**2. Competition between weeds and soybean by abiotic and biotic factors**

Plants genetically improved by human action, aiming increases in productivity, lost part of their aggressive nature and therefore the ability to survive and compete against adversities imposed by the environment. Thus, most of the weeds show higher extraction capacity and utilization of environmental resources compared to cultivated species. The competition for limited resources or not, directly or indirectly, can be described as: *spatial* competition, which is generated by the physical dominance of a given species over another, simultaneously; a second classification that could be addressed is *temporal* competition, that results from competition over the time in which the crop is under development [5].

The various aspects of competition occurring between weeds and crops may also be named *ecological*, being classified as to their nature in biotic or abiotic [5]. The former are those from the live action elements of the ecosystem, such as predation, parasitism, commensalism, morphophysiological factors among others. The latter is a result of the action of non-living environmental factors, such as climatic and soil factors.

#### **2.1. Competition between soybean and weeds for biotic factors**

Competition is defined as the condition that exists when requirements of one or more organ‐ isms living in a community cannot be obtained from available resources. Because competition involves many direct and indirect factors, it is often, preferable to consider it as interference of a plant community on another one, rather than competition. Interference is a natural phenomenon in a plant community where limited resources exist, and tends to be more harmful to competitors as more equal are the environmental demands and vegetative habit

In agricultural ecosystems, weeds show competitive advantages over crop plants, because the aim of crop breeding is to increase the economic productivity, and this is almost always accompanied by a decrease in the competitive potential. Another important aspect in weed interference is the capacity of weeds in reducing or preventing cultivated plants to get access to resources. Thus, when those are limited, weeds almost always stand out, due to its higher efficiency in either capturing or using them. It is up to farmers and agronomists to use weed control methods and cultural practices in order to increase the chances of the crop overcoming

Reduction in weed competition is perfectly achievable with the wide spectrum of tools and herbicides existing in the market, but weed management strategies are not related solely to the use of herbicides [2]. Weed control consists in suppressing the development and/or decreasing the number of weeds per area, until an acceptable levels for the coexistence between the species involved is reached, with minimum damages to both. In soybean crop, weed control can be achieved by using one or more control methods that are: preventive, mechanical, chemical, biological and cultural. Farmers can also use the integrated weed management

The IWM approaches incorporate multiple tactics of prevention, avoidance, monitoring and suppression of weeds, undergirded by the knowledge of the agroecosystem biology [3]. The development of IWM was motivated by a desire to provide farmers with systematic ap‐ proaches to reduce reliance upon herbicides [4] and, consequently, retard the selection of herbicide-resistant biotypes. The use of integrated control facilitates weed control during all crop cycle. The cultural practices, like soil tillage, fertilization, cultivar choice, sowing time, number of plants per area and crop rotation should be done in order to benefit crop develop‐ ment, and in some cases can reduce or eliminate the need of using other control methods.

The aim of this chapter is to summarize basic information about weed interference and weed management in the soybean crop, subsidizing technicians in the adoption of suitable positions

**2. Competition between weeds and soybean by abiotic and biotic factors**

Plants genetically improved by human action, aiming increases in productivity, lost part of their aggressive nature and therefore the ability to survive and compete against adversities imposed by the environment. Thus, most of the weeds show higher extraction capacity and

between them.

86 Soybean - Pest Resistance

weeds in the competition for resources.

regarding problems with weed control.

(IWM), in which two or more of these methods are adopted.

The biotic factors that determine the increased competitiveness of certain species over others are: plant size and architecture, growth rate, extension of root system, dry mass production, increased susceptibility to environmental elements (such as frost and dry spells), greater leaf area index and greater capacity for production and release of chemicals with allelopathic properties [6].

Morphophysiological traits of plants influence the competitive relationship between crop and weeds. Plant height and development cycle, for example, are features that have been positively associated with competitive ability in soybean; cultivars with higher cycle length and height reduce seeds production and size of weed species due to the increase in competitiveness of the crop [7].

