ACCase-inhibiting herbicides: mechanism of action, resistance evolution and stewardship

Herbicides play an important role in preventing crop yield losses due to both their weed interference ability and their capacity for increasing soil conservation in no-till systems. Group A herbicides or acetyl-CoA carboxylase (ACCase) are essential tools the selective management of glyphosate resistance in grass weed species. In this review, we describe important aspects of ACCase biology and herbicides targeting this enzyme, along with a discussion on stewardship programs to delay the evolution of herbicide resistance which can evolve either through target site and/or non-target site mechanisms. Sixteen-point mutations have been reported to confer resistance to ACCase inhibitors. Each mutation confers cross resistance to a different group of herbicides. Metabolic resistance can result in resistance to multiple herbicides with different mechanisms of action (MoA), and herbicide detoxification is often conferred by cytochrome P450 monooxigenases and glutathione-Stransferases. Regardless of whether resistance mechanisms are target or non-target site, using herbicides with the same MoA will result in resistance evolution. Therefore, while field surveys and resistance mechanism studies are crucial for designing reactive management strategies, integrated weed management plays a central role in both reactive and proactive mitigation of herbicide resistance evolution

Monitoramento e baseline da resistência de capim-amargoso ao herbicida glifosato nas principais regiões produtoras de soja no Brasil

Frequent application of glyphosate for consecutive years has enhanced the selection pressure on sourgrass (Digitaria insularis) populations, which resulted in the development of glyphosate-resistant biotypes. Therefore, this work was developed with the objective of monitoring sourgrass resistance to glyphosate, develop a baseline of sourgrass susceptibility to this molecule and, consequently, identify the discriminatory dose between resistant and susceptible populations.  This work was divided into three steps. The first step consisted of identifying and sorting sourgrass resistant and susceptible biotypes among 30 samples. In the second step, glyphosate baseline was elaborated considering exclusively the glyphosate-susceptible biotypes, which allowed the definition of a discriminatory dose. At the end, the third step, monitoring of glyphosate-resistant biotypes was achieved, considering five growing seasons (2016 – 2020) and 809 samples of sourgrass populations, collected throughout 12 states of Brazil.  Glyphosate baseline was elaborated to sourgrass and ideal discriminatory rate was identified as 960 g ha-1. Glyphosate-resistant populations of sourgrass were found in all soybean growing regions sampled. Among 809 populations, 25.96% were considered resistant to glyphosate. The states with the highest frequency of glyphosate-resistant populations were Rio Grande do Sul, Mato Grosso do Sul, Bahia, Mato Grosso and Paraná.  Este trabalho foi desenvolvido com o objetivo de monitorar a resistência de capim-amargoso ao herbicida glyphosate desenvolver uma baseline de suscetibilidade da espécie e, por consequência, identificar a dose discriminatória de glifosato entre populações resistentes e suscetíveis de capim-amargoso.  Todo o trabalho foi dividido em três fases.  A primeira fase consistiu da análise de 30 amostras de capim-amargoso, identificando-as e classificando-as em resistentes ou suscetíveis. Na segunda fase, foi elaborada uma baseline para suscetibilidade do capim-amargoso ao herbicida glyphosate, o que permitiu a definição de uma dose discriminatória.  Ao final, na terceira fase, obteve-se o monitoramento dos biótipos de capim-amargoso quanto à resistência, considerando-se cinco safras (2016 – 2020) e 809 amostras de capim-amargoso, oriundas de 12 estados brasileiros. Assim sendo, a baseline de suscetibilidade de capim-amargoso ao glyphosate foi estimada, cuja dose discriminatória ideal foi de 960 g ha-1. Populações de capim-amargoso resistentes ao glyphosate foram encontradas em todas as regiões produtoras de soja amostradas.  Dentre 809 populações, 25,96% foram consideradas resistentes ao glyphosate. Os estados com maior frequência de populações resistentes foram: Rio Grande do Sul, Mato Grosso do Sul, Bahia, Mato Grosso e Paraná.

