**4. Weed control methods in soybeans**

plant species with higher leaf area increasing following N applications and soybean accumu‐ lated the highest biomass in its root system, which tended to decrease with the addition of N. *B. pilosa* and *E. heterophylla* increased its biomass as N was increased [46]. The total content of N in soybean leaves decreased as the dose of N was increased; however, for all weed species, N content in leaves increased according to the doses of N. The higher efficiency of roots in N uptake was found for bean plants. *B. pilosa* and *E. heterophylla* were the most efficient species in N use. The supply of N favored more the weed species not belonging to the legume family than soybean and bean; therefore, an inadequate management of N in these crops may

For the same species evaluated in the previous experiment, soybean was the species that showed the largest increase of P in root biomass as the dose of this nutrient was increased. *Desmodium tortuosum*, soybean and *B. pilosa* showed greater response to the addition of increasing doses of P in relation to dry matter accumulation [50]. The efficiency of P uptake by *D. tortuosum*, soybean and common bean decreased as the dose was increased. *E. hetero‐*

The critical period of weed control (CPWC) has been defined by Silva et al. [51] as a window in the crop growth cycle during which weeds must be controlled to prevent quantitative and qualitative yield losses. In essence, the CPWC represents the time interval between two separately measured crop-weed competition components: (1) the critical timing of weed removal (CTWR) or the maximum amount of time early-season weed competition can be tolerated by the crop before it suffers irrevocable yield reduction, and (2) the critical weed-free period (CWFP) or the minimum weed-free period required from the moment of planting, to prevent unacceptable yield reductions [52]. The former component is estimated to determine the beginning of the CPWC, whereas the latter determines its end. Results from both compo‐ nents are combined to determine the CPWC. Theoretically, weed control before and after the

The beginning and end of the CPWC determined using the functional approach will depend on the level of acceptable yield loss (AYL) used to predict its beginning and end. Many studies report 5% as the maximum AYL. But it can be adjusted depending on the cost of weed control

Silva et al. [53], evaluated the CTWR in soybean, cv. BRS-244 RR in low, medium and high weed density and observed that the CTWR was 17 days after emergence (DAE) in low infestation area and 11 DAE in medium and high infestation area, considering 5% of tolerance of crop yield decrease. According to the authors, weed interference during the full crop cycle reduced soybean grain yield in 73%, 82% and 92%, for low, medium and high weed density, respectively. Meschede et al. [54], evaluated the CPWC of *Euphorbia heterophylla* in soybean crop, cv BRS 133, under low seeding rate, and observed that the presence of weeds caused daily yield loss of 5.15 kg ha-1, whereas their absence provided a daily yield gain of 7.27 kg

*phylla* and bean, performed worst on the efficient use of available P in soil.

CPWC may not contribute to the conservation of the crop yield potential.

exacerbate the problem of weed interference [46].

92 Soybean - Pest Resistance

**3. Critical period of weed interference**

and the anticipated financial gain [52].

According to Hart [56], the population of weeds may be divided into three components: the active seed, the inactive/dormant seeds and plants.

The active seed (ready to germinate) can come from three sources: production by plants, seeds from outside the system and seeds that were dormant and that, for some reason, have become active. The dormant seed can also come from three sources: active seeds, plants and outside the system.

Weed management involves activities directed at the weeds (direct management) and, or, the system formed by soil and crop (indirect management). The direct management refers to the direct elimination of weeds using herbicides, manual or mechanical action and biological action. In soil management (indirect management) the relationship active and inactive seed can be worked. In this case, germination of the weeds should be increased before controlling them, using techniques such as the sequential application of desiccants.

According to Silva et al. [7], weed control possibilities include preventive, cultural, mechanical, biological and chemical methods. However, to maintain the sustainability of agricultural systems, it is important to integrate these control measures by observing the characteristics of soil, climate and socioeconomic aspects of the producer. The achievement of an environmen‐ tally and economically compatible integration requires deep knowledge of the available strategies, promoting balance with the management measures of soil and water, as well as the control of pests and diseases. To adopt any measure of control, the medium in which the weeds are should be treated as an ecosystem that can respond to any changes imposed, thus, not limited to the application of herbicides or using any other method alone. Furthermore, efforts will encourage the improvement of the quality of life, both of the farmer directly involved, as the whole population which will benefit from the supply chain.