Moreover, yield losses due to competition tend to be higher the more similar are the individ‐ uals, i.e. their morphophysiological traits, reaching maximum stress within the same species, because in this case neighboring plants compete for the same resources and occupy the same ecological niche [5].

### *2.1.1. Plant traits indicators of higher competitive ability*

The competitive ability of crops can be expressed according to the crop ability to compete with weeds, reducing the production of seeds and dry mass accummulation by weeds, which is called *suppressive ability*. There is also the crop ability to *tolerate competition* with weeds, when under competition the crop is capable of maintaining yields almost unchanged [8, 9]. For Jordan [10], the suppressive ability should be preferred because it reduces seeds production by weeds and its benefits remain for subsequent growing seasons, while tolerance to weeds limits its benefits only to the current growing season. It is worth noting that in case crops do not have the ability to suppress weeds, the probability of yield reduction is increased, regard‐ less of crop tolerance to competition.

Olofsdotter [11] remarks that several traits which confer competitive ability are genetically changeable, and can be manipulated by plant breeding, as they are elucidated by research. According to the author, it is necessary to identify one or more traits as well as their genetic variability in the crop. After demonstrating its variability, studies are needed to indicate the mechanisms involved and the environmental effect on the expression of these traits. Finally, it is necessary to involve geneticists and breeders in the identification of genes coding for the desired trait, as well as to evaluate the usefulness of indicators in the selection, i.e. if the character can be selected.

Plants that have rapid and uniform emergence can compete more effectively for environ‐ mental resources [13]. These authors reported that the emergence rate is positively corre‐ lated with the ability of soybean cultivars to compete with weeds. In this sense, Fischer & Miles [19] formulated theoretical principles in which the greater the rate of development of a plant, the higher the shoot and edaphic volume explored. The first seedlings to emerge, probably present higher yields because they have priority in using water, light

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 89

Plants that use resources earlier will shade the others, reducing the amount and quality of light available for the neighbors [20]. Weeds which establish before the crop, with big size and high number of seeds, will increase its frequence in the soil seed bank and keep infesting subsequent crops [6]. Another problem resulting from the establishment of crops later in relation to weeds

Ecologically, weeds are less demanding in true growth factors in relation to crop plants, which confers great competitive ability for them [6]. In a study conducted by Carranza et al. [22], it was found that the relative intraspecific competition (yield loss per weed unit) decreased when weed population increased. According to the authors, plants that emerged earlier were 1.5

Crop management practices such as use of high quality seeds, appropriate management of soil and planting at the recommended time and depth significantly increase chances of crop plants to be more competitive. The adoption of these practices, along with the use of cultivars with fast establishment, are key points to accelerate crop growth and focus on their success in

The better arrangement of crop plants may be more important for those species with less potential for branching or tillering. The increase in grain yield of soybean with narrow row spacing has been demonstrated in several studies [23, 24, 16]. Positive results are obtained with this practice especially in wet years with the use of early maturing cultivars [16], in soil well

In the case of planting soybean after the recommended period, Board et al. [25] found that the reduced spacing resulted in higher dry mass of branches of plants at maturity (R8), which was highly correlated with grain yield. They also observed that the yield components of the branches, such as number, length and number of nodes in the branches, were higher in the smaller spacing, justifying the greater yield in reduced spacing system in late planting.

The dry weight of soybean can be used as a criterion to choose between wider or narrower spacings between rows. For Board & Harville [26], if plant dry mass of late-maturing cultivars, in the stage R8, is at least 800 g m-2 in wide spacing, probably no benefit will be obtained by reducing row spacing. However, this value should be used carefully because it may cause lower levels of total dry mass, for example, if planting is accomplished out of the indicated

and nutrients, i.e., they occupy the niche early [20].

is the need for increasing herbicide rates for their control [21].

times more competitive than those who had delayed emergence.

