitrogen source for primary crops, legume covers provide a weed control potential. Due to the rapid degradation of legume residue on the soil surface in comparison to cereal grain residue, weed control through a physical barrier may not last

Fig. 5. Legume cover crops, such as white lupin (in mixture with black oats), provide weed

Determining allelopathic effects of legume cover crop extracts concluded that legume covers did inhibit radish and cotton radicle elongation; however, cotton root exhibited less inhibition than that of radish for all included crops (Price et al. 2008) (Figure 6). In our research, hairy vetch had the greatest inhibition while winter pea had the least effect on germinating seedlings. It is important to note that different varieties of cover crops are

suppression and nitrogen benefits to the subsequent cash crop.

and cereal grains.

as long into the season as other cover crops.

sites (Nimbal et al. 1996; Rimando et al. 1998). Thus far, characteristics of sorgoleone show that it is a promising compound for development into a natural herbicide as an alternative to synthetic herbicides.

#### **4. Weed control through allelopathy**

Ongoing research into allelopathy seeks to better understand the mechanisms of allelopathy in order to make use of these naturally occurring weed suppressants within agricultural areas. Benefits offered by employing allelopathy as some form of weed control could aid in developing more sustainable agricultural systems for future generations (Einhellig 1994a). Current efforts focus primarily on natural herbicide production and cover crops. Although these concepts are being utilized to some degree, there remains a great deal of research to fully utilize the potential of allelopathy.

The role of naturally derived compounds, or synthetically produced mimics, for use as pesticides has been widely adopted, particularly for insect control. Several plant derived compounds, such as pyrethrum, neem, and nicotine, are important chemicals for insect control in many areas (Isman 2006). Herbicide potentials of isolated plant extracts have been indicated by a number of researchers but to date, few have been marketed. Synthetic compounds, such as cinmethylin, and mesotrione, were developed based upon plantderived allelochemicals, but release of subsequent plant-based herbicides has lagged (Lee et al. 1997; Macias et al. 2004; Secor 1994; Vyvyan 2002). Slow production and release of herbicides developed in this manner are most likely attributed to limited understanding of the modes of action for many identified allelochemicals. To date, a number of allelochemicals have been isolated and investigated to develop natural herbicides with these compounds. Understanding the mode of action for plant inhibition may aid in the development of new products for the market.

A great deal of research has been devoted to the use of cover crops for weed control. Until recently, however, the allelopathic potential of cover crops has received less attention due, in part, to the lack of knowledge about allelopathy in general. As the functions of allelopathic compounds are beginning to be understood, more focus is being given to the allelochemicals within cover crops. In agricultural settings, cover crops have been in use for a number of years as a ground cover to slow erosion and water runoff as well as to impede germination of weed seed by providing a physical barrier (Kaspar et al. 2001; Price et al. 2008; Sarrantonio and Gallandt 2003). The growing need for sustainable agricultural systems has necessitated increased cover crop research to better utilize these covers for effective weed control. As a result, recent investigations have sought to understand the role of allelopathy for weed suppression within various cover crops (Burgos and Talbert 2000; Khanh et al. 2005; Price et al. 2008; Walters and Young 2008).

#### **5. Allelopathic potential of cover crops**

Determining allelopathic potential of exudates of plant species can be difficult and time consuming to complete. Bioassays are generally conducted to identify allelopathic properties of compounds in order to differentiate between allelopathy and mulching effects. Our research has focused on determining the extent of allelopathic effects of available cover crops on weed species as well as crop species. Extract-agar bioassays conducted with radish

sites (Nimbal et al. 1996; Rimando et al. 1998). Thus far, characteristics of sorgoleone show that it is a promising compound for development into a natural herbicide as an alternative

Ongoing research into allelopathy seeks to better understand the mechanisms of allelopathy in order to make use of these naturally occurring weed suppressants within agricultural areas. Benefits offered by employing allelopathy as some form of weed control could aid in developing more sustainable agricultural systems for future generations (Einhellig 1994a). Current efforts focus primarily on natural herbicide production and cover crops. Although these concepts are being utilized to some degree, there remains a great deal of research to