#### **4.1. Preventive control**

It is harder to control weeds once they establish themselves, so preventing foreign weeds from entering a new area is usually easier and costs less than controlling after they have spread.

According to Silva et al. [57], the preventive control of weeds is the use of practices aimed at preventing the introduction, establishment and, or, spread of certain problematic species in areas not yet infested by them. These areas can be a country, a state, a municipality or a piece of land inside the farm.

cover crops etc. Amending the soil, neutralizing the aluminum content and increasing the pH, favors the crop and not certain weed species adapted to acid soils conditions and high contents of Al. Fertilization applied at the planting furrow is a common practice, and also favors soybean, so the fertilizer do not stand so close to the weeds in the inter-rows. These practices help to reduce the seed bank of weeds. It consists, therefore, in using their own ecological traits, both from crops and weeds, in order to benefit the establishment and development of crops.

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 95

One of the main practices is crop rotation. Its benefits depend on the selection of crops and their sequence in the system. Continuous cultivation of a single crop or crops having similar management practices allows certain weed species to become dominant in the sys‐ tem and, over time, these weed species become hard to control [58]. According to Kelley et al. [62], soybean production is improved by using crop rotation as a management prac‐ tice. Numerous studies have shown decreased yield when soybean was grown continu‐ ously in monoculture than when rotated with another crop [63, 64, 65]. In the short-term, benefit of crop rotation was increased soybean yield, which would likely increase soybean profitability. In the long-term, rotations with high residue-producing crops, such as wheat and grain sorghum, significantly increase total soil C and N concentrations over time,

Variation of the spacing or plant density in the row is another practice that can contribute to the reduction of weed interference on the crop, depending on the architecture of the cultivated plants and weed species. The reduction of spacing between rows often provides competitive advantage for most crops over shading sensitive weeds. In this case, by reducing the spacing between rows, provided it does not exceed the minimum limit, there is increased light interception by the canopy of cultivated plants. This effect is dependent on factors like the type of species to be cultivated, morphophysiological traits of genotypes, weed species present in the area and season and weather conditions at the time of its emergence, as well as environ‐

The main goal of using cover crops for weed control is replacing an unmanageable weed population with a manageable cover crop. This is accomplished by selecting the phenology of the cover crop to preempt the niche occupied by weed populations [69]. They have been used to manage weeds in soybean [70, 71, 72, 73]. According to Silva et al. [57], green covers are crops that usually are very competitive with weeds. Lupine, vetch, ryegrass, turnips, oats and rye are used in southern Brazil. In the subtropics, velvetbean, crotalarias, pigeon pea, jack-bean and lab-lab can be used. Its main effect is to reduce the seed bank and also improve soil physical-chemical conditions. However, these plants may also have inhibitory effects over others and can reduce infestations of some weed species after desiccation or incorporated in soil, and must be carefully chosen in each case. The presence of the mulch creates conditions for the installation of a dense and diverse microbiote in the soil, especially in the surface layer, with a high amount of microorganisms responsible for the elimination of dormant seeds by

Both the composition and the population density of a weed community are influenced by the level of mulching in the production system [74]. The mulch has physical (interference on

which may further improve soil productivity [62].

mental conditions [66, 67, 68].

deterioration and loss of viability.

In federal and state levels, there are laws regulating the entry of seeds into the country or state and its internal commercialization. Under these laws are the tolerable limits of seeds of each weed species and also the list of prohibited seeds per crop or crop group.

Locally, it is the responsibility of individual farmers or cooperatives, to prevent the entry and spread of one or more weed species that may become serious problems for the region. In summary, *the human element is the key to preventive control*. The efficient occupation of the agroecosystem space by the crop reduces the availability of appropriate factors for growth and development of weeds, and can be considered an integration between preventive and cultural method.

Choosing the right cultivars is actually the first step in successfully establishing a crop. In the soybean case, there is a large number of cultivars adapted to different regions of the world.

Some of the measures that can prevent the introduction of the species are: use of high purity seeds, clean thoroughly machines, harrows and harvesters; carefully inspect seedlings acquired with soil and also all the organic matter (manure and compost) from other areas; clean irrigation canals; quarantine of introduced animals, etc. [5].