*2.1.5. Soybean plants arrangement and its relationship with weeds*

supplied with nutrients [24], and also with late planting [25].

competition with weeds.

time interval (early or late planting).

Differences in competitive ability between soybean cultivars with weeds have been reported by Bussan et al. [12]; Jannink et al. [9]; Lamego et al. [13]; Bianchi et al. [14] and Fleck et al. [15]. Suitable conditions for crop planting, such as moist soil, proper and uniform planting depth, close contact between seed and soil, as well as certified quality seeds, are essential to ensure competitive advantage to the crop by promoting the rapid emergence and establishment of uniform populations. In a study with soybeans, higher size of seeds resulted in seedlings with higher hypocotyl expansion rates, which may constitute a favorable feature in adverse conditions of emergence as in the case of soil crusting following heavy rainfalls [16].

#### *2.1.2. Exploring competitive traits*

The use of cultural methods for weed management can minimize weeds interference on soybean. Among the most efficient management practices for the suppression of weeds, the population density of the crop can be highlighted, as well as equal plants arrangement, development cycle and root growth of the crop.

#### *2.1.3. Population density*

In areas of agricultural production, the density of cultivated plants is kept constant along the field while weeds density varies with the degree of infestation, which is determined by the soil seed bank richness [17, 5]. According to these authors a variation occurs in the crop/weeds density ratio, making important to understand in competition studies not only the influence of density in the competition process – additive studies, but also the influence of the variation in the species proportion in the population - substitutive studies [5].

The duration of the period planting-emergence is also affected by seeding rate, temperature and soil moisture, planting depth and seed traits [18]. The duration of this period changes seedling height and subsequently, the intra-specific competitive ability. According to this author, the effects on the duration of this period are more evident under high plant densities.

#### *2.1.4. Emergence speed*

The use of high vigor seeds, which provide immediate plant emergence after planting, is important for the cultural management of weeds. In the dispute for limited environmental resources, the advantage is granted for plants that exhibit early establishment. A growing plant must quickly seize space and other resources, and its competitive success depends on the anticipated use of them. Plants stop growing when its area is restricted by competitors, so that the last individuals appear to grow very little due to shading. Thus, a fast emergence is often more important than the spatial arrangement of individuals in determining the competitive‐ ness of the population [19].

Plants that have rapid and uniform emergence can compete more effectively for environ‐ mental resources [13]. These authors reported that the emergence rate is positively corre‐ lated with the ability of soybean cultivars to compete with weeds. In this sense, Fischer & Miles [19] formulated theoretical principles in which the greater the rate of development of a plant, the higher the shoot and edaphic volume explored. The first seedlings to emerge, probably present higher yields because they have priority in using water, light and nutrients, i.e., they occupy the niche early [20].

Plants that use resources earlier will shade the others, reducing the amount and quality of light available for the neighbors [20]. Weeds which establish before the crop, with big size and high number of seeds, will increase its frequence in the soil seed bank and keep infesting subsequent crops [6]. Another problem resulting from the establishment of crops later in relation to weeds is the need for increasing herbicide rates for their control [21].

Ecologically, weeds are less demanding in true growth factors in relation to crop plants, which confers great competitive ability for them [6]. In a study conducted by Carranza et al. [22], it was found that the relative intraspecific competition (yield loss per weed unit) decreased when weed population increased. According to the authors, plants that emerged earlier were 1.5 times more competitive than those who had delayed emergence.

Crop management practices such as use of high quality seeds, appropriate management of soil and planting at the recommended time and depth significantly increase chances of crop plants to be more competitive. The adoption of these practices, along with the use of cultivars with fast establishment, are key points to accelerate crop growth and focus on their success in competition with weeds.

