**4. Management of weeds in soybean areas: Argentina, Brazil and the USA**

#### **4.1. Weed management in Argentina**

The first recorded experience with soybeans in Argentina was in 1862, just a few years after their introduction to the US, but back then the country was a stronghold of cattle production, and there was little interest in agriculture. The first variety trials and commercial harvests occurred during the 60s. At the turn of the century, soybeans in Argentina were reaching the 10,000,000 hectares mark, coinciding with the adoption of transgenic GR soybeans. Soybean production increased over 1000-fold to a record of 52 million metric tons in 2010. The most productive area for soybeans is comprised by the northern portion of the province of Buenos Aires, the central and southern part of the province of Santa Fe, and the southeastern part of Córdoba (humid pampas), but in recent years the expansion has been more noticeable in other provinces, like Entre Ríos, Santiago del Estero, Tucumán, Salta and Chaco, in the northern part of Argentina. Another factor influenced by the adoption of GR soybeans was the oversimpli‐ fication of the weed control programs, which eventually led to the selection of resistant biotypes and hard-to-control weeds.

The development and early expansion of the crop in Argentina was accompanied by the constant introduction of new herbicide molecules. During the 70s, as farmers in Argentina were learning how to grow this crop, the most common weed control methods in soybeans were a combination of tillage and pre-emergent (PRE) herbicides such as trifluralin, dinitra‐ mine (dinitroanilines), cloramben (benzoic acid), naptalam (amide), flucloralin (chloroanilin), vernolate (thiocarbamate), metribuzin, prometrin (triazines), alaclor (chloracetamide), and linuron (phenylurea). Bentazon, one of the first post-emergent options, did not become available until the end of that decade. The dinitroanlinies, flucloralin, and vernolate were used on pre-planting incorporated (PPI) for annual grasses and broadleaves control, clormaben was one of the few burndown options for broadleaves, naptalam was applied PRE for annual grasses and broadleaves, the triazines also PRE, for small seeded broadleaves, often in combination with alachlor to improve annual grass control, and linuron offered broad spectrum control also applied PRE.

As a result of the limited choices in herbicides in soybean, there were several weed problems, such as the perennial grasses *Sorghum halepense* (L.) Pers. and *Cynodon dactylon* (L.) Pers., several annual grasses, such as *Digitaria sanguinalis* (L.) Scop.*, Echinochloa crus-galli* (L.) Beauv.*, E. colonum* (L.) Moench*, Eleusine indica* (L.) Gaertn., and the typical broadleaf weeds of summer crops — *Amaranthus* sp., *Chenopodium album* L.*, C. cordobense* Aellen*, C. pumilio* R. Br., *Datura ferox* auct. non L., *Tagetes minuta* L.*, Ipomoea* spp.. It was mention at least 6 species of Ipomoea*, Xanthium strumarium* L.*, X. cavanillesii* Shouw*, Anoda cristata* (L.) Schlecht. and *Portulaca oleracea* L. [71] *—* among the broadleaf weeds in the humid pampas (Table 4). These plants represented a challenge and slowed the initial expansion of the crop. Most of the weeds described here are the same or very similar to the weeds commonly found in conventionaltillage systems around the world. A very interesting point is that none of the broadleaf weeds that are posing a challenge today to glyphosate in the temperate region is in this list, and most of the emerging weeds are local weeds, not common in other regions.


\*Sunflower was a common component of the rotation systems.

The outlook is that the main crops (soybean, corn, cotton) from Brazil, the USA and Argentina will be resistant to glyphosate. In this context, succession and crop rotation with conventional seeds is a strong chance in the field. There is the necessity to convince farmers that repeated and continuous use of glyphosate-resistant crops in few years could cripple the weed control

Evolution of glyphosate-resistant populations is an imminent threat in areas where there is dominance of glyphosate-resistant crops, intense selection pressure and no diversity [70]. Certainly other glyphosate-resistant weeds will be identified in the coming years. But when and how it is related to use of glyphosate-resistant crops? The use of practices to reduce selection pressure and switch mechanisms is important to protect and prolong the use of

**4. Management of weeds in soybean areas: Argentina, Brazil and the USA**

The first recorded experience with soybeans in Argentina was in 1862, just a few years after their introduction to the US, but back then the country was a stronghold of cattle production, and there was little interest in agriculture. The first variety trials and commercial harvests occurred during the 60s. At the turn of the century, soybeans in Argentina were reaching the 10,000,000 hectares mark, coinciding with the adoption of transgenic GR soybeans. Soybean production increased over 1000-fold to a record of 52 million metric tons in 2010. The most productive area for soybeans is comprised by the northern portion of the province of Buenos Aires, the central and southern part of the province of Santa Fe, and the southeastern part of Córdoba (humid pampas), but in recent years the expansion has been more noticeable in other provinces, like Entre Ríos, Santiago del Estero, Tucumán, Salta and Chaco, in the northern part of Argentina. Another factor influenced by the adoption of GR soybeans was the oversimpli‐ fication of the weed control programs, which eventually led to the selection of resistant

The development and early expansion of the crop in Argentina was accompanied by the constant introduction of new herbicide molecules. During the 70s, as farmers in Argentina were learning how to grow this crop, the most common weed control methods in soybeans were a combination of tillage and pre-emergent (PRE) herbicides such as trifluralin, dinitra‐ mine (dinitroanilines), cloramben (benzoic acid), naptalam (amide), flucloralin (chloroanilin), vernolate (thiocarbamate), metribuzin, prometrin (triazines), alaclor (chloracetamide), and linuron (phenylurea). Bentazon, one of the first post-emergent options, did not become available until the end of that decade. The dinitroanlinies, flucloralin, and vernolate were used on pre-planting incorporated (PPI) for annual grasses and broadleaves control, clormaben was one of the few burndown options for broadleaves, naptalam was applied PRE for annual grasses and broadleaves, the triazines also PRE, for small seeded broadleaves, often in combination with alachlor to improve annual grass control, and linuron offered broad

important molecules such as triazines, ALS inhibitors, ACCase, and glycines.

with the use of glyphosate-based products.

60 Soybean - Pest Resistance

**4.1. Weed management in Argentina**

biotypes and hard-to-control weeds.

spectrum control also applied PRE.

**Table 4.** Most important weeds in the humid pampas in 1997, before the adoption of GR soybeans [72].

Usually a moldboard plow was used in the fall to incorporate the previous crop residue and destroy existing vegetation. Herbicides were part of the control methods from the beginning, given the timing of the introduction of soybeans in Argentina, so a mechanical-only technology was never developed for the region, except for specific purposes, like organic soybeans. In the spring, residual herbicides were applied after the preparation of the seedbed, incorporating them if needed. There were several escape problems given the limitation of POST options, especially with large seeded broadleaf weeds like *D. ferox*, *A. cristata*, and *Ipomoea* spp. The problem was so common that in many areas a special device called "Chamiquera" (Figure 3) was used to separate the harvested soybeans from "Chamico" (*D. ferox*) and sometimes "Bejucos" (*Ipomoea* spp.) seeds before the beans could be delivered at the grain elevators.

first approach to no tillage in soybeans. The resistance problems associated with this group were not noticeable in Argentina, although the first resistant weed in the country is resistant to this herbicide group (ALS inhibitors), because it coincided with the introduction of the GR tech‐