Chauhan et al. [58] affirm that most crops have their seeds contaminated with weeds, especially when weed seeds resemble the size and shape of crop seeds. Contamination usually happens during the time of crop harvesting when weeds that have life cycles similar to those of crops set seeds. When even a small amount of weed seeds is present, it may be enough for a serious infestation in the next season. The idea should be to minimize the weed infestation area and decrease the dissemination of weed seeds from one area to another or from one crop to another. Control of weed species is achieved by reducing plants and propagules to the point at which their presence does not seriously interfere with an area of economic use. The planning of postinfested weed control programs should be done in such a way that the build-up of weed seeds is reduced drastically within a short period. Proper care should be taken to restrict the weed seed bank size in the area by using integrated methods of weed control. In undisturbed or notill systems, seeds of weeds and volunteer crops are deposited in the topsoil [59, 60, 61]. Therefore, an appropriate strategy is needed to avoid high weed infestations and to prevent unacceptable competition with the emerging crop [60].

#### **4.2. Cultural control**

The competitive ability of weeds largely depends on the time of emergence in relation to the soybean, in such a way that, if the crop germinates faster, and also occurs a delay on the emergence of weeds, competition will be reduced [5].

According to Silva et al [57], cultural control is the use of common practices for the proper management of water and soil as crop rotation, variation of crop row spacing, living mulches, cover crops etc. Amending the soil, neutralizing the aluminum content and increasing the pH, favors the crop and not certain weed species adapted to acid soils conditions and high contents of Al. Fertilization applied at the planting furrow is a common practice, and also favors soybean, so the fertilizer do not stand so close to the weeds in the inter-rows. These practices help to reduce the seed bank of weeds. It consists, therefore, in using their own ecological traits, both from crops and weeds, in order to benefit the establishment and development of crops.

According to Silva et al. [57], the preventive control of weeds is the use of practices aimed at preventing the introduction, establishment and, or, spread of certain problematic species in areas not yet infested by them. These areas can be a country, a state, a municipality or a piece

In federal and state levels, there are laws regulating the entry of seeds into the country or state and its internal commercialization. Under these laws are the tolerable limits of seeds of each

Locally, it is the responsibility of individual farmers or cooperatives, to prevent the entry and spread of one or more weed species that may become serious problems for the region. In summary, *the human element is the key to preventive control*. The efficient occupation of the agroecosystem space by the crop reduces the availability of appropriate factors for growth and development of weeds, and can be considered an integration between preventive and cultural

Choosing the right cultivars is actually the first step in successfully establishing a crop. In the soybean case, there is a large number of cultivars adapted to different regions of the world.

Some of the measures that can prevent the introduction of the species are: use of high purity seeds, clean thoroughly machines, harrows and harvesters; carefully inspect seedlings acquired with soil and also all the organic matter (manure and compost) from other areas;

Chauhan et al. [58] affirm that most crops have their seeds contaminated with weeds, especially when weed seeds resemble the size and shape of crop seeds. Contamination usually happens during the time of crop harvesting when weeds that have life cycles similar to those of crops set seeds. When even a small amount of weed seeds is present, it may be enough for a serious infestation in the next season. The idea should be to minimize the weed infestation area and decrease the dissemination of weed seeds from one area to another or from one crop to another. Control of weed species is achieved by reducing plants and propagules to the point at which their presence does not seriously interfere with an area of economic use. The planning of postinfested weed control programs should be done in such a way that the build-up of weed seeds is reduced drastically within a short period. Proper care should be taken to restrict the weed seed bank size in the area by using integrated methods of weed control. In undisturbed or notill systems, seeds of weeds and volunteer crops are deposited in the topsoil [59, 60, 61]. Therefore, an appropriate strategy is needed to avoid high weed infestations and to prevent

The competitive ability of weeds largely depends on the time of emergence in relation to the soybean, in such a way that, if the crop germinates faster, and also occurs a delay on the

According to Silva et al [57], cultural control is the use of common practices for the proper management of water and soil as crop rotation, variation of crop row spacing, living mulches,

weed species and also the list of prohibited seeds per crop or crop group.

clean irrigation canals; quarantine of introduced animals, etc. [5].

unacceptable competition with the emerging crop [60].

emergence of weeds, competition will be reduced [5].

