**3. Weed interference and critical period**

#### **3.1 Introduction**

Weeds compete with crop plants for water, light nutrients and space and cause considerable yield losses. Integrate weed management (IWM) involves a combination of cultural, mechanical, biological, genetic and chemical methods for effective and economical weed control (Swanton and Weise, 1991). The principles of IWM should provide the foundation for developing optimum weed control systems and efficient use of herbicides. The critical period for weed control (CPWC) is a key component of an IWM program. Weeds are limiting factors in sugar beet production (Cooke and Scott, 1993). Integrated weed control management is necessary for minimizing weeds interference and maximizing the crop yield (Schweizer, 1983; Cooke and Scott, 1993).

Sugar Beet Weeds in Tadla Region (Morocco):

Fisher's protected LSD (α = 0.05).

were measured at the regional sugar factory.

**3.3.1 Effect of Weed free and weedy periods on weed dry matter** 

**3.3 Results and discussion** 

significant difference (Not shown).

generally in low weeds density.

**3.2.4 Measurements** 

**3.2.3 Experimental design and statistical analysis** 

10 m long.

Species Encountered, Interference and Chemical Control 389

9, 11, 13, 17 and 21 weeks after emergence (WAE) of sugar beet. In weed infested treatments, weeds were allowed to interfere with sugar beet crop 4, 7, 9, 11, 13, 17 and 21 weeks after emergence sugar beet crop. Two control treatments (full-season control of weeds and fullseason interference of weeds) were also included. Individual plots consisted of 10 rows, each

The experiment was a randomized complete block design with four replicates. Data on weeds and on sugar beet growth parameters and yield components were subjected to an analysis of variance using statistical STATITCF software. The means were compared using

Weed density is not as reliable as biomass to assess weed interference in a crop (Scott et aI., 1979; Tomer et aI., 1991; Wilson and Peters, 1982), especially for species which have a high capacity to compensate for low densities through tillering and branching. Therefore, the impact of weed-free and weedy duration on weed growth and on crop growth and crop yield was assessed through weed dry weight. Weed dry weight were measured during the entire growing season for all individual plots. Four 0.5 m x 0.5m quadrates per plot were placed randomly over the plot. Weeds within the sampling area were removed by hand, taken to laboratory and dried at 60 C for 48 h to determine total weed dry weight. Sugar beet growth was assessed at the same time as weed sampling. Six sugar beet plants without root were taken randomly in plot but not on central rows that served for estimating yield. The number of leaf per plant, leaf area and dry matter was determined. Because of unavailability of an electronic leaf area meter, a graduated table was used for measuring leaf area. Sucrose percentage and the concentration of impurities (sodium, potassium, amino-N)

The dominant weeds observed in 2003 were volunteer wheat (ADI = 4), *Phalaris brachystachys* Link.(3), *Avena sterilis* L. (2), *Cichorium endivia* L. (4), *Papaver rhoeas* L. (3), *Ridolfia segetum* L. (3), *Sinapis arvensis* L. (3), and *Galium tricornitum* Dandy (2). With the exception of field bindweed (*Convolvulus arvensis* L.) (3), the same weed species were dominant in 2004. Weed free periods resulted in lower weed dry matter and weedy periods resulted in high weed dry matter (Figure 2). Maximum total weed dry weight generally decreased as weed-free duration was increased. The statistical analysis showed a highly

These findings are similar to those observed by Salehi et al. (2006), Rzozi (1993) and Alaoui et al. (2003). Weed growth was reduced drastically after a weed free duration greater than 17 WAE in both years. Same results were obtained for all the two years 2003 an 2004. For the later, weed dry matter was relatively lower because the later date of sowing results

The critical period of weed interference refers to the period during which a crop must be kept free of weeds in order to prevent yield loss. It represents the time interval falling between two separate components: (a) the minimum length of time after seeding that a crop must be kept weed-free so that later-emerging weeds do not reduce yield, and (b) the maximum length of time that weeds which emerge with the crop can remain before they become large enough to compete for growth resources (Radosevich and Holt, 1984; Zimdahl, 1988; Weaver *et al*., 1992; Baziramakenga and Leroux, 1994; Ghadiri, 1996).