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Sources: 1979-2004, Secretaría de Agricultura, Ganadería, Pesca y Alimentos de la República Argentina. 2005-2012,

**Figure 4.** Soybean production, in million metric tons, from 1979 to 2012. In red: first year with commercial GR soy‐

New inhibitors of the protoporphyrinogen oxidase enzime herbicides were introduced during the late 90s. Carfentrazone, sulfentrazone (aryl triazinones) and flumioxazin (N-phenylphta‐ limides derivative) offered new options for burndown (carfentrazone) and residual control (sulfentrazone, flumioxazin), but the introduction of the GR soybean varieties prevented its adoption, thus the most dramatic expansion of soybean production in Argentina was the

Nearly all the soybeans in Argentina are transgenic (GR1). Argentina had the fastest adoption of glyphosate-resistant soybeans in the world. This fast adoption coincided with the expansion of no tillage technology in the region, fueling a synergism between GR soybeans and no tillage. AAPRESID, the national association of no tillage farmers, had held its first national symposium a few years prior to the launching of this technology, and its members welcomed and quickly embraced a new biotech development that allowed them to fully implement their preferred

Until the adoption of GR soybeans, tillage was an important weed control method, comple‐ menting chemical control options, but it had a negative impact on erosion, soil structure and or‐ ganic matter mineralization. The introduction of herbicide-resistant varieties increased not only the use of glyphosate, but also the practice of no tillage as well, replacing mechanical con‐

Diario La Nación, May 24, 2012.

technology.

beans. In yellow: droughts of the 08-09 and 11-12 seasons.

introduction of the glyphosate resistant varieties in 1996.

nology, and the quick adoption of the new varieties masked the problem.

**Figure 3.** Special device "Chamiquera", Rojas, Buenos Aires, circa 1980.

During the 80s and 90s, until the introduction of GR soybeans, the development of several new molecules improved the control of many weeds, but still in combination with mechanical methods, leading to a steady expansion of both the area planted with soybeans and the average yields (Figure 4). Gradually, new herbicides allowed technology developments that replaced, at least in part, mechanical control methods with chemical ones. The need of field cultivators was reduced or replaced by the application of pre-emergent combinations of alachlor and metribuzin that offered a wide spectrum of control and proven residuality, replacing other herbicides — like trifluralin — that required mechanical incorporation.

Acifluorfen became a common tool for rescuing treatment, even though it caused severe crop injury. This herbicide allowed the control of large seeded broadleaf weeds — *Xanthium* spp., *D. ferox*, late flushes of *Ipomoea* spp. —, all common problems in most of the soybean area, but the injury it caused to the crop was something the farmer was not used to dealing with. It was re‐ placed in part by another diphenylether, fomesafen, although it did not have the same efficacy or control spectrum. The registration of ALS inhibiting herbicides (sulfonylureas, imidazoli‐ nones and triazolopyrimidines) ushered a new era of weed control in soybeans in Argentina, al‐ lowing for PRE/POST combinations that offered effective and lasting control of the most important weeds with less crop injury than the previous options. Imazaquin, imazethapyr, chlorimuron, diclosulam and flumetsulam were launched in Argentina during the second half of the 80s (the first registration of imazaquin was actually in Argentina, in 1984) and allowed the first approach to no tillage in soybeans. The resistance problems associated with this group were not noticeable in Argentina, although the first resistant weed in the country is resistant to this herbicide group (ALS inhibitors), because it coincided with the introduction of the GR tech‐ nology, and the quick adoption of the new varieties masked the problem.

was used to separate the harvested soybeans from "Chamico" (*D. ferox*) and sometimes "Bejucos" (*Ipomoea* spp.) seeds before the beans could be delivered at the grain elevators.

During the 80s and 90s, until the introduction of GR soybeans, the development of several new molecules improved the control of many weeds, but still in combination with mechanical methods, leading to a steady expansion of both the area planted with soybeans and the average yields (Figure 4). Gradually, new herbicides allowed technology developments that replaced, at least in part, mechanical control methods with chemical ones. The need of field cultivators was reduced or replaced by the application of pre-emergent combinations of alachlor and metribuzin that offered a wide spectrum of control and proven residuality, replacing other

Acifluorfen became a common tool for rescuing treatment, even though it caused severe crop injury. This herbicide allowed the control of large seeded broadleaf weeds — *Xanthium* spp., *D. ferox*, late flushes of *Ipomoea* spp. —, all common problems in most of the soybean area, but the injury it caused to the crop was something the farmer was not used to dealing with. It was re‐ placed in part by another diphenylether, fomesafen, although it did not have the same efficacy or control spectrum. The registration of ALS inhibiting herbicides (sulfonylureas, imidazoli‐ nones and triazolopyrimidines) ushered a new era of weed control in soybeans in Argentina, al‐ lowing for PRE/POST combinations that offered effective and lasting control of the most important weeds with less crop injury than the previous options. Imazaquin, imazethapyr, chlorimuron, diclosulam and flumetsulam were launched in Argentina during the second half of the 80s (the first registration of imazaquin was actually in Argentina, in 1984) and allowed the

**Figure 3.** Special device "Chamiquera", Rojas, Buenos Aires, circa 1980.

62 Soybean - Pest Resistance

herbicides — like trifluralin — that required mechanical incorporation.

Sources: 1979-2004, Secretaría de Agricultura, Ganadería, Pesca y Alimentos de la República Argentina. 2005-2012, Diario La Nación, May 24, 2012.

**Figure 4.** Soybean production, in million metric tons, from 1979 to 2012. In red: first year with commercial GR soy‐ beans. In yellow: droughts of the 08-09 and 11-12 seasons.

New inhibitors of the protoporphyrinogen oxidase enzime herbicides were introduced during the late 90s. Carfentrazone, sulfentrazone (aryl triazinones) and flumioxazin (N-phenylphta‐ limides derivative) offered new options for burndown (carfentrazone) and residual control (sulfentrazone, flumioxazin), but the introduction of the GR soybean varieties prevented its adoption, thus the most dramatic expansion of soybean production in Argentina was the introduction of the glyphosate resistant varieties in 1996.

Nearly all the soybeans in Argentina are transgenic (GR1). Argentina had the fastest adoption of glyphosate-resistant soybeans in the world. This fast adoption coincided with the expansion of no tillage technology in the region, fueling a synergism between GR soybeans and no tillage. AAPRESID, the national association of no tillage farmers, had held its first national symposium a few years prior to the launching of this technology, and its members welcomed and quickly embraced a new biotech development that allowed them to fully implement their preferred technology.