**4.2. Cultural control**

of land inside the farm.

94 Soybean - Pest Resistance

method.

One of the main practices is crop rotation. Its benefits depend on the selection of crops and their sequence in the system. Continuous cultivation of a single crop or crops having similar management practices allows certain weed species to become dominant in the sys‐ tem and, over time, these weed species become hard to control [58]. According to Kelley et al. [62], soybean production is improved by using crop rotation as a management prac‐ tice. Numerous studies have shown decreased yield when soybean was grown continu‐ ously in monoculture than when rotated with another crop [63, 64, 65]. In the short-term, benefit of crop rotation was increased soybean yield, which would likely increase soybean profitability. In the long-term, rotations with high residue-producing crops, such as wheat and grain sorghum, significantly increase total soil C and N concentrations over time, which may further improve soil productivity [62].

Variation of the spacing or plant density in the row is another practice that can contribute to the reduction of weed interference on the crop, depending on the architecture of the cultivated plants and weed species. The reduction of spacing between rows often provides competitive advantage for most crops over shading sensitive weeds. In this case, by reducing the spacing between rows, provided it does not exceed the minimum limit, there is increased light interception by the canopy of cultivated plants. This effect is dependent on factors like the type of species to be cultivated, morphophysiological traits of genotypes, weed species present in the area and season and weather conditions at the time of its emergence, as well as environ‐ mental conditions [66, 67, 68].

The main goal of using cover crops for weed control is replacing an unmanageable weed population with a manageable cover crop. This is accomplished by selecting the phenology of the cover crop to preempt the niche occupied by weed populations [69]. They have been used to manage weeds in soybean [70, 71, 72, 73]. According to Silva et al. [57], green covers are crops that usually are very competitive with weeds. Lupine, vetch, ryegrass, turnips, oats and rye are used in southern Brazil. In the subtropics, velvetbean, crotalarias, pigeon pea, jack-bean and lab-lab can be used. Its main effect is to reduce the seed bank and also improve soil physical-chemical conditions. However, these plants may also have inhibitory effects over others and can reduce infestations of some weed species after desiccation or incorporated in soil, and must be carefully chosen in each case. The presence of the mulch creates conditions for the installation of a dense and diverse microbiote in the soil, especially in the surface layer, with a high amount of microorganisms responsible for the elimination of dormant seeds by deterioration and loss of viability.

Both the composition and the population density of a weed community are influenced by the level of mulching in the production system [74]. The mulch has physical (interference on germination and seedling survival rate), chemical (allelopathic effect) and biological (instal‐ lation of a dense and diverse microbiocenose in the topsoil) effects on weeds [75,76].

in conditions of heat and dry soil. Cultivation breaks the intimate relationship between root and soil, suspending the absorption of water, and exposes the roots to unfavorable environ‐ mental conditions. Depending on the relative size of weeds and crops, the displacement of the soil on the row, using special hoe cultivators, can cause the burial of seedlings and thereby

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 97

Biological control is the use of natural enemies (fungi, bacteria, viruses, insects, birds, fish, etc.) capable of reducing weed populations, reducing their ability to compete. This is maintained by the population balance between the natural enemy and the host plant. It should also be

According to Charudattan & Dinoor [80], bioherbicide is defined as a plant pathogen used as a weed-control agent through inundative and repeated applications of its inoculum. In the United States and many other countries, the prescriptive use of plant pathogens as weed control agents is regarded as a "pesticidal use" and therefore these pathogens must be registered or approved as biopesticides by appropriate governmental agencies. Currently, one fungus species is registered as bioherbicide in the United States for use in soybeans. Collego®, based on *Colletotrichum gloeosporioides* f.sp. *aeschynomene*, is used to control *Aeschynomene virginica* (northern jointvetch), a leguminous weed, in soybean and rice crops in Arkansas,