#### *2.1.5. Soybean plants arrangement and its relationship with weeds*

variability in the crop. After demonstrating its variability, studies are needed to indicate the mechanisms involved and the environmental effect on the expression of these traits. Finally, it is necessary to involve geneticists and breeders in the identification of genes coding for the desired trait, as well as to evaluate the usefulness of indicators in the selection, i.e. if the

Differences in competitive ability between soybean cultivars with weeds have been reported by Bussan et al. [12]; Jannink et al. [9]; Lamego et al. [13]; Bianchi et al. [14] and Fleck et al. [15]. Suitable conditions for crop planting, such as moist soil, proper and uniform planting depth, close contact between seed and soil, as well as certified quality seeds, are essential to ensure competitive advantage to the crop by promoting the rapid emergence and establishment of uniform populations. In a study with soybeans, higher size of seeds resulted in seedlings with higher hypocotyl expansion rates, which may constitute a favorable feature in adverse

The use of cultural methods for weed management can minimize weeds interference on soybean. Among the most efficient management practices for the suppression of weeds, the population density of the crop can be highlighted, as well as equal plants arrangement,

In areas of agricultural production, the density of cultivated plants is kept constant along the field while weeds density varies with the degree of infestation, which is determined by the soil seed bank richness [17, 5]. According to these authors a variation occurs in the crop/weeds density ratio, making important to understand in competition studies not only the influence of density in the competition process – additive studies, but also the influence of the variation

The duration of the period planting-emergence is also affected by seeding rate, temperature and soil moisture, planting depth and seed traits [18]. The duration of this period changes seedling height and subsequently, the intra-specific competitive ability. According to this author, the effects on the duration of this period are more evident under high plant densities.

The use of high vigor seeds, which provide immediate plant emergence after planting, is important for the cultural management of weeds. In the dispute for limited environmental resources, the advantage is granted for plants that exhibit early establishment. A growing plant must quickly seize space and other resources, and its competitive success depends on the anticipated use of them. Plants stop growing when its area is restricted by competitors, so that the last individuals appear to grow very little due to shading. Thus, a fast emergence is often more important than the spatial arrangement of individuals in determining the competitive‐

conditions of emergence as in the case of soil crusting following heavy rainfalls [16].

character can be selected.

88 Soybean - Pest Resistance

*2.1.2. Exploring competitive traits*

*2.1.3. Population density*

*2.1.4. Emergence speed*

ness of the population [19].

development cycle and root growth of the crop.

in the species proportion in the population - substitutive studies [5].

The better arrangement of crop plants may be more important for those species with less potential for branching or tillering. The increase in grain yield of soybean with narrow row spacing has been demonstrated in several studies [23, 24, 16]. Positive results are obtained with this practice especially in wet years with the use of early maturing cultivars [16], in soil well supplied with nutrients [24], and also with late planting [25].

In the case of planting soybean after the recommended period, Board et al. [25] found that the reduced spacing resulted in higher dry mass of branches of plants at maturity (R8), which was highly correlated with grain yield. They also observed that the yield components of the branches, such as number, length and number of nodes in the branches, were higher in the smaller spacing, justifying the greater yield in reduced spacing system in late planting.

The dry weight of soybean can be used as a criterion to choose between wider or narrower spacings between rows. For Board & Harville [26], if plant dry mass of late-maturing cultivars, in the stage R8, is at least 800 g m-2 in wide spacing, probably no benefit will be obtained by reducing row spacing. However, this value should be used carefully because it may cause lower levels of total dry mass, for example, if planting is accomplished out of the indicated time interval (early or late planting).

The removal of weeds by using reduced spacing was evaluated by Legere & Schreiber [27]. These authors found that in the middle of the soybean growing season, the contribution of pigweed (*Amaranthus retroflexus*) for total dry mass per area was 43% in soybean row spacing of 76 cm, but was only 24% for rows spaced in 25 cm. In this work, in any situation of weed infestation, grain yield was always higher in the smaller row spacing.

