Weed resistance management

Weeds currently represent a huge challenge for farmers around the world and are responsible for losses in crop productivity in different crops. In addition to competing for vital factors for crop development, they can also cause a loss in the agricultural quality of the final product. Several authors demonstrate that the average losses caused by weed interference are between 20% and 80%. This fact is not only due to the growth in the number of species that infest agricultural properties, but also to the change in the existing flora, containing different biotypes of the same species, including plants resistant to herbicides for one or multiple mechanisms of action.

In terms of sensitivity to herbicides, weeds can be divided into three groups: susceptible, tolerant and resistant. Susceptible weeds are those that, once a recommended standard dose of herbicide has been applied, their population is reduced to a form that is unable to complete its development. By tolerant, we consider plants that have a species-specific ability to tolerate the application of the herbicide and, even if it presents some type of damage, will be able to complete its cycle, even if it presents some limitation.

Resistant plants are those that, in some way, demonstrate the ability to survive the application of herbicides, even at high doses. Plant resistance can result from genetic mutations in plants (difficult to occur in the environment), biotechnological modifications of agricultural interest (introduction of a specific herbicide tolerance gene) or the selection of biotypes, this being the most common.

RESISTANCE FACTOR

The first cases of resistance found occurred in the United States (USA) and Canada in the 50s, however, they were only officially reported in 1998 by the American Weed Science Society (WSSA). In less than 60 years after the first record, there was a jump to 434 cases cataloged worldwide, with 237 species being reported in 82 crops in 65 countries (doctor Ian Heap - International Research on Herbicide-Resistant Weeds).

The scenario worsens when figures of more than 80% reduction in crop productivity are found when infested by weeds, which compete with crops for water and nutrients. Some studies carried out by Brazilian research institutions report that approximately 18 horseweed plants (Conyza ssp.) per m² can reduce soybean productivity by up to 50%, and this number becomes even more worrying when analyzing the plant's ability to reproduce: just one horseweed plant can produce up to 200 seeds.

IMPORTANCE OF MANAGEMENT

The need for complete control of weeds to prevent their spread becomes an essential requirement when thinking about weed management. Using the USA as an example, it appears that, in the last two years, the number of hectares infested with weeds reported as resistant to glyphosate increased by 80%, and the low control efficiency allows the plants to multiply year after year.

Current weed control systems show signs of the need to use new technologies. And to make the weed control situation even more complex, most herbicide-tolerant crops are based on a single gene or product: glyphosate. Considering the issues addressed, the appearance of resistant weeds, in addition to the spread of others that are difficult to control in crops such as soybeans, corn and cotton, points to the fact that, in most cases, glyphosate, alone, is no longer a efficient alternative.

Graph - Number of cases of resistance to a single mechanism of action

However, not only resistance to the herbicide glyphosate (EPSPs) has farmers worried. In 2014, more than 45 herbicide-resistant weeds in Brazil and approximately 160 in the USA are registered. More than that, today plants with multiple resistance are found, that is, resistant to more than one herbicide action mechanism. O Amaranthus tuberculatus (a species of pigweed, present on American properties) can now be found with resistance to four different mechanisms of action.

Resistance levels are unlikely to return to past levels, once resistant plants have been selected, however, the management of agricultural areas is an extremely important tool to maintain weed control levels.

Some points must be followed and will certainly help in the management of these weeds. Starting by accepting that difficult to control and resistant weeds are a reality around your property is the farmer's starting point. From there, applying sustainable management tactics, predicting risks and balancing investments are actions that will help prevent and reduce losses caused by weeds.

The development of a medium/long-term action plan is also key in this context. It is important to keep in mind that not only the sown/implanted crop is the source of success, but also other activities throughout the year, such as the use of cover crops, crop rotation, weed management during the off-season. (fallow), avoiding “seed rain” – which increases the soil seed bank for multiple years –, cleaning equipment, rotating herbicide action mechanisms, adopting different cultural practices and using seeds high quality and performance.

In addition to implementing all items over a long period, it is also necessary to monitor the action plan and check its effects, adjusting as necessary. Success in managing difficult-to-control or resistant weeds lies not only in the correct use of the practices adopted, but also in validating what was used.

Another crucial factor concerns the correct understanding of herbicides and their application. Understand the different action mechanisms used, adapting to the reality of the weed plant and the area to be treated, correctly use the indicated dose of the product for the weed stage, use regulated application equipment consistent with the area of ​​application and the crop -target – taking into account the environmental varieties and the combination of herbicides – are basic factors to be successful in controlling these pests.

NEW TECHNOLOGIES

The new tools for weed control that some companies have been working to make available to farmers come through the use of biotechnology in the development of plants tolerant to the application of more than one mechanism of action, helping to manage resistance. Among the most recent technological solutions that are in the approval phase for commercialization is the Enlist™ Weed Control System, technology developed by Dow AgroSciences to provide sustainability and improve traditional agricultural practices, such as direct planting and integrated crop management, with efficiency gains, with broad flexibility of use, and thus delivering productivity in the field in crops such as soybeans, corn and cotton.

This technology offers a new option for weed management and will also offer control of caterpillars in soybeans, containing the characteristics derived from biotechnology of tolerance to multiple herbicide groups and resistance to insects, combined with an innovative herbicide solution.

The system includes a herbicide tolerance technological package with an innovative formulation of Sal Colina (quaternary ammonium salt). It contains the proprietary Colex-D technology, a new formulation of traditional 2,4-D, which will offer important benefits to the farmer, including: ultra-low volatility, which reduces the possibility of the product turning into gas and disperse; anti-drift agent, which reduces the chance of the herbicide reaching anything other than its target; and reduced odor, as well as improved management characteristics and ready-mix availability with glyphosate, maintaining broad-spectrum control of broadleaf weeds.

The Enlist technology has already been submitted for registration, regulation and authorization for commercialization in Brazil by the responsible regulatory agencies, and will be made available to the Brazilian market as soon as all registration stages have been completed.

BOX

Enlist System - Also an ally in the fight against caterpillars

In Brazil, in addition to containing genes tolerant to three different types of herbicides, the Enlist™ Weed Control System for soybean expresses two proteins Bt (Bacillus thuringiensis), which also makes it resistant to the most common species of caterpillars that attack Brazilian crops. Among them, soybean (Anticarsia gemmatalis), apple (Heliothis virescens), of the false-measurement (Chrisodeixis includens) and cartridge (Spodoptera frugiperda).

Enlist corn is expected to add, in the future, Powercore technology, a platform already successfully adopted in the Brazilian market. In this way, in addition to tolerance to herbicides, corn will add resistance to lepidopteran insects, such as fall armyworm (Spodoptera frugiperda), stem borer (Diatraea saccharalis), earworm (Helicoverpa zea), caterpillar-elasmus (Elasmopalpus lignosellus) and screwworm (Agrotis ipsilon).

This article was published in issue 184 of Cultivar Grandes Culturas magazine. Click here to read the edition.