**2. Implication of weed management in soybean**

Despite differences between soybean cultivars used worldwide and the main weed species which attack these cultivars, there are many resemblances in management practices and control. The species hairy fleabane, *Conyza bonariensis* (L.) Cronq., horseweed, *Conyza cana‐ densis* (L.) Cronq., goosegrass, *Eleusine indica (L.)* Gaertn., barnyardgrass, *Echinochloa crusgalli (L.)* Beauv., johnsongrass, *Sorghum halepense* (L.) Pers., beggarticks, *Bidens pilosa* L. and common ragweed, *Ambrosia artemisiifolia* L., are common weeds in Argentine, Brazilian and American soybean crops. The burndown and subsequent post-emergence (POST) spraying of crop with glyphosate usually occur from south to north in the American continent, with some distinctions among products used in mixture with glyphosate for managing resistant weeds.

The introduction of GR (glyphosate-resistant) soybean, genetically modified (GM), contribut‐ ed to standardization of weed management. With a large adoption of this technology, there are many concerns regarding the control and the high selection pressure on common weed species in soybean. In the US, more than 93% of soybean has the GR technology. In Brazil and

The use of very similar technologies as well as the facility of proliferation of weeds has intensified reported herbicide resistance. Since the first report of *E. indica* resistance, in Malasia (1997), 22 species (biotypes) are already not controlled by glyphosate and 10 show multiple resistance. The number of reports increases every year and, in 2011, 7 weed resistance cases were recorded. The evolution of weed resistance to glyphosate also worries members of the Weed Science Society of America, mainly by the spread rate and

New technologies derived from genetic alteration of cultivars resistant to herbicides are part of management alternatives to glyphosate. Many of them still under test should be available on short notice. In Brazil, both soybean resistant to ALS (acetolactate synthase) inhibitors and those resistant to 2,4-D should take up areas with a history of weed glyphosate resistance. In the US, besides soybean resistant to dicamba and that resistant to glyphosate + ALS, mixtures are used on crop pre-emergence (PRE), for example, dimethenamid and saflufenacil (new active ingredient). Spraying of encapsulated ingredients (acetochlor) at soybean POST and at

Despite efforts on weed control in soybeans, the benefits of IWM based on preventive and cultural controls will always be fundamental to the maintenance of monocultures. However, it appears that much of what is discussed about IWM is slightly practical, with corrective measures mostly. This chapter aims to present some focal issues related to weed management in soybean growing areas, which include weed potential to cause severe damages and yield losses by weeds, the evolution of resistant weeds in GR soy‐ bean monoculture, the soybean management characterization in the main producing countries and discussions about the benefits of IWM use as an accurate control measure. It presents a set of information for researchers and experts on weed management service area, reporting clear and objectively the major impacts of the current management used

All these factors increase the selection pressure even more.

Argentina, these values represent 80% and 99%, respectively.

weed PRE also come up as management alternatives.

and the outlook for soybean farming.

by the impact on ecosystems.

48 Soybean - Pest Resistance

Weed control is a practice of great importance for obtaining high soybean yields. Weed species is a serious problem for the soybean crops and its control is needed especially in infested sides. Therefore, weed management is an integral part of soybean production. Recently, research has reported that the density and distribution of weed species in the soybean plantations are significant parameters on yield losses. This happens because the weed species competes with the sunlight, water and nutrients, and may, depending on the level of infestation and species, hamper harvesting operations and compromise the quality of soybean grains [2]. Current studies on weed biology are changing, largely due to the effects of agricultural practices on weeds, cropping systems, and the environment. Research emphasis has been altered based on the need to understand basic weed biology [3]. It is our job to predict how weed species, populations, and biotypes evolve in response to selection pressure primarily due to agricul‐ tural practices. This knowledge helps developing weed management practices in the soybean crops. Other important biological factors in weed management decisions include weed and crop density, seedbank processes, demographic variation, weed-crop competition, and reproductive biology [4]. Development of economic thresholds for weed species made significant progress in the last decade. Integrated weed management has focused on the effects of crop planting dates, row spacing, cultivators, use of cover crops and reduced herbicide rates.

