**6. Herbicide resistant weeds in transgenic soybeans**

Soybean is a crop characterized by the high consumption of herbicides. Chemical control is the most usual, given the characteristics of practicability, efficiency and speed on its execution.

Most of the farmers in Brazil and the world adopt the chemical method for weed control. This is because this technology is very efficient, has attractive cost compared to alternative methods, is easy to use and is professionally developed. However, most producers have only an immediatist and economical vision of weed control and this could lead to environmental problems in the medium and long-term. Although it is public domain that repeated applica‐ tions of herbicides with the same mechanism of action on a genetically diverse population of weeds may cause strong selection pressure and evolution of resistance [95], it has been a common practice in many parts of the world. As a consequence, the population of herbicideresistant weeds has expanded rapidly in several regions, making it a hard solution problem in many areas with intensive agriculture. Evidence suggests that the appearance of resistance to a herbicide, in a plant population, is due to the selection of pre-existent resistant biotypes, because of the selection pressure exerted by repeated applications of the same active ingredi‐ ent, finding conditions for propagation and prevalence [96].

The strategies for the integrated weed management in different weed species can be divided as short or long-term. Measures such as weeding or direct employment of herbicides (chemical control) can be considered as short-term, accounting for only temporary control, requiring new applications to each crop season. In the case of long-term measures, the use of cultural practices and control by other biological agents, has permanent character and take into account more pronounced changes in different agronomic practices. From this, results the integrated management, which should integrate prevention and other control methods that promote short (mechanical and chemical methods) and medium and long-term (cultural and biological

According to Chauhan et al. [58], any single method of weed control cannot provide seasonlong and effective weed control. Therefore, a combination of different weed management strategies should be evaluated for widening the weed control spectrum and efficacy for sustainable crop production. The use of clean crop seeds and seeders and field sanitation (irrigation canals and bunds free from weeds) should be integrated for effective weed man‐ agement. Combining good agronomic practices, timeliness of operations, fertilizer and water management, and retaining crop residues on the soil surface improve the weed control efficiency of applied herbicides and competitiveness against weeds. In Canada, for example, integrating superior cultivars with a high seeding rate and the earliest time of weed removal led to a 40% yield increase compared with the combination of a weaker cultivar, the lowest

According to Bernards et al. [94], the development of an IWM program is based on a few

**a.** use agronomic practices that limit the introduction and spread of weeds, preventing weed

Combining agronomic practices based on these rules will allow the farmer to design an IWM program for his reality. There is not a single recipe for all conditions and years. The plan will need to be changed and adjusted to a particular farming operation and season. The goal is to

Soybean is a crop characterized by the high consumption of herbicides. Chemical control is the most usual, given the characteristics of practicability, efficiency and speed on its execution. Most of the farmers in Brazil and the world adopt the chemical method for weed control. This is because this technology is very efficient, has attractive cost compared to alternative methods, is easy to use and is professionally developed. However, most producers have only an immediatist and economical vision of weed control and this could lead to environmental

**c.** use practices that keep weeds off balance and do not allow weeds to adapt.

**6. Herbicide resistant weeds in transgenic soybeans**

methods) control.

100 Soybean - Pest Resistance

seeding rate, and the latest time of weed removal [93].

general rules that can be used at any farm:

**b.** help the crop compete with weeds; and

problems before they started;

manage, not eradicate weeds.

In 2005, transgenic soybean was officially released for planting in Brazil. From this moment on, several products and product combinations have been replaced by a single active ingre‐ dient, the glyphosate. Glyphosate is a systemic herbicide used for postemergence control of grasses and broadleaved weeds [97]. In transgenic soybean, it is used in single or sequential applications, at doses and times that will vary according to each scenario.

Currently, the technology of glyphosate-resistant soybean, readily accepted and adopted by the producers caused the use of this herbicide to expand, with average of three applications of glyphosate per cycle of soybean, at desiccation and two after crop emergence. Furthermore, the glyphosate is the primary herbicide for several crops such as fruits, coffee, eucalyptus and desiccation for no-tillage [96].

The technology of glyphosate-resistant soybean allows to reduce or eliminate the need to apply other herbicides for the management of different weed species, which contributes to increased selection pressure and emergence of resistant biotypes. Moreover, some aspects of population dynamics of weeds and the possibility of selecting glyphosate-tolerant spe‐ cies must be considered. The type of management and herbicides used in an area cause changes in the type and proportion of species which compose the local population. This is explained by the fact that herbicides do not control evenly the species in the area; so, some end up being benefited and multiply. In these situations, a low occurrence of plants in the area can become a serious problem for the producer. Thus, the repeated and contin‐ uous use of the same herbicide or herbicides with the same mechanism of action, makes the selection of species inevitable [98].

