Herbicide association applied to control weeds in glyphosate-resistant soybean

Abstract

In order to bypass cases of glyphosate-resistant weeds, it becomes necessary to use herbicides with other action mechanisms. The goal of this work was to evaluate the effectiveness and selectivity of herbicides with isolated or tank mix application for the management of soybean-infesting glyphosate-resistant weeds. Treatments were applied sequentially, during pre-emergence: clomazone (900 g ha^{-1 a.i.); flumioxazin (50 g ha-1 a.i.); diclosulam (35.02 g ha-1 a.i.); s-metolachlor (1152 g ha-1 a.i.); sulfometuron + chlorimuron-ethyl (18.75 + 18.75 g ha-1 a.i.); pendimethalin (1250 g ha-1 a.i.); sulfentrazone (250 g ha-1 a.i.); sulfentrazone + chlorimuron-ethyl (250 + 20 g ha-1 a.i.); imazethapyr + sulfentrazone (100+250 g ha-1 a.i.) and imazaquin (150 g ha-1 a.i.) and, during post-emergence, glyphosate was used over these treatments (1080 g ha-1 a.e.). During post-emergence, glyphosate was applied in an isolated manner (1080 g ha-1 a.e.) and, mixed in the sprayer tank with glyphosate (1080 g ha-1 a.e.), imazethapyr (100 g ha-1 a.i.), clethodim (96 g ha-1 a.i.) and chlorimuron-ethyl (20 g ha-1 a.i.), plus the infested and weeded control samples. The experimental area was infested with 75% of creeping signalgrass plants (Urochloa plantaginea) and 25% of summergrass (Digitaria ciliaris), at the average densities of 133 and 45 plants m-2}, respectively. At 14 and 21 DAT, the application of glyphosate + [sulfometuron + chlorimuron-ethyl] caused a 67 and 62% phytotoxicity to soybean, respectively. In order to control summergrass and creeping signalgrass, this very mixture presented an 88% index at 21 DAT, whereas control for the other treatments exceeded 91% during all evaluated periods. All the evaluated treatments caused over 91% control of both weeds, except for glyphosate + [sulfometuron + chlorimuron-ethyl]. The associations of herbicides to glyphosate caused weed control and were selective to soybean, except for sulfometuron + chlorimuron-ethyl, which presented lower control and high phytotoxicity. Some herbicides damaged the yield components of soybean, but only the sulfometuron + chlorimuron-ethyl mixture caused a reduction in grain productivity.