**3. Evolution of weed resistance in soybean**

Herbicide-resistant weeds represent the evolution of plants as a consequence of environmental changes, which are usually caused by human action. This process is aligned with the theory of evolution. The process of natural selection, according to Darwin's theory of evolution, may be summarized by three guiding principles: i) principle of variation – there are variations in physiology, morphology and between behavior of individuals of any population, ii) principle of heredity – descendents are more similar to their parents than unrelated individuals, and iii) principle of selection – some individuals are more successful at survival and reproduction than others in a particular environment [56].

Considerable evidence suggests that the appearance of herbicide resistance in a plant popu‐ lation comes with the selection of a resistant biotype, which is pre-existing. According to the selection pressure, this individual finds favorable conditions to reproduce [58]. The perception of resistance is only possible when the number of resistant plants or failure in control are clearly identified (Table 1). Unfortunately, for most cases, the seedbank already has seedlings of the resistant biotype in this time and eradication becomes arduous and expensive. The resistant biotypes may exhibit less ecological adaptation in these environments and become predomi‐ nant due to elimination of sensitive plants. In terms of natural selection, biotypes with greater

**No sensitive plants Control (%) Progress**

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ecological adaptation reveal greater production than less adapted biotypes [59].

**Table 1.** Evolution of resistance in a population of resistant weed biotypes [60].

 1 1,000,000 99.9999 unnoticeable 1 100,000 99.999 unnoticeable 1 10,000 99.99 unnoticeable 1 1,000 99.9 unnoticeable 1 100 99.0 unnoticeable 1 10 90.0 barely noticeable 1 5 80.0 noticeable 1 2 50.0 apparent

Most of the ecological issues associated with evolution of herbicide resistance involve the understanding of relationship between adaptation, gene frequency, inheritance and gene flow [61] because the interactions among these factors shall determine the time required for resistant

The time for resistant plants' appearance and resistant and non-resistant weed proportion change frequently with herbicide use and its biological effects, which may be fairly short (two years from commercial use — ALS inhibitors) or take more than 20 years, as happened with glyphosate (EPSP – 5-enolpyruvylshikimate-3 phosphate synthase inhibitors) [62] (Table 2). Weeds resistant to sulfonylureas were identified after four or five years of a continuous use of this herbicide group [63]. In Australia, *Lolium rigidum* Gaudin biotypes resistant to diclofopp-methyl have been selected into three generations, starting from a sensitive population and

Herbicides with a high level of safety, i.e., high efficiency and specificity play a huge selective pressure. Examples include inhibitors of the enzymes ALS and ACCase (acetyl coA carboxy‐ lase), which have great chances to select resistant weed biotypes, since any change in its action

point (enzyme) may result on activity losses and resistant weed increase.

**Years**

**No resistant plants**

biotypes to become predominant.

by using a normal herbicide dose.

Therefore, a whole species keeps changing its composition because the individuals evolve in the same direction. The next generation will have a higher frequency of individuals that have been most successful in surviving and multiplying on environmental conditions. Frequencies of individuals within a population will change over time and those better adapted to the environment become predominant [56]. The biotype selection in a population by the same repeated herbicide application and its multiplying are shown bellow (Figure 1).

**Figure 1.** Illustration of a resistant biotype selection of a sensitive species [57].

Considerable evidence suggests that the appearance of herbicide resistance in a plant popu‐ lation comes with the selection of a resistant biotype, which is pre-existing. According to the selection pressure, this individual finds favorable conditions to reproduce [58]. The perception of resistance is only possible when the number of resistant plants or failure in control are clearly identified (Table 1). Unfortunately, for most cases, the seedbank already has seedlings of the resistant biotype in this time and eradication becomes arduous and expensive. The resistant biotypes may exhibit less ecological adaptation in these environments and become predomi‐ nant due to elimination of sensitive plants. In terms of natural selection, biotypes with greater ecological adaptation reveal greater production than less adapted biotypes [59].


**Table 1.** Evolution of resistance in a population of resistant weed biotypes [60].

**3. Evolution of weed resistance in soybean**

others in a particular environment [56].