Charudattan & Dinoor [80] also state that, among the limitations of biocontrol of weeds by plant pathogens, the most important are the limited commercial interest in this approach to weed control due to the fact that markets for biocontrol agents are typically small, fragmented, highly specialized, and consequently the financial returns from biocontrol agents are too small to be of interest to big industries; and the complexities in production and assurance of efficacy and shelf-life of inoculum can further stifle bioherbicide development. For instance, the inability to mass-produce inoculum needed for large-scale use is a serious limitation that has led to the abandonment of several promising agents. The authors conclude that plant patho‐ gens hold enormous potential as weed biocontrol agents. In addition to the use of plant pathogens as biocontrol agents, it is likely that pathogen-derived genes, gene products, and genetic mechanisms (e.g., hypersensitive plant cell death and herbicidal biochemicals) will be exploited in the near future to provide novel weed management systems. On the other hand, the present over-reliance on chemical herbicides and the tendency to base weed-management decisions purely on economic considerations, at expense of the exclusion of ecological and

There are several advantages in using herbicides: pre-emergence control, eliminating the weeds precociously; hits targets that the hoe or cultivator does not reach, like the weeds in the crop row; reduces or eliminates the risk of damage to the roots and to young plants; do not alter soil structure and, therefore, reduces risk of erosion; controls more efficiently the

considered as biological control the allelopathic inhibition of weeds [6].

societal benefits, is a serious limitation that could stifle biological control.

promote weed control even in the rows of the crop.

**4.4. Biological control**

Mississippi, and Louisiana [80].

**4.5. Chemical control**

Thus, of the numerous known advantages of no-tillage - a practice that keeps the soil covered by crop residues - stands out the improvement in weed control. Trezzi & Vidal [77] found that the presence of residues of sorghum shoot (4 t ha-1) was sufficient to reduce 91, 96 and 59% the population of *Sida rhombifolia, Brachiaria plantaginea* and *Bidens pilosa*, respectively.

According to Silva et al. [57], in no-tillage, using systemic herbicides as desiccants, together with not revolving the soil, whether to produce corn for grain or silage, excellent results were found in the management of purple nutsedge (*Cyperus rotundus*). In two years in this system, it is possible to reduce population levels of nutsedge in favor of no-tillage, compared to conventional tillage, for both corn and beans, to the order of 90 to 95%, being that in three years, the reduction on the bank of tubers in the soil can reach more than 90%. The greatest benefits of no-tillage system in the integrated management of purple nutsedge (*Cyperus rotundus*) are obtained due to the integration of chemical control provided by the use of the systemic herbicide for desiccation of the vegetation at pre-sowing, to the cultural control exercised by the lack of soil disturbance and consequent lack of fragmentation of the vegetative structures of the nutsedge, and to the adoption of highly competitive crops, mainly by light, such as corn and beans. Thus, the population levels of purple nutsedge can be reduced, especially during the crop development period that is sensitive to weed interference, or approximately 45 days after emergence, as not to cause reductions in infested crop yields. Furthermore, the ability of sprouting of the nutsedge tubers collected in the soil under integrated management is diminished over time, remaining dormant [78].

#### **4.3. Mechanical control**

According to Silva et al. [57], weed plucking, or weeding, is the oldest method of weed control. It is still used to control weeds in home gardens and in the removal of weeds between crop rows, when the main method of control is the use of a hoe.

The manual weeding made with a hoe is very effective and still widely used in our agriculture, especially in mountainous regions, where there is subsistence agriculture, and for many families, this is the only source of work. However, in a more intensive agriculture in larger areas, the high cost of manpower and the difficulty of finding workers when necessary and in the desired quantity, make this method only complementary to others, and should be done when the weeds are still young and the soil is not too humid. It can assume great importance in seed production fields, being a good alternative for using isolated or as a complement for other control methods [79].

According to Silva et al. [57], mechanized cultivation, made by cultivators pulled by animals or tractors, is widely accepted in Brazilian agriculture, being one of the main methods of weed control on properties with smaller areas planted. The main limitations of this method are the difficulty of controlling weeds in the crop rows, low efficiency when performed in wet conditions (wet soil), and it is also inefficient to control weeds that reproduce by vegetative parts. However, all the annual species, when young (2-4 pairs of leaves), are easily controlled in conditions of heat and dry soil. Cultivation breaks the intimate relationship between root and soil, suspending the absorption of water, and exposes the roots to unfavorable environ‐ mental conditions. Depending on the relative size of weeds and crops, the displacement of the soil on the row, using special hoe cultivators, can cause the burial of seedlings and thereby promote weed control even in the rows of the crop.