*Conyza canadensis* is an example of problematic weed in soybeans, in which were detected cases of resistance of biotypes from this species to glyphosate in various parts of the world in transgenic soybeans fields. Experiments conducted by Vargas et al. [99], Moreira et al. [100] and Lamego & Vidal [101] demonstrated that application of 360 g a.e. ha-1 of glyphosate is enough, under greenhouse studies, to distinguish between resistant or susceptible biotypes of Conyza bonariensis and C. canadensis.

The resistance factors (GR50) ranged between 7 and 11 for *C. canadensis* [100] and between 10 and 15 [100] and 2.4 [101] for *C. bonariensis*. It is noteworthy that determining the resistance level of suspect populations supports the decisions on strategies to control these biotypes.

Up to date, 23 cases of glyphosate resistant weeds were found in weed species worldwide, described in Table 1.


**6.1. Management of herbicide resistant weeds in soybean**

**Weed Alternative herbicides**

allogamous plants.

emergence

sistant species in the area.

*Conyza bonariensis* and *Conyza canadensis*/ post-

The rational management of herbicides with different mechanisms of action is a very important practice. Furthermore, the use of herbicides with little soil residual activity and optimization of doses and number of applications reduces the selection pressure, decreasing the risks of selection of plant resistance to herbicides. Another very efficient technique for the management of weeds consists in using mixtures of herbicides with different mechanisms of action. In this case, the prevention of resistance is based on the fact that the active ingredients efficiently control both biotypes of the same species, i.e., the biotype resistant to a herbicide is controlled by another active ingredient of the mixture [98]. It is noteworthy that the herbicide mixture of different mechanisms of action as a means of management and prevention of resistance is more efficient when the reproductive system of the weed is self pollination, since the genetic recombination of different alleles which confer resistance is less likely to occur in relation to

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 103

Due to the numerous cases of herbicide-resistant weed biotypes in Brazil, several studies were performed looking for alternatives to control these plants, finding that the use of herbicides with different mechanisms of action is a viable alternative for managing resist‐ ance [103]. Table 02 shows alternative herbicides suitable for soybean according to the re‐

*Lolium multiflorum*/ post-emergence Fluazifop-p, Haloxyfop-r, Clethodim, Sethoxydim

Clorimuron-ethyl

Clorimuron-ethyl and 2,4-D

*Lolim multiflorum*/ desiccation Paraquat and Ammonium-Glufosinate

**Table 2.** Alternative herbicides to control glyphosate resistant weeds in soybean crop used in Brazil.

*Conyza bonariensis* and *Conyza canadensis* / desiccation Paraquat + Diuron, Ammonium-Glufosinate,

preventing the appearance of resistant biotypes as in managing installed resistance.

mitigated and the likelihood of the emergence of new cases minimized [98].

Only with a rational management and using several control methods will the resistance be

Weed resistance is an evolving phenomenon in world and, in certain cases, may restrain the use of some herbicides. Therefore, weed resistance to herbicides should be managed through the use of alternative strategies associated to the application of herbicides. Crop rotation is a good strategy to break the life cycle of weed, preventing its dominance in the area. When the same cultural techniques are applied, year after year, in the same soil, the interference of these weeds is greatly increased. When the main goal is the weed control, the choice of the rotating crop should fall on plants with very contrasting growth habits and cultural characteristics [98]. Thus, when using crops with different physiological needs, a change occurs in weed species from one crop to another and, if it becomes necessary to use herbicides, there is a greater chance they will have different mechanisms of action. The rotation is an effective method both in

**Table 1.** Glyphosate (EPSPs inhibitor) resistant weed species, countries of occurrence and type of resistance.

#### **6.1. Management of herbicide resistant weeds in soybean**

**Weed species Occurrence/observation Type of resistance** *1. Amaranthus palmeri* USA/2005\* Multiple resistance to ALS and EPSPs

*2. Amaranthus tuberculatus* USA/2005 Multiple resistance, triple (ALS, Protox

*3. Ambrosia artemisiifolia* USA/2004 Multiple resistance to ALS and EPSPs

*4. Ambrosia trifida* USA/2004 Multiple resistance to ALS and EPSPs

Colombia, USA/2003, Australia, Greece, Portugal

Republic, Poland and Italy

*13. Eleusine indica* Malaysia/1997, Colombia and USA Multiple resistance to ALS and EPSPs