54 Soybean - Pest Resistance

Herbicide-resistant weeds represent the evolution of plants as a consequence of environmental changes, which are usually caused by human action. This process is aligned with the theory of evolution. The process of natural selection, according to Darwin's theory of evolution, may be summarized by three guiding principles: i) principle of variation – there are variations in physiology, morphology and between behavior of individuals of any population, ii) principle of heredity – descendents are more similar to their parents than unrelated individuals, and iii) principle of selection – some individuals are more successful at survival and reproduction than

Therefore, a whole species keeps changing its composition because the individuals evolve in the same direction. The next generation will have a higher frequency of individuals that have been most successful in surviving and multiplying on environmental conditions. Frequencies of individuals within a population will change over time and those better adapted to the environment become predominant [56]. The biotype selection in a population by the same

repeated herbicide application and its multiplying are shown bellow (Figure 1).

**Figure 1.** Illustration of a resistant biotype selection of a sensitive species [57].

Most of the ecological issues associated with evolution of herbicide resistance involve the understanding of relationship between adaptation, gene frequency, inheritance and gene flow [61] because the interactions among these factors shall determine the time required for resistant biotypes to become predominant.

The time for resistant plants' appearance and resistant and non-resistant weed proportion change frequently with herbicide use and its biological effects, which may be fairly short (two years from commercial use — ALS inhibitors) or take more than 20 years, as happened with glyphosate (EPSP – 5-enolpyruvylshikimate-3 phosphate synthase inhibitors) [62] (Table 2). Weeds resistant to sulfonylureas were identified after four or five years of a continuous use of this herbicide group [63]. In Australia, *Lolium rigidum* Gaudin biotypes resistant to diclofopp-methyl have been selected into three generations, starting from a sensitive population and by using a normal herbicide dose.

Herbicides with a high level of safety, i.e., high efficiency and specificity play a huge selective pressure. Examples include inhibitors of the enzymes ALS and ACCase (acetyl coA carboxy‐ lase), which have great chances to select resistant weed biotypes, since any change in its action point (enzyme) may result on activity losses and resistant weed increase.


The **reproductive traits**, such as pollen scattering and number of propagules generated, influence directly the spread of resistant plants. Dispersal of resistance by pollen is affected by

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The **cross rate between resistant and sensitive biotypes** determines the spread of resistant alleles in a population. Pollen exchange between resistant and sensitive plants allows dispersion of the resistance, mainly in plants with high cross-fertilization rate, since the contribution of seed displacement is relatively small [59]. Gene flow is correlated with pollen flow distribution and varies on the species, with pollination mechanism and climatic conditions during flowering [68]. Species which presents more resistant biotypes and effective propagule dispersion may spread itself quickly, even though the inheritance of this resist‐

Repeated use of herbicides to plant control exerts high **selection pressure**, causing changes on flora of some regions, especially those with predominance of monoculture, such as soybean in major producing countries. Usually, better biotypes of a species adapted to a particular practice are selected and then they multiply rapidly [69]. Species exhibit different features and several responses to herbicide treatment. Therefore, an association of species characteristics with those of herbicides creates different periods needed for selection of resistant biotypes (Table 3).

**Weed Herbicide sprayed Years** *Alopecurus myosuroides* Huds. Chlortoluron 10 *Avena fatua* L. Diclofop methyl 4-6 *Avena fatua* L. Triallate 18-20 *Carduus nutans* L. 2,4-D or MCPA 20 *Hordeum leporinum* Link Paraquat or Diquat 25 *Kochia scoparia* (L.) Schrad Sulfonylurea 3-5 *Lolium multiflorum* Lam. Diclofop methyl 7 *Lolium rigidum* Gaudin Diclofop methyl 4 *Lolium rigidum* Gaudin Amitrole + Atrazine 10 *Lolium rigidum* Gaudin Sethoxydim 3 *Senecio vulgaris* L. Simazine 10 *Setaria viridis* (L.) Beauv. Trifluralin 15

**Table 3.** Number of years required for natural selection of resistant biotypes of a weed population according to the

In summary, the evolution process of herbicide resistance goes through three stages: removal of biotypes highly sensitive, remaining only the most tolerant and resistant; elimination of all biotypes except those resistant and selecting them in a population with high tolerance; intercrossing among survivors biotypes, generating new individuals with higher level of

scattering efficiency and pollen longevity [67].

ance is maternal.

herbicide used [61].

