Herbicides: How they interact

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

Pre-emergence chemical control of bittergrass is highly efficient. However, in post-emergence there are few herbicides registered for applications, among which EPSPs inhibitors (glyphosate) and ACCase inhibitors (graminicides) are those that have the ability to translocate to the rhizomes, an important characteristic for efficient control. 
In addition to the unfavorable control characteristics of this weed, there is currently 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 of bittergrass to the herbicides fenoxaprop and haloxyfop-p-methyl, both belonging to the ACCase mechanism of action, was also reported. However, resistance to graminicides is still sporadic 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 glyphosate-resistant bittergrass is the use of herbicides that inhibit the ACCase enzyme.
The problem worsens when there are infestations of bittergrass together with glyphosate-resistant/tolerant broad-leaved plants, such as horseweed, caruru and trapoeraba, in the same production area. A study carried out by Embrapa last year evaluated that in soybean crops with infestations of horseweed and bittergrass resistant to herbicides, production costs can increase on average by up to 222%. As they are grasses and broad leaves, the 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 mixing or sequential herbicides is a common management practice, several precautions must be taken to ensure that unfavorable interactions do not occur between the products used, which could negatively interfere with weed control.

INTERACTIONS BETWEEN HERBICIDES

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

Camila Ferreira de Pinho

Ana Claudia Langaro

Jessica Ferreira Lourenço Leal

Amanda dos Santos Souza

Gabriella Francisco Pereira Borges de Oliveira

Gabriela de Souza da Silva

Rúbia de Moura Carneiro

Research Group on Weeds and Pesticides in the Environment - UFRRJ