Abstract Background Areas simultaneously infested with both glyphosate-resistant sourgrass (Digitaria insularis L.) and fleabane (Conyza spp.) are usual in Brazil. However, there are no effective management strategies to control both species concurrently. Objective The aim was to evaluate the interaction and interval of application of haloxyfop-P-methyl and cloransulam-methyl to effective control sourgrass (from 3- to 4-tillers to flowering stages) and fleabane (12- to 15-cm at the vegetative stage). Methods The experiment was conducted twice in a randomized complete blocks design with four biological replicates. The treatments were composed of sequential applications of haloxyfop-P-methyl (firstly applied) and cloransulam-methyl (secondly applied) and vice-versa. After the first application, the second one was applied in intervals of 3, 6 and 12 days. Haloxyfop-P-methyl and cloransulam-methyl were also applied in sequential (without interval), stand-alone applications of both herbicides and an untreated check. Results All treatments effectively controlled all fleabane plants. Haloxyfop-P-methyl applied ≥ 6 days before cloransulam-methyl controlled 100% of 3- to 4-tillers sourgrass and 60% of flowering sourgrass. Conclusions Therefore, haloxyfop-P-methyl may be applied ≥ 6 days before cloransulam-methyl in a sequential application structure to effectively control fleabane and sourgrass (3- to 4-tillers stage), when present concurrently.

RESUMO O longo período residual dos herbicidas pré-emergentes pode interferir em culturas sucessoras, ocasionando o fenômeno denominado de efeito residual (carryover). O trabalho teve como objetivo avaliar o efeito residual dos herbicidas imazapir + imazapic na cultura de algodão em sucessão à soja. O experimento foi conduzido entre setembro de 2016 a março de 2017, no delineamento em blocos, com quatro repetições, sendo os tratamentos quatro doses de imazapir + imazapic, 75, 150, 225 e 300 g i.a. ha-1 e a testemunha. As variáveis analisadas foram fluorescência da clorofila a, altura da planta, comprimento e volume da raiz, massa seca da parte aérea e massa seca da raiz. Os dados gerados foram submetidos à análise da variância e ao teste de Tukey, a p ≤ 0,05 de probabilidade. A diminuição nos índices de desempenho fotossintético no algodão nos tratamentos com doses de 150, 225 e 300 g i.a. ha-1 aos 15 dias após emergência (DAE), e com dose de 75 g i.a. ha-1 aos 60 DAE, indica o efeito fotoinibitório destes herbicidas sobre as plantas, entretanto, as variáveis altura da planta, comprimento, volume e massa seca da raiz não foram afetados nos tratamentos testados. Portanto, a persistência do herbicida no solo não foi suficiente para caracterizar o efeito residual (carryover) em algodão 135 dias após a aplicação (DAA).

ABSTRACT The long residual period of pre-emergent herbicides can interfere with successor crops, causing a phenomenon named carryover. This study evaluated the residual effect of imazapyr + imazapic herbicides on cotton crop grown in succession to soybean. The experiment was conducted between September 2016 and March 2017, in a randomized block design with 4 replicates, where the treatments were four doses of imazapyr + imazapic, 75, 150, 225 and 300 g a.i. ha-1, and the control. The analyzed variables were chlorophyll a fluorescence, shoot height, root length and volume, dry shoot mass and root dry mass. The data were submitted to analysis of variance and Tukey’s test, at p ≤ 0.05. The reduction in photosynthetic performance indexes for treatments of 150, 225 and 300 g a.i. ha-1 at 15 days after emergence (DAE) and of 75 g a.i ha-1 at 60 DAE indicated a photoinhibitory effect of these herbicides on plants. However, shoot height, length, volume and root dry mass were not affected in the treatments tested. Therefore, the persistence of the herbicide in the soil was not enough to characterize a carryover effect in cotton 135 days after application (DAA).