Until the adoption of GR soybeans, tillage was an important weed control method, comple‐ menting chemical control options, but it had a negative impact on erosion, soil structure and or‐ ganic matter mineralization. The introduction of herbicide-resistant varieties increased not only the use of glyphosate, but also the practice of no tillage as well, replacing mechanical con‐ trol almost completely in soybean production. The high efficacy of this herbicide combined with the simplicity of the system resulted in a quick replacement of other herbicides used in soy‐ beans, both over the top applications and during the chemical fallow period. In 2005, over 92% of the herbicide volume used in chemical fallow was glyphosate, while some hormonal herbi‐ cides were commonly tank-mixed with glyphosate to improve the control of thistles and other "new" weeds. Overall costs of weed control in soybeans decreased dramatically as new generic glyphosate brands entered the Argentine market. Another aspect that contributed to the simpli‐ fication of the system, including soybean monoculture, was the general economic situation of the country. Corn required a higher investment, while soybeans, especially GR soybeans, as de‐ scribed above, allowed farmers to plan their soybean season with less financial requirements (in Argentina, the law allows farmers to save seeds for their own use) in times when the prices of commodities were uncertain and financial means were limited, or expensive.

and only glyphosate over the crop. From the three herbicides mentioned in the surveys, there were no specific graminicides (Table 5), so it is not a surprise that all the weeds that have been

> Flumetsulam 2,4-D Metribuzin Metsulfuron

> > Fallow applications

PRE/Over the top

Based on individual responses to surveys, % for each answer is omitted. FOPs (aryloxyphenoxypropionate herbicide

**Table 5.** Herbicides, other than glyphosate, used in soybean production before and after the introduction of GR

Apart from the confirmed cases of glyphosate-resistant weeds, there are several problems caused by the excessive use of glyphosate. To better understand the problem, it is important to state that in Argentina about 70% of the farming is done in rented land, and during the last decade the rental price has increased constantly. In many cases, this situation prevented the traditional early fallow procedures and resorted to burndown practices with weeds that had grown beyond their optimal control stage. One particular case is *C. bonariensis*, which has been confirmed to be resistant to glyphosate in Brazil, although the resistant biotype is not present in Argentina yet. When treated at the rosette stage, the plant is susceptible to be controlled with glyphosate, but when it has elongated (early in the spring), it becomes resilient, even

Atrazine

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confirmed as resistant to glyphosate are grasses.

Fallow applications

PRE/Over the top

group)

**Before 1997 (1995-1997) After 1997**

Picloram

MCPA Dicamba Atrazine 2,4-D Metsulfuron Other herbicides No applications

Flumioxazim

Clorimuron 2,4-DB Imazaquin Acetochlor

Graminicides (FOP's) Flumetsulam Diclosulam Imazetapyr Other herbicides No applications

Source: courtesy of Professor Daniel Tuesca, Universidad Nacional de Rosario, AR.

soybeans in Argentina, according to surveys with farmers.

Glyphosate effectively controlled not only the most problematic weeds in soybean fields; it replaced herbicide combinations that required a deep knowledge of the weed spectrum, careful planning to avoid escapes, tank mix problems, timing concerns and crop injury, and still did not offer the satisfaction of a field completely clean of weeds. RR technology simplified the business of growing soybeans like no other technology ever developed. Today, soybean system is characterized by over-reliance on glyphosate, low crop rotation, absence of mechan‐ ical control methods and limited monitoring (of both weeds present at the time of application and results). The lack of monitoring practices is a direct result of the high efficacy of glyphosate control in the early years of the biotech age. As a result, the weed spectrum has shifted and there are several glyphosate-resistant weeds, combined with hard-to-control ones, while the presence of weeds with resistance to other modes of action is still limited.

Glyphosate is still a very valuable weed control tool, in spite of the weed shift that Argentina has experienced due to its over-use. In [73] it was studied the effectiveness of glyphosate applications at two stages (vegetative and reproductive) on 31 weeds that represented the typical weed spectrum of the region. The herbicide had complete control on 58% of the species at both stages, complete control at the vegetative stage but deficient control at the reproductive stage on 32% of the species and poor control on only 10% of the species at either stage. Disregarding the poor control at the reproductive stage-only, which is not recommended, it is clear that glyphosate satisfactorily controlled 90% of the weeds. The remaining 10% can be managed easily combining glyphosate with the proper herbicides, providing a cost-effective complement. The control of some of these difficult weeds is improved when glyphosate is combined with atrazine or metsulfuron applied during fall [74]. For example, *Bowlesia incana* Ruiz & Pavón and *Parietaria debilis* G. Forst., increased when glyphosate was applied as a tank mix to these herbicides, compared to glyphosate by itself. These herbicides are readily available and are cost-effective alternatives to combine with glyphosate.

The selection of herbicide-resistant biotypes was a consequence of lack of crop and herbicide rotations. Daniel Tuesca, a weed scientist in the University of Rosario, states that, in the years preceding the introduction of the GR soybean, farmers mentioned, on surveys, the use of 16 different herbicides in the fallow process and 13 on the crop (either PRE or POST), but a few years after the introduction of the technology, there were only 3 herbicides applied in fallow, and only glyphosate over the crop. From the three herbicides mentioned in the surveys, there were no specific graminicides (Table 5), so it is not a surprise that all the weeds that have been confirmed as resistant to glyphosate are grasses.

trol almost completely in soybean production. The high efficacy of this herbicide combined with the simplicity of the system resulted in a quick replacement of other herbicides used in soy‐ beans, both over the top applications and during the chemical fallow period. In 2005, over 92% of the herbicide volume used in chemical fallow was glyphosate, while some hormonal herbi‐ cides were commonly tank-mixed with glyphosate to improve the control of thistles and other "new" weeds. Overall costs of weed control in soybeans decreased dramatically as new generic glyphosate brands entered the Argentine market. Another aspect that contributed to the simpli‐ fication of the system, including soybean monoculture, was the general economic situation of the country. Corn required a higher investment, while soybeans, especially GR soybeans, as de‐ scribed above, allowed farmers to plan their soybean season with less financial requirements (in Argentina, the law allows farmers to save seeds for their own use) in times when the prices of

Glyphosate effectively controlled not only the most problematic weeds in soybean fields; it replaced herbicide combinations that required a deep knowledge of the weed spectrum, careful planning to avoid escapes, tank mix problems, timing concerns and crop injury, and still did not offer the satisfaction of a field completely clean of weeds. RR technology simplified the business of growing soybeans like no other technology ever developed. Today, soybean system is characterized by over-reliance on glyphosate, low crop rotation, absence of mechan‐ ical control methods and limited monitoring (of both weeds present at the time of application and results). The lack of monitoring practices is a direct result of the high efficacy of glyphosate control in the early years of the biotech age. As a result, the weed spectrum has shifted and there are several glyphosate-resistant weeds, combined with hard-to-control ones, while the

Glyphosate is still a very valuable weed control tool, in spite of the weed shift that Argentina has experienced due to its over-use. In [73] it was studied the effectiveness of glyphosate applications at two stages (vegetative and reproductive) on 31 weeds that represented the typical weed spectrum of the region. The herbicide had complete control on 58% of the species at both stages, complete control at the vegetative stage but deficient control at the reproductive stage on 32% of the species and poor control on only 10% of the species at either stage. Disregarding the poor control at the reproductive stage-only, which is not recommended, it is clear that glyphosate satisfactorily controlled 90% of the weeds. The remaining 10% can be managed easily combining glyphosate with the proper herbicides, providing a cost-effective complement. The control of some of these difficult weeds is improved when glyphosate is combined with atrazine or metsulfuron applied during fall [74]. For example, *Bowlesia incana* Ruiz & Pavón and *Parietaria debilis* G. Forst., increased when glyphosate was applied as a tank mix to these herbicides, compared to glyphosate by itself. These herbicides are readily available

The selection of herbicide-resistant biotypes was a consequence of lack of crop and herbicide rotations. Daniel Tuesca, a weed scientist in the University of Rosario, states that, in the years preceding the introduction of the GR soybean, farmers mentioned, on surveys, the use of 16 different herbicides in the fallow process and 13 on the crop (either PRE or POST), but a few years after the introduction of the technology, there were only 3 herbicides applied in fallow,

commodities were uncertain and financial means were limited, or expensive.