#### **4.4. Biological control**

germination and seedling survival rate), chemical (allelopathic effect) and biological (instal‐

Thus, of the numerous known advantages of no-tillage - a practice that keeps the soil covered by crop residues - stands out the improvement in weed control. Trezzi & Vidal [77] found that the presence of residues of sorghum shoot (4 t ha-1) was sufficient to reduce 91, 96 and 59% the

According to Silva et al. [57], in no-tillage, using systemic herbicides as desiccants, together with not revolving the soil, whether to produce corn for grain or silage, excellent results were found in the management of purple nutsedge (*Cyperus rotundus*). In two years in this system, it is possible to reduce population levels of nutsedge in favor of no-tillage, compared to conventional tillage, for both corn and beans, to the order of 90 to 95%, being that in three years, the reduction on the bank of tubers in the soil can reach more than 90%. The greatest benefits of no-tillage system in the integrated management of purple nutsedge (*Cyperus rotundus*) are obtained due to the integration of chemical control provided by the use of the systemic herbicide for desiccation of the vegetation at pre-sowing, to the cultural control exercised by the lack of soil disturbance and consequent lack of fragmentation of the vegetative structures of the nutsedge, and to the adoption of highly competitive crops, mainly by light, such as corn and beans. Thus, the population levels of purple nutsedge can be reduced, especially during the crop development period that is sensitive to weed interference, or approximately 45 days after emergence, as not to cause reductions in infested crop yields. Furthermore, the ability of sprouting of the nutsedge tubers collected in the soil under

According to Silva et al. [57], weed plucking, or weeding, is the oldest method of weed control. It is still used to control weeds in home gardens and in the removal of weeds between crop

The manual weeding made with a hoe is very effective and still widely used in our agriculture, especially in mountainous regions, where there is subsistence agriculture, and for many families, this is the only source of work. However, in a more intensive agriculture in larger areas, the high cost of manpower and the difficulty of finding workers when necessary and in the desired quantity, make this method only complementary to others, and should be done when the weeds are still young and the soil is not too humid. It can assume great importance in seed production fields, being a good alternative for using isolated or as a complement for

According to Silva et al. [57], mechanized cultivation, made by cultivators pulled by animals or tractors, is widely accepted in Brazilian agriculture, being one of the main methods of weed control on properties with smaller areas planted. The main limitations of this method are the difficulty of controlling weeds in the crop rows, low efficiency when performed in wet conditions (wet soil), and it is also inefficient to control weeds that reproduce by vegetative parts. However, all the annual species, when young (2-4 pairs of leaves), are easily controlled

lation of a dense and diverse microbiocenose in the topsoil) effects on weeds [75,76].

population of *Sida rhombifolia, Brachiaria plantaginea* and *Bidens pilosa*, respectively.

integrated management is diminished over time, remaining dormant [78].

rows, when the main method of control is the use of a hoe.

**4.3. Mechanical control**

96 Soybean - Pest Resistance

other control methods [79].

Biological control is the use of natural enemies (fungi, bacteria, viruses, insects, birds, fish, etc.) capable of reducing weed populations, reducing their ability to compete. This is maintained by the population balance between the natural enemy and the host plant. It should also be considered as biological control the allelopathic inhibition of weeds [6].

According to Charudattan & Dinoor [80], bioherbicide is defined as a plant pathogen used as a weed-control agent through inundative and repeated applications of its inoculum. In the United States and many other countries, the prescriptive use of plant pathogens as weed control agents is regarded as a "pesticidal use" and therefore these pathogens must be registered or approved as biopesticides by appropriate governmental agencies. Currently, one fungus species is registered as bioherbicide in the United States for use in soybeans. Collego®, based on *Colletotrichum gloeosporioides* f.sp. *aeschynomene*, is used to control *Aeschynomene virginica* (northern jointvetch), a leguminous weed, in soybean and rice crops in Arkansas, Mississippi, and Louisiana [80].