Species with carbon metabolism by the cycle C4 are usually more efficient in the use of water (higher WUE); as a consequence, they produce more biomass per unit of water consumed. According to Patterson and Flint [39], *Amaranthus hybridus*, with C4 metabolism, showed higher WUE compared to soybean plants. When soybean was compared to beans and with some weed species (*Euphorbia heterophylla*, *Bidens pilosa* and *Desmodium tortuosum*), bean was the plant which used the water more efficiently from the beginning of the cycle; soybean was the plant with the highest biomass accumulation rate and greater WUE along the cycle; *Desmodium tortuosum* was the most efficient in the capture and use of water during the vegetative stage and *Bidens pilosa* after flowering [40]. In another study, soybean and the weed *Xanthium*

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 91

Light is the most disputed factor in competition, highlighting the importance of plant height in defining the competitive ability of crops [5]. The high ability of plants to intercept the incident light in the canopy is a desirable feature when crop is under competition with weeds [42]. Light interception by the canopy is dependent on plant density and arrangement, branching rate, plant height, leaf area, distribution of leaves, leaf angle, angle of leaf blades and dry mass accumulation [42]. Cultivars that concentrate photosynthates in leaves, i.e., high leaf area ratio (LAR), have greater potential for ground cover [43] and consequently the greater

The initial growth rate is directly related to light interception and use in earlier stages of the plant cycle, allowing a greater leaf area development which provides to crop a higher com‐

The rate of biomass accumulation in shoots becomes a key factor for competitive success [40]. Earlier emergence of weeds in soybean, in relation to crop emergence, increased grain yield losses of soybean [13]. Evaluating the efficiency of capture and utilization of light by soybean and bean against the weeds *Bidens pilosa*, *Euphorbia heterophylla* and *Desmodium tortuosum*, Santos et al. [45] observed the highest accumulation rate of dry mass and the largest leaf area index for soybean, indicating its greater ability to capture light and shade the competing plants. Bean, especially after flowering, was more effective in draining its photosynthates for leaf

Nitrogen (N), phosphorus (P) and potassium (K), are of great importance for understanding yield losses by crops [46]. According to Anguinoni et al. [47] the capacity for absorption of nutrients in plants depends on the magnitude and the morphology of the root system and its efficiency in absorption of these elements. Crops with fast root growth maximizes the use of water and nutrients [48] so an accelerated growth of the root system constitutes a desirable

Under field conditions, in a study of competition for nutrients between soybean or bean with the weeds *Euphorbia heterophylla, Bidens pilosa* and *Desmodium tortuosum*, Bidens pilosa was the

*strumarium* showed similar WUEs [41].

will be their competitive ability with weeds.

*2.2.2. Competition for light*

petitive ability [32, 44].

*2.2.3. Competition for nutrients*

feature for better nutrient use [49].

formation.

In some situations, however, an adequate suppression of weeds may not occur. Burnside [28] found no difference in yields between spacings of 38 and 76 cm in the presence of weeds during different periods of coexistence. Also, Nice et al. [29] found no effect on the population of sicklepod (*Senna obtusifolia*) by reducing soybean row spacing, when there was no increase in the population of the crop. Under higher populations and smaller row spacings, a reduction in population and seed production of sicklepod was observed.

Another advantage of the smaller row spacing is the possibility of using lower doses of certain herbicides due to the effect of the additional shading of weeds by crop plants. Young et al. [30] observed that the reduction in row spacing from 76 cm to 38 cm, increased weed control after herbicide application. In contrast, glyphosate presented weed control superior to 90% at row spacing of 19 cm, controlling between 75% and 90% of the weeds when crop was planted at a spacing of 76 cm.

The set of morphological and physiological traits of cultivars defines its ability to compete with weeds for environmental resources [31, 14]. However, the competitive ability of cultivars can be altered by agronomic practices [32]. Weed population and its emergence delay in relation to the crop, often define the relationships of competition between species [33].