**Table 2.** Year of introduction and its first confirmation of weed resistance to different herbicide action mode [64].

There are six factors related to plant population, which interact and determine the probability as well as the time of resistance evolution. They are the following: *number of alleles* involved in strength expression, *resistant allele frequency* in an initially sensitive population, mode of *resistance inheritance* (cytoplasmatic or nuclear), *reproductive traits* of species, *rate crosses between resistant and sensitive biotypes* and *selection pressure* [65].

The **number of genes** that confer resistance is important because, when inheritance is poly‐ genic, the likelihood of the resistance to appear is low. However, when a single gene is responsible for resistance (monogenic), there is a high probability of occurrence. Most cases of resistance are conferred on a single gene. It is due to two factors. First of all, modern herbicides are specific, acting upon specific enzymes in metabolic pathways. Incidence of gene mutations responsible for coding that enzyme may change the plant sensitivity to the product, resulting in resistance. The second factor refers to the high selection pressure exerted by high efficiency of these herbicides. In order to occur polygenic resistance, the recombination between individuals for several generations would be necessary to obtain adequate number of alleles and to confer high plant resistance level [66].

**Frequency of resistant allele(s)** in sensitive population is usually between 10-16 and 10-6 [65]. So, the higher is the frequency of these alleles, the greater is the probability of selecting a resistant biotype. The frequency of resistant allele in the population becomes more significant in the evolutionary process when herbicide requires low selection pressure. However, if allele frequency is high, evolution of resistance may be faster, regardless of selection pressure.

**Inheritance resistance typ**e is fundamental for the establishment of resistance in a plant population. There are two basic types of inheritance: cytoplasmatic (maternal) and nuclear. Cytoplasmatic inheritance happens when hereditary traits are transmitted by cytoplasm, so only the mother plant can pass the trait to the descendants, as an example, resistance to triazines. On the other hand, if the inheritance is nuclear, transmission is by chromosomes, and both father and mother might forward its resistance, such as resistant plants to ALS inhibitors. In case of maternally inherited resistance, allelic migration between adjacent populations does not occur [66], so the development of this type of resistance is slower than nuclear, where migration of alleles occurs via pollen.

The **reproductive traits**, such as pollen scattering and number of propagules generated, influence directly the spread of resistant plants. Dispersal of resistance by pollen is affected by scattering efficiency and pollen longevity [67].

**Herbicide Introduction year Confirmation year Place**

Triazines 1959 1970 EUA Propanil 1962 1991 EUA Paraquat 1966 1980 Japan EPSP syntase inhibitor 1974 1996 Australia ACCase inhibitor 1977 1982 Australia ALS inhibitor 1982 1984 Australia

*resistant and sensitive biotypes* and *selection pressure* [65].

56 Soybean - Pest Resistance

and to confer high plant resistance level [66].

nuclear, where migration of alleles occurs via pollen.

2,4-D 1948 1957 EUA and Canada

**Table 2.** Year of introduction and its first confirmation of weed resistance to different herbicide action mode [64].

There are six factors related to plant population, which interact and determine the probability as well as the time of resistance evolution. They are the following: *number of alleles* involved in strength expression, *resistant allele frequency* in an initially sensitive population, mode of *resistance inheritance* (cytoplasmatic or nuclear), *reproductive traits* of species, *rate crosses between*

