**5. Principles of integrated weed management**

perennial weeds; reduces the need for labor; increases the speed and efficiency of the control operation per unit area, reducing the cost per treated area; controls the weeds for a longer period, when the use of a cultivator is impossible in view of the crop growth; and can be used in rainy periods, when the mechanical control is not efficient and when labor is required for other activities. However, it has the disadvantage of requiring skilled labor, because, if done improperly, can poison the crop, the environment and, especially, the applicator himself. Although herbicides are very effective in controlling weeds, they may promote the develop‐ ment of resistant biotypes, a fact that would further exacerbate the problem within an area [81].

According to Oliveira Jr. et al. [82], the most common strategies used in the management of both cover crops and weed vegetation in areas of no-tillage are reduced to three: desiccation immediately before sowing, between seven and ten days before sowing or anticipated drying.

These authors undertook a study aimed to evaluate the interaction between tillage systems and weed control in post emergence in soybean with these three strategies. They concluded that, although desiccation in different management systems have been effective, the anticipa‐ tion of desiccation in anticipated management favored the emergence and initial soybean development, providing greater productivity gains, given the infestation conditions. The management system also affected the flow of weed emergence after soybean emergence, with fewer reinfestations in the anticipated management system, due to the control of initial flows given by the second application of this management system. Management applied at planting and ten days before planting, hindered the development of soybean, resulting in lower

Procópio et al. [83] carried out a study in which they compared the effects of tillage systems on the control of the weeds *Digitaria insularis*, *Synedrellopsis grisebachii* and *Leptochloa filifor‐ mis* before soybean planted in no-till. The authors found satisfactory control and prevention of regrowth of *D. insularis* and *L. filiformis* when glyphosate was applied five days prior to soybean planting or when the sequential application of glyphosate and paraquat + diuron was done. Sequential applications of the mixture paraquat + diuron were not effective in controlling or preventing the regrowth of *D. insularis* and *L. filiformis* and the weed *S. grisebachii* proved to be tolerant to glyphosate. The use of a non-residual herbicide such as glyphosate fails by not controlling weeds emerged after application, and eventually produce seeds that can easily replenish the seed bank [84]. Adding a residual herbicide to glyphosate can be a consistent management to control the weeds as they germinate and promotes long-term activity which

According to Arregui et al. [86], there are several soil-applied broadleaf herbicides that effectively control weeds like *Ipomoea* spp., *Commelina* spp. and *Sida spinosa*. Chlorimuron and sulfentrazone reduce *Ipomoea* spp. density [87]; *S. spinosa* density decreased with imazaquin, metribuzin and sulfentrazone applications [87] and with cloransulam and diclosulam [88].

The same authors [86] affirm that soil-applied herbicides as metribuzin and imazaquin may be beneficial reducing early season competition of weeds, particularly those inherently more tolerant to glyphosate such as *Parietaria debilis* or *Commelina erecta*, which survive pre-planting glyphosate applications. Likewise, when dry conditions are observed during vegetative

productivity, while anticipated management provided the highest yield.

controls plants which emerge later [85].

98 Soybean - Pest Resistance

The concept of Integrated Weed Management (IWM), a component of Integrated Pest Management, has been proposed (i) to decrease the density of weeds emerging in crops, (ii) to reduce their relative competitive ability (in order both to preserve crop yields and to limit the replenishment of weed seed bank), and (iii) to control emerged weeds using non chemical techniques, with the overall aim of reducing the need for herbicide applica‐ tion at the cropping system level [91]. IWM advocates the use of all available weed con‐ trol options such as: plant breeding, fertilization, crop rotation, tillage practices, planting pattern, cover crops and mechanical, biological and chemical controls. To define the cor‐ rect weed management strategies, it is necessary to know the ability of the weed species, in relation to the crop, to compete for water, light and nutrients, which are factors respon‐ sible for decreasing crop yield [6].

Usually, it is not taken into consideration that a good program of weed management should allow for maximum production in the shortest time, the maximum sustainable production and minimal environmental and economic risk. Wilson et al. [92] in a study to compare the Ohio farmer model to a weed scientist decision model about management of weeds, concluded that farmers understand but do not practice IWM. The failure to adopt may be attributed to gaps in their understanding of the human role in weed dispersal, their focus on the risks associated with weeds without recognition of their ecological benefits, and the tendency to overlook risks associated with management.

Therefore, to accomplish the IWM, it is required knowledge in botany, plant physiology, molecular biology, climatology and application technology, among others.

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 methods) control.

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‐

Weed Management in the Soybean Crop http://dx.doi.org/10.5772/54596 101

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

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

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

*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

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,

ent, finding conditions for propagation and prevalence [96].

desiccation for no-tillage [96].

the selection of species inevitable [98].

Conyza bonariensis and C. canadensis.

described in Table 1.

applications, at doses and times that will vary according to each scenario.

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

According to Bernards et al. [94], the development of an IWM program is based on a few general rules that can be used at any farm:


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 manage, not eradicate weeds.
