Cloransulam-methyl efficiency in postemergence control of Conyza bonariensis in RRTM soybeans crops

Abstract

With the introduction of transgenic soybean resistant to glyphosate, which was widespread in southern Brazil, the number of applications and the selection of biotypes of horseweed (Conyza bonariensis) resistant to herbicide have stepped up. Thus, new chemical alternatives are essential for the efficient control of horseweed. Thus, the aim of this study was to evaluate the effectiveness of cloransulam-methyl (Pacto^TM), applied in postemergence, in controlling C. bonariensis and SYN 1059 RR Vtop^TM soybean yield. The experiment was conducted in the Brazilian municipality of Assis Chateaubriand, PR, in an experimental design of randomized blocks with four replications. The treatments consisted in doses of cloransulam-methyl (25, 30, 35 and 40 g a.i. ha^-1), chlorimuron-ethyl (20 g a.i. ha^-1) and weeded and unweeded controls. The horseweed plants were between 5 to 10 cm high and had an average density of 35 plants per m². Cloransulam-methyl, at doses of 25 and 30 g ha^-1, did not manage horseweed efficiently (< 80%) from 28 DAA (days after application), being inferior to chlorimuron-ethyl. However, cloransulam-methyl at 40 to 35 g ha^-1 showed horseweed control (87.0% and 90.7%), which did not differ significantly from chlorimuron-ethyl at 42 DAA (85.7%) and it did not cause symptoms of injuries and soybean productivity reduction either. Cloransulam-methyl, despite not having been effective in controlling C. bonariensis at 30 g ha^-1, showed soybean yield similar to the weeded control, and superior to chlorimuron-ethyl. The weed interference caused by the horseweed infestation reduced soybean yield in 47.5%.