The **number of genes** that confer resistance is important because, when inheritance is poly‐ genic, the likelihood of the resistance to appear is low. However, when a single gene is responsible for resistance (monogenic), there is a high probability of occurrence. Most cases of resistance are conferred on a single gene. It is due to two factors. First of all, modern herbicides are specific, acting upon specific enzymes in metabolic pathways. Incidence of gene mutations responsible for coding that enzyme may change the plant sensitivity to the product, resulting in resistance. The second factor refers to the high selection pressure exerted by high efficiency of these herbicides. In order to occur polygenic resistance, the recombination between individuals for several generations would be necessary to obtain adequate number of alleles

**Frequency of resistant allele(s)** in sensitive population is usually between 10-16 and 10-6 [65]. So, the higher is the frequency of these alleles, the greater is the probability of selecting a resistant biotype. The frequency of resistant allele in the population becomes more significant in the evolutionary process when herbicide requires low selection pressure. However, if allele frequency is high, evolution of resistance may be faster, regardless of selection pressure.

**Inheritance resistance typ**e is fundamental for the establishment of resistance in a plant population. There are two basic types of inheritance: cytoplasmatic (maternal) and nuclear. Cytoplasmatic inheritance happens when hereditary traits are transmitted by cytoplasm, so only the mother plant can pass the trait to the descendants, as an example, resistance to triazines. On the other hand, if the inheritance is nuclear, transmission is by chromosomes, and both father and mother might forward its resistance, such as resistant plants to ALS inhibitors. In case of maternally inherited resistance, allelic migration between adjacent populations does not occur [66], so the development of this type of resistance is slower than The **cross rate between resistant and sensitive biotypes** determines the spread of resistant alleles in a population. Pollen exchange between resistant and sensitive plants allows dispersion of the resistance, mainly in plants with high cross-fertilization rate, since the contribution of seed displacement is relatively small [59]. Gene flow is correlated with pollen flow distribution and varies on the species, with pollination mechanism and climatic conditions during flowering [68]. Species which presents more resistant biotypes and effective propagule dispersion may spread itself quickly, even though the inheritance of this resist‐ ance is maternal.

Repeated use of herbicides to plant control exerts high **selection pressure**, causing changes on flora of some regions, especially those with predominance of monoculture, such as soybean in major producing countries. Usually, better biotypes of a species adapted to a particular practice are selected and then they multiply rapidly [69]. Species exhibit different features and several responses to herbicide treatment. Therefore, an association of species characteristics with those of herbicides creates different periods needed for selection of resistant biotypes (Table 3).


**Table 3.** Number of years required for natural selection of resistant biotypes of a weed population according to the herbicide used [61].

In summary, the evolution process of herbicide resistance goes through three stages: removal of biotypes highly sensitive, remaining only the most tolerant and resistant; elimination of all biotypes except those resistant and selecting them in a population with high tolerance; intercrossing among survivors biotypes, generating new individuals with higher level of resistance, which may be selected later [65]. This process resulted in 383 resistant biotypes, 208 species (122 dicotyledonous and 86 monocotyledonous) and over 570,000 fields [62].

use of ALS inhibitors (iodosulfuron-methyl) and ACCase to *L. multiflorum* control resulted in biotypes resistant to ALS and ACCase in 2010 and in 2011, respectively. These biotypes have multiple resistance to glyphosate, glyphosate + ALS and glyphosate + ACCase. Certainly, the

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For soybeans and wheat, ACCase inhibitors are the main alternative to *L. multiflorum* control. Thus, impact selection of resistant and tolerant species in Brazil is mainly focused on cost production, since the farmer will have to use alternative herbicides in the area, usually more

(a)Photograph: Leandro Vargas, Embrapa Wheat, (b)Photograph: Marlene Lazzaretti, Unnoba,

**Figure 2.** (a) Illustration of *Conyza* sp. resistant to glyphosate in Brazilian soybean field; b) rossettes of *Conyza bonariensis* (L.) Cronq. (smaller, smooth lobes) and *C. sumatrensis* (wider, serrated lobes), germinating in the fall,

