Susceptibility of Echinochloa populations to cyhalofop-butyl in Southern region of Brazil and impact of the weed phenology on its efficacy of control

portuguesCyhalofop-butyl destaca-se dentre os herbicidas usados no manejo de populacoes de capim-arroz resistente as imidazolinonas, mas nem sempre o orizicultor fica satisfeito com o resultado obtido com este herbicida. Objetivou-se avaliar a suscetibilidade de populacoes de capim-arroz (Echinochloa spp.) ao cyhalofop-butyl e, quantificar o efeito da fenologia da infestante sobre a eficacia do seu controle. Para isto, tres experimentos foram realizados em casa de vegetacao com delineamento inteiramente casualizado, utilizando-se populacoes de capim-arroz coletadas na regiao Sul do Brasil. Em dois experimentos, a suscetibilidade de 156 (safra 2012/13) e 103 (safra 2013/14) populacoes foram avaliadas frente a aplicacao do herbicida cyhalofop-butyl na dose de 360g ha-1. Em outro experimento, em que se arranjaram os tratamentos em esquema bi-fatorial, foram avaliados seis doses do herbicida e cinco estadios fenologicos de populacoes de E. crus-galli. Foi verificada variacao na resposta das populacoes de capim-arroz ao cyhalofop-butyl, em especial na safra 2013/14, em que 20, das 103 populacoes testadas, tiveram controle menor que 90%. A eficacia do controle foi inversamente proporcional ao estadio fenologico e, o atraso no controle da infestante contribui diretamente para a diminuicao da sua suscetibilidade ao herbicida. Cyhalofop-butyl e uma alternativa eficaz para controlar capim-arroz resistente as imidazolinonas, desde que a aplicacao ocorra nos estadios iniciais de desenvolvimento (2 a 4 folhas). Palavras-chave: capim-arroz; estadio de desenvolvimento; imidazolinonas; Echinochloa crus-galli; Echinochloa colona EnglishCyhalofop-butyl stands out among the herbicides in the control of imidazolinone-resistant Echinochloa species; but, rice farmers are not always satisfied with the control achieved with this herbicide. The objectives were to evaluate in regional scale the susceptibility of Echinochloa populations to cyhalofop-butyl, and quantify the effect of the weed phenology on its efficacy of control. For this, three trials were carried out under greenhouse conditions with a fully random design, using Echinochloa populations collected in rice fields in the southern region of Brazil. In two trials, the susceptibility level of 156 (2012/13 growth season) and 103 (2013/14 growth season) populations were evaluated with the application of cyhalofop-butyl at 360g ha-1. In other trial, in which treatments were arranged in a bi-factorial design (A = 6 x B = 5), it was evaluated six cyhalofop-butyl rates and five phenological stages of E. crus-galli populations. Echinochloa populations had showed differential susceptibility to cyhalofop-butyl, especially in the 2013/14 growth season, where 20 out of the 103 populations had control lower than 90%. The efficacy of this herbicide was inversely proportional to the phenological stage, and the application timing delay contributed directly to the decrease of susceptibility to the herbicide. Cyhalofop-butyl is an effective alternative to control imidazolinone-resistant Echinochloa populations, as long as the application timing occurs in the early phenological stages (2 to 4 leaves). Key words: barnyardgrass; development stage; imidazolinones; Echinochloa crus-galli; Echinochloa colona