According to Lamego et al. [13] soybean cultivars with early emergence, fast leaf area expan‐ sion, high growth rate, and higher plant height in early stages, are more capable of competing with weeds. On the other hand, weed species with fast emergence, like the ones from Genus *Brachiaria*, *Digitaria*, *Euphorbia, Bidens* and *Raphanus*, among others, are able to compete earlier for environmental resources in relation to soybean [34, 13, 14, 35]. Rizzardi et al. [36], studying competition between soybean and *Euphorbia heterophylla*, *Ipomoea ramosissima, Bidens pilosa* or *Sida rhombifolia*, reported that several management practices can minimize their interference on soybeans, like the use of more competitive cultivars, correction of soil fertility and the adequacy of the arrangement of plants.

#### **2.2. Competition between soybean and weeds for abiotic factors**

#### *2.2.1. Competition for water*

Water is the most limiting factor essential for plant growth and production [37]. The rainfall and soil moisture strongly influence the growth of weeds, affecting, therefore, competition with crops [38]. Certain morphological and physiological traits determine the ability of plants to compete for soil water. In nature, species with C3 metabolism predominate in temperate regions, while the C4 are prevalent in tropical and subtropical regions. The relative distribution of C3 and C4 species depends on the temperature during the growing season of the plants [5]. Species with carbon metabolism by the cycle C4 are usually more efficient in the use of water (higher WUE); as a consequence, they produce more biomass per unit of water consumed.

According to Patterson and Flint [39], *Amaranthus hybridus*, with C4 metabolism, showed higher WUE compared to soybean plants. When soybean was compared to beans and with some weed species (*Euphorbia heterophylla*, *Bidens pilosa* and *Desmodium tortuosum*), bean was the plant which used the water more efficiently from the beginning of the cycle; soybean was the plant with the highest biomass accumulation rate and greater WUE along the cycle; *Desmodium tortuosum* was the most efficient in the capture and use of water during the vegetative stage and *Bidens pilosa* after flowering [40]. In another study, soybean and the weed *Xanthium strumarium* showed similar WUEs [41].

#### *2.2.2. Competition for light*

The removal of weeds by using reduced spacing was evaluated by Legere & Schreiber [27]. These authors found that in the middle of the soybean growing season, the contribution of pigweed (*Amaranthus retroflexus*) for total dry mass per area was 43% in soybean row spacing of 76 cm, but was only 24% for rows spaced in 25 cm. In this work, in any situation of weed

In some situations, however, an adequate suppression of weeds may not occur. Burnside [28] found no difference in yields between spacings of 38 and 76 cm in the presence of weeds during different periods of coexistence. Also, Nice et al. [29] found no effect on the population of sicklepod (*Senna obtusifolia*) by reducing soybean row spacing, when there was no increase in the population of the crop. Under higher populations and smaller row spacings, a reduction

Another advantage of the smaller row spacing is the possibility of using lower doses of certain herbicides due to the effect of the additional shading of weeds by crop plants. Young et al. [30] observed that the reduction in row spacing from 76 cm to 38 cm, increased weed control after herbicide application. In contrast, glyphosate presented weed control superior to 90% at row spacing of 19 cm, controlling between 75% and 90% of the weeds when crop was planted at a

The set of morphological and physiological traits of cultivars defines its ability to compete with weeds for environmental resources [31, 14]. However, the competitive ability of cultivars can be altered by agronomic practices [32]. Weed population and its emergence delay in relation

According to Lamego et al. [13] soybean cultivars with early emergence, fast leaf area expan‐ sion, high growth rate, and higher plant height in early stages, are more capable of competing with weeds. On the other hand, weed species with fast emergence, like the ones from Genus *Brachiaria*, *Digitaria*, *Euphorbia, Bidens* and *Raphanus*, among others, are able to compete earlier for environmental resources in relation to soybean [34, 13, 14, 35]. Rizzardi et al. [36], studying competition between soybean and *Euphorbia heterophylla*, *Ipomoea ramosissima, Bidens pilosa* or *Sida rhombifolia*, reported that several management practices can minimize their interference on soybeans, like the use of more competitive cultivars, correction of soil fertility and the

Water is the most limiting factor essential for plant growth and production [37]. The rainfall and soil moisture strongly influence the growth of weeds, affecting, therefore, competition with crops [38]. Certain morphological and physiological traits determine the ability of plants to compete for soil water. In nature, species with C3 metabolism predominate in temperate regions, while the C4 are prevalent in tropical and subtropical regions. The relative distribution of C3 and C4 species depends on the temperature during the growing season of the plants [5].

to the crop, often define the relationships of competition between species [33].

