Management of resistant weeds

Project developed in Mato Grosso visits properties in several cotton producing regions to collect seeds of weed biotypes suspected of being resistant to herbicides. The work serves as a basis to guide actions in an attempt to preserve available chemical control technologies.

Weed control with herbicides is widely used in agriculture, as it is not only efficient, it is agile and has a good cost/benefit ratio when compared to other control methods. However, herbicides are strong selection agents for weed species and biotypes, and their inappropriate use has accelerated the emergence of tolerant species and resistant biotypes.

If, in a weed community, there is a species that is not well controlled by successive herbicide treatments that are applied in the area (tolerant species), the consequence is that this species, already problematic for the management adopted, has its population increased. Among other conditions, the choice of herbicide treatment must take into account efficiency on weed species with greater density and competitiveness, but also have good efficiency on species that have recently shown a rapid increase in population, or that are doing so constantly. .

If in a weed community there is a species whose population has always been well controlled by a certain herbicide treatment, and we begin to observe individuals within this population that are no longer controlled like the others, they are suspected of being resistant biotypes. Confirming resistance requires studies using appropriate methods, since, for other reasons, control failures may occur in susceptible individuals. Once resistance is confirmed, management must be changed so that the resistant biotype is effectively controlled.

There are some known mechanisms by which plants become resistant to herbicides. Of these, the most common so far, considering all herbicides in use, has been the occurrence of genetic modification that promotes changes in one or more points of the substance where the herbicide acts, called changes in the site of action. In these cases, knowledge of the site of action of the herbicide to which the biotype has become resistant is necessary, as one of the measures to control it is the use of herbicides that are efficient on the species, but that act on another site of action. That is why, among the measures for managing and/or preventing weed resistance to herbicides, it is recommended that the use of herbicides respects the principle of alternating sites of action.

There are other important resistance mechanisms, sometimes more common for certain types of herbicides, and others that are of great concern due to their ability to generate biotypes with simultaneous resistance to herbicides with different sites of action (multiple resistance), with severe implications for management by reducing herbicide options or even make chemical control unfeasible.

The problem of weed resistance to herbicides is of interest to all sectors involved in the production and use of plant species that have chemical control as an important tool. Management of this problem depends on information and knowledge, including the formation of an updated database on the occurrence of resistant biotypes, with information on detection sites, herbicides to which they have become resistant, efficient herbicides for their control, etc. . One of the ways to contribute with this information are surveys carried out regionally, or on a larger scale, where individuals identified as suspected of having resistance are collected and tested. In the state of Mato Grosso, a project is being implemented with this purpose.

The project

The project is an initiative of the Mato-Grossense Cotton Institute (IMAmt) and the Mato-Grossense Association of Cotton Producers (AMPA), financed by the Brazilian Cotton Institute (IBA), with Univag - Cento Universitário de Várzea as partner institutions. Grande and the Federal University of Mato Grosso (UFMT). In this work, visits are made to properties in various regions of the state, which have cotton as one of the crops in the production system, and seeds of weed biotypes suspected of being resistant to herbicides are collected from them. These seeds are used in batteries of tests under controlled conditions with the aim of confirming or ruling out resistance. At the same time, seed samples from unvisited properties are received and tested, regardless of region or culture.

The project began in the 2011-12 harvest and is ongoing, at this stage prioritizing suspected biotypes that could constitute new cases.

Results

The weed biotypes, whose resistance to herbicides has already been confirmed by the project team in the agrosystems of Mato Grosso, are listed in Table 1. All herbicides were applied post-emergence of the weeds.

In addition to the biotypes listed, a biotype of Amaranthus retroflexus resistant to atrazine and promethrin, which are inhibitors of photosystem II, and is also resistant to trifloxysulfuron sodium. But in this case only when applied in the pre-emergence modality (Francischini, 2011).

Considerations on identifying resistant biotypes

To reduce the number of negative results in the resistance assessment, some points must be observed:

- Is there effective control of other weed species susceptible to the herbicide used?

- In the population of the investigated species, are there dead (susceptible) weeds alongside uncontrolled ones (suspected biotype)? If the answer is yes, check that these live plants are not re-infestation of the area (this can be seen by the difference in development between the plants).

- Is there an occurrence of weeds with confirmed resistance in neighboring areas or plots?

- Is there a characteristic visual pattern of resistant weeds in the area, such as reboleiras with a high density of plants in the center, which appear every year in the same point of the field, or escapes in different directions?

- Check that there were no problems when applying the herbicide at a dose below the recommended dose; inadequate volume of syrup, adjuvants and nozzles; spraying failures; umbrella effect exerted by the culture itself; climatic problems such as rain, winds, high temperature and low humidity and plants outside the recommended development stage.

Actions to prevent and control resistant weeds

 In the same area, rotate or combine herbicides with different sites of action. For this practice to be effective, weeds must be susceptible to at least two herbicides with different sites of action, used in rotation or combined.

It is important to control remaining plants after applications of herbicide treatments - control must be carried out before these remaining plants are adults, avoiding seed production.

Crop rotation is another important aspect, mainly to facilitate the use of other control methods and the rotation of herbicides with different sites of action.

Rotating herbicide-resistant varieties is another measure. Although these varieties constitute useful tools, technologies that allow the use of herbicides with different sites of action must be alternated in the same areas, as repeatedly using the herbicides for which the varieties were developed will lead to the selection of resistant biotypes and the loss of technology.

It is necessary to make appropriate use of varieties with resistance to herbicides as they are important tools for the prevention and control of resistant biotypes, but their inappropriate use has favored the selection of tolerant species and resistant biotypes. This has occurred because only one active ingredient has been applied to the area, successive times, to control the same group of weeds. Therefore, it is necessary to plan the use of these varieties to allow the rotation of herbicides in the same area, so that they have different sites of action and the same weed control spectrum.

In a scenario of multiple resistance, or involving resistance mechanisms unrelated to the site of action, more specific actions are necessary and it is recommended to consult an experienced specialist in the subject.

Edson de Andrade Junior, Instituto Mato-Grossense do Algodão; Anderson Luis Cavenaghi, Univag – Centro Universitário de Várzea Grande; Sebastião Carneiro Guimarães, Federal University of Mato Grosso

Article published in issue 207 of Cultivar Grandes Culturas.