In general, weed resistance to herbicides in Argentina became important after 2005 and is also related to the intensive use of glyphosate in GR soybean crop. The introduction of the RR technology in 1996 quickly masked the incipient problem of herbicide resistance in the country, marked by the appearance, in the northern part of Argentina, of an *Amaranthus* sp. resistant to ALS inhibitors herbicide (sulfonylureas and imidazolinones), officially con‐ firmed as resistant in 1996. For many years, the problem faded into obscurity and farmers enjoyed the efficacy of an herbicide that seemed to elude the perils of resistance selection, again ignoring the advice of the few experts that protested against the practice of monocul‐ ture and lack of herbicide rotation. Reports of *Sorghum halepense* (L.) Pers. escapes in the province of Salta (NW Argentina), even after repeated applications of glyphosate, started in 2003, and the resistance was confirmed in 2006. This was the first case of resistance to glyphosate in Argentina, followed by *Lolium rigidum* Gaudin (and *L. multiflorum*) in 2007. In 2010, it was reported the case of *Avena fatua* L. resistant to ACCase inhibitors and two cases of multiple resistance *L. multiflorum*, resistant to ALS inhibitors plus glycine and ACCase inhibitors plus glycine as well [62], followed by *Echinochloa colonum* (L.) Moench (2011) and

resistance to the three mechanisms in the same biotype will not take long to happen.

expensive than glyphosate and less efficient.

in Argentina.

*Cynodon hirsutum* (2012).

There are no doubts that selection pressure by use of herbicides at cultivated soybean areas contributed to the increasing of resistant weeds. Among the main representative countries, Argentina, Brazil and the USA, there is a positive correlation between soybean expansion areas and intensive use of herbicides, as well as between the increasing of re‐ sistance incidence and massive adoption by the same technology in these countries, i.e., one or few herbicide action modes.

In the USA, country with the largest number of resistance cases, 139 occurrences have been recorded, approximately 119 resistant species to different states and mechanism ac‐ tions. From the 139 cases, around 25.9% are resistant species to two or more herbicide mechanism actions [62]. The first resistance case in the US, to auxin herbicides, was *Com‐ melina diffusa* Burm. f., in 1957. Then, in 1964, it was reported a *Convolvus arvensis* L. case, resistant to 2,4-D. In the years 1970 and 1972, resistance cases of *Senecio vulgaris* L. and *Amaranthus hybridus* L. to PSII inhibitors were reported. In 1973, it was recorded *Eleusine indica* (L.) Gaertn resistant to dinitroanilines and, in 1979, *Chenopodium album* L. resistant to PSII inhibitors. With soybean advance in the 80s, the resistance cases in‐ creased to 28 reports with PSII inhibitors and 10 cases with ALS inhibitors. In the 90s, the intensive use of ALS and ACCase inhibitors in soybean contributed to 68 ALS resist‐ ance events and 26 to ACCase. From 2000, resistance cases to glyphosate became more common. Between 2000 and 2011, it was registered more than 70 resistance events to gly‐ cine group as the result of larger glyphosate use at GR soybean, genetically modified to glyphosate resistance (RR1). Among reports so far, the largest number of species is relat‐ ed to ALS inhibitors (44), triazines (25), ACCase inhibitors (15) and glycines (13).

In Brazil, selection of tolerant or resistant species started in the 70s, with repeated metribuzin use. This herbicide was introduced to control *Bidens pilosa* L., but it had low efficiency against *Euphorbia heterophylla* L.. *E. heterophylla* showed tolerance to metribuzin and so was selected and became a major weed to be fought in crops. Concerns with *E. heterophylla* control were solved by imazaquin herbicide (ALS inhibitor) in the 80s, which had been used widely, becoming the main herbicide used in soybean fields. But at the end of the 90s, *E. heterophylla* and *B. pilosa* became resistant to imazaquin, including the selection of *Cardiospermum halica‐ cabum* L..

