Effects of Dissection with Glyphosate and Chlorimuron-Ethyl on Weed Community and Soybean Yield

The effects of burndown herbicides on the period before weed interference (PBI) may provide support to Weed management decision-making. The objective of this research was to verify whether the PBI is affected by the application of glyphosate plus chlorimuron-ethyl to presowing burndown in soybean. The experiment was carried out in Jaboticabal-SP, Brazil, submitting the cultivar Monsoy 7908RR to eight coexistence periods with weeds, maintaining weedy and weed-free checks, which were applied to two groups of treatments: glyphosate and glyphosate + chlorimuron-ethyl. At each period, relative importance, diversity and equitability indexes were calculated. The PBI was determined based on regression analysis of grain yield. Digitaria insularis, Acanthospermum hispidum, Raphanus raphanistrum and Commelina benghalensis showed the highest values of relative importance. The diversity and equitability indexes oscillated during the periods, and dry mass accumulation was the basis of the difference among the weeds. The PBI in soybean was 37 days after sowing (DAS) in the treatment with glyphosate and 51 DAS in the glyphosate + chlorimuron-ethyl treatment. The addition of chlorimuronethyl to glyphosate allows the crop to coexist with weeds for a longer time without any significant yield reduction.