64 Soybean - Pest Resistance

presence of weeds with resistance to other modes of action is still limited.

and are cost-effective alternatives to combine with glyphosate.


Based on individual responses to surveys, % for each answer is omitted. FOPs (aryloxyphenoxypropionate herbicide group)

Source: courtesy of Professor Daniel Tuesca, Universidad Nacional de Rosario, AR.

**Table 5.** Herbicides, other than glyphosate, used in soybean production before and after the introduction of GR soybeans in Argentina, according to surveys with farmers.

Apart from the confirmed cases of glyphosate-resistant weeds, there are several problems caused by the excessive use of glyphosate. To better understand the problem, it is important to state that in Argentina about 70% of the farming is done in rented land, and during the last decade the rental price has increased constantly. In many cases, this situation prevented the traditional early fallow procedures and resorted to burndown practices with weeds that had grown beyond their optimal control stage. One particular case is *C. bonariensis*, which has been confirmed to be resistant to glyphosate in Brazil, although the resistant biotype is not present in Argentina yet. When treated at the rosette stage, the plant is susceptible to be controlled with glyphosate, but when it has elongated (early in the spring), it becomes resilient, even when using 2 and 3 times the dose of glyphosate. The situation changes when residual herbicides are applied in the fall (flumioxazin, metsulfuron, atrazine, and diclosulam have proved to be effective). There was a lot of confusion when this weed began to emerge as a problem because it co-exists with another species, *C. sumatrensis*, more susceptible to be controlled by glyphosate applications at later stages, leading to a general belief that there are resistant biotypes that escape control. Again, the lack of monitoring practices is evident here. These weeds are strongly associated with no-tillage practices, since they do not progress at all in tilled soil.

Few herbicides used previously restricted the implementation period, affecting more specific actions for managing the weeds emerged in advanced stages of the crop. The launch of bentazon POST herbicide revolutionized the market, allowing the control of major dicotyled‐ onous weeds on soybean. Introduction of new molecules from the 80s and 90s afforded efficiency on the control of several species, in particular those belonging to genders Amaran‐ thus, Digitaria, Brachiaria, Euphorbia and Bidens. The main herbicides applied belonged to the chemical groups ALS and ACCase inhibitors, with monocotyledonous and dicotyledonous

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Since the introduction of no tillage system, weed management has changed and, as a conse‐ quence, moved to consider factors other than chemical control on the production system. The main benefit of no tillage system is the reduction of weed germination over time [75] and greater use of crop control. Furthermore, species not commonly observed in the conventional system demand better preparation and expertise of producers. Such modifications are related to the absence of soil disturbance, favoring perennial cycle weeds, as well as changes in patterns of temperature and light incidence, influencing seeds' mechanisms of dormancy. Cover crops result in greater amount of organic residue, with higher C/N ratios, and are more efficient in weed management, by composing a thicker layer of mulch on surface soil [76]. The weed density decreases linearly with organic residues increasing on surface soil, mainly by reduction

Originally, no tillage system in Brazil used 2,4-D and paraquat herbicides as burndown to prepare cultivation areas. At the time there was no product like glyphosate, with non-selective and desiccant action. Despite the effective action, there were limited control with paraquat and some residual effects of 2,4-D on soybeans, hindering the sowing immediately after spraying. With glyphosate releasing in Brazil in 1982, the technology suited local and producers' needs, gaining the market by its control efficiency. But the POST application was still limited to the same herbicides (bentazon, imazethapyr, setoxydin, tepraloxydym, etyl-chlorimuron, diclo‐ sulam, clorasulan-methyl, etc.). Doses were necessarily higher and the number of resistance cases to ALS inhibitors started to increase, since the first record of *Bidens pilosa* L., which is

With the introduction of GR soybean, most of the herbicides were replaced in 2003/2004 harvest in Brazil. The system that provides a single application of glyphosate at early stages of the crop gained market for its easy adoption, undeniable efficiency in weed control and guarantee of profitability. According to data, nearly 81% of all soybeans cultivated area in Brazil is GR and its contribution to farmers is unquestionable (Figure 5). The impact of using GR soybeans has been similar to that identified in the US and Argentina, although the net savings on herbicide costs are larger in Brazil, due to higher average costs of weed control [77]. The average cost savings originated from a combination of reduced herbicide use, fewer spray runs, labor and machinery savings, were between US\$30/ha and US\$81/ha in the period 2003-2010, which means that the net cost saving after deduction of the technology fee (assumed to be about US \$19/ha in 2010) has been between US\$9/ha and US\$61/ha in recent years, with increased farm

resistant to imazaquin and chlorimuron-ethyl, appeared in 1993.

income levels of US\$694 million in 2010 by the GR soybean adoption.

actions.

on weed germination.

Today, there are many efforts to revert the reliance on glyphosate and the selection of resistant biotypes and hard-to-control weeds. Universities, professional associations and the industry are advocating the rational use of herbicides with different sites of action, in a crop rotation program, to prevent the selection of new resistant biotypes, not only to glyphosate but to others as well, especially biotypes with multiple resistance. It is only fair to mention that academics from different institutions such as INTA, Universidad Nacional de Buenos Aires, Universidad Nacional de Rosario, Universidad Católica de Córdoba, Universidad Nacional de Tucumán, Estación Obispo Colombres, just to mention a few, have been working hard on this matter in the previous years, when glyphosate was still the undisputed weed control method of choice. Argentina is shifting from a simple and effective system to a more complex one that requires a stronger commitment from farmers, advisors, the academic sector and the industry. The soybean sector is facing a turning point, and this new reality will have to include more crop rotations, more herbicides and also mechanical and cultural weed control methods.

#### **4.2. Weed management in Brazil**

According to professor Gustavo Dutra, from Cruz das Almas, Bahia, it may be inferred that, since its introduction in the country in 1882, soybean crop has transformed the Brazilian agriculture. Initially planted in the state of Rio Grande do Sul, first recorded in 1914, in Santa Rosa, the soybean "tropicalization" has found space coming out from southern pampas to the midwestern region of the country. While only 2% of national soybean production had been recorded in this region in the 70s, more than 47% of national production was reported in midwestern region in 2010/2011 harvest. Hence, Brazil represents one of the most important regions with a growing potential in soybean production. Probable areas to produce soybean ponder between 20˚ S and 20˚ N. However, the largest portion of this production belt is concentrated in the Brazilian lands, with estimated increases of 2.3% up to the year 2020.

Weed control has bothered growers from the beginning of soybean cultivation, especially since 1950, with the expansion of southern region. Adaptation of production system allowed the satisfactory management, even when using only mechanical tools to control. Cost constraints and limitations set by this control led to its quick replacement by the chemical control, which became a primary tool of weed management. Due to its importance, Brazilian pesticide market has expanded from 1977 to 2006, on average, 10% per year. Even after many decades, the use of soybean herbicide has been restricted to spraying in incorporated pre-plant (eg triflurallin) and pre-emergence (eg metribuzin, alachlor and linuron) along with plowing and harrowing, to prepare conventional soybean field.