The control of resistant species to ALS inhibitors was solved with GR soybean. History repeated itself with glyphosate and this has become practically the only herbicide hold on soybeans, imposing great selection pressure of tolerant and resistant species. Thus, the continuous glyphosate spraying has selected tolerant weeds such as *Ipomoea* sp., *E. heterophyl‐ la*, *Richardia brasiliensis* (Moq.) Gomez and *Commelina* sp., as those resistant species, such as *Lolium multiflorum* Lam., *Conyza bonariensis (L.)* Cronq., *C. canadensis*, *C. sumatrensis* (Figure 2) and *Digitaria insularis* (L.) Mez ex Ekman.

Resistance of *L. multiflorum* to glyphosate was identified in Brazil in 2003, and this forced ALS inhibitors and ACCase to become the main control options for this species. The continuous use of ALS inhibitors (iodosulfuron-methyl) and ACCase to *L. multiflorum* control resulted in biotypes resistant to ALS and ACCase in 2010 and in 2011, respectively. These biotypes have multiple resistance to glyphosate, glyphosate + ALS and glyphosate + ACCase. Certainly, the resistance to the three mechanisms in the same biotype will not take long to happen.

For soybeans and wheat, ACCase inhibitors are the main alternative to *L. multiflorum* control. Thus, impact selection of resistant and tolerant species in Brazil is mainly focused on cost production, since the farmer will have to use alternative herbicides in the area, usually more expensive than glyphosate and less efficient.

(a)Photograph: Leandro Vargas, Embrapa Wheat, (b)Photograph: Marlene Lazzaretti, Unnoba,

resistance, which may be selected later [65]. This process resulted in 383 resistant biotypes, 208

There are no doubts that selection pressure by use of herbicides at cultivated soybean areas contributed to the increasing of resistant weeds. Among the main representative countries, Argentina, Brazil and the USA, there is a positive correlation between soybean expansion areas and intensive use of herbicides, as well as between the increasing of re‐ sistance incidence and massive adoption by the same technology in these countries, i.e.,

In the USA, country with the largest number of resistance cases, 139 occurrences have been recorded, approximately 119 resistant species to different states and mechanism ac‐ tions. From the 139 cases, around 25.9% are resistant species to two or more herbicide mechanism actions [62]. The first resistance case in the US, to auxin herbicides, was *Com‐ melina diffusa* Burm. f., in 1957. Then, in 1964, it was reported a *Convolvus arvensis* L. case, resistant to 2,4-D. In the years 1970 and 1972, resistance cases of *Senecio vulgaris* L. and *Amaranthus hybridus* L. to PSII inhibitors were reported. In 1973, it was recorded *Eleusine indica* (L.) Gaertn resistant to dinitroanilines and, in 1979, *Chenopodium album* L. resistant to PSII inhibitors. With soybean advance in the 80s, the resistance cases in‐ creased to 28 reports with PSII inhibitors and 10 cases with ALS inhibitors. In the 90s, the intensive use of ALS and ACCase inhibitors in soybean contributed to 68 ALS resist‐ ance events and 26 to ACCase. From 2000, resistance cases to glyphosate became more common. Between 2000 and 2011, it was registered more than 70 resistance events to gly‐ cine group as the result of larger glyphosate use at GR soybean, genetically modified to glyphosate resistance (RR1). Among reports so far, the largest number of species is relat‐

ed to ALS inhibitors (44), triazines (25), ACCase inhibitors (15) and glycines (13).

In Brazil, selection of tolerant or resistant species started in the 70s, with repeated metribuzin use. This herbicide was introduced to control *Bidens pilosa* L., but it had low efficiency against *Euphorbia heterophylla* L.. *E. heterophylla* showed tolerance to metribuzin and so was selected and became a major weed to be fought in crops. Concerns with *E. heterophylla* control were solved by imazaquin herbicide (ALS inhibitor) in the 80s, which had been used widely, becoming the main herbicide used in soybean fields. But at the end of the 90s, *E. heterophylla* and *B. pilosa* became resistant to imazaquin, including the selection of *Cardiospermum halica‐*