Sugar beet can tolerate weeds until 2-8 weeks after emergence, depending on the weed species, planting date, the time of weed emergence relative to crop and environmental conditions (Cooke and Scott, 1993). The presence of weeds can decrease sugar beet yield by 90%. For example, a single presence of barnyardgrass *Echinochloa crus-galli* (L.) *Beauv.* plant per 1.5 m2 resulted in yield reduction of 5 to 15 % (Norris, 1996). The earliest date at which weeding could cease in sugar beet without significant yield loss has been shown to be between 4 and 12 weeks, depending on sowing date, rainfall and weed infestation (Link and Koch, 1984; Scott et aI., 1979; Singh et aI., 1996). Studies on the competitive effect of weeds in sugar beet have been numerous under temperate climates (Dawson, 1965; Farahbakhsh and Murphy, 1986; Schweizer and Dexter, 1987; Scott et aI., 1979; Zimdahl and Fertig, 1967). Continuous post-planting hand-weeding for 17 weeks and 15 weeks in 1990, and for 15 weeks and 12.5 weeks in 1991 were required to limit sugar beet root yield loss to 5% and 10%, respectively In Gharb region (Alaoui et al., 2003). Based on 10% loss of yield, the beginning of the critical period of weed control (CPWC) was 25 and 5 days after planting for the first year and the second year, respectively. On this basis, the end of the critical period of weed control was 78 days for the first year and 88 after planting for the second year (Salehi et al., 2006).

This research was conducted to study (i) the effect of weed competition on sugar beet growth parameters and (ii) determine the minimum period sugar beet should be kept weed-free after planting (CPWC) in the Tadla region to limit yield loss from late emerging weeds

#### **3.2 Material and methods**

#### **3.2.1 Experimental site localization and characterization**

Field experiment was conducted during two growth seasons 2003- 2004 and 2004-2005 at Afourer experimental station of the National Institute of Agricultural Research in Tadla region. The soil characteristic are as follows: 2.72 % organic matter, 11% sand, 37.2% silt, 51.6 % clay, and pH 8.1. Plots were plowed, disked three times and harrowed for seedbed preparation. Sugar beet cv. 'lydia', a mono germ variety, was seeded manually in a 2 cm deep in 70-cm wide rows with a spacing of 10 cm between seeds (population of 83,000 plants/ha) on October 15 in 2003 and November 25 in 2004.

Fertilization, irrigation and diseases and predators control were achieved in experimental plots according to those recommended by the sugar regional comity.

#### **3.2.2 Competition duration**

To determine the critical period of weed control in sugar beet, an experiment was conducted and consisting of 16 treatments. Weed free treatments included the removal of weeds at 4, 7, 9, 11, 13, 17 and 21 weeks after emergence (WAE) of sugar beet. In weed infested treatments, weeds were allowed to interfere with sugar beet crop 4, 7, 9, 11, 13, 17 and 21 weeks after emergence sugar beet crop. Two control treatments (full-season control of weeds and fullseason interference of weeds) were also included. Individual plots consisted of 10 rows, each 10 m long.

#### **3.2.3 Experimental design and statistical analysis**

The experiment was a randomized complete block design with four replicates. Data on weeds and on sugar beet growth parameters and yield components were subjected to an analysis of variance using statistical STATITCF software. The means were compared using Fisher's protected LSD (α = 0.05).

#### **3.2.4 Measurements**

388 Herbicides – Properties, Synthesis and Control of Weeds

The critical period of weed interference refers to the period during which a crop must be kept free of weeds in order to prevent yield loss. It represents the time interval falling between two separate components: (a) the minimum length of time after seeding that a crop must be kept weed-free so that later-emerging weeds do not reduce yield, and (b) the maximum length of time that weeds which emerge with the crop can remain before they become large enough to compete for growth resources (Radosevich and Holt, 1984;