Few herbicides used previously restricted the implementation period, affecting more specific actions for managing the weeds emerged in advanced stages of the crop. The launch of bentazon POST herbicide revolutionized the market, allowing the control of major dicotyled‐ onous weeds on soybean. Introduction of new molecules from the 80s and 90s afforded efficiency on the control of several species, in particular those belonging to genders Amaran‐ thus, Digitaria, Brachiaria, Euphorbia and Bidens. The main herbicides applied belonged to the chemical groups ALS and ACCase inhibitors, with monocotyledonous and dicotyledonous actions.

when using 2 and 3 times the dose of glyphosate. The situation changes when residual herbicides are applied in the fall (flumioxazin, metsulfuron, atrazine, and diclosulam have proved to be effective). There was a lot of confusion when this weed began to emerge as a problem because it co-exists with another species, *C. sumatrensis*, more susceptible to be controlled by glyphosate applications at later stages, leading to a general belief that there are resistant biotypes that escape control. Again, the lack of monitoring practices is evident here. These weeds are strongly associated with no-tillage practices, since they do not progress at all

Today, there are many efforts to revert the reliance on glyphosate and the selection of resistant biotypes and hard-to-control weeds. Universities, professional associations and the industry are advocating the rational use of herbicides with different sites of action, in a crop rotation program, to prevent the selection of new resistant biotypes, not only to glyphosate but to others as well, especially biotypes with multiple resistance. It is only fair to mention that academics from different institutions such as INTA, Universidad Nacional de Buenos Aires, Universidad Nacional de Rosario, Universidad Católica de Córdoba, Universidad Nacional de Tucumán, Estación Obispo Colombres, just to mention a few, have been working hard on this matter in the previous years, when glyphosate was still the undisputed weed control method of choice. Argentina is shifting from a simple and effective system to a more complex one that requires a stronger commitment from farmers, advisors, the academic sector and the industry. The soybean sector is facing a turning point, and this new reality will have to include more crop

rotations, more herbicides and also mechanical and cultural weed control methods.

According to professor Gustavo Dutra, from Cruz das Almas, Bahia, it may be inferred that, since its introduction in the country in 1882, soybean crop has transformed the Brazilian agriculture. Initially planted in the state of Rio Grande do Sul, first recorded in 1914, in Santa Rosa, the soybean "tropicalization" has found space coming out from southern pampas to the midwestern region of the country. While only 2% of national soybean production had been recorded in this region in the 70s, more than 47% of national production was reported in midwestern region in 2010/2011 harvest. Hence, Brazil represents one of the most important regions with a growing potential in soybean production. Probable areas to produce soybean ponder between 20˚ S and 20˚ N. However, the largest portion of this production belt is concentrated in the Brazilian lands, with estimated increases of 2.3% up to the year 2020.

Weed control has bothered growers from the beginning of soybean cultivation, especially since 1950, with the expansion of southern region. Adaptation of production system allowed the satisfactory management, even when using only mechanical tools to control. Cost constraints and limitations set by this control led to its quick replacement by the chemical control, which became a primary tool of weed management. Due to its importance, Brazilian pesticide market has expanded from 1977 to 2006, on average, 10% per year. Even after many decades, the use of soybean herbicide has been restricted to spraying in incorporated pre-plant (eg triflurallin) and pre-emergence (eg metribuzin, alachlor and linuron) along with plowing and harrowing,

in tilled soil.

66 Soybean - Pest Resistance

**4.2. Weed management in Brazil**

to prepare conventional soybean field.

Since the introduction of no tillage system, weed management has changed and, as a conse‐ quence, moved to consider factors other than chemical control on the production system. The main benefit of no tillage system is the reduction of weed germination over time [75] and greater use of crop control. Furthermore, species not commonly observed in the conventional system demand better preparation and expertise of producers. Such modifications are related to the absence of soil disturbance, favoring perennial cycle weeds, as well as changes in patterns of temperature and light incidence, influencing seeds' mechanisms of dormancy. Cover crops result in greater amount of organic residue, with higher C/N ratios, and are more efficient in weed management, by composing a thicker layer of mulch on surface soil [76]. The weed density decreases linearly with organic residues increasing on surface soil, mainly by reduction on weed germination.

Originally, no tillage system in Brazil used 2,4-D and paraquat herbicides as burndown to prepare cultivation areas. At the time there was no product like glyphosate, with non-selective and desiccant action. Despite the effective action, there were limited control with paraquat and some residual effects of 2,4-D on soybeans, hindering the sowing immediately after spraying. With glyphosate releasing in Brazil in 1982, the technology suited local and producers' needs, gaining the market by its control efficiency. But the POST application was still limited to the same herbicides (bentazon, imazethapyr, setoxydin, tepraloxydym, etyl-chlorimuron, diclo‐ sulam, clorasulan-methyl, etc.). Doses were necessarily higher and the number of resistance cases to ALS inhibitors started to increase, since the first record of *Bidens pilosa* L., which is resistant to imazaquin and chlorimuron-ethyl, appeared in 1993.

With the introduction of GR soybean, most of the herbicides were replaced in 2003/2004 harvest in Brazil. The system that provides a single application of glyphosate at early stages of the crop gained market for its easy adoption, undeniable efficiency in weed control and guarantee of profitability. According to data, nearly 81% of all soybeans cultivated area in Brazil is GR and its contribution to farmers is unquestionable (Figure 5). The impact of using GR soybeans has been similar to that identified in the US and Argentina, although the net savings on herbicide costs are larger in Brazil, due to higher average costs of weed control [77]. The average cost savings originated from a combination of reduced herbicide use, fewer spray runs, labor and machinery savings, were between US\$30/ha and US\$81/ha in the period 2003-2010, which means that the net cost saving after deduction of the technology fee (assumed to be about US \$19/ha in 2010) has been between US\$9/ha and US\$61/ha in recent years, with increased farm income levels of US\$694 million in 2010 by the GR soybean adoption.

**Scientific name Common name Scientific name Common name** *Acanthospermum hispidum* DC. Starbur *Eleusine indica* (L.) Gaertn Goosegrass *Amaranthus retroflexus* L. Pigweed *Euphorbia heterophylla* (L.) Wild poinsettia *Bidens pilosa* L. Hairy beggarticks *Galinsoga parviflora* Cav. Smallflower *Brachiaria plantaginea* L. Alexandergrass *Ipomoea purpurea* (L.) Roth Morningglory *Cenchrus echinatus* L. Sandbur *Panicum maximum* Jacq Urochloa maxima *Commelina benghalensis* L. Dayflower *Pennisetum setosum* Rich Bufflegrass

Beauv.

Foxtail

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Weed Management in Soybean — Issues and Practices

*Cynodon dactylon* (L.) Pers Bermudagrass *Setaria geniculata* auct. non (Willd.)

*Digitaria horizontalis* Willd. Jamaica crabgrass

**Table 6.** Some weed species on soybean Brazilian crop [2].

management must be made with young plants.

market and it contributed to glyphosate use indefinitely.

