How to deal with crow's foot grass resistance

A weed that is among the glyphosate-resistant species, crow's foot grass, is a serious challenge faced by soybean and corn producers. Alternative herbicides are an option for use in rotational systems, but other control methods should also be considered to avoid the selection of multiple resistance.

Crow's foot grass is a of the most important weeds in the world. Its cycle characteristics photosynthetic type C4, fast growth and high seed production that can germinate in different soil conditions (high salinity, high or low pH, compaction and temperature), make this species extremely competitive in the environment. Occurs mainly on the edges of roads and machinery tracks due to soil compaction in these locations and to the absence of other plants around it.

Plant resistance weeds to glyphosate constitutes the main problem of herbicide resistance in the world. The selection of species resistant to this herbicide has occurred in response to current management practices used in the field, such as the use repeated use of herbicides with the same mechanism of action. Therefore, management effective herbicide resistance must consider all available options of cultural, mechanical and chemical control, with the aim of minimizing pressure of selection caused by a single tool.

The use of recommended doses of herbicides, the association of active ingredients, as well as the rotation of Different mechanisms of action are fundamental for plant management resistant weeds. In this sense, four experiments were conducted by the State University of Maringá (UEM - Maringá/PR) with the objective of evaluating chemical alternatives for the control of crow's foot grass resistant to glyphosate. These experiments included recommended herbicides for soybean and corn crops in different application modalities. The doses of herbicides that were used are those registered in the leaflet of commercial products.

At the case of soybeans and corn, since the application of the herbicide in pre-emergence is carried out at the time of sowing, residual control up to 20 days after sowing application (DAA) may be sufficient to keep the culture clean between sowing and post-emergence application. Even though residual control of some herbicides is lower after 35 DAA or 60 DAA, the suppression imposed by these products allows post-emergence application occurs in a calmer situation (lower infestation and plants in the initial growth stage). Another important point lies in the fact that the use of residual herbicides is crucial for the management of grass populations glyphosate-resistant crow's feet. Considering all these aspects, among the herbicides evaluated, sulfentrazone, pendimethalin, s-metolachlor and trifluralin are the herbicides with the highest efficiency. However, flumioxazin, medosafen, clomazone, isoxaflutole, diuron and [flumioxazin + imazethapyr] can also be used to control grass glyphosate-resistant crow's foot in pre-emergence.

For post-emergence, both paraquat and [paraquat + diuron] herbicides are effective for controlling crow's foot grass with up to one tiller. Paraquat (600 g/ ha) and [paraquat + diuron] control plants with up to four tillers in more than 80%. These herbicides are recommended for application in desiccation pre-planting for both soybean and corn crops, with viable alternatives for sowing crops in the open.

In Generally speaking, most ACCase inhibitor herbicides applied alone provide greater or similar control to that obtained by the combination of these glyphosate herbicides. In the case of small plants (one tiller), except for fenoxaprop and sethoxydim, even though the control is slower, the association of these graminicides with glyphosate was effective. On the other hand, in plants larger (four tillers), ultimate control of crow's foot grass is limited in most cases, especially in combinations with glyphosate. Even the ACCase inhibitor herbicides showing efficacy on some populations of crow's foot grass, it is important to remember that other resistant populations This mechanism of action has already been documented in Brazil. Therefore, the use of these herbicides must be done in a sustainable way, to preserve the its effectiveness on the species.

Among the herbicides evaluated in this experiment, glyphosate alone presents maximum 65% control on small plants and 55% control on small plants big ones. All graminicides isolated or associated with glyphosate present efficacy above 80% on crow's foot grass with one tiller, with the exception of glyphosate + fenoxaprop (960 g/ ha + 110 g/ ha and 960 g/ ha + 220 g/ ha) and glyphosate + sethoxydim (960 g/ ha + 384 g/ ha). For plants with four tillers, effective options are clethodim (108 g/ ha and 192 g/ ha), haloxyfop (60 and 120 g/ ha), quizalofop-tefuril (70 g/ ha and 120 g /ha), quizalofop-methyl (75 g/ ha and 100 g/ ha), glyphosate + haloxyfop (960 g/ ha + 120 g/ ha), glyphosate + quizalofop-tefuril (960 g/ ha + 120 g/ ha), glyphosate + quizalofop-methyl (960 g/ ha + 100 g/ ha), glyphosate + fluazifop (960 g/ ha + 250 g/ ha and 960 + 500 g/ ha).  

At the In the case of corn, nicosulfuron (50 g/ ha and 60 g/ ha), glyphosate + nicosulfuron (960 g/ ha + 50 g/ ha and 960 g/ ha + 60 g/ ha), as well as all associations of atrazine with mesotrione, tembotrione and nicosulfuron are effective in controlling plants with up to one tiller. On the other hand, in plants with four tillers the options are extremely restricted. Even though these herbicides are recommended for corn cultivation, control of crow's foot grass at an advanced stage becomes quite limited in this culture.

In In a generalized way, the number of effective treatment options for controlling of crow's foot grass at a more advanced stage is always smaller than for plants with even a tiller. For this species, the time between emergence and the beginning tillering takes just nine days, which means little time for the application is made at the appropriate stage. There is an important hypothesis for explain the lower effectiveness of herbicides on crow's foot grass plants in advanced stadium. The cuticle of plants with more than three tillers has greater accumulation of waxes, which can limit absorption by the plant. These waxes act as a herbicide accumulation compartment and therefore prevent partially the entry of the active ingredient into epidermal cells and consequently in the phloem. In this sense, for the effective control of plants of crow's foot grass at an advanced stage, complementary applications or sequential applications are required.

Other important fact is that unlike bittergrass, glyphosate mixtures with ACCase inhibitor herbicides, apparently do not have a synergistic effect in controlling crow's foot grass. The combination of these herbicides constitutes one of the main tools for the control of bittergrass resistant to glyphosate.

A The use of residual herbicides is crucial for the management of this species, since most herbicides applied post-emergence control just small plants. The rotation of herbicide action mechanisms, as well as mixtures of these products, generally have a retardant effect on selection of resistant populations, especially for species that are self-pollinated, which have a resistance mechanism based on changes in site of action and the dispersal of its seeds is restricted. This series of characteristics suit precisely the crow's foot grass, which is a species autogamous, whose dispersion occurs strictly by seeds and the mechanism of Glyphosate resistance is a mutation in the EPSPS gene.

Therefore, alternative herbicides to glyphosate effective in grass control crow's feet can be recommended in rotation systems of gear mechanisms. action with the aim of managing this case of resistance. In addition to the rotation of mechanisms of action of herbicides, other control methods must be considered to avoid the selection of multiple resistance in the short term. O use of cover crops, decompaction of the soil and rapid closure of between the lines are alternative methods to herbicides that also reduce the infestation of these plants in crops. Finally, the use of residual herbicides for the control of glyphsate-resistant crow's foot grass constitutes a extremely important tool for its effective management, and the stage of application is crucial for achieving post-emergence effectiveness (at most a tiller).

Article published in issue 220 of Cultivar Grandes Culturas, September, 2017.