Selection and adaptation of weed populations occur at the level of the individual. Weeds interfere with crop production, and the yield losses incurred are the aggregate consequence of competition between heterogeneous weed phenotypes and homogeneous crop phenotype [5]. Because weed selection results in diversity, a population of weeds on a field consists of a heterogeneous collection of genotypes and phenotypes that allows exploitation of many niches left available by crops. Weed species respond to these opportunities with an impressive array of adaptions: phenotypes plasticity in response to microsite resource availability, somatic polymorphism of plant and seed form and function, density-dependent mortality (population size adjustment), density-independent mortality (disease, predator, stress resistances), and chemical inhibition of neighbors by allelopathic interference [6]. When all else fails, many weed seeds can remain dormant and extend their life for several years in the soil seedbank, waiting for the right opportunity to grow [7].

Weed populations possess considerable heterogeneity at many levels, consequence of adap‐ tation for colonization and survival. In order to select the most appropriate herbicides or devise the optimum weed control system, one must be able to properly identify the weeds present within a field. Weed identification immediately following emergence is essential since the effectiveness of most herbicides depends on weed size. Maps of weeds by species in fields prior to harvest will aid in the choice of herbicide program for the following year.

#### **2.1. Issues on weed management**

All the characteristics cited are essential for soybean weed management. However, starting from the identification of species, three leading questions must be answered in order to suitably handle weeds: i) What are the available tools for weed management? ii) How should one use them for reducing weed interference? and iii) When should one use them?

many cover crops allow using selective herbicide in the fallow period, reducing hard-to-control species. Despite the high costs, it saves on using herbicides along cultivation years for the primary crop, as the infestation plant is reduced by ongoing practice of this system. Nutrient cycling is also favored by means of cover crops, especially for those who exhibit high mobility on the ground, such as nitrogen [19]. For other nutrients, arbuscural mycorhizal development is favored in areas in which cover plants are used. This arbuscural mycorhizal promotes phosphorus absorption [20]. Nitrate loss in annual row crops could also be significantly mitigated by the adoption of no tillage and cover crops or greater reliance on biologically based inputs, according to [21]. In general, cover crops increase the primary productivity of the

Weed Management in Soybean — Issues and Practices

http://dx.doi.org/10.5772/54595

51

However, the selection of proper cover crop is essential for the success of the system. Plantfeeding nematodes, for example, were less abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affects soil food web structure [22]. Other species, such as those from genus Desmodium, may be used suppressing *Striga hermonthica* (Del.) Benth. by means of an allelopathic mechanism. Their root exudates contain novel flavonoid compounds, which stimulate suicidal germination

Herbicides, in the broad action spectrum, are and will be essential tools in weed management, even for those with a great number of resistant weeds. But the trend is that using different herbicide is increasingly related to GM crops which show resistance to more than one active ingredient. For new GM soybean, 2,4-D and dicamba resistance traits will always be used in stacks with at least one other herbicide-resistant trait. Glyphosate and ALS trait stack, recently deregulated in the US, possibly will allow the use of ALS-inhibiting herbicides with soil residual that are too phytotoxic to use on conventional crop cultivars [24]. In reference [25], diversification may make weed management more complex, but growers must not use new GM crop resistant to herbicides in the same way that some used initial GM crops, in order to rely only on one herbicide until it is no longer effective and then switch herbicides. Research alerts that "if growers use the new GM crops and the herbicides that they enable properly, GM crops will expand the utility of currently available herbicides and provide long-term solutions

Answering the question related to the period when control tools should be used, different opinions arise. Many specialists recommend to use tools, especially chemical control, only when economic loss level is reached, ie, when population density finds a minimum threshold at which costs of controlling are lower than economic damage coming from losses by weed interference. Nevertheless, by following the concept of integrated management, it is recom‐ mended the use of many available tools, even at fallow periods or at low weed densities. In reference [26], as opposed to pest and pathogens which attack crops in epidemic cycles, weeds are endemic, regenerating from the seed and/or vegetative propagules that are introduced into the soil; thus, the continuous management allows the best result. Besides, confining weed management to a narrow temporal window increases the risk of unsatisfying weed manage‐ ment outcomes due to unfavorable weather [27]. Coupled with this agreement, good man‐

of *S. hermonthica* seeds and dramatically inhibit its attachment to host roots [23].

system and diversify basal resources for higher trophic levels.

to manage resistant weeds".

