Sour grass and herbicide interactions

In great weed potential and high capacity to form clumps, the grass bitter becomes even more challenging when there are infestations in conjunction with glyphosate-resistant/tolerant broad-leaved plants. The necessary The use of mixtures or sequential herbicides to combat these weeds demands a lot of attention and care so that unfavorable interactions between products do not occur used.

O bittergrass (Digitaria insularis) It is a perennial, bushy grass that reproduces by rhizomes and seeds. This species presents slow initial growth. However, from 45 days to 105 days after emergency, its growth is accelerated, presenting an exponential increase in dry matter. It is a plant with great pest potential, due to the rapid and aggressive development, its type of reproduction and ability to form clumps.

O Pre-emergence chemical control of bittergrass is highly efficient. However, in post-emergence there are few herbicides registered for applications, including EPSPs inhibitors (glyphosate) and ACCase inhibitors (graminicides) are those that have the ability to translocate to the rhizomes, a characteristic important for efficient control.

In addition of the unfavorable control characteristics of this weed, currently there is still a record of bittergrass resistance to the herbicide glyphosate in Argentina, Paraguay and Brazil (first reports in the state of Paraná in 2008). In 2016, the first case of cross-resistance in bittergrass to the herbicides fenoxaprop and haloxyfop-p-methyl, both belonging to the ACCase mechanism of action. However, resistance to graminicides is still punctual and, in areas where it does not occur, these herbicides are effective for controlling bittergrass when applied correctly. Therefore, the most effective chemical alternative for controlling bittergrass resistant to Glyphosate is the use of herbicides that inhibit the ACCase enzyme.

O The problem worsens when there are bittergrass infestations in conjunction with glyphosate-resistant/tolerant broad-leaved plants, such as horseweed, caruru and trapoeraba, in the same production area. Study carried out by Embrapa in last year, assessed that in soybean crops with horseweed and herbicide-resistant bittergrass, the production costs can suffer an average increase of up to 222%. As they are grasses and broad leaves, chemical control must be specific for each species, requiring the use of a mixture or sequence of herbicides to control both resistant plants. However, although the mixing or sequential herbicides is a common management practice, several precautions must be taken taken to prevent unfavorable interactions between products used, which can negatively interfere with the control of plants weeds.

Interactions between herbicides

A association or mixture of herbicides is based on the simultaneous use or sequence of two or more products applied to the same area or crop. Currently, tank mixing is not a prohibited practice, but it is responsibility of farmers and cannot be prescribed in revenue agronomic analysis by a qualified professional. However, the release of the mixture in tank is being processed under ordinance number 148 at the Ministry of Agriculture, Livestock and Supply (Mapa), which, if approved, will allow the professional recommend this type of activity. In addition to the use of mixtures in tank, the application of herbicides in sequential mode has been shown to be a interesting alternative for weed control. This practice increases control efficiency, in addition to being an opportunity to eliminate several emergency flows, reducing weed density.

Interactions due to mixtures/sequential herbicides may occur before, during or after the app. Herbicides have the ability to interact physically or chemically in the spray solution (syrup) or biologically on the plant. These interactions can generate incompatibility in the syrup and/or synergistic effect, additive or antagonistic on the plant in relation to the effect of each herbicide used in isolation.

Before application, the physical interaction is associated with the inert ingredients present in commercial products, while the chemical interaction is associated with the molecule of the herbicide. Physical incompatibilities can occur in tank mixes, through the formation of precipitates or lumps, which can lead to clogging of the tips and filters. Chemical incompatibility occurs when the mixture in tank changes the efficiency of the active ingredients mixed, which can generate increased phytotoxicity, causing damage to crops of economic interest. In this sense, when information about the mixture is unknown, a pre-mix (jar test) is recommended to check undesirable physicochemical reactions. If this occurs, you should seek means to replace incompatible products or carry out the application of isolated form. However, it is worth highlighting that incompatibilities Biological factors are not revealed by these tests.

After application, when the effect of mixing/sequential on weeds is greater than the expected effect for herbicides used separately, the mixture is synergistic. When it is smaller than the Expectedly, it is antagonistic. Additives occur when the effect of herbicides mixed is equal to the sum of their effects when applied separately.

Antagonism and synergism

Interactions antagonistic interactions between herbicides often cause inefficiency in plant control. weed plants. Negative interactions between herbicides in mixing/sequential can be attributed to changes in the amount of a herbicide that reaches its site of action through changes in absorption, translocation or metabolism caused by the presence of the other herbicide. A interaction between herbicides may affect their binding at the site of action, reducing its effect. Approximately 80% of antagonistic interactions occur in species of the Poaceae family (grasses).

On the other hand, synergistic interactions can help weed management by increasing the control spectrum and reducing costs. Most of the time, synergism occurs due to the action of constituents of the formulation of one herbicide over another, improving its absorption and/or translocation.

The association between mixed herbicides or sequentially depends on the doses used, the plant species, the time of application, the stage of plant development and the biochemical compatibility between the mechanisms of action of herbicides (Table 1 and 2).

Considering that bittergrass and horseweed are among the weeds of greatest concern in grain production areas in Brazil, the Plantas Daninhas and Pesticides in the Environment (PDPA/UFRRJ), has been carrying out research that demonstrates have the mixture of graminicides (ACCase) with latifolicides (2,4-D or ALS) effect antagonistic in the control of bittergrass and an additive effect in the fight against horseweed. However, the interval between applications can nullify the antagonism between herbicides. in bittergrass control. In these cases, the recommendation is restricted to function of antagonism for bittergrass.

When latifolicide (2,4-D or cloransulam) was applied on the same day as the graminicide (haloxyfop-p-methyl), or when the latifolicide was applied before the graminicide, regardless of the interval between applications, the interaction between herbicides was antagonistic and There was no efficient control of the plants (control less than 80%). However, sequential application of dehaloxyfop-p-methyl followed by latifolicide (2,4-D or cloransulam), with a minimum interval of 6 days between applications, presented additive interaction in the control of bittergrass, nullifying the antagonistic effect observed previously. In another test it was observed that in application glyphosate+haloxyfop-p-methyl sequence (first and second application with 21 days apart) the control of bittergrass plants was greater than 95%. However, when 2,4-D was added in the first application, the control became approximately 70%, highlighting the antagonism between herbicides.

Therefore, in areas infested with plants of bittergrass and glyphosate-resistant/tolerant broadleaf plants, the recommendation is to apply the graminicide at least 6 days before the latifolicide. Control of both plants in the initial stage of development, given the difficulty of control in advanced stages.

Practical recommendations for producer for correct management of bittergrass in production areas agricultural:

• Use of herbicides pre-emergent plants in order to reduce the seed bank and initial competition from bittergrass with a crop of interest;

• Carry out control of the bittergrass in the initial stages of development, preventing the plants form clumps;

• When present in the same area, bittergrass and resistant/tolerant broadleaf plants, carry out sequential application of graminicide +latifolicide, with a minimum interval of 6 days;

• Rotate action mechanisms in order to avoid the selection of bittergrass biotypes resistant to herbicides;

• Maintain good soil cover (including the consortium) in production areas, to suppress the germination of plants weeds.

Camila Ferreira de Pinho, Ana Claudia Langaro, Jéssica Ferreira Lourenço Leal, Amanda dos Santos Souza, Gabriella Francisco Pereira Borges de Oliveira, Gabriela de Souza da Silva, Rúbia de Moura Carneiro, UFRRJ

Article published in issue 230 of Cultivar Grandes Culturas, July 2018.