Another weed resists glyphosate in Brazil

Weed known as milkweed - wild peanut or devil's coffee (Euphorbia heterophylla) - is the tenth weed in grain crops that has acquired resistance to glyphosate in Brazil. The fact was reported this week by Embrapa Soja (PR) and Cocari - Cooperativa Agropecuária e Industrial, in the Ivaí Valley region (PR). Although restricted to just one area, it is worrying researchers, technicians and producers, especially due to the economic impact it could cause if similar cases are found in other regions of the country.

To give you an idea, production costs in soybean crops with glyphosate-resistant weeds can rise from 42% to 222%, due to spending on herbicides and also because of the loss of soybean productivity.

According to Embrapa researcher Fernando Adegas, the values ​​increase, on average, between 42% and 48%, for isolated infestations of buva and ryegrass, respectively, and up to 165%, if there is resistant sourgrass.

The scientist calculates that the average cost, in reais, in Brazil, for weed control is R$120,00, per hectare. In mixed infestations of glyphosate-resistant weed species, the increase in control costs is greater. In areas with horseweed and bittergrass infestation, the control cost can reach R$386,00 per hectare, that is, a 222% increase in production costs. Therefore, the researcher advocates a broad discussion on the issue of resistance in Brazil. “In addition to the economic impact that is already felt, the producer needs to take measures to minimize or live with this resistance,” says Adegas.

“In a scenario of glyphosate-resistant ryegrass infestation, for example, there is a need to use an alternative herbicide associated with the product to control the weed. In this case, the cost per hectare is between R$118,60 and R$236,70, which represents an average increase in spending on herbicides of R$57,65 per hectare”, he explains.

In areas infested with glyphosate-resistant bittergrass, Adegas says that the control alternative becomes the use of graminicidal herbicides, on average, between two and four applications. With the increase in spraying, the average cost for control increases from R$120,00 to approximately R$318,00. “This has an additional 165% impact on production costs”, she assesses.

The discovery of resistance

The first suspicion of the milkman's resistance (Euphorbia heterophylla) was observed in the 2018/2019 harvest, when the weeds survived, even after glyphosate applications, on a property in the Ivaí Valley region. From then on, Cocari informed the technical team at Embrapa Soja, which began studies to prove the case of resistance.

Adegas explains that firstly, a new application of glyphosate was carried out in the affected area, with the recommended leaflet dose and application technology, but the glyphosate plants Euphorbia heterophylla again were not controlled by the herbicide.

The second stage of the work was the collection of seeds from the surviving plants in the field for evaluation by Embrapa. “It is important to highlight that we followed the entire protocol for reporting cases of weed resistance to herbicides, proposed by the Resistance Committee of the Brazilian Society of Plant Science (SBCPD), and the case was also reported to the Herbicide Resistance Action Committee (HRAC-Brazil),” explains Adegas.

Complementary studies have been carried out by Embrapa Soja in partnership with the State University of Maringá (UEM) and Colorado State University, with the aim of studying the resistance mechanism of this population.

In addition to the work to verify resistance, management studies of the species were also carried out, both in a greenhouse and in the field, in the area with suspected resistance. “In these studies, the hypothesis of the existence of multiple resistance to acetolactate synthase (ALS) inhibitors was observed, which is being verified through complementary experiments”, says Adegas. 

Plant has great competitive power. Milkweed is an annual weed widespread in the main agricultural regions of Brazil, especially in grain production systems, which include soybean crops. Adegas reports that before the introduction of genetically modified soybeans for resistance to the herbicide glyphosate, this weed was considered one of the main infesters of soybean crops.

“This occurred because the plant reproduces by seeds that have long viability and high germination potential, in addition to rapid vegetative growth, among other characteristics that provided this weed with great competitive power with crops,” he explains.

Plant has great competitive power

Milkweed is an annual weed widespread in the main agricultural regions of Brazil, especially in grain production systems, which include soybean crops. Adegas reports that before the introduction of genetically modified soybeans for resistance to the herbicide glyphosate, this weed was considered one of the main infesters of soybean crops.

“This occurred because the plant reproduces by seeds that have long viability and high germination potential, in addition to rapid vegetative growth, among other characteristics that provided this weed with great competitive power with crops,” he explains.

Embrapa researcher Dionísio Gazziero recalls that from the 1990s onwards, milkweed also became resistant to acetolactate synthase (ALS)-inhibiting herbicides, which were the group with the main mechanism of action used for control at the time. “This made controlling this weed even more difficult and complicated for Brazilian soybean farmers,” he recalls. 

How to overcome the resistance problem

Adegas recommends preventive methods to minimize the issue of resistance in weeds. Among them are: the acquisition of weed-free seeds; cleaning machines and equipment, especially harvesters and maintaining roadsides, paths and terraces free of weeds. In relation to mechanical control, weeding and mowing are recommended.

In the case of chemical control, the researcher remembers that the main action is the use of herbicides with different mechanisms of action, in different control systems. There are also cultural methods that can be allies for producers, such as reducing fallow periods, investing in straw production for soil coverage and the use of adapted cultivars in row spacing, in addition to crop rotation.

History of resistance

From the 2005/2006 harvest, with the introduction of transgenic soybeans, the management of populations of Euphorbia heterophylla was made much easier. “Weed control was basically carried out with two to three applications of glyphosate in the post-emergence of the crop,” says Adegas. This management system, with the almost exclusive use of glyphosate in the chemical control of weeds in soybean crops, has resulted in strong pressure to select species that are resistant to the herbicide.

In addition to milkweed, there are nine resistant weeds in Brazil: Amaranthus palmeri, Amaranthus hybrydus, Chloris elata, Conyza bonariensis, Conyza canadensis, conyza sumatrensis, Digitaria insularis, Eleusine indica e Annual ryegrass.

To prevent the resistance problem from occurring in other regions of Brazil, the researchers warn producers to adopt different anti-resistance strategies.

“It is very important to diversify products, with groups of herbicides with different mechanisms of action and to adopt crop rotation, emphasizes the scientist. It is also necessary to pay attention to the weeds remaining after chemical control so that the producer can identify whether there has been a failure in control or whether there may be dairy resistance in his crop”, he highlights.