*Conyza bonariensis* (L.) Cronq. Hairy fleabane *Sida rhombifolia* L. Sida

*Conyza canadensis* (L.) Cronq. Horseweed *Sorghum halepense* (L.) Pers. Johsongrass *Digitaria insularis* (L.) Mez ex Ekman Sourgrass *Spermacoce latifolia* Aubl. Buttonweed

For the management of weeds resistant to glyphosate, the alternative control, besides herbicide mixtures, includes crop rotation, autumnal management or even return of non transgenic soybeans, as well as herbicides spray recommended in 80s and 90s. To reduce *Conyza* spp. competition, which can cause yield losses above 70% for soybean [78] it is recommended winter management by mixing residual herbicides and glyphosate + 2,4-D [79] ever sprayed on initial growth stage and on plant less than 10 cm-height. For *L.* multiflorum control in the south region, clethodin or haloxyfop-p-methyl herbicides in a glyphosate mix can be used. *S. halepense* is another glyphosate-resistant species which has a reasonable control with haloxyfop-p-methyl application. Nevertheless, this last

In the US, saflufenacil is being used as a major product mixed to glyphosate for controlling resistant weeds. This PPO inhibitor empowers the action of glyphosate as desiccant and it is applied on off-season management or before crop sowing. Though, its release in Brazil has not occurred yet and it should be soon on the market to assist the producers. One of its advantages is the low residual rate in the soil at recommended doses, which allows its implementation

The steady cost increase in weed control by intensive herbicides use and their mixtures emphasizes the need of changing. Since introduction of GR soybean technology in 2003, until 2006, there has been a reduction in herbicide application in soybeans in the country, deriving mainly from efficiency control and range of action of the glyphosate (Table 7). However, amount of active ingredients utilized on crop has risen since 2006, as a result of the intense use of glyphosate and other herbicides. New generic glyphosate brands entered the Brazilian

and subsequent planting without requiring longer intervals before sowing.

#### Source: adapted [77].

Unfortunately, the overuse of the technology (GR soybean + glyphosate) in tillage and no tillage system led to strong selection pressure. Apart from the variation of biotypes selectivity, the level of herbicide application also contributes to the tolerance of species. It was checked the Brazilian herbicide usage data for the periods 2001-2003 and 2007-2009, as well as information from industry and extension advisers and was conclud‐ ed that the annual average use of herbicide active ingredient per ha in the early years of GR soybean was lesser than 2007-2009, an estimated difference of 0.22 kg/ha [77]. From 2007-2009 data, it was observed an average active ingredient use of 2.37 kg/ha for GR soybean compared to 1.96 kg/ha for conventional soybeans.

These data clearly illustrate the current weed management in soybean in the country. Nowa‐ days, Brazilian producers are using sequential spraying of glyphosate in order to control species which are difficult to manage in crops, such as *Bidens* spp., *Chamaesyce hirta* (L.), *Spermacoce latifolia* Aubl., *Chloris polydactyla* (L.) Sw., *Ipomoea grandifolia* (Dammer) O'Donell, *Commelina benghalensis* L., etc. along with glyphosate herbicides. They also associate herbicides of other chemical control groups and, especially on southern and southeastern regions, producers are using the autumn management, in areas where these species are present [2]. Other herbicides — such as imazethapyr and imazapic — are frequently applied to reduce the emergence of weeds during the fallow period and/or associated with the herbicide 2,4-D on burndown, about 15-20 days before the sowing, for dicotyledonous management of complex control by glyphosate. A relevant number of not highlighted weed species are worrying Brazilian soybean producers. *Borreria* spp.*, Tridax procumbens* L. and *Alternathera tenella* Colla, among others (Table 6), are species with high adaptability to different ecological niches throughout the national territory and they are on the list of species likely to be capable of developing resistance to herbicides used in cultivation, being it a GMO or not.


**Table 6.** Some weed species on soybean Brazilian crop [2].

<sup>4</sup> <sup>21</sup> <sup>44</sup> <sup>44</sup> <sup>59</sup> <sup>67</sup>

soybean compared to 1.96 kg/ha for conventional soybeans.

**Figure 5.** Impact of using GR soybean on farm income (IFI), at a national level. Brazil, 1997-2010.

Source: adapted [77].

68 Soybean - Pest Resistance

215

**IFI (US\$ millions)**

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Unfortunately, the overuse of the technology (GR soybean + glyphosate) in tillage and no tillage system led to strong selection pressure. Apart from the variation of biotypes selectivity, the level of herbicide application also contributes to the tolerance of species. It was checked the Brazilian herbicide usage data for the periods 2001-2003 and 2007-2009, as well as information from industry and extension advisers and was conclud‐ ed that the annual average use of herbicide active ingredient per ha in the early years of GR soybean was lesser than 2007-2009, an estimated difference of 0.22 kg/ha [77]. From 2007-2009 data, it was observed an average active ingredient use of 2.37 kg/ha for GR

These data clearly illustrate the current weed management in soybean in the country. Nowa‐ days, Brazilian producers are using sequential spraying of glyphosate in order to control species which are difficult to manage in crops, such as *Bidens* spp., *Chamaesyce hirta* (L.), *Spermacoce latifolia* Aubl., *Chloris polydactyla* (L.) Sw., *Ipomoea grandifolia* (Dammer) O'Donell, *Commelina benghalensis* L., etc. along with glyphosate herbicides. They also associate herbicides of other chemical control groups and, especially on southern and southeastern regions, producers are using the autumn management, in areas where these species are present [2]. Other herbicides — such as imazethapyr and imazapic — are frequently applied to reduce the emergence of weeds during the fallow period and/or associated with the herbicide 2,4-D on burndown, about 15-20 days before the sowing, for dicotyledonous management of complex control by glyphosate. A relevant number of not highlighted weed species are worrying Brazilian soybean producers. *Borreria* spp.*, Tridax procumbens* L. and *Alternathera tenella* Colla, among others (Table 6), are species with high adaptability to different ecological niches throughout the national territory and they are on the list of species likely to be capable of

developing resistance to herbicides used in cultivation, being it a GMO or not.

321

535

731

116

592

448

694

For the management of weeds resistant to glyphosate, the alternative control, besides herbicide mixtures, includes crop rotation, autumnal management or even return of non transgenic soybeans, as well as herbicides spray recommended in 80s and 90s. To reduce *Conyza* spp. competition, which can cause yield losses above 70% for soybean [78] it is recommended winter management by mixing residual herbicides and glyphosate + 2,4-D [79] ever sprayed on initial growth stage and on plant less than 10 cm-height. For *L.* multiflorum control in the south region, clethodin or haloxyfop-p-methyl herbicides in a glyphosate mix can be used. *S. halepense* is another glyphosate-resistant species which has a reasonable control with haloxyfop-p-methyl application. Nevertheless, this last management must be made with young plants.

In the US, saflufenacil is being used as a major product mixed to glyphosate for controlling resistant weeds. This PPO inhibitor empowers the action of glyphosate as desiccant and it is applied on off-season management or before crop sowing. Though, its release in Brazil has not occurred yet and it should be soon on the market to assist the producers. One of its advantages is the low residual rate in the soil at recommended doses, which allows its implementation and subsequent planting without requiring longer intervals before sowing.