The control of resistant species to ALS inhibitors was solved with GR soybean. History repeated itself with glyphosate and this has become practically the only herbicide hold on soybeans, imposing great selection pressure of tolerant and resistant species. Thus, the continuous glyphosate spraying has selected tolerant weeds such as *Ipomoea* sp., *E. heterophyl‐ la*, *Richardia brasiliensis* (Moq.) Gomez and *Commelina* sp., as those resistant species, such as *Lolium multiflorum* Lam., *Conyza bonariensis (L.)* Cronq., *C. canadensis*, *C. sumatrensis* (Figure

Resistance of *L. multiflorum* to glyphosate was identified in Brazil in 2003, and this forced ALS inhibitors and ACCase to become the main control options for this species. The continuous

species (122 dicotyledonous and 86 monocotyledonous) and over 570,000 fields [62].

one or few herbicide action modes.

58 Soybean - Pest Resistance

*cabum* L..

2) and *Digitaria insularis* (L.) Mez ex Ekman.

**Figure 2.** (a) Illustration of *Conyza* sp. resistant to glyphosate in Brazilian soybean field; b) rossettes of *Conyza bonariensis* (L.) Cronq. (smaller, smooth lobes) and *C. sumatrensis* (wider, serrated lobes), germinating in the fall, in Argentina.

In general, weed resistance to herbicides in Argentina became important after 2005 and is also related to the intensive use of glyphosate in GR soybean crop. The introduction of the RR technology in 1996 quickly masked the incipient problem of herbicide resistance in the country, marked by the appearance, in the northern part of Argentina, of an *Amaranthus* sp. resistant to ALS inhibitors herbicide (sulfonylureas and imidazolinones), officially con‐ firmed as resistant in 1996. For many years, the problem faded into obscurity and farmers enjoyed the efficacy of an herbicide that seemed to elude the perils of resistance selection, again ignoring the advice of the few experts that protested against the practice of monocul‐ ture and lack of herbicide rotation. Reports of *Sorghum halepense* (L.) Pers. escapes in the province of Salta (NW Argentina), even after repeated applications of glyphosate, started in 2003, and the resistance was confirmed in 2006. This was the first case of resistance to glyphosate in Argentina, followed by *Lolium rigidum* Gaudin (and *L. multiflorum*) in 2007. In 2010, it was reported the case of *Avena fatua* L. resistant to ACCase inhibitors and two cases of multiple resistance *L. multiflorum*, resistant to ALS inhibitors plus glycine and ACCase inhibitors plus glycine as well [62], followed by *Echinochloa colonum* (L.) Moench (2011) and *Cynodon hirsutum* (2012).

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 with the use of glyphosate-based products.

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.

*Cynodon dactylon* (L.) Pers *Ipomoea nil* (L.) Roth.*, I. purpurea* (L.) Roth.

*Anoda cristata* (L.) Schlecht *Setaria viridis* (L.) Beauv.*, S. verticillata*

*Portulaca oleracea* L. *Alternanthera philoxeroides* (*Mart.)*

*Amaranthus quitensis* Kunth *Euphorbia heterophylla* L. *Sorghum halepense* (L.) Pers. *Wedelia glauca* (Ort.) Hoffm. ex

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

(L.) Beauv.

*Griseb.*

Hicken

*Echinochloa crus-galli* (L.) Beauv; *E.*

*colonum* (L.) Moench

**Economically important weeds Secondary weeds Emerging weeds**

*Cyperus rotundus* L. *Sida rhombifolia* L.*, S. spinosa* L. *Solanum sisymbriifolium* Lam. *Datura ferox* auct. *non* L. *Galinsoga parviflora* Cav. *Aeschynomene virginica (L.) B.S.P. Tagetes minuta* L. *Bidens* spp. *Nicandra physalodes (L.) Gaertn.*

*Digitaria sanguinalis* (L.) Scop. *Helianthus annuus* L. (volunteer)\* *Acanthospermum hispidum* DC. *Chenopodium album* L. *Eleusine indica* (L.) Gaertn. *Flaveria bidentis* (L.) Kuntze

**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)

*Xanthium* spp. *Physallis angulata* L.

*Abutilion theophrasti* Medik

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L.

*Solanum chacoense* Bitter*, S. nigrum*

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