The role of naturally derived compounds, or synthetically produced mimics, for use as pesticides has been widely adopted, particularly for insect control. Several plant derived compounds, such as pyrethrum, neem, and nicotine, are important chemicals for insect control in many areas (Isman 2006). Herbicide potentials of isolated plant extracts have been indicated by a number of researchers but to date, few have been marketed. Synthetic compounds, such as cinmethylin, and mesotrione, were developed based upon plantderived allelochemicals, but release of subsequent plant-based herbicides has lagged (Lee et al. 1997; Macias et al. 2004; Secor 1994; Vyvyan 2002). Slow production and release of herbicides developed in this manner are most likely attributed to limited understanding of the modes of action for many identified allelochemicals. To date, a number of allelochemicals have been isolated and investigated to develop natural herbicides with these compounds. Understanding the mode of action for plant inhibition may aid in the

A great deal of research has been devoted to the use of cover crops for weed control. Until recently, however, the allelopathic potential of cover crops has received less attention due, in part, to the lack of knowledge about allelopathy in general. As the functions of allelopathic compounds are beginning to be understood, more focus is being given to the allelochemicals within cover crops. In agricultural settings, cover crops have been in use for a number of years as a ground cover to slow erosion and water runoff as well as to impede germination of weed seed by providing a physical barrier (Kaspar et al. 2001; Price et al. 2008; Sarrantonio and Gallandt 2003). The growing need for sustainable agricultural systems has necessitated increased cover crop research to better utilize these covers for effective weed control. As a result, recent investigations have sought to understand the role of allelopathy for weed suppression within various cover crops (Burgos and Talbert 2000;

Determining allelopathic potential of exudates of plant species can be difficult and time consuming to complete. Bioassays are generally conducted to identify allelopathic properties of compounds in order to differentiate between allelopathy and mulching effects. Our research has focused on determining the extent of allelopathic effects of available cover crops on weed species as well as crop species. Extract-agar bioassays conducted with radish

to synthetic herbicides.

**4. Weed control through allelopathy** 

fully utilize the potential of allelopathy.

development of new products for the market.

Khanh et al. 2005; Price et al. 2008; Walters and Young 2008).

**5. Allelopathic potential of cover crops** 

(*Raphanus sativus* L.), an indicator species, and cotton (*Gossypium hirsutum* L.) established levels of inhibition for radicle elongation by extracts from cover crops, primarily legumes and cereal grains.

Legume cover crops have the ability to fix atmospheric nitrogen that potentially provides a nitrogen source to the subsequent crop without the need for additional fertilizer applications (Balkcom et al. 2007; Hartwig and Ammon 2002). Legume species such as vetch (*Vicia villosa* Roth), clover (*Trifolium spp.*), black medic (*Medicago lupulina* L.), and winter pea (*Pisum sativum* L.) are typically used as cover crops in agricultural production in the United States (Figure 5) (SARE 2007). Other legume crops beginning to be researched as possible choices for cover crops are sunn hemp (*Crotalaria juncea* L.) and white lupin (*Lupinus albus*  L.); however, their availability and use are not as widespread as the previously mentioned legumes. In addition to being a nitrogen source for primary crops, legume covers provide a weed control potential. Due to the rapid degradation of legume residue on the soil surface in comparison to cereal grain residue, weed control through a physical barrier may not last as long into the season as other cover crops.

Fig. 5. Legume cover crops, such as white lupin (in mixture with black oats), provide weed suppression and nitrogen benefits to the subsequent cash crop.

Determining allelopathic effects of legume cover crop extracts concluded that legume covers did inhibit radish and cotton radicle elongation; however, cotton root exhibited less inhibition than that of radish for all included crops (Price et al. 2008) (Figure 6). In our research, hairy vetch had the greatest inhibition while winter pea had the least effect on germinating seedlings. It is important to note that different varieties of cover crops are

Allelopathic Weed Suppression Through the Use of Cover Crops 125

Fig. 7. Cotton growing in rolled black oat residue. Cereal grain cover crops, like black oat and rye, can be utilized to achieve a large quantity of plant residue on the soil surface.

> Radish Radicle (mm) Cotton Radicle (mm)

Fig. 8. Radish and cotton radicle elongation is reduced by cereal grain cover crops.

Control Black oat Rye Triticale Wheat

10

15

20

25

30

35

available for use in agricultural systems and the varieties of one species may differ in level of allelopathy. Although under field conditions, allelopathic performance of these species may fluctuate, it is apparent that these cover crops can provide additional weed control measures over systems that do not include a cover crop.