The steady cost increase in weed control by intensive herbicides use and their mixtures emphasizes the need of changing. Since introduction of GR soybean technology in 2003, until 2006, there has been a reduction in herbicide application in soybeans in the country, deriving mainly from efficiency control and range of action of the glyphosate (Table 7). However, amount of active ingredients utilized on crop has risen since 2006, as a result of the intense use of glyphosate and other herbicides. New generic glyphosate brands entered the Brazilian market and it contributed to glyphosate use indefinitely.


forced soybean to expand rapidly and occupy many areas previously cultivated with corn, in

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71

Despite high yields, the country also passed through difficulties at the beginning of crop establishment. Even with great advances in farmland during the 50s, farming tools were limited, especially the ones related to weed management. There was no PRE or POST herbi‐ cides. Usual control practices were restricted to the use of mechanical weeding, fundamental on conventional crop system. Wide-row spacings were used in order to provide effective mechanical weeding and post-sowing. The 2,4-D was used over-the-top at the end of crop growing, prior to the harvest. This allowed reduction on dicotyledonous weeds and on subsequent crops, but did not control the monocotyledonous ones. These have become the main weeds and *Sorghum halepense* (L.) Pers was a major problem weed in many fields.

Until glyphosate and, mainly, GR soybean advents, weed management in the US was restricted to mechanical control and some PRE and POST herbicides to monocotyledonous and dicotyled‐ onous control. Trifularin was a major narrowleaf herbicide used for years, which was applied in autumn or in spring before sowing. Its use requires tillage system but did not aid weed manage‐ ment in early-season, especially *S. halepense* and *Amaranthus* sp. control. Between the 70s and 80s, glyphosate and paraquat came into use as preplant burndown, being helpful on no tillage sys‐ tem. These herbicides replaced preplant tillage and fostered the currently used stale seedbed planting system. Not so far, PRE and POST selective herbicides became available to most mono‐ cotyledonous and dicotyledonous weed control. Narrow-row and no tillage system challenged soybean farmers to introduce a new management concept. In the US, the first POST herbicide for‐ mulations were available years before their release in Argentina and Brazil and some chemical alternatives on weed management in the country had always been more accessible. Neverthe‐ less, the order of release was followed, initially by bentazon, with a broad spectrum of action, and after, ACCase inhibitors, diphenylethers, imidazolinones and sulfonylureas (ALS inhibitors).

Traditionally, soybean is the rotational crop with rice in most farming areas, particularly in midsouth region. Prior to rapid rice expansion area in the 70s, the common rotation involved 2-year soybean and 1-year rice. Today, rice is often grown for 2 or 3 years before another crop, especially where the land is unsuited for other crops, and soybean is predominant. Major conventional herbicides that have been used in soybean include trifluralin, pendimethalin, metolachlor, alachlor, dimethenamid, clomazone, imazethapyr, sethoxydim, fluazifop,

The main herbicides such as trifluralin, pendimethalin, imazethapyr and imazaquin were widespread until the mid 90s, but with glyphosate effectiveness, mainly linked to GR soybean, there was a massive replacement of the "out-of-fashion" herbicides. During the period considered, 1995-2006, the treated areas with pendimethalin decreased from 26% to 3%; areas treated with imazethapyr suffered a reduction from 44% to 3% [83]. Especially for imazethapyr, whose decrease was greater than pendimethalin, many resistant weeds had been selected, even

Many advantages provided by glyphosate on GR soybean weed control overlapped other man‐ agement tools, leading to a replacement of herbicides and conventional soybean for the new

quizalofop, and clethodim [82]; many of them are useful against *S. halepense*.

in the first using years, encouraging technology exchangings.

the extensive Corn Belt.

Sources: Kleffmann & AMIS Global;

\* Including herbicides (mostly glyphosate) used in no/low tillage production systems for burndown.

**Table 7.** National level changes in herbicide use (active ingredient – ai) by GR soybean. Brazil, 1997-2010 [77].

In spite of weed shift in Brazil, glyphosate is still a helpful weed control tool. To extend its use as a major tool in chemical control strategies on tillage and no tillage sowing, GR and no-GR soybean, current management in soybean aims to integrate methods that minimize the effects to the environment and offer adequate security control. Therefore, in addition to new tech‐ nologies afforded by the chemical industry, producers should also cooperate in the process, even though this implies the return of already used tools, as the conventional soybean (no GM). Among the alternatives, there is the rotation area with conventional soybeans, the use of offseason management (autumn), the spraying of non-selective herbicides that reduce shifts on further glyphosate applications, the advanced management in spraying installment being the first 30 days before sowing and the second between five and seven days prior of planting —, the sowing of cover crops in fallow period and the spraying of recommended herbicide doses in order to avoid progressive biotypes selection [30,80].

#### **4.3. Weed management in the USA**

Soybean production in the US is undoubtedly part of the greatest productions worldwide and it has an expressive occupation of agriculture area in the country. According to the USDA projections, last average yield was around 3.7 tons/ha crop; in 2012, there will be about 29.9 million hectares crop in the country. Most of soybean cultivated area in the US (93%) uses GR soybeans. The first scientific record of soybean cultivation in the US took place in 1879 at the Rutgers Agricultural College, in New Jersey [81]. Initially, the crop was mainly used as animal fodder. However, the growing interest in culture, sponsored by the demand for oil and meat, forced soybean to expand rapidly and occupy many areas previously cultivated with corn, in the extensive Corn Belt.

**Year ai saving (kg; negative sign denotes**

Sources: Kleffmann & AMIS Global;

70 Soybean - Pest Resistance

**4.3. Weed management in the USA**

 22,333 0.1 111,667 0.3 263,533 0.7 290,333 0.7 292,790 0.7 389,145 0.8 670,000 1.2 1,116,667 1.7 2,010,000 2.9 2,546,000 4.0 -5,808,563 -8.8 -5,704,705 -17.6 -6,642,000 -18.7 -7,529,650 -20.0

\* Including herbicides (mostly glyphosate) used in no/low tillage production systems for burndown.

herbicide doses in order to avoid progressive biotypes selection [30,80].

**Table 7.** National level changes in herbicide use (active ingredient – ai) by GR soybean. Brazil, 1997-2010 [77].

In spite of weed shift in Brazil, glyphosate is still a helpful weed control tool. To extend its use as a major tool in chemical control strategies on tillage and no tillage sowing, GR and no-GR soybean, current management in soybean aims to integrate methods that minimize the effects to the environment and offer adequate security control. Therefore, in addition to new tech‐ nologies afforded by the chemical industry, producers should also cooperate in the process, even though this implies the return of already used tools, as the conventional soybean (no GM). Among the alternatives, there is the rotation area with conventional soybeans, the use of offseason management (autumn), the spraying of non-selective herbicides that reduce shifts on further glyphosate applications, the advanced management in spraying installment being the first 30 days before sowing and the second between five and seven days prior of planting —, the sowing of cover crops in fallow period and the spraying of recommended

Soybean production in the US is undoubtedly part of the greatest productions worldwide and it has an expressive occupation of agriculture area in the country. According to the USDA projections, last average yield was around 3.7 tons/ha crop; in 2012, there will be about 29.9 million hectares crop in the country. Most of soybean cultivated area in the US (93%) uses GR soybeans. The first scientific record of soybean cultivation in the US took place in 1879 at the Rutgers Agricultural College, in New Jersey [81]. Initially, the crop was mainly used as animal fodder. However, the growing interest in culture, sponsored by the demand for oil and meat,

**increase in ai use)\* % decrease in ai (- = increase)**

Despite high yields, the country also passed through difficulties at the beginning of crop establishment. Even with great advances in farmland during the 50s, farming tools were limited, especially the ones related to weed management. There was no PRE or POST herbi‐ cides. Usual control practices were restricted to the use of mechanical weeding, fundamental on conventional crop system. Wide-row spacings were used in order to provide effective mechanical weeding and post-sowing. The 2,4-D was used over-the-top at the end of crop growing, prior to the harvest. This allowed reduction on dicotyledonous weeds and on subsequent crops, but did not control the monocotyledonous ones. These have become the main weeds and *Sorghum halepense* (L.) Pers was a major problem weed in many fields.

