ABSTRACT 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- S -transferases. 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.

ABSTRACT Glyphosate-resistant (GR) Italian ryegrass (LOLMU) and Alexandergrass (URPLA) are troublesome weeds in corn cropping systems in Southern Brazil. The emergence pattern of those weeds is not uniform and may change according to the season’s environmental characteristics. Also, herbicide resistance has been diminishing the success of the weed control programs. The objectives of this study were to evaluate the influence of corn-sowing date on LOLMU and URPLA densities and their control provided by pre- and post-emergent herbicides. Field trials were conducted in two crop seasons in Southern Brazil consisting of three corn sowing date (August, September, and October) and the application of atrazine + S-metolachlor (residual) in corn pre-emergence in different post-emergence weed control programs with glyphosate, ammonium-glufosinate, nicosulfuron, and atrazine. The results indicated that the sowing date had a significant influence on LOLMU and URPLA densities. Corn sown in the earliest period was exposed to a higher LOLMU density, whereas corn sown in the latest period had a higher density of URPLA. Also, the application of residual herbicide at corn pre-emergence reduced both weed species densities and decreased the pressure for the control of glyphosate-resistant LOLMU for the post-emergence herbicides. The use of residual herbicides in corn pre-emergence is an efficient strategy to be considered in the LOLMU and URPLA control programs, followed by post-emergence application of glyphosate, ammonium-glufosinate, atrazine, and nicosulfuron.

RESUMO A evolução de plantas daninhas resistentes e tolerantes ao glyphosate tem causado mudanças no manejo de plantas daninhas em todo o mundo. Herbicidas residuais são cruciais para o manejo dessas plantas daninhas, porém a taxa de adoção dessas ferramentas ainda é significativamente baixa no Brasil. Nesta pesquisa, Ipomoea triloba tolerante ao glyphosate foi utilizada como uma espécie modelo para avaliar, em múltiplos anos e localidades do Brasil, as vantagens de se utilizar herbicidas residuais em soja, comparado ao controle realizado apenas em pós-emergência. A maioria dos herbicidas residuais proporcionaram atividade residual suficiente para uma janela maior de aplicação em pós-emergência. Tratamentos com herbicidas residuais também aumentaram o controle final de plantas daninhas, prevenindo a interferência inicial com a soja, o que refletiu em maior produtividade. Maiores porcentagens de controle sobre I. triloba foram observadas quando dois herbicidas residuais foram utilizados, em comparação à apenas um, especialmente nos anos e localidades com alta infestação. O uso de herbicidas utilizados em pré-emergência permite uma janela de aplicação maior em pós-emergência, o que pode ser importante em grandes propriedades onde logística constitui um entrave.

ABSTRACT The evolution of glyphosate-resistant and -tolerant weeds has caused changes in weed management around the world. Residual herbicides are crucial tools for weed management, but the rate of adoption by soybean growers remains very low in Brazil. In this research, we used glyphosate tolerant Ipomoea triloba as a model weed species to evaluate the advantages of using residual herbicides on soybeans in multiple years and locations of transition and Cerrado regions of Brazil, rather than relying only on post-emergence control. Most residual herbicides provided enough residual activity to allow a longer application window in post-emergence. Treatments with residual herbicides increased overall weed control, preventing weed interference and increasing soybean yield. When two residual herbicides were used as opposed to only one, a better I. triloba control was achieved, reflecting in higher crop yield, especially in conditions of high weed infestation. The use of pre-emergence herbicides allows growers to have a longer application window for the post-emergence treatment, which is particularly important in Brazilian Cerrado large fields when logistic could be an issue.

RESUMO A resistência a herbicidas tem se tornado um dos principais problemas que os produtores têm enfrentado, e as estratégias de manejo integrado de plantas daninhas são as mais importantes ferramentas para a sua mitigação. O objetivo deste trabalho foi investigar o efeito do cultivo de braquiária (Urochloa ruziziensis) associado a diferentes programas de rotação de herbicidas no manejo de buva (Conyza sumatrensis) e capim-amargoso (Digitaria insularis) resistentes ao glyphosate no sistema de produção de soja e milho. Um fatorial 3x3 foi avaliado durante quatro safras consecutivas de soja no verão e milho no inverno. O Fator A foi constituído de diferentes sistemas de manejo de inverno: milho solteiro, milho integrado com braquiária e braquiária solteira. O Fator foi baseado em programas de rotação de herbicidas com diferentes níveis de diversidade em mecanismos de ação ao longo das quatro safras. O cultivo de braquiária no inverno incrementou significativamente a biomassa residual no solo. Essa cobertura do solo foi mais eficiente para controlar buva do que capim-amargoso, embora ambas plantas daninhas foram suprimidas pela cultura de cobertura. O uso de programas com maior rotação de mecanismos de ação de herbicidas foi mais eficiente para controlar capim-amargoso do que buva. O uso isolado de cultura de cobertura ou de rotação de herbicidas não foi efetivo para o manejo de buva e amargoso. No entanto, integrando essas duas opções de manejo, foi possível controlar efetivamente ambas as espécies resistentes ao glyphosate e consequentemente aumentar a produtividade das culturas.

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.

Resumo: O objetivo deste trabalho foi avaliar o crescimento e o desenvolvimento de uma população de Conyza bonariensis resistente ao herbicida glifosato em diferentes datas de semeadura (outono, inverno e primavera) e dois ambientes agrícolas, com base em dias ou unidades térmicas. Foram desenvolvidos cinco experimentos em dois ambientes agrícolas: dois no Município de Não-Me-Toque, RS - com semeaduras em julho e setembro de 2011; e três no Município de Santa Cruz das Palmeiras, SP ‒ com semeaduras em abril, julho e setembro de 2011. Em cada experimento, realizaram-se dez avaliações do desenvolvimento fenológico e do acúmulo de massa de matéria seca total de C. bonariensis, com ajuste das variáveis à contagem de tempo em dias ou graus-dia acumulados. O desenvolvimento fenológico da espécie apresentou melhor ajuste com temperatura basal de 8,4ºC, e foi influenciado pelo ambiente agrícola e pela data de semeadura. No outono, com temperatura e fotoperíodo decrescentes, as plantas permanecem aptas a acumular massa de matéria seca, porém sem indução floral. Na primavera, com temperatura e fotoperíodo crescentes, o acúmulo de massa de matéria seca é menor, mas o desenvolvimento fenológico é mais rápido, com indução floral significativa.

Abstract: The objective of this work was to evaluate the growth and development of a glyphosate-resistant population of Conyza bonariensis in different sowing dates (autumn, winter, and spring) and in two agricultural environments, based on days or thermal units. Five experiments were performed in two agricultural environments in Brazil: two in the municipality of Não-Me-Toque, in the state of Rio Grande do Sul - with sowings in July and September 2011; and three in the municipality of Santa Cruz das Palmeiras, in the state of São Paulo ‒ with sowings in April, July, and September 2011. In each trial, ten evaluations of the phenological development and total dry mass of C.bonariensis were performed, fitting these variables to a unit of time in days or growing degree days. The phenological development of C.bonariensis had the best adjustment at the base temperature of 8.4oC, and was affected by sowing date and agricultural environment. In autumn, with decreasing temperature and photoperiod, plants are still able to accumulate dry mass, but without floral induction. In spring, with increasing temperature and photoperiod, dry mass accumulation is lower, but phenological development is faster, with significant floral induction.