Glyphosate-resistant grasses

Ryegrass, bitter grass, white grass and crow's foot grass are the four grasses that represent greater concern for Brazilian farmers, due to show resistance to glyphosate. The use of herbicides applied in pre-emergence is one of the options that is gaining ground in control of these weeds.

Although weed resistance to herbicides is a global problem, the particularities of a country intensive tropical agriculture like Brazil leads to the development of problems specific to such environments. Selection of weed species glyphosate-resistant species comprises 22 broad-leaved species and 19 of grasses. In Brazil, the number of glyphosate-resistant species currently It has eight, four of which are broad leaves and four are grassy. However, among the broad leaves, three of the four species are from the genus Conyza (horseweed) and the fourth case is Amaranthus palmeri, which even the moment remains confined to specific areas of Mato Grosso under severe surveillance of regulatory authorities. The hot climate and the supply of water and light throughout most of the year favors the dispersion, development and multiplication of grasses. This is one of the items that can help explain the growing problem of glyphosate-resistant grasses. The concern worsens when it is found that the availability of herbicides intended for control specifically of grasses is much more limited than that of intended for broadleaf control. In areas with resistance problems from grasses to glyphosate, the first control alternative for farmers It is normally one of the ACCase inhibitor herbicides (DIM's and FOP's). In function In addition, three of the four species that already show resistance to glyphosate They have also developed resistance to some of the herbicides in this group.

All resistance reports from around the world can be found at the email address www.weedscience.org. Browsing the website, you can find information about resistance to weed plants, updated and broken down by species, herbicides, countries, cultures and year of occurrence, not only in Brazil but also in other countries around the world.

The four grasses that represent a concern for the Brazilian farmer, due to their resistance to glyphosate are ryegrass, bitter grass, white grass and crow's foot grass.

1 - Ryegrass (Lolium multiflorum)

This species is typical of environments temperate, therefore problems with this weed are limited to regions with a milder climate, in the southern region of Brazil.

Ryegrass was the first species to develop resistance to glyphosate in Brazil. The first populations resistant were identified in RR soybean crops in Rio Grande do Sul in 2005. Although it is a species typically present in areas with a more temperate region of the country, concern has increased following reports of resistance multiple use of glyphosate and ACCase inhibitors in soybeans (2010), as well as glyphosate and ALS inhibitors in soy/wheat succession areas (2016).

2 - Bittergrass (Digitaria insularis)

Bittergrass is, without a doubt, the case resistance to glyphosate, which has become more widespread in the country. This fact has been attributed not only to the intensive use of this herbicide, but also to the fact that is a perennial species, with seed transport through machines and wind. The first cases of glyphosate resistance were registered in Paraguay in 2005, in Brazil in 2008 and in Argentina in 2014. Brazil, from the first observation in Western Paraná in 2008, spread quickly to practically all states grain producers. The breadth of dissemination of bittergrass areas with resistance to glyphosate has led to the intensification of the use of ACCase inhibitor herbicides. Recently, in 2016, it was registered the first case of resistance to ACCase inhibitors (fenoxaprop and haloxyfop).

3 - White grass (Chloris elata)

The first report of glyphosate-resistant whitegrass occurred in 2014, in a population from soybean areas. Apparently, the mechanisms of resistance of this population are related to lower absorption of glyphosate and to greater retention of the herbicide in the treated leaves. Of the four cases described In this work, it is the case of resistance to glyphosate with less dissemination until the moment.

4 - Crow's foot grass (Eleusine indicates)

Crow's foot grass is one of the plants most problematic weeds, found in many regions of the world. Is annual plant, fast growing and with high competition potential interspecific, autogamous, with C4 cycle and high seed production, disseminated by the wind. Its center of origin is probably Asia and it was distributed across tropical, subtropical and temperate regions of the world, with a greater presence in regions between the tropics. Each plant is capable of producing more than 120 thousand seeds.

In Brazil, the first case of resistance from chicken foot grass to ACCase (Acetyl-CoA carboxylase) inhibitors, it was registered in 2003 in Rio Grande do Sul and later in Paraná. With the introduction of RR soybeans and the increased use of glyphosate in post-emergence, Such problems were apparently considered to have little impact, due to the effectiveness of glyphosate to control this plant. In recent years many complaints of escape from chicken-foot grass plants were registered in the Wax, mainly in cotton-producing areas. Researcher's work Anderson Cavenaghi together with IMA-MT indicate that more than 60% of samples of chicken-foot grass evaluated are currently resistant to clethodim. In 2016, NAPD/UEM reported the first case of resistance to glyphosate in crow's feet in Brazil, in the region of Campo Mourão (PR). Finally, in 2018, researcher Núbia Correia (Embrapa Hortaliças) reported the first case of multiple resistance (glyphosate, fenoxaprop-P-ethyl and haloxyfop-methyl).

Control

Until now There are no herbicides recommended for controlling whitegrass. To the desiccation of areas containing these glyphosate-resistant grasses, the options are limited to contact products (paraquat, [paraquat+diuron] and ammonium-glufosinate), which present limitations regarding the size of plants they control. Despite cases of resistance, ACCase inhibitors are still the most used herbicides to control these populations, both in desiccation and powders, as they are systemic and enable the control of adult plants.

Another option interesting and which has acquired greater importance lies in the use of herbicides applied in pre-emergence. The options available on the Brazilian market are listed in Table 1. It is worth noting that not all options for herbicides listed in Table 1 are selective for all crops greater importance, and it is necessary to consult technical assistance to evaluate the best option for each case.

Is important point out that there are also alternative options to herbicides that can be used together, such as crop rotation, use of living cover or dead on the ground, mechanical operations such as mowing and weeding, plowing and harrowing, among others. Not just for the control itself, but these methods are also considered fundamental techniques in the prevention and management of resistance.

Article published in issue 228 of Cultivar Grandes Culturas, May, 2018.