Management of resistant weeds in soybeans

The misuse of technology and the repeated application of herbicides with the same mechanism of action have helped to increase the selection pressure for weed resistance in Brazilian crops. The resumption of integrated management, with crop and pesticide rotation, is one of the main recommendations to help prevent the shortening of the useful life of control tools and avoid worsening the situation.

Brazilian agricultural production systems are quite diversified from one region to another, however, weed management, in general, is based on the use of glyphosate and crops tolerant to this herbicide, given the wide acceptance of genetically modified soybeans. modified for tolerance to glyphosate (RR Soya), later RR cotton and later RR corn.

The use of glyphosate in a total area on the already established crop brought greater practicality in the management of weeds, due to the fact that it is a herbicide with a broad spectrum of control, without observable phytotoxicity and with numerous commercial product options on the market.

The success of the technology and the lower cost of weed management have led glyphosate to become the only herbicide used in crops such as soybeans, making the use of pre-emergents and other glyphosate partners in post-emergence unnecessary. The use of glyphosate repeatedly over the years, however, acted as selection pressure on the weed community, resulting in the selection of weed biotypes resistant to this herbicide, such as ryegrass (Annual ryegrass), horseweed (Conyza spp.), bittergrass (Digitaria insularis) and white grass (Chloris spp.). Furthermore, species such as guitar strings (Ipomoea spp) and crow's foot grass (Eleusine indica) have become very difficult to manage with the use of glyphosate alone.

Horseweed (Conyza spp.)

The species of Conyza of greatest importance in Brazil are C. canadensis, C. bonariensis e C. sumatrensis, the first two being predominant. The first differs morphologically from the other species, however C. bonariensis e C. sumatrensis have a lower degree of differentiation from each other. The main morphological distinction between the species is the insertion of the inflorescences and the margin of the leaves, although other morphological characters can be considered in the identification, such as greater branching in the upper part of the stem (C. canadensis), lateral branches taller than the inflorescences (C. bonariensis) and size regression of the upper leaves in relation to the lower ones (C. sumatrensis).

There is evidence of resistance of biotypes of Conyza in more than 20 countries and for diverse herbicide mechanisms of action, such as EPSPs (glyphosate), FSI (paraquat / diquat), FSII (atrazine, diuron) and ALS (Chlorimuron / imazethapyr). In addition to multiple resistance between the mechanisms of action ALS + EPSPs, ALS + FSII and EPSPs + FSI. These cases of resistance make horseweed a weed that is difficult to control in soil conservation systems, with a high use of glyphosate-based herbicides.

For good management of fleabane species in soybean crops, good desiccation and planting in a clean environment are recommended, since there are few pre- and post-emergent herbicides that are effective in controlling weeds. Conyza spp. and selective for soybean cultivation. One of the most effective options for desiccation is the application of glyphosate associated with 2,4-D or chlorimuron, with sequential application of saflufenacil, paraquat, ammonium-gluphosinate or paraquat + diuron. Diclosulan or flumioxazine can be used pre-emergence, the dose being based on the soil texture of the area. In post-emergence of the crop, the options are medosafen (Protox), cloransulan (ALS), chlorimuron (ALS) and imazethapyr (ALS), the last three being useless in areas with the presence of horseweed populations with multiple resistance to ALS-inhibiting herbicides. and glyphosate. In areas of crop-livestock integration, the dense layer of straw deposited on the soil makes the emergence of this species difficult. Another alternative is crop rotation, which can provide the use of other herbicide action mechanisms, such as corn, which in the off-season allows the use of herbicides in the season with the highest incidence of this species, preventing its reproduction and reducing the occurrence. during the next harvests. Therefore, the association of different control methods provides greater effectiveness and aims to achieve greater sustainability through integrated management.

Bittergrass (Digitaria insularis)

The first reported case of a glyphosate-resistant bittergrass biotype came from Paraguay, in 2005, and in Brazil the first case was reported in 2008. In both cases, the species presents biotypes resistant only to EPSPS-inhibiting herbicides and were found in corn and soybean crops, in addition to sunflower in Paraguay. However, the resistance of bittergrass can be confused with the greater tolerance to herbicides in the plant after its perennialization (rhizome formation), since the susceptibility of the population to herbicides reduces in advanced stages of development.

As an alternative control, some post-emergent herbicides stand out, such as ACCase inhibitor herbicides (quizalofop, haloxifop, clethodim and sethoxydim). Studies show that the use alone or in association with glyphosate of the herbicides quizalofop, haloxifop, clethodim and sethoxydim, when applied in the initial stages, prove to be excellent control alternatives for resistant bittergrass. However, the massive use of ACCase inhibitor herbicides to control D. insularis it can increase selection pressure, causing this control strategy to be compromised, and other control practices must be adopted.

Good results were also observed in the control of resistant bittergrass (3 to 5 tillers) when using herbicides such as paraquat alone or in association with diuron and ammonium-glufosinate, being carried out as a sequential application to the glyphosate herbicide. Furthermore, chemical control associated with the use of cover crops can be a promising strategy for bittergrass management. Thus, it is clear that the recommendation for the management of bittergrass is not unique and specific, as best practices involve knowledge about the behavior of this weed in the area throughout the year.