Charudattan & Dinoor [80] also state that, among the limitations of biocontrol of weeds by plant pathogens, the most important are the limited commercial interest in this approach to weed control due to the fact that markets for biocontrol agents are typically small, fragmented, highly specialized, and consequently the financial returns from biocontrol agents are too small to be of interest to big industries; and the complexities in production and assurance of efficacy and shelf-life of inoculum can further stifle bioherbicide development. For instance, the inability to mass-produce inoculum needed for large-scale use is a serious limitation that has led to the abandonment of several promising agents. The authors conclude that plant patho‐ gens hold enormous potential as weed biocontrol agents. In addition to the use of plant pathogens as biocontrol agents, it is likely that pathogen-derived genes, gene products, and genetic mechanisms (e.g., hypersensitive plant cell death and herbicidal biochemicals) will be exploited in the near future to provide novel weed management systems. On the other hand, the present over-reliance on chemical herbicides and the tendency to base weed-management decisions purely on economic considerations, at expense of the exclusion of ecological and societal benefits, is a serious limitation that could stifle biological control.

#### **4.5. Chemical control**

There are several advantages in using herbicides: pre-emergence control, eliminating the weeds precociously; hits targets that the hoe or cultivator does not reach, like the weeds in the crop row; reduces or eliminates the risk of damage to the roots and to young plants; do not alter soil structure and, therefore, reduces risk of erosion; controls more efficiently the perennial weeds; reduces the need for labor; increases the speed and efficiency of the control operation per unit area, reducing the cost per treated area; controls the weeds for a longer period, when the use of a cultivator is impossible in view of the crop growth; and can be used in rainy periods, when the mechanical control is not efficient and when labor is required for other activities. However, it has the disadvantage of requiring skilled labor, because, if done improperly, can poison the crop, the environment and, especially, the applicator himself. Although herbicides are very effective in controlling weeds, they may promote the develop‐ ment of resistant biotypes, a fact that would further exacerbate the problem within an area [81].

soybean growth, glyphosate applications could be less effective for weed control and the

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 99

Hager et al. [89], in a study to examine the influence of herbicide application timing and dose on efficacy of six soil-applied herbicides for common waterhemp (*Amaranthus rudis*) control in soybean, found that sulfentrazone controlled this weed better and reduced its density more

Nosworthy [90], evaluating broadleaved weed control and economics of conventional and glyphosate-containing herbicide programmes in glyphosate-resistant soybean planted in wide rows, found that pre-emergence herbicides followed by glyphosate, controlled *Ipomoea lacunosa* L. eight weeks after emergence (WAE). *I. hederacea* var. *integriuscula* Gray control with pre-emergence herbicides followed by glyphosate was 100% with similar control from chlorimuron plus sulfentrazone followed by lactofen, whereas control following the single glyphosate application was 84%. *Amaranthus palmeri* S. Wats. control nine WAE was 100% following single or sequential glyphosate applications, while control ranged from 76% to 96% with pre-emergence herbicides followed by lactofen. However, early season weed interference when a single application of glyphosate was delayed until four WAE reduced soybean yields an average of 389 kg ha-1 compared to pre-emergence herbicides followed by glyphosate.

The concept of Integrated Weed Management (IWM), a component of Integrated Pest Management, has been proposed (i) to decrease the density of weeds emerging in crops, (ii) to reduce their relative competitive ability (in order both to preserve crop yields and to limit the replenishment of weed seed bank), and (iii) to control emerged weeds using non chemical techniques, with the overall aim of reducing the need for herbicide applica‐ tion at the cropping system level [91]. IWM advocates the use of all available weed con‐ trol options such as: plant breeding, fertilization, crop rotation, tillage practices, planting pattern, cover crops and mechanical, biological and chemical controls. To define the cor‐ rect weed management strategies, it is necessary to know the ability of the weed species, in relation to the crop, to compete for water, light and nutrients, which are factors respon‐

Usually, it is not taken into consideration that a good program of weed management should allow for maximum production in the shortest time, the maximum sustainable production and minimal environmental and economic risk. Wilson et al. [92] in a study to compare the Ohio farmer model to a weed scientist decision model about management of weeds, concluded that farmers understand but do not practice IWM. The failure to adopt may be attributed to gaps in their understanding of the human role in weed dispersal, their focus on the risks associated with weeds without recognition of their ecological benefits, and the tendency to overlook risks

Therefore, to accomplish the IWM, it is required knowledge in botany, plant physiology,

molecular biology, climatology and application technology, among others.

resulting competition could reduce soybean yields.

