Control of weeds resistant to glyphosate herbicide

The growing number of crops experiencing failures in controlling pigweed with the herbicide glyphosate is a concern for both growers and researchers. In 2015, in Mato Grosso, Brazil, the first report of glyphosate-resistant pigweed occurred. It involved the Amaranthus palmeri species, considered an exotic plant in Brazil, requiring strict control measures, such as isolating areas and conducting periodic inspections to eliminate these plants. Since then, farmers have treated pigweed control failures with trepidation, "fear and concern" that these plants were of the A. palmeri species. Currently, A. palmeri is considered a quarantine pest, isolated in the state of Mato Grosso.

In 2018, Rio Grande do Sul reported the second case of glyphosate-resistant pigweed, this time in a different species, Amaranthus hybridus (Oliveira et al., 2019), a species considered native to Brazil. In January 2019, in the Campos Gerais region of Paraná, the Herbology department of the ABC Foundation identified A. hybridus plants suspected of being resistant to the herbicide glyphosate and ALS inhibitors.

From then on, field work was carried out by the ABC Foundation, which found that doses of the herbicide glyphosate of 500 to 16 thousand grams of acid equivalent per hectare did not result in control of these caruru plants, even at the highest dose of the herbicide (Figure 1).

Samples of the plants were collected and sent to different institutes and laboratories in the country, which identified them as being of the species Amaranthus hybridus, which coincidentally presented the same characteristics as the caruru biotypes initially found in Rio Grande do Sul and those existing in Argentina.

In Brazil, 46 cases of pigweed plants of the species A. hybridus suspected of resistance to glyphosate and ALS inhibitor herbicides were recently reported (Figure 2). Most of the reported cases are in the southern region of the country, mainly in Rio Grande do Sul, where pigweed residues from post-emergence glyphosate applications on soybeans became more frequent in the last three harvests. In the 2019/20 harvest, these residues attracted more attention due to their higher frequency than in previous harvests. It is worth noting that there was a period of drought from mid-November to the end of December 2019, which may have contributed to the increase in observed failures.

Possible hypotheses for the introduction and dissemination of populations found in these areas to date are that seeds from infested regions have entered through: commercialization of winter cover crop seeds (mainly ryegrass), acquisition of cattle at auctions, animal manure, feed, birds and agricultural machinery traffic.

Since then, the number of cases of A. hybridus control failures with the herbicide glyphosate has been increasing in the states of Rio Grande do Sul, Santa Catarina and Paraná.

Pigweed species

The Amaranthus genus has approximately 60 species worldwide, found mainly in tropical and subtropical regions.

In Brazil, the species hybridus It is found in virtually every state. Amaranthus hybridus, known as purple caruru, wild caruru, or simply caruru, is an annual, monoecious, herbaceous plant with an erect stem that exhibits a wide range of colors, from green to purple-red. The leaves are simple lanceolate, arranged in a helical pattern, with inflorescences bearing male and female flowers. Maturity occurs when plants reach 20 cm to 2 m in height (Kissmann & Groth, 1999).

Regarding its growth habit, it is considered an aggressive weed, with a C4 photosynthetic cycle, which gives it a high level of competition for water, light, and nutrients, especially when compared with C3 crops such as soybeans, beans, and cotton (Carvalho et al., 2015). This species of pigweed can produce 200 to 600 seeds per plant. It is dispersed through seeds, which can be spread by agricultural machinery, irrigation canals, inputs, animal manure, birds, mammals, and cover crops.

The similarity between the species makes them difficult to identify in the field, but some aspects can be taken into account when differentiating Amaranthus species. Since there are reports of glyphosate resistance in two species in Brazil, this article will focus on their differentiation.

The inverted "V"-shaped watermark may be present in some Amaranthus hybridus plants (Figure 3), but it is also a characteristic present in A. palmeri plants. The petiole being longer than the leaf blade (Figure 4), the presence of hairs on the leaf tips (Figure 5), and the growth of the leaves in a "rosette" (Figure 6) are also characteristics that can occur in both A. hybridus and A. palmeri plants. Therefore, the differentiation of the species occurs when the plants are in flowering, as A. palmeri is considered a dioecious plant, with the presence of male and female flowers on separate plants, while A. hybridus is a monoecious plant, with the presence of male and female flowers occurring in the same inflorescence (Figure 7).

cause for concern

In Argentina, there have been cases of resistance of this species to the herbicide glyphosate since 2013, and currently, there are populations of A. hybridus that present multiple resistance to the herbicides that inhibit EPSPS (glyphosate), ALS (chlorimuron-ethyl), and Auxins (2,4-D and dicamba) (Heap, 2020). Therefore, the application of glyphosate in the post-emergence of RR soybean or RR corn proves ineffective, requiring the use of other herbicides, both pre- and post-emergence of the crop, increasing the cost of control.

One of the characteristics that facilitates rapid Amaranthus infestation in crops is its high seed production capacity (Figure 8), with a single plant capable of producing up to 600 seeds. These seeds are small, spherical, and do not have a wind-dispersal structure (Photo 9).

The ideal average temperature for germination of these seeds is 20°C and can occur even in the absence of light, so the months of May to July may present low germination flows; the peak of emergence of Amaranthus hybridus occurs in the months of September-October, extending with lower flows between December and February.

Understanding this emergence flow is an important tool for defining the best control strategies. In this case, the use of residual herbicides in pre-emergence soybeans will be the basis for recommendations for controlling this weed.

Another point that must be considered is that during the harvest several emergency flows will occur (Figure 10), depending on the water supply in the field.

This makes it possible to have weeds at different stages of development, from adult plants to seedlings, which grow rapidly, 2cm to 3cm per day, and can reach up to 3m in height (Figure 11).

Again, with this information, it is clear that in addition to the residual herbicide, it is necessary to carry out further control in the post-emergence of the crop, and this will depend on the stage of the weed.

And what is the impact of this weed on soybean crops? The Amaranthus viridis species, Amaranthus viridis, is easily controlled with glyphosate. Susceptible biotypes of Amaranthus hybridus and Amaranthus palmeri were also managed with glyphosate. For glyphosate-resistant biotypes, post-emergence control only occurs on very small plants (two to six leaves), which will make control in RR soybeans more difficult and more expensive.

The presence of this weed in corn and soybean crops can reduce yields by up to 80%, in addition to making mechanical harvesting impossible. Another important point is that the plants naturally hybridize, and therefore, herbicide resistance from A. hybridus can be transferred to other pigweed species.

It is important to highlight that in areas already infested, the dissemination of these seeds to neighboring areas must be avoided, mainly through the transit of machines and animals, and, above all, for areas not yet infested, that permanent monitoring of crops is carried out, and if they are If pigweed plants are observed with control failures against the herbicide glyphosate, immediately contact the research areas of the institutions that prepared this statement: Fundação ABC, CCGL, Coamo, Embrapa, UEM, UFSM and UPF.

By Luis Henrique Penckowski, Evandro H.G. Maschietto and Eliana Fernandes Borsato, ABC Foundation; Fernando S. Adegas, Embrapa Soybeans; Lucas S.O. Moreira, Coamo; Mario A. Bianchi, CCGL; Mauro A. Rizzardi, UPF; Rubem S. Oliveira Jr., UEM; Sylvio H.B. Dornelles, UFS