Until glyphosate and, mainly, GR soybean advents, weed management in the US was restricted to mechanical control and some PRE and POST herbicides to monocotyledonous and dicotyled‐ onous control. Trifularin was a major narrowleaf herbicide used for years, which was applied in autumn or in spring before sowing. Its use requires tillage system but did not aid weed manage‐ ment in early-season, especially *S. halepense* and *Amaranthus* sp. control. Between the 70s and 80s, glyphosate and paraquat came into use as preplant burndown, being helpful on no tillage sys‐ tem. These herbicides replaced preplant tillage and fostered the currently used stale seedbed planting system. Not so far, PRE and POST selective herbicides became available to most mono‐ cotyledonous and dicotyledonous weed control. Narrow-row and no tillage system challenged soybean farmers to introduce a new management concept. In the US, the first POST herbicide for‐ mulations were available years before their release in Argentina and Brazil and some chemical alternatives on weed management in the country had always been more accessible. Neverthe‐ less, the order of release was followed, initially by bentazon, with a broad spectrum of action, and after, ACCase inhibitors, diphenylethers, imidazolinones and sulfonylureas (ALS inhibitors).

Traditionally, soybean is the rotational crop with rice in most farming areas, particularly in midsouth region. Prior to rapid rice expansion area in the 70s, the common rotation involved 2-year soybean and 1-year rice. Today, rice is often grown for 2 or 3 years before another crop, especially where the land is unsuited for other crops, and soybean is predominant. Major conventional herbicides that have been used in soybean include trifluralin, pendimethalin, metolachlor, alachlor, dimethenamid, clomazone, imazethapyr, sethoxydim, fluazifop, quizalofop, and clethodim [82]; many of them are useful against *S. halepense*.

The main herbicides such as trifluralin, pendimethalin, imazethapyr and imazaquin were widespread until the mid 90s, but with glyphosate effectiveness, mainly linked to GR soybean, there was a massive replacement of the "out-of-fashion" herbicides. During the period considered, 1995-2006, the treated areas with pendimethalin decreased from 26% to 3%; areas treated with imazethapyr suffered a reduction from 44% to 3% [83]. Especially for imazethapyr, whose decrease was greater than pendimethalin, many resistant weeds had been selected, even in the first using years, encouraging technology exchangings.

Many advantages provided by glyphosate on GR soybean weed control overlapped other man‐ agement tools, leading to a replacement of herbicides and conventional soybean for the new technology. No tillage systems became widely used and weed control costs were lowered. Total applied herbicides and labor inputs declined initially and narrow-row on soybean became the standard. In 1995 the GR soybean areas treated with glyphosate were only 20%, but they took over 96% in 2006 [84]. Currently, the GR soybean represents over 94% of the soybeans grown in the US, and more than 90% of soybeans produced worldwide are considered GR.

to billions of dollars in global crop losses annually. Because of their ability to persist and spread through the production and dispersal of dormant seeds or vegetative propagules, weeds are virtually impossible to eliminate from any given field. The importance of weed management to successful farming systems is demonstrated by the fact that herbicides account for the large majority of pesticides used in agriculture, eclipsing inputs for all other major pest groups. To no small extent, the success and sustainability of our weed management systems shapes the

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Integrated pest management (IPM) concept was introduced in the 60s comprising many definitions from then. The primary goals of IPM programs are to reduce pesticide use and the subsequent environmental impact and to rely more on alternative strategies to control pests [87]. Integrated weed management (IWM) comes as a secondary effect of IPM, but it has similar proposal of using multiple management tactics and incorporating the knowledge of weed biology and crop physiology into the weed management system. The goals of IWM range from maximizing profit margins to safeguarding natural resources and minimizing the negative

Integrated Weed Management combines multiple management tools (biological, chemical, mechanical and others) to reduce a pest population to an acceptable level while preserving the quality of existing habitat, water, and other natural resources. The integrated management provides connection of all the involved organisms, whether weeds, pests or diseases, and should focus on decision-making with case studies. There are many practices set out in the integrated management systems, whose benefits have been extensively studied by several authors (Table 8). These studies demonstrate many benefits and the efficiency of integrated

success and sustainability of agriculture as a whole [86].

impact of weed control practices on the environment [88].

**Practices evaluated in IWM Study** Monitoring weeds in crop fields [90,91] Use economic thresholds to determine when to apply herbicides [91-93] Crop rotation [80,91] Using the biological and chemical control [94,95] Using cultural and chemical control [96] Using mechanical and chemical control [97] Using rotation of herbicides [90,91] Plant cover crops [90,98] Using the tillage, no-tillage or reduced tillage system [90,92]

**Table 8.** Practices evaluated in previous studies as part of an Integrated Weed Management (IWM).

However, there are no more ready-made and generalized solutions without risk of errors. IWM is characterized by reliance on multiple weed management approaches that are firmly

tools in crop management systems.

The initial advice for GR soybean system was only one spray and its late application would not undermine crop yield. In extremely wet sites with late sowing — Iowa, for example —, weeds emerged early and single POST glyphosate spray was enough for effective control till the end of the cycle [85]. But for the midwest region, the sowing scheduled occurred earlier, thus only one application was unsuitable for weed control, usually requiring additional sprays.

Concerns about the definition of better periods of spraying, along with the appearance of the first glyphosate resistance case, registered for *Lolium rigidum* in 1998, have collaborated with gradual increase in herbicide use. For the period 2003-2009, herbicides applied to GR soybean increased 30%, whereas consumption remained stable for conventional soybeans [83]. Among changes observed in the global agricultural production, there is the search for socioeconomic and environmental efficiency. Farmers want new tools for weed management. New GM crops have allowed simple and effective solutions, but if producers keep outdated manners when using new tools with GR soybean and glyphosate, these tools will soon become obsolete [25].

As a result, a second generation of GR soybean was launched recently in the US in 2009. Although this technology offers the same soybean resistance to glyphosate as the first gener‐ ation (RR1), it has a higher yield potential, between 7% and 11%. Some farmers reported no increasing yield in relation to first GR soybean generation; perhaps others found positive yield effect. In 2010, soybean farmers pointed that second GR soybean generation has, on average, about 5% of yield improvement.

Many soybean farmers currently use glyphosate mixed with residual herbicides employed previously. The increase of these mixtures permits earlier glyphosate sprays promoting weed management for a larger period. Using conventional herbicides into new GM soybeans are also essential to ensure its resilience, since new traits will be released to use with former herbicides. New technologies include GM soybeans resistant to glufosinate "Liberty Link", to 2,4-D "Optimum GAT", to dicamba and also to glyphosate plus ALS inhibitors. Despite the creation of technologies for landing efficiency and easy management on weed control, good practices at all soybean crop system are rather necessary. Also, weaknesses and difficulties on weed management in many regions of the US have attracted the interest for non-GM soybeans. Differentiated prices in the international market have also stimulated this substitution, yet it is constrained to small and middle producers.