**5. Principles of integrated weed management**

sible for decreasing crop yield [6].

associated with management.

than other herbicides.

According to Oliveira Jr. et al. [82], the most common strategies used in the management of both cover crops and weed vegetation in areas of no-tillage are reduced to three: desiccation immediately before sowing, between seven and ten days before sowing or anticipated drying.

These authors undertook a study aimed to evaluate the interaction between tillage systems and weed control in post emergence in soybean with these three strategies. They concluded that, although desiccation in different management systems have been effective, the anticipa‐ tion of desiccation in anticipated management favored the emergence and initial soybean development, providing greater productivity gains, given the infestation conditions. The management system also affected the flow of weed emergence after soybean emergence, with fewer reinfestations in the anticipated management system, due to the control of initial flows given by the second application of this management system. Management applied at planting and ten days before planting, hindered the development of soybean, resulting in lower productivity, while anticipated management provided the highest yield.

Procópio et al. [83] carried out a study in which they compared the effects of tillage systems on the control of the weeds *Digitaria insularis*, *Synedrellopsis grisebachii* and *Leptochloa filifor‐ mis* before soybean planted in no-till. The authors found satisfactory control and prevention of regrowth of *D. insularis* and *L. filiformis* when glyphosate was applied five days prior to soybean planting or when the sequential application of glyphosate and paraquat + diuron was done. Sequential applications of the mixture paraquat + diuron were not effective in controlling or preventing the regrowth of *D. insularis* and *L. filiformis* and the weed *S. grisebachii* proved to be tolerant to glyphosate. The use of a non-residual herbicide such as glyphosate fails by not controlling weeds emerged after application, and eventually produce seeds that can easily replenish the seed bank [84]. Adding a residual herbicide to glyphosate can be a consistent management to control the weeds as they germinate and promotes long-term activity which controls plants which emerge later [85].

According to Arregui et al. [86], there are several soil-applied broadleaf herbicides that effectively control weeds like *Ipomoea* spp., *Commelina* spp. and *Sida spinosa*. Chlorimuron and sulfentrazone reduce *Ipomoea* spp. density [87]; *S. spinosa* density decreased with imazaquin, metribuzin and sulfentrazone applications [87] and with cloransulam and diclosulam [88].

The same authors [86] affirm that soil-applied herbicides as metribuzin and imazaquin may be beneficial reducing early season competition of weeds, particularly those inherently more tolerant to glyphosate such as *Parietaria debilis* or *Commelina erecta*, which survive pre-planting glyphosate applications. Likewise, when dry conditions are observed during vegetative soybean growth, glyphosate applications could be less effective for weed control and the resulting competition could reduce soybean yields.

Hager et al. [89], in a study to examine the influence of herbicide application timing and dose on efficacy of six soil-applied herbicides for common waterhemp (*Amaranthus rudis*) control in soybean, found that sulfentrazone controlled this weed better and reduced its density more than other herbicides.

Nosworthy [90], evaluating broadleaved weed control and economics of conventional and glyphosate-containing herbicide programmes in glyphosate-resistant soybean planted in wide rows, found that pre-emergence herbicides followed by glyphosate, controlled *Ipomoea lacunosa* L. eight weeks after emergence (WAE). *I. hederacea* var. *integriuscula* Gray control with pre-emergence herbicides followed by glyphosate was 100% with similar control from chlorimuron plus sulfentrazone followed by lactofen, whereas control following the single glyphosate application was 84%. *Amaranthus palmeri* S. Wats. control nine WAE was 100% following single or sequential glyphosate applications, while control ranged from 76% to 96% with pre-emergence herbicides followed by lactofen. However, early season weed interference when a single application of glyphosate was delayed until four WAE reduced soybean yields an average of 389 kg ha-1 compared to pre-emergence herbicides followed by glyphosate.