**2.2. Competition between soybean and weeds for abiotic factors**

infestation, grain yield was always higher in the smaller row spacing.

in population and seed production of sicklepod was observed.

spacing of 76 cm.

90 Soybean - Pest Resistance

adequacy of the arrangement of plants.

*2.2.1. Competition for water*

Light is the most disputed factor in competition, highlighting the importance of plant height in defining the competitive ability of crops [5]. The high ability of plants to intercept the incident light in the canopy is a desirable feature when crop is under competition with weeds [42]. Light interception by the canopy is dependent on plant density and arrangement, branching rate, plant height, leaf area, distribution of leaves, leaf angle, angle of leaf blades and dry mass accumulation [42]. Cultivars that concentrate photosynthates in leaves, i.e., high leaf area ratio (LAR), have greater potential for ground cover [43] and consequently the greater will be their competitive ability with weeds.

The initial growth rate is directly related to light interception and use in earlier stages of the plant cycle, allowing a greater leaf area development which provides to crop a higher com‐ petitive ability [32, 44].

The rate of biomass accumulation in shoots becomes a key factor for competitive success [40]. Earlier emergence of weeds in soybean, in relation to crop emergence, increased grain yield losses of soybean [13]. Evaluating the efficiency of capture and utilization of light by soybean and bean against the weeds *Bidens pilosa*, *Euphorbia heterophylla* and *Desmodium tortuosum*, Santos et al. [45] observed the highest accumulation rate of dry mass and the largest leaf area index for soybean, indicating its greater ability to capture light and shade the competing plants. Bean, especially after flowering, was more effective in draining its photosynthates for leaf formation.

#### *2.2.3. Competition for nutrients*

Nitrogen (N), phosphorus (P) and potassium (K), are of great importance for understanding yield losses by crops [46]. According to Anguinoni et al. [47] the capacity for absorption of nutrients in plants depends on the magnitude and the morphology of the root system and its efficiency in absorption of these elements. Crops with fast root growth maximizes the use of water and nutrients [48] so an accelerated growth of the root system constitutes a desirable feature for better nutrient use [49].

Under field conditions, in a study of competition for nutrients between soybean or bean with the weeds *Euphorbia heterophylla, Bidens pilosa* and *Desmodium tortuosum*, Bidens pilosa was the 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 exacerbate the problem of weed interference [46].

ha-1. According to the authors, weed-crop coexistence for up to 17 DAE did not cause any negative effect on crop yield, and the maximum length of time in which weeds had to be controlled to prevent crop yield losses was 44 DAE; the CPWC was, therefore, from 17 to 44 DAE. Carvalho and Velini [55], observed that weeds germinated 20 days after the emergence

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 93

Different results of CPWC showed that the degree of weed interference on crops depends on the infesting plant community (species, density and population), on the crop (cultivar, spacing and density) and environment (soil, climate and management). Thus, it is necessary a greater number of studies to create a data base and in the future create models to predict the adequate

According to Hart [56], the population of weeds may be divided into three components: the

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

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

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

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.

them, using techniques such as the sequential application of desiccants.

the whole population which will benefit from the supply chain.

of soybean, cv. IAC-11, did not affect crop yield.

moment of weed control for each situation.

**4. Weed control methods in soybeans**

the system.

**4.1. Preventive control**

active seed, the inactive/dormant seeds and plants.

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