Sugar beet can tolerate weeds until 2-8 weeks after emergence, depending on the weed species, planting date, the time of weed emergence relative to crop and environmental conditions (Cooke and Scott, 1993). The presence of weeds can decrease sugar beet yield by 90%. For example, a single presence of barnyardgrass *Echinochloa crus-galli* (L.) *Beauv.* plant per 1.5 m2 resulted in yield reduction of 5 to 15 % (Norris, 1996). The earliest date at which weeding could cease in sugar beet without significant yield loss has been shown to be between 4 and 12 weeks, depending on sowing date, rainfall and weed infestation (Link and Koch, 1984; Scott et aI., 1979; Singh et aI., 1996). Studies on the competitive effect of weeds in sugar beet have been numerous under temperate climates (Dawson, 1965; Farahbakhsh and Murphy, 1986; Schweizer and Dexter, 1987; Scott et aI., 1979; Zimdahl and Fertig, 1967). Continuous post-planting hand-weeding for 17 weeks and 15 weeks in 1990, and for 15 weeks and 12.5 weeks in 1991 were required to limit sugar beet root yield loss to 5% and 10%, respectively In Gharb region (Alaoui et al., 2003). Based on 10% loss of yield, the beginning of the critical period of weed control (CPWC) was 25 and 5 days after planting for the first year and the second year, respectively. On this basis, the end of the critical period of weed control was 78 days for the first year and 88 after planting for the second year (Salehi

This research was conducted to study (i) the effect of weed competition on sugar beet growth parameters and (ii) determine the minimum period sugar beet should be kept weed-free after planting (CPWC) in the Tadla region to limit yield loss from late emerging

Field experiment was conducted during two growth seasons 2003- 2004 and 2004-2005 at Afourer experimental station of the National Institute of Agricultural Research in Tadla region. The soil characteristic are as follows: 2.72 % organic matter, 11% sand, 37.2% silt, 51.6 % clay, and pH 8.1. Plots were plowed, disked three times and harrowed for seedbed preparation. Sugar beet cv. 'lydia', a mono germ variety, was seeded manually in a 2 cm deep in 70-cm wide rows with a spacing of 10 cm between seeds (population of 83,000

Fertilization, irrigation and diseases and predators control were achieved in experimental

To determine the critical period of weed control in sugar beet, an experiment was conducted and consisting of 16 treatments. Weed free treatments included the removal of weeds at 4, 7,

Zimdahl, 1988; Weaver *et al*., 1992; Baziramakenga and Leroux, 1994; Ghadiri, 1996).

et al., 2006).

**3.2 Material and methods** 

**3.2.2 Competition duration** 

**3.2.1 Experimental site localization and characterization** 

plants/ha) on October 15 in 2003 and November 25 in 2004.

plots according to those recommended by the sugar regional comity.

weeds

Weed density is not as reliable as biomass to assess weed interference in a crop (Scott et aI., 1979; Tomer et aI., 1991; Wilson and Peters, 1982), especially for species which have a high capacity to compensate for low densities through tillering and branching. Therefore, the impact of weed-free and weedy duration on weed growth and on crop growth and crop yield was assessed through weed dry weight. Weed dry weight were measured during the entire growing season for all individual plots. Four 0.5 m x 0.5m quadrates per plot were placed randomly over the plot. Weeds within the sampling area were removed by hand, taken to laboratory and dried at 60 C for 48 h to determine total weed dry weight. Sugar beet growth was assessed at the same time as weed sampling. Six sugar beet plants without root were taken randomly in plot but not on central rows that served for estimating yield. The number of leaf per plant, leaf area and dry matter was determined. Because of unavailability of an electronic leaf area meter, a graduated table was used for measuring leaf area. Sucrose percentage and the concentration of impurities (sodium, potassium, amino-N) were measured at the regional sugar factory.

#### **3.3 Results and discussion**

#### **3.3.1 Effect of Weed free and weedy periods on weed dry matter**

The dominant weeds observed in 2003 were volunteer wheat (ADI = 4), *Phalaris brachystachys* Link.(3), *Avena sterilis* L. (2), *Cichorium endivia* L. (4), *Papaver rhoeas* L. (3), *Ridolfia segetum* L. (3), *Sinapis arvensis* L. (3), and *Galium tricornitum* Dandy (2). With the exception of field bindweed (*Convolvulus arvensis* L.) (3), the same weed species were dominant in 2004. Weed free periods resulted in lower weed dry matter and weedy periods resulted in high weed dry matter (Figure 2). Maximum total weed dry weight generally decreased as weed-free duration was increased. The statistical analysis showed a highly significant difference (Not shown).

These findings are similar to those observed by Salehi et al. (2006), Rzozi (1993) and Alaoui et al. (2003). Weed growth was reduced drastically after a weed free duration greater than 17 WAE in both years. Same results were obtained for all the two years 2003 an 2004. For the later, weed dry matter was relatively lower because the later date of sowing results generally in low weeds density.