The available tools are those that enable the reduction of weed-crop competition. It integrates all traditional control — cultural, physical, chemical, among others — and it should be evaluated in accordance with locally grown system. Currently, due to countless resistance cases, preferences are for those that integrate cultural and physical controls together with chemical ones, and the following ones can be cited: no tillage system, crop rotation, using of cover crops, autumnal herbicide management directed to key-weeds, and new GM soybean resistant to herbicide from different modes of action.

All tools should be adapted to use availability, particularly considering the ratio income/ investment. Many of these tools are easy to be used and have high impact. The no tillage system, for example, changes weed management completely, so that the mulch formed reduces weed survival [8] and also encourages the germination of negative photoblastic species [9], in addition to all other benefits found in the tropical regions of soybean production [10]. The advantages of no tillage over conventional tillage systems in improving soil quality are generally accepted, resulting in benefits for physical, chemical and biological properties of the soil [11]. Nowadays, no tillage is practiced on over 100 million ha worldwide, mostly in North and South America, but also in Australia and in Europe, Asia and Africa [12,13]. Among the advantages, one can cite the control of soil erosion, moisture conservation, favorable soil temperatures, increased efficiency in nutrient cycling, improvement on soil structure, machi‐ nery conservation and time saving in terms of human and animal labor [12,14]. The system also ensured changing among the population of arthropods, which are usually favored by the system because they find greater protection to natural enemies or use many of weed seeds as a feed source.

The crop rotation system constitutes another important management tool, often over‐ looked by producers. It allows the variation primarily at chemical control. Corn rotating, despite inconvenient profitability decreases, compared with soybean, allows an important POST emergent apply against glyphosate-tolerant weeds, in areas where GR corn is not used. Several studies carried out from 1970 to 1990, associated with cultivation of soybeans in crop rotation systems with diverse grasses (rice, maize, sorghum, wheat, sugar cane) and cotton, have shown that nitrogen residual effect, fixed by soybean crop and its residues, replaces partial the nitrogen on following crop, resulting in field optimization and alleviating part of the production costs [15]. In China, for example, soybean is commonly grown continuous‐ ly in monoculture rather than rotated with other crops, like maize or wheat. The soybean monoculture results in yield decline, as well as its quality. The yield reduction on soybean in 2, 3 and 4-year monoculture was 15%, 20%, and 30%, respectively [16,17], highlighting the significance of rotational system in the preservation of crop production. Furthermore, several experiments suggest that carbon and nitrogen from microbial biomass (particularly nitro‐ gen) are sensitively affected by soil- and crop-management regimens, being directly influenced by crop rotation [18].

Using cover crops between the main crops (fallow period) is also part of conservation practices and it represents a breakthrough in weed management. Besides, competing against weeds, many cover crops allow using selective herbicide in the fallow period, reducing hard-to-control species. Despite the high costs, it saves on using herbicides along cultivation years for the primary crop, as the infestation plant is reduced by ongoing practice of this system. Nutrient cycling is also favored by means of cover crops, especially for those who exhibit high mobility on the ground, such as nitrogen [19]. For other nutrients, arbuscural mycorhizal development is favored in areas in which cover plants are used. This arbuscural mycorhizal promotes phosphorus absorption [20]. Nitrate loss in annual row crops could also be significantly mitigated by the adoption of no tillage and cover crops or greater reliance on biologically based inputs, according to [21]. In general, cover crops increase the primary productivity of the system and diversify basal resources for higher trophic levels.

handle weeds: i) What are the available tools for weed management? ii) How should one use

The available tools are those that enable the reduction of weed-crop competition. It integrates all traditional control — cultural, physical, chemical, among others — and it should be evaluated in accordance with locally grown system. Currently, due to countless resistance cases, preferences are for those that integrate cultural and physical controls together with chemical ones, and the following ones can be cited: no tillage system, crop rotation, using of cover crops, autumnal herbicide management directed to key-weeds, and new GM soybean