Fig. 6. Legume cover crops affect radicle elongation of different plant species to varying degrees.

Cereal grain crops such as black oat (*Avena strigosa* Schreb), rye, triticale (*X Triticosecale*  Wittmack), and wheat, are utilized frequently in conservation systems as cover crops with effective ground cover and weed suppression (Figure 7). Rye is a commonly used cereal cover crop due to its ability to be sown later in the season while maintaining successful growth and its biomass production capability. With increased biomass on the soil surface, weed suppression will be increased as well. Cereal crops will also decay more slowly than more herbaceous plant species and provide some ground cover, and allelochemical release, further into the growing season. Additionally, rye has been noted to be less affected by plant diseases than other cover crops, and aids in reducing insect pests within a system (Wingard 1996).

Like legumes, cereal grain crop exudates in our study were able to significantly inhibit radicle elongation compared to the control (Figure 8). The disparity between radish and cotton radicle inhibition for each cover crop studied suggests that minimized interference with primary crops and increased weed suppression potential could be achieved with the use of cereal grain crops. These allelopathic effects, however, may be amplified or diminished depending on the field environment, plant stress levels, cover crop variety, and a number of other factors involved in determining allelochemical levels. Nevertheless, this research provides a base of allelopathic concentrations and impacts from various cover crops and may be an initial consideration when choosing a cover crop for inclusion in a system.

available for use in agricultural systems and the varieties of one species may differ in level of allelopathy. Although under field conditions, allelopathic performance of these species may fluctuate, it is apparent that these cover crops can provide additional weed control

Fig. 6. Legume cover crops affect radicle elongation of different plant species to varying

Cereal grain crops such as black oat (*Avena strigosa* Schreb), rye, triticale (*X Triticosecale*  Wittmack), and wheat, are utilized frequently in conservation systems as cover crops with effective ground cover and weed suppression (Figure 7). Rye is a commonly used cereal cover crop due to its ability to be sown later in the season while maintaining successful growth and its biomass production capability. With increased biomass on the soil surface, weed suppression will be increased as well. Cereal crops will also decay more slowly than more herbaceous plant species and provide some ground cover, and allelochemical release, further into the growing season. Additionally, rye has been noted to be less affected by plant diseases than other cover crops, and aids in reducing insect pests within a system (Wingard 1996).

Lupin Sunn

Hemp

Winter pea

Radish Radicle (mm) Cotton Radicle (mm)

Like legumes, cereal grain crop exudates in our study were able to significantly inhibit radicle elongation compared to the control (Figure 8). The disparity between radish and cotton radicle inhibition for each cover crop studied suggests that minimized interference with primary crops and increased weed suppression potential could be achieved with the use of cereal grain crops. These allelopathic effects, however, may be amplified or diminished depending on the field environment, plant stress levels, cover crop variety, and a number of other factors involved in determining allelochemical levels. Nevertheless, this research provides a base of allelopathic concentrations and impacts from various cover crops and may be an initial

consideration when choosing a cover crop for inclusion in a system.

measures over systems that do not include a cover crop.

degrees.

10

Control Black

medic

Crimson clover

Hairy vetch

15

20

25

30

35

Fig. 7. Cotton growing in rolled black oat residue. Cereal grain cover crops, like black oat and rye, can be utilized to achieve a large quantity of plant residue on the soil surface.

Fig. 8. Radish and cotton radicle elongation is reduced by cereal grain cover crops.

Allelopathic Weed Suppression Through the Use of Cover Crops 127

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#### **6. Conclusions**

The growing demand for sustainable agricultural systems requires that researchers reevaluate current production methods and inputs. To ensure continued productivity and potentially reduce synthetic herbicide requirements, allelopathy has become a focal point for research in the agricultural community. Although, many questions have yet to be resolved, the utilization of allelochemicals for weed suppression remains a promising avenue for reducing herbicide usage. Whether through the development of natural herbicides from isolated allelochemicals or through the application of cover crops with allelopathic properties, allelopathy will most likely be a factor in providing sustainable systems in the future.

#### **7. References**


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**6. Conclusions** 

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**Part 2** 

**Integrated Chemical Weed Management** 