Sugar Beet Weeds in Tadla Region (Morocco):

Fig. 3. Effect of weeds on sugar beet leaf number.

Fig. 4. Effect of weeds on sugar beet leaf surface.

1987).

Species Encountered, Interference and Chemical Control 391

other countries, weeds also seriously suppress sugar beet yield (Schweizer and Dexter,

Fig. 2. Effect of competition duration on weed dry matter.

#### **3.3.2 Effect of weed free and weedy periods on sugar beet growth parameters**

All sugar beet growth parameters were affected by the presence of weeds. Effectively, the sugar beet leaf number decreased as weedy periods increased and in contrast it increased as weed free periods increased (Figure 3). Also, the leaf area decreased as weedy periods increased. This parameter was highly significantly reduced because of the important competitive effect of weeds. (Figure 4). The crop leaf dry matter was also significantly reduced by the weed competitive effect. The longer the weedy period the lower sugar beet dry matter. The later increased as the weed free period increased (Figure 5). These results confirm those of Alaoui et al. (2003) reporting that the leaf area and the other growth parameters are vigorously decreased by the competitive effect of weeds.

#### **3.3.3 Effect of weed free and weedy periods on sugar beet yield, on sugar yield and sugar content**

Weed infestation reduced root yield in all treatments. The presence of weeds during the entire growing season decreased root yield by 97.6 % and 68.9 % in 2003 and 2004, respectively, as compared to full season weed free check. Although sugar content did not show any significant difference between various treatments in both years, weed infestation decreased sugar yield, their corresponding yields decreased considerably in infested treatments. For example, season-long weed infestation decreased sugar yield by 89.8% and 81.1 % in 2003 and 2004, respectively, as compared to weed free check (data not shown). The concentration of sugar beet impurities such as potassium, sodium and amino nitrogen were not affected by weed competition (data not shown).

In most years in Morocco, weeds can cause more than 75% yield reduction (Rzozi, unpublished data; Rzozi et al., 1990). Such reductions indicate complete crop failure because small sugar beet roots produced under severe weed competition cannot be processed. In

Fig. 2. Effect of competition duration on weed dry matter.

not affected by weed competition (data not shown).

**sugar content** 

**3.3.2 Effect of weed free and weedy periods on sugar beet growth parameters** 

parameters are vigorously decreased by the competitive effect of weeds.

All sugar beet growth parameters were affected by the presence of weeds. Effectively, the sugar beet leaf number decreased as weedy periods increased and in contrast it increased as weed free periods increased (Figure 3). Also, the leaf area decreased as weedy periods increased. This parameter was highly significantly reduced because of the important competitive effect of weeds. (Figure 4). The crop leaf dry matter was also significantly reduced by the weed competitive effect. The longer the weedy period the lower sugar beet dry matter. The later increased as the weed free period increased (Figure 5). These results confirm those of Alaoui et al. (2003) reporting that the leaf area and the other growth

**3.3.3 Effect of weed free and weedy periods on sugar beet yield, on sugar yield and** 

Weed infestation reduced root yield in all treatments. The presence of weeds during the entire growing season decreased root yield by 97.6 % and 68.9 % in 2003 and 2004, respectively, as compared to full season weed free check. Although sugar content did not show any significant difference between various treatments in both years, weed infestation decreased sugar yield, their corresponding yields decreased considerably in infested treatments. For example, season-long weed infestation decreased sugar yield by 89.8% and 81.1 % in 2003 and 2004, respectively, as compared to weed free check (data not shown). The concentration of sugar beet impurities such as potassium, sodium and amino nitrogen were

In most years in Morocco, weeds can cause more than 75% yield reduction (Rzozi, unpublished data; Rzozi et al., 1990). Such reductions indicate complete crop failure because small sugar beet roots produced under severe weed competition cannot be processed. In other countries, weeds also seriously suppress sugar beet yield (Schweizer and Dexter, 1987).

Fig. 3. Effect of weeds on sugar beet leaf number.

Fig. 4. Effect of weeds on sugar beet leaf surface.

Sugar Beet Weeds in Tadla Region (Morocco):

sugar beet crop.

**3.4 Conclusion** 

reduced by weed infestation.

Fig. 6. Critical period of weed control (2003/2004).