All tools should be adapted to use availability, particularly considering the ratio income/ investment. Many of these tools are easy to be used and have high impact. The no tillage system, for example, changes weed management completely, so that the mulch formed reduces weed survival [8] and also encourages the germination of negative photoblastic species [9], in addition to all other benefits found in the tropical regions of soybean production [10]. The advantages of no tillage over conventional tillage systems in improving soil quality are generally accepted, resulting in benefits for physical, chemical and biological properties of the soil [11]. Nowadays, no tillage is practiced on over 100 million ha worldwide, mostly in North and South America, but also in Australia and in Europe, Asia and Africa [12,13]. Among the advantages, one can cite the control of soil erosion, moisture conservation, favorable soil temperatures, increased efficiency in nutrient cycling, improvement on soil structure, machi‐ nery conservation and time saving in terms of human and animal labor [12,14]. The system also ensured changing among the population of arthropods, which are usually favored by the system because they find greater protection to natural enemies or use many of weed seeds as

The crop rotation system constitutes another important management tool, often over‐ looked by producers. It allows the variation primarily at chemical control. Corn rotating, despite inconvenient profitability decreases, compared with soybean, allows an important POST emergent apply against glyphosate-tolerant weeds, in areas where GR corn is not used. Several studies carried out from 1970 to 1990, associated with cultivation of soybeans in crop rotation systems with diverse grasses (rice, maize, sorghum, wheat, sugar cane) and cotton, have shown that nitrogen residual effect, fixed by soybean crop and its residues, replaces partial the nitrogen on following crop, resulting in field optimization and alleviating part of the production costs [15]. In China, for example, soybean is commonly grown continuous‐ ly in monoculture rather than rotated with other crops, like maize or wheat. The soybean monoculture results in yield decline, as well as its quality. The yield reduction on soybean in 2, 3 and 4-year monoculture was 15%, 20%, and 30%, respectively [16,17], highlighting the significance of rotational system in the preservation of crop production. Furthermore, several experiments suggest that carbon and nitrogen from microbial biomass (particularly nitro‐ gen) are sensitively affected by soil- and crop-management regimens, being directly

Using cover crops between the main crops (fallow period) is also part of conservation practices and it represents a breakthrough in weed management. Besides, competing against weeds,

them for reducing weed interference? and iii) When should one use them?

resistant to herbicide from different modes of action.

a feed source.

50 Soybean - Pest Resistance

influenced by crop rotation [18].

However, the selection of proper cover crop is essential for the success of the system. Plantfeeding nematodes, for example, were less abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affects soil food web structure [22]. Other species, such as those from genus Desmodium, may be used suppressing *Striga hermonthica* (Del.) Benth. by means of an allelopathic mechanism. Their root exudates contain novel flavonoid compounds, which stimulate suicidal germination of *S. hermonthica* seeds and dramatically inhibit its attachment to host roots [23].

Herbicides, in the broad action spectrum, are and will be essential tools in weed management, even for those with a great number of resistant weeds. But the trend is that using different herbicide is increasingly related to GM crops which show resistance to more than one active ingredient. For new GM soybean, 2,4-D and dicamba resistance traits will always be used in stacks with at least one other herbicide-resistant trait. Glyphosate and ALS trait stack, recently deregulated in the US, possibly will allow the use of ALS-inhibiting herbicides with soil residual that are too phytotoxic to use on conventional crop cultivars [24]. In reference [25], diversification may make weed management more complex, but growers must not use new GM crop resistant to herbicides in the same way that some used initial GM crops, in order to rely only on one herbicide until it is no longer effective and then switch herbicides. Research alerts that "if growers use the new GM crops and the herbicides that they enable properly, GM crops will expand the utility of currently available herbicides and provide long-term solutions to manage resistant weeds".

Answering the question related to the period when control tools should be used, different opinions arise. Many specialists recommend to use tools, especially chemical control, only when economic loss level is reached, ie, when population density finds a minimum threshold at which costs of controlling are lower than economic damage coming from losses by weed interference. Nevertheless, by following the concept of integrated management, it is recom‐ mended the use of many available tools, even at fallow periods or at low weed densities. In reference [26], as opposed to pest and pathogens which attack crops in epidemic cycles, weeds are endemic, regenerating from the seed and/or vegetative propagules that are introduced into the soil; thus, the continuous management allows the best result. Besides, confining weed management to a narrow temporal window increases the risk of unsatisfying weed manage‐ ment outcomes due to unfavorable weather [27]. Coupled with this agreement, good man‐ agement models for weed control may join forces to the definition of weed control periods according to their competitive ability and the local crop conditions set out during the growing (climate, cultivar, sowing density, etc).

