Research confirms resistance of new weed to glyphosate

The first occurrence of resistance of the picão-preto weed (Bidens subalternans) to the herbicide, glyphosate, was identified in Brazil in the 2022/2023 harvest, which was reported to Embrapa Soja by technicians from Coamo Agroindustrial Cooperativa, who observed the survival of a population of the species in the municipality of Juranda (PR). ″Even after sequential applications of glyphosate in the doses recommended on labels and leaflets and carried out correctly, the product did not control picão-preto plants in soybean crops,″ said Embrapa researcher Fernando Adegas.

To evaluate the case, a group composed of technicians from Coamo and researchers from Embrapa began studies to prove resistance under controlled conditions. ″Firstly, we collected surviving plants in the area to begin our studies. In additional analyses, samples of seeds and plants from populations of picão-preto also with suspected resistance to glyphosate and from other properties were evaluated, assisted by the Lar cooperative,″ Adegas added.

″While carrying out resistance studies, we also carried out management work on this population of weeds, both in the greenhouse and in the field,″ he further added. The report is available in the publication, ″New case of weed resistance to glyphosate in Brazil: black picão (Bidens subalternans).

In 2018, a team of researchers from Embrapa Soja and the State University of Maringá identified, in Paraguay, the first and until then the only case of resistance of the species to glyphosate in the world. Paraguay and Brazil now have reports of the occurrence of glyphosate-resistant walleye.

Embrapa and the Coamo and Lar cooperatives are developing monitoring, management, mitigation and containment procedures for the resistant picão-preto population. ″We are also planning studies to determine the mechanism of resistance to glyphosate in this population and whether there is multiple resistance to other herbicides,″ Adegas said. These studies are being carried out in partnership with the State University of Maringá ( UEM ) and Pennsylvania State University ( PSU ) in the United States.

Picão-preto weed 

The picão-preto is a weed that occurs in Brazil, with the two main species being Bidens pilosa and Bidens subalternans. ″Because they are very similar and are generally present at the same time in cultivation areas, producers do not differentiate between species, calling the entire population ″pião-preto,″ Adegas explained.

Despite the identification of Bidens subalternans resistance to glyphosate, picão-preto populations were already identified as resistant to other herbicides in the 1990s. During this period, the main herbicides used to control this weed were inhibitors of the enzyme, acetolactate synthase (ALS). ″Due to the continued and practically exclusive use of this group of herbicides, resistant populations were selected,″ he added.

History of resistance in Brazil

In Brazil, there are two moments related to weed resistance. The first cases of resistance to herbicides in the country were reported in 1993, relating to the species Bidens pilosa and Euphorbia heterophylla that were resistant to ALS-inhibiting herbicides.

Currently, producers are experiencing a second stage of weeds resistant to the herbicide, glyphosate. ″This resistance process is related to the constant use of the same active ingredient in the same area and for a long period of time,″ Adegas explained. 

The intensive use of glyphosate placed great selection pressure on weeds, resulting in the selection of 12 resistant species, which are ryegrass (Lolium perene spp multiflorum), three species of horseweed (Conyza bonariensis, C. canadensis and C sumatrensis), bittergrass (Digitaria insularis), palmeri pigweed (Amaranthus palmeri), giant pigweed (Amaranthus hybridus), white grass (Chloris elata), crow's foot grass (Eleusine indica), milkweed (Euphorbia heterophylla), barnyard grass (Echinochloa crusgalli), and now picão-preto (Bidens subalternans).

A study carried out by Embrapa showed that production costs in soybean crops with weeds resistant to glyphosate could rise, on average, from 42% to 222%, mainly due to the rise in spending on herbicides and the loss of soybean productivity. According to Adegas, values rise, on average, between 42% and 48% for isolated infestations of horseweed and ryegrass, respectively, and up to 165% if there is resistant bittergrass. In cases of mixed infestations of horseweed and bittergrass, for example, the average increase is 222%.

Resistance process

Adegas defines resistance as the inherited ability of a weed plant to survive and reproduce after exposure to a dose of herbicide normally lethal at the label dose to the natural population. ″It is a natural occurrence, as weeds evolve and adapt to changes in the environment and the use of agricultural technologies. In practice, the emergence of resistance occurs through the process of selecting resistant individuals that already exist in the population and are present in production areas, due to repeated and continued applications of the same herbicide or herbicides with the same mechanisms of action during a given period of time,″ he explained.

How to prevent the selection of resistant plants

Among the recommended preventive methods, Adegas highlighted the acquisition of weed-free seeds and cleaning machines and equipment, especially harvesters, and maintaining roadsides, paths and terraces free of weeds.

In terms of mechanical control, the recommendation is weeding and mowing. In the case of chemical control, the main action is the use of herbicides with different mechanisms of action under different control systems.

Among cultural methods, the reduction of fallow periods, investment in straw production for soil coverage and the use of adapted cultivars in row spacing, in addition to crop rotation, are recommended.

The study was carried out following the protocol for reporting cases of weed resistance to herbicides, proposed by the Resistance Committee of the Brazilian Society of Weed Science (SBCPD) and approved by the Brazilian Herbicide Resistance Action Committee (HRAC- BR) and International (IRAC).

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