Resistance management of the crow's foot grass weed

crow's foot grass, Eleusine indica, is the most recent case of resistance to the application of glyphosate in Brazil. Herbicide associations, use of pre-emergents, rotation of mechanisms of action and crops are among the complementary measures being studied for the management of these resistant populations.

Herbicide resistance has been the center of attention of professionals involved in weed management. This characteristic can be briefly defined as the ability of a biotype to survive exposure to a dose of herbicide that is normally lethal for the susceptible biotype. Worldwide, more than 470 cases of herbicide resistance have already been documented, with 41 of these episodes occurring in Brazil.

Glyphosate is the most used herbicide in Brazil and around the world, therefore, cases of resistance involving this pesticide deserve attention. Initially, it was believed that resistance to this herbicide would be unlikely to occur, due to the product's characteristics such as a broad spectrum of weed control and low or no residual effect on the soil. However, in 1996 the first case of resistance to glyphosate was reported, and after the start of cultivation of soybeans resistant to this herbicide, known as RR soybeans, there was a large increase in the number of resistant species, due to the excessive use of glyphosate, depending on the possibility of post-emergence application of the crop.

In Brazil, the species resistant to glyphosate already reported are horseweed (Conyza sp.), ryegrass (Perennial Lolium ssp. multiflorum), bittergrass (Digitaria insularis), white grass (Chloris elata) and the caruru (Amaranthus palmeri). In addition to these species, in recent harvests, problems with crow's foot grass escapes (Eleusine indica) after the application of glyphosate have been observed in the field, in different regions of the country.

Chicken foot grass is an annual grass, with rapid growth, high competition capacity with crops and which is present in practically all regions of Brazil and the rest of the world. The wide distribution area of ​​this weed is mainly due to its adaptation to different environments, the production of a high number of seeds and, more recently, its resistance to herbicides.

Several cases of resistance of this species have been reported in other countries such as Argentina, the United States, Australia, Malaysia, China, Japan, Bolivia, the Philippines and Colombia. These cases can be simple resistance or multiple resistance, with the mechanisms involved being ACCase inhibitors, EPSPS inhibitors, GS-GOGAT inhibitors, photosystem I inhibitors, ALS inhibitors and microtubule formation inhibitors. In Brazil, the only case of resistance reported for this species, until then, involved ACCase inhibitor herbicides.

In order to assess whether such control failures in the field were really related to the resistance of crow's foot grass to glyphosate, the State University of Maringá has been conducting several research works since 2013.

In these works, henfoot grass seeds were collected in the 2013/2014 and 2014/2015 harvests in areas with a history of glyphosate applications and cultivated in a soybean/corn succession system, in all cases being soybean varieties RR. In these areas, the collection was carried out on plants not controlled by the application of the herbicide in post-emergence of soybeans. In the first year, 14 different populations from locations in the states of Paraná, Santa Catarina and Goiás were evaluated, while in the second year, 16 locations were evaluated in Paraná alone. Increasing doses of glyphosate (0, 60, 120, 240, 480, 960, 1920, 3840, 7680 and 15360 g a.e./ ha) were applied at two weed stages: with two to three tillers and on plants with five to six tillers , evaluating the percentage of control and dry mass at 28 days after application.

The results obtained showed that lower percentages of control were observed for plants at a more advanced stage, requiring higher doses for control in these situations. Based on these results, two populations from the 2013/2014 harvest and 12 populations from 2014/2015 were considered potentially resistant to glyphosate (resistance factor between 2 and 8). Among these populations, one from Luiziânia and another from Campo Mourão presented all the attributes to prove resistance, as the dose necessary to obtain 80% control (C₈₀) was greater than that recorded for the control of this species, and the progenies of these populations also showed the same characteristics.

In addition to dose response experiments, the resistance mechanism was also investigated by comparing resistant and susceptible plants regarding shikimate accumulation after glyphosate application. Shikimate is a substrate for the reaction in which the herbicide acts, therefore, after application of the pesticide on susceptible plants. This route is interrupted and shikimate is accumulated, which does not occur in resistant plants. The results revealed significant differences between susceptible and resistant plants, corroborating the hypothesis that control failures would be associated with the selection of resistant populations. Subsequently, fragments of the gene that encodes the EPSPS enzyme were sequenced, and a substitution of an amino acid was found in position 106 of the resistant biotype sequence. This demonstrates that resistance is caused by a mutation in the glyphosate site of action, which prevents the herbicide from performing its function.

Therefore, it was proven that crow's foot grass is a new case of resistance to glyphosate in Brazil and the mechanism that confers resistance involves a mutation in the herbicide's site of action. In this sense, new research is being developed jointly by the State University of Maringá and the technical team of the Coamo cooperative, with the aim of providing complementary measures for managing these resistant populations, such as herbicide associations, use of pre-emergent herbicides, rotation of action mechanisms, crop rotation and cover crops, among others.

Hudson Takano, Rubem Silvério de Oliveira JR. and Jamil Constantin, State University of Maringá; Fernando Storniolo Wineries, Embrapa Soja

Article published in issue 208 of Cultivar Grandes Culturas.