*delianum* (L.) Garcke against four bacterial species, *Xanthomonas axonopodis*, *Pseudomonas syrin‐ gae*, *Corynebacterium minutissium*, *Clostridium difficile* and major seed-born fungi *Aspergillus niger*, *Alternaria alternata*, *Drechslera biseptata*, *Fusarium solani* in vitro. Leaf extracts of these weeds exhibit antimicrobial effects and all were moderately active against seed-born fungi.

Weed Management in Soybean — Issues and Practices

http://dx.doi.org/10.5772/54595

53

Some experiments found preliminary details, which suggest that the presence of weeds that serve as hosts of both tobacco rattle virus (Corky ringspot disease) and *Paratrichodorus allius* (root nematode) may nullify the positive effects of growing alfalfa or Scotch spearmint for Corky ringspot control conducted [43]. For all species researched, *Solanum sarrachoides* Sendtn

Weed management is also associated with most pests on crop cultivation; ecological relation‐ ships set out among organisms (weeds, insects, mites, etc.) allow their maintenance and proliferation. Examples of pest and weed interactions established in soybean has been reported by [44], who found anticipation of 14 days at critical period of weed control when crop was 60% defoliated by insects. Increasing of *Anticarsia gemmatalis* Hübner oviposition was also logged in [45] when soybean presented a high infestation of *Sesbania exaltata* (Raf.) Rydb. ex A.W. Hill. Thus, S. exaltata management reduces *A. Gemmatalis* population. Overall, mono‐ culture areas tend to present higher mites and pest infestation and reduced biological diversity when maintained free of weeds. At the same time, weeds help insect diversity and natural biological control [46]. Mites, important arthropods in agricultural systems, currently consti‐ tute themselves key pests for soybeans in regions of hot and dry weather. Some predatory mites can be used against them into the biological management scope. Therefore, a funda‐ mental aspect is the alternative feed sources for predatory mites during periods in which mite pests are at low populations. Among feed sources, there are many weeds, especially *Ageratum conyzoides* L., commonly encountered in citrus orchards and further agricultural areas. Overall, dicotyledonous weeds that produce a lot of pollen are preferred by predatory mites, in particular the genus Euseius [47]. Phytophagous mites, especially the web mite family Tetranychidae, were traditionally considered secondary pests in soybean. However, in recent years, it has been recorded severe and frequent attacks of these in different producing regions in Brazil [48]. Into surveys about GR soybean carried out in the state of Rio Grande do Sul, six phytophagous mite species were identified, five tetranychid — *Mononychellus planki* (McGre‐ gor), *Tetranychus desertorum* (Banks), *T. gigas* (Pritchard & Baker), *T. ludeni* (Zacher) and *T. urticae* (Koch) — and white mite tarsonemid *Polyphagotarsonemus latus* (Banks) [49,50]. In most of the sampled sites, more than one tetranychid were reported, being directly influenced by

The integrated management of weeds and pests, despite essential, is not easy to be performed on extensive production systems, especially because there are interactions of many species having various relations, either symbiosis, predation or parasitism. Knowing the interactions and the organisms that comprise production system is the great challenge and it can bring good results. Examples can be viewed in [51], with the reporting of lepidopterous in corn, in cotton [52], in *Heliothis zea* [53] and *H. virescens* [54] with *Bemisia tabaci* (Genn), among others. Maintaining biodiversity and sustainable production are some of the main advantages of using

presented positive correlation with Corky ringspot disease.

weed management.

these systems [55].

So far, absence of management or misuse of control tools may undermine the productivity, the sustainability of system production and the agricultural activity, also interfering in the preservation and balance between species. Thus, interactions among weeds and further organisms (fungi, viruses, bacteria, mites, insects, nematodes, etc.) as well as their handling may have a direct or indirect impact into the production system.