*14. Kochia scoparia* USA/2007 and Canada Multiple resistance to ALS and EPSPs

Argentina

Spain, Israel and Italy

**Table 1.** Glyphosate (EPSPs inhibitor) resistant weed species, countries of occurrence and type of resistance.

*5. Bromus diandrus* Australia/2011 *6. Chloris truncata* Australia/2010 *7. Conyza bonariensis* South Africa, Spain, Brazil, Israel,

102 Soybean - Pest Resistance

*8. Conyza canadensis* USA/2000, Brazil, China, Spain, Czech

*9. Conyza sumatrensis* Spain and Brazil/2009 *10. Cynodon hirsutus* Argentina/2008 *11. Digitaria insularis* Paraguay and Brazil/2005 *12. Echinochloa colona* Australia/2007, USA and Argentina

*15. Leptochloa virgata* Mexico/2010

*17. Lolium perenne* Argentina/2008 *18. Lolium rigidum* Australia/1996, USA, South Africa,

*19. Parthenium hysterophorus* Colombia/2004 *20. Plantago lanceolata* South Africa/2003 *21. Poa annua* USA/2010 *22. Sorghum halepense* Argentina/2005 and USA *23. Urochloa panicoides* Australia/2008

\* Observation year of the first resistance case. Source: Weed Science [103]

*16. Lolium multiflorum* Chile/2001, Brazil, USA, Spain and

inhibitors

inhibitors

inhibitors

and EPSPs inhibitors)

and EPSPs inhibitors

EPSPs inhibitors

inhibitors

inhibitors

EPSPs inhibitors

Multiple Resistance, double (Photosystem II and EPSPs inhibitors), triple (ACCase, Photosystem I and EPSPs inhibitors), quadruple ( ALS , ACCase, EPSPs and dinitroanilines inhibitors)

and EPSPs inhibitors) and double (ALS

Multiple resistance to Photosystem I

Multiple resistance to ALS and EPSPs inhibitors and to Photosystem I and

Multiple resistance to ALS and EPSPs inhibitors, ACCase and EPSPs inhibitors, triple resistance to ALS, ACCase and

The rational management of herbicides with different mechanisms of action is a very important practice. Furthermore, the use of herbicides with little soil residual activity and optimization of doses and number of applications reduces the selection pressure, decreasing the risks of selection of plant resistance to herbicides. Another very efficient technique for the management of weeds consists in using mixtures of herbicides with different mechanisms of action. In this case, the prevention of resistance is based on the fact that the active ingredients efficiently control both biotypes of the same species, i.e., the biotype resistant to a herbicide is controlled by another active ingredient of the mixture [98]. It is noteworthy that the herbicide mixture of different mechanisms of action as a means of management and prevention of resistance is more efficient when the reproductive system of the weed is self pollination, since the genetic recombination of different alleles which confer resistance is less likely to occur in relation to allogamous plants.

Due to the numerous cases of herbicide-resistant weed biotypes in Brazil, several studies were performed looking for alternatives to control these plants, finding that the use of herbicides with different mechanisms of action is a viable alternative for managing resist‐ ance [103]. Table 02 shows alternative herbicides suitable for soybean according to the re‐ sistant species in the area.


**Table 2.** Alternative herbicides to control glyphosate resistant weeds in soybean crop used in Brazil.

Weed resistance is an evolving phenomenon in world and, in certain cases, may restrain the use of some herbicides. Therefore, weed resistance to herbicides should be managed through the use of alternative strategies associated to the application of herbicides. Crop rotation is a good strategy to break the life cycle of weed, preventing its dominance in the area. When the same cultural techniques are applied, year after year, in the same soil, the interference of these weeds is greatly increased. When the main goal is the weed control, the choice of the rotating crop should fall on plants with very contrasting growth habits and cultural characteristics [98]. Thus, when using crops with different physiological needs, a change occurs in weed species from one crop to another and, if it becomes necessary to use herbicides, there is a greater chance they will have different mechanisms of action. The rotation is an effective method both in preventing the appearance of resistant biotypes as in managing installed resistance.

Only with a rational management and using several control methods will the resistance be mitigated and the likelihood of the emergence of new cases minimized [98].