In general, desiccations involving flumioxazin, trifluralin, imazethapyr and s-metolachlor are management options for bittergrass sowing. The herbicides flumioxazin and s-metolachlor are also options for pre-emergence applications of soybean crops. Invariably, post-emergence of the crop, the aforementioned ACCase inhibitor herbicides are also management options in association with glyphosate. It should be noted that if ACCase inhibitors are used every year in the desiccation of perennial plants and in the post-emergence of soybeans, this management option will also be lost, due to the possible multiple resistance between ACCase inhibitors and glyphosate in the case of grass. -bitter.

White grass (Chloris spp.)

White grass (Chloris polydactyla, sin: Chloris she's) is another weed that, similar to bittergrass, was found more sparsely and in unmanaged areas, but its importance in soybean areas has been increasing.

The species also presents tolerance to several herbicides, requiring an increase in the dose of glyphosate herbicide for effective control in more advanced stages, even in susceptible biotypes. The first report of whitegrass resistant to the herbicide glyphosate in Brazil in soybean areas occurred in 2014, which may explain the rapid increase in frequency and density of the species.

The use of pre-emergence herbicides (chlorimuron-ethyl, clomazone, diclosulan, imazethapyr, s-metolachlor, sulfentrazone and metribuzin) is effective in controlling resistant or susceptible whitegrass. In post-emergence, studies show that the herbicides haloxyfop-p-methyl and fenoxaprop-p-ethyl have been the most effective in controlling the species. For whitegrass control, it can be said that management cannot be unique and punctual, requiring attention to the behavior of the weed and herbicides throughout the year.

In general, the management of whitegrass is very similar to that of bittergrass, with the preventive use of residual herbicides in soybean areas being a way to prevent infestations of both these species.

Ryegrass (Lolium multiflorum)

The first report of resistant ryegrass in Brazil was in 2003 in orchards and soybean crops, showing resistance to glyphosate. Biotypes of Annual ryegrass resistant to ALS-inhibiting herbicides in wheat, in addition to multiple resistance to the herbicides clethodim (ACCase inhibitor) and glyphosate in corn, soybean and wheat in 2010.

However, herbicides that inhibit the ACCase enzyme, such as haloxyfop and quizalofop, are still widely used to control glyphosate-resistant ryegrass in the desiccation (pre-sowing) of crops such as soybeans, as long as the herbicide dose is adjusted to the phenological stage. of the plant, Complementary application with paraquat, regardless of the dose, herbicide and ryegrass phenological stage is also a viable practice.

Another valid practice is the use of crop succession, which can significantly reduce the seed bank in the area. In this way, it is clear that the management of Annual ryegrass in soybean cultivation requires attention and joint measures, such as rotation of herbicide action mechanisms and crop rotation, with the objective of prevention and effective control of herbicide-resistant biotypes.

Ryegrass with problems of multiple resistance between ACCase inhibitors and glyphosate is already a reality in areas of the south of the country, which clearly points to the chance of developing the same problem for the weeds-grass (crowsfoot grass) (Eleusine indica) white grass (Chloris spp.) and bittergrass (D. insularis).

General considerations

The agricultural system based on the use of glyphosate and resistant crops initially provided high practicality and flexibility in the management of weeds in soybean crops. However, the selection of several resistant species, whether monocotyledons or dicotyledons, and the diversity of agricultural systems, made management based solely on this method unfeasible.

With the diversity of the weed community, it is more appropriate to obtain good weed control by combining chemical and cultural methods, such as the use of residual herbicides, green manures, cover crops, and crop rotation.

The use of pre-emergent herbicides has become essential for chemical management to provide good control effectiveness in soybean crops, in addition to reducing production losses resulting from weed competition at the beginning of the crop cycle.

The use of straw through the direct planting system, in addition to intercropping and crop rotation with green manures, aims to reduce the pressure caused by the existing seed bank in the area, as they hinder the emergence of numerous species and make it unviable germination over time.

The association of more than one control method significantly increases management effectiveness and brings benefits in the medium and long term. Integrated management, in which different practices are combined, aims to achieve high effectiveness in controlling herbicide-resistant weed species, as they provide better control, in addition to hindering the establishment and, consequently, the reproduction of weeds in the culture, in the medium and long term, offering greater sustainability to agricultural activity.

The uncontrolled use of ALS inhibitors to control horseweed, as well as ACCase inhibitor herbicides to control grasses, should be reconsidered by farmers before selecting biotypes in their areas with multiple resistance to these mechanisms of action with glyphosate.

By Acacio Goncalves Netto, Ednaldo Alexandre Borgato, Maiara Maria Franzoni, Pedro Jacob Christoffoleti (Esalq/Usp); Marcelo Nicolai (Agrocon Agronomic Consulting)

Article published in issue 207 of Cultivar Grandes Culturas