#### **2.2. Impact of weed management on nontarget organisms**

Many studies have attempted to relate the intensification of certain pathogenic diseases of shoot plants in areas annually treated with herbicides, being placed on proof the intensive use of those mainly in no tillage system. Glyphosate, for example, is a highly effective broadspectrum herbicide that is phytotoxically active on a large number of weeds and crop species across a wide range of taxa [28]. Glyphosate inhibits the biosynthesis of aromatic aminoacids, thereby reducing biosynthesis of proteins, auxins, pathogen defense compounds, phytoalex‐ ins, folic acid, precursors of lignins, flavonoids, plastoquinone, and hundreds of other phenolic and alkaloid compounds [29]. These effects could increase the susceptibility of glyphosatesensitive plants to pathogens or other stress agents [30]. Engineered to express enzymes that are insensitive to or are able to metabolize glyphosate, GR crops have enabled farmers to easily apply this herbicide in soybean, corn, cotton, canola, sugar beet and alfalfa, besides controlling problematic weeds without harming the crop [28].

For glyphosate and its interspecific transfer from weeds to nontarget organisms, in [31] it was related the increasing remark number of plant diseases growing in long term [32]. But the herbicide influence on disease incidence at glyphosate-resistant crops has varied. While in [33,34] it was observed an increase of *Fusarium solani* (Mart.) Sacc. in soybean, others showed a reduction of *Phakopsora pachyrhizi* Sidow at this crop [35]. For nitrogen-fixing microorganisms in soybean, negative interference of glyphosate has been proven by different authors [36-39], usually in laboratory experiments, with clear differences among rhizobial strains, as well as among glyphosate formulations, having roughly deleterious effects according to combinations of these.

Disease caused by *Sclerotinia sclerotiorum* (Lib.) de Bary, for example, occurs in numerous weeds considered plant hosts. Crop rotation is essential in this case, specially when it uses non host crops and some herbicides with effects over the weed hosts and, consequently, the disease. In [40], the use of chemical weed management with sethoxydim, an important herbicide on soybean system, had the biggest toxicity rate together with cycloxydim. Other herbicides tested, such as cycloxidim and haloxyfop-ethoxy-ethyl, had less impact on *S. sclerotiorum*, but negative action on *Trichoderma* sp..

In other cases, not only herbicides, but also weeds, can supply the decrease of several crop dis‐ eases, so that their management is extremely important. In [41] it was investigate the efficacy of three common weeds, i.e., *Amaranthus viridis* (L.), *Lantana camara* (L.) and *Malvastrum coroman‐* *delianum* (L.) Garcke against four bacterial species, *Xanthomonas axonopodis*, *Pseudomonas syrin‐ gae*, *Corynebacterium minutissium*, *Clostridium difficile* and major seed-born fungi *Aspergillus niger*, *Alternaria alternata*, *Drechslera biseptata*, *Fusarium solani* in vitro. Leaf extracts of these weeds exhibit antimicrobial effects and all were moderately active against seed-born fungi.

agement models for weed control may join forces to the definition of weed control periods according to their competitive ability and the local crop conditions set out during the growing

So far, absence of management or misuse of control tools may undermine the productivity, the sustainability of system production and the agricultural activity, also interfering in the preservation and balance between species. Thus, interactions among weeds and further organisms (fungi, viruses, bacteria, mites, insects, nematodes, etc.) as well as their handling

Many studies have attempted to relate the intensification of certain pathogenic diseases of shoot plants in areas annually treated with herbicides, being placed on proof the intensive use of those mainly in no tillage system. Glyphosate, for example, is a highly effective broadspectrum herbicide that is phytotoxically active on a large number of weeds and crop species across a wide range of taxa [28]. Glyphosate inhibits the biosynthesis of aromatic aminoacids, thereby reducing biosynthesis of proteins, auxins, pathogen defense compounds, phytoalex‐ ins, folic acid, precursors of lignins, flavonoids, plastoquinone, and hundreds of other phenolic and alkaloid compounds [29]. These effects could increase the susceptibility of glyphosatesensitive plants to pathogens or other stress agents [30]. Engineered to express enzymes that are insensitive to or are able to metabolize glyphosate, GR crops have enabled farmers to easily apply this herbicide in soybean, corn, cotton, canola, sugar beet and alfalfa, besides controlling

For glyphosate and its interspecific transfer from weeds to nontarget organisms, in [31] it was related the increasing remark number of plant diseases growing in long term [32]. But the herbicide influence on disease incidence at glyphosate-resistant crops has varied. While in [33,34] it was observed an increase of *Fusarium solani* (Mart.) Sacc. in soybean, others showed a reduction of *Phakopsora pachyrhizi* Sidow at this crop [35]. For nitrogen-fixing microorganisms in soybean, negative interference of glyphosate has been proven by different authors [36-39], usually in laboratory experiments, with clear differences among rhizobial strains, as well as among glyphosate formulations, having roughly deleterious effects according to combinations