Species Encountered, Interference and Chemical Control 393

also observed by Shahbazi and Rashed Mohassel (2000). Dawson (1977) showed that annual weeds that germinate during a 2-week period after planting or a 4-week period after twoleaf stage in sugar beet reduce root yield by 26 to 100%. Therefore, effective control of weeds at early stages seems to be more important than that of later developed stages. The closure of crop canopy at later growth stages suppresses the late-emerging weeds. The increased

(Zimdahl, 1987; Ghadiri, 1996). Longer presence of weeds caused more use of environmental resources (light, water, and nutrients) and more accumulation of dry matter in weeds, making the critical period longer and, therefore reducing root and white sugar yield of the

A field experiment was conducted during two growing seasons 2003/2004 and 2004/2005 to assess the effect of weeds on sugar beet growth parameters and sugar beet yield and to determine the critical period of weed control (CPWC). Weed free treatments and weed infested treatments included the removal (or not) of weeds at 4, 7, 9, 11, 13, 17 and 21 weeks after emergence of sugar beet. Dry matter of weed, sugar beet leaves/plant, sugar beet leaf area and sugar beet dry weight was measured during all growing season. Weed free periods resulted in lower weed dry matter and weedy periods resulted in high weed dry matter. Maximum total weed dry weight generally decreased as weed-free duration was increased. The presence of weeds during the entire growing season decreased root yield by 97.6 % and 68.9 % in 2003 and 2004, respectively. All crop growth parameters were significantly

The critical period of weed control began at 4 and 7 weeks after sugar beet emergence (WAE) and continued until 15 and 12 WAE in 2003/2004 and 2004/2005 respectively depending on sowing period. It was concluded that the CPWC is longer in 2003/2004 than in 2004/2005.

CPWC

period of weed competition reduces the photosynthesis and crop growth

Fig. 5. Effect of weeds on sugar beet leaf dry matter.

#### **3.3.4 Critical period of weed control**

Weed interference caused a sharp decline in sugar beet root yield in both years (Figure 6 and 7). Based on 10 % permissible decrease in root yield, weeding should start from 4 WAE and 7 WAE in 2003 and 2004, respectively (Figure 6 and 7). For the given 10% root yield reduction, weed control should be continued until 15 WAE and 12 WAE in 2003 and 2004, respectively (Figure 5 and 6). Weed interference caused a sharp decline in sugar yield (data not shown). Based on 10 % permissible decrease in root yield, weeding should start from 3.5 WAE and 7 WAE and must be continued until 15 WAE and 11 WAE in 2003 and 2004 respectively.

The results show that the critical period begins earlier in 2003 and its duration is longer comparatively to that observed in 2004 which is shorter and begins relatively later. This may be due to date of sowing. Effectively, in 2003, sugar beet was sown October 15 and this allows to many weed species, particularly gramineous including volunteer wheat, to germinate and emerge in great number and vigorously at the same time of the crop germination and emergence. In 2004, sugar beet was sown 25 November. At this time, a great number of weed species (mainly gramineous) has germinated and emerged from soil and destructed during the seedbed preparation.

Emergence time of weeds influences the critical period of weed control (Zimdahl, 1987; Weaver *et al*., 1992; Mesbah *et al*., 1994; Ghadiri, 1996). In Shahrekord, sugar beet is planted in May and June; this delay in seedbed preparation and planting may lead to earlier germination of weeds over the sugar beet crop. Therefore, critical period of weed control starts earlier and its duration is longer. At early growth stages, sugar beet has a low competitive ability against weeds; as a result critical period would start sooner. In 2003, presence of weeds for the entire growing season reduced root yield by 97.6% relative to weed free control. In 2004, the reduction was 68.6 %. A similar 71% root yield reduction was also observed by Shahbazi and Rashed Mohassel (2000). Dawson (1977) showed that annual weeds that germinate during a 2-week period after planting or a 4-week period after twoleaf stage in sugar beet reduce root yield by 26 to 100%. Therefore, effective control of weeds at early stages seems to be more important than that of later developed stages. The closure of crop canopy at later growth stages suppresses the late-emerging weeds. The increased period of weed competition reduces the photosynthesis and crop growth

(Zimdahl, 1987; Ghadiri, 1996). Longer presence of weeds caused more use of environmental resources (light, water, and nutrients) and more accumulation of dry matter in weeds, making the critical period longer and, therefore reducing root and white sugar yield of the sugar beet crop.