Managing glyphosate-resistant weeds with cover crop associated with herbicide rotation and mixture

ABSTRACT Herbicide resistance has become a major concern for agricultural systems, and integrating weed management practices seem to be the most promise way for its mitigation. The effects of cropping systems including ruzigrass (Urochloa ruziziensis) associated with herbicide rotation programs were evaluated for the management of glyphosate resistant conyza (Conyza sumatrensis) and sourgrass (Digitaria insularis) in soybean and corn production systems. A 3x3 factorial was evaluated in four consecutive soybean (spring-summer) and corn (fall-winter) double cropping seasons. Factor A consisted of three combinations of cropping systems in the fall-winter: corn, corn plus ruzigrass, and ruzigrass alone. Factor B was based on increasing levels of diversity in herbicide mode of action over the four-year period (5, 8, and 11 MoAs). The results indicate that using ruzigrass significantly increased soil coverage by cover crop residues. In general, the cover crop was more efficient for conyza than for sourgrass control, whereas herbicide programs provided greater control on sourgrass than on conyza. Besides the weed suppression effect, the accumulation of ruzigrass biomass on the soil surface during the fall-winter also improved yield of soybean in the spring-summer. The integration of ruzigrass as a cover crop and the use of herbicide programs with multiple modes of action can provide efficient control of glyphosate-resistant conyza and sourgrass. The use of only one of these strategies was not effective in most cases, especially for the long term

Volatilization of Dicamba Diglycolamine Salt in Combination with Glyphosate Formulations and Volatility Reducers in Brazil

Dicamba can be included in weed management programs for Brazilian agricultural crops, such as Conyza spp. and Amaranthus spp., and it is essential to implement good management practices that include salts with low volatility levels and appropriate associations to maintain volatility at acceptable levels. The objective of this study was to evaluate the volatilization of dicamba diglycolamine (DGA) salt associated with different glyphosate salts and volatility reducers. Laboratory and field studies were conducted with the application of DGA alone and in mixtures with three glyphosate formulations (potassium salt, ammonium salt, and di-ammonium salt) with and without a volatility reducer. Under laboratory conditions, the sprayed targets (corn straw) were sent to a vapor collection system for subsequent determination of the amount of volatilized dicamba. In the field, the treatments were applied in a tray containing clay soil, and then, these trays were arranged in soybean rows for 48 h under plastic tunnels. The plant injury and the concentrations of the dicamba in the soybean plants at different distances from the tray were determined. The methodologies used in this study were adequate for understanding the volatility of DGA. The volatility of the dicamba DGA salt used was reduced and was managed through the use of volatility reducers and the correct formulation of glyphosate in the mixture. The VR was efficient in reducing the volatility for dicamba alone and DGA in combination with all glyphosate salts. The combination of dicamba DGA salt with glyphosate potassium salt and a volatility reducer was the mixture with the lowest volatility and is the most suitable combination to recommend to farmers

Volatilization of Dicamba Diglycolamine Salt in Combination with Glyphosate Formulations and Volatility Reducers in Brazil

Dicamba can be included in weed management programs for Brazilian agricultural crops, such as Conyza spp. and Amaranthus spp., and it is essential to implement good management practices that include salts with low volatility levels and appropriate associations to maintain volatility at acceptable levels. The objective of this study was to evaluate the volatilization of dicamba diglycolamine (DGA) salt associated with different glyphosate salts and volatility reducers. Laboratory and field studies were conducted with the application of DGA alone and in mixtures with three glyphosate formulations (potassium salt, ammonium salt, and di-ammonium salt) with and without a volatility reducer. Under laboratory conditions, the sprayed targets (corn straw) were sent to a vapor collection system for subsequent determination of the amount of volatilized dicamba. In the field, the treatments were applied in a tray containing clay soil, and then, these trays were arranged in soybean rows for 48 h under plastic tunnels. The plant injury and the concentrations of the dicamba in the soybean plants at different distances from the tray were determined. The methodologies used in this study were adequate for understanding the volatility of DGA. The volatility of the dicamba DGA salt used was reduced and was managed through the use of volatility reducers and the correct formulation of glyphosate in the mixture. The VR was efficient in reducing the volatility for dicamba alone and DGA in combination with all glyphosate salts. The combination of dicamba DGA salt with glyphosate potassium salt and a volatility reducer was the mixture with the lowest volatility and is the most suitable combination to recommend to farmers