Disease caused by *Sclerotinia sclerotiorum* (Lib.) de Bary, for example, occurs in numerous weeds considered plant hosts. Crop rotation is essential in this case, specially when it uses non host crops and some herbicides with effects over the weed hosts and, consequently, the disease. In [40], the use of chemical weed management with sethoxydim, an important herbicide on soybean system, had the biggest toxicity rate together with cycloxydim. Other herbicides tested, such as cycloxidim and haloxyfop-ethoxy-ethyl, had less impact on *S. sclerotiorum*, but

In other cases, not only herbicides, but also weeds, can supply the decrease of several crop dis‐ eases, so that their management is extremely important. In [41] it was investigate the efficacy of three common weeds, i.e., *Amaranthus viridis* (L.), *Lantana camara* (L.) and *Malvastrum coroman‐*

(climate, cultivar, sowing density, etc).

52 Soybean - Pest Resistance

may have a direct or indirect impact into the production system.

**2.2. Impact of weed management on nontarget organisms**

problematic weeds without harming the crop [28].

of these.

negative action on *Trichoderma* sp..

Some experiments found preliminary details, which suggest that the presence of weeds that serve as hosts of both tobacco rattle virus (Corky ringspot disease) and *Paratrichodorus allius* (root nematode) may nullify the positive effects of growing alfalfa or Scotch spearmint for Corky ringspot control conducted [43]. For all species researched, *Solanum sarrachoides* Sendtn presented positive correlation with Corky ringspot disease.

Weed management is also associated with most pests on crop cultivation; ecological relation‐ ships set out among organisms (weeds, insects, mites, etc.) allow their maintenance and proliferation. Examples of pest and weed interactions established in soybean has been reported by [44], who found anticipation of 14 days at critical period of weed control when crop was 60% defoliated by insects. Increasing of *Anticarsia gemmatalis* Hübner oviposition was also logged in [45] when soybean presented a high infestation of *Sesbania exaltata* (Raf.) Rydb. ex A.W. Hill. Thus, S. exaltata management reduces *A. Gemmatalis* population. Overall, mono‐ culture areas tend to present higher mites and pest infestation and reduced biological diversity when maintained free of weeds. At the same time, weeds help insect diversity and natural biological control [46]. Mites, important arthropods in agricultural systems, currently consti‐ tute themselves key pests for soybeans in regions of hot and dry weather. Some predatory mites can be used against them into the biological management scope. Therefore, a funda‐ mental aspect is the alternative feed sources for predatory mites during periods in which mite pests are at low populations. Among feed sources, there are many weeds, especially *Ageratum conyzoides* L., commonly encountered in citrus orchards and further agricultural areas. Overall, dicotyledonous weeds that produce a lot of pollen are preferred by predatory mites, in particular the genus Euseius [47]. Phytophagous mites, especially the web mite family Tetranychidae, were traditionally considered secondary pests in soybean. However, in recent years, it has been recorded severe and frequent attacks of these in different producing regions in Brazil [48]. Into surveys about GR soybean carried out in the state of Rio Grande do Sul, six phytophagous mite species were identified, five tetranychid — *Mononychellus planki* (McGre‐ gor), *Tetranychus desertorum* (Banks), *T. gigas* (Pritchard & Baker), *T. ludeni* (Zacher) and *T. urticae* (Koch) — and white mite tarsonemid *Polyphagotarsonemus latus* (Banks) [49,50]. In most of the sampled sites, more than one tetranychid were reported, being directly influenced by weed management.

The integrated management of weeds and pests, despite essential, is not easy to be performed on extensive production systems, especially because there are interactions of many species having various relations, either symbiosis, predation or parasitism. Knowing the interactions and the organisms that comprise production system is the great challenge and it can bring good results. Examples can be viewed in [51], with the reporting of lepidopterous in corn, in cotton [52], in *Heliothis zea* [53] and *H. virescens* [54] with *Bemisia tabaci* (Genn), among others. Maintaining biodiversity and sustainable production are some of the main advantages of using these systems [55].
