Effect of Repeated Glyphosate Use on Weed Communities in a Soybean‐Corn Rotation

Five weed management systems, ranging from total reliance on glyphosate to no glyphosate, were evaluated over a four-year period (1998-2001) in a soybean [Glycine max (L.) Merr.]-corn (Zea mays L.) rotation in chisel and no-tillage systems. All treatments provided acceptable levels of weed control during the 4-year period, with a maximum end-of-season weed density of less than 27 weeds per 100 ft2 in the glyphosate-only treatment during the fourth year of the study. The dominant weed was dandelion. While occasional differences in control were observed among treatments, they were minor and weed escapes were not sufficient to impact crop yields. The glyphosate-only treatment in soybean outyielded other treatments in 2000, but yields were not affected by weed management system in the other years. The weed management systems imposed during the four years of the study did not influence weed density or composition that emerged during 2002 when no herbicides were applied. Giant foxtail, velvetleaf, and Amaranthus spp. were more prevalent in the chisel-plow system than in no-tillage at the conclusion of the experiment, whereas dandelion was present at higher densities in the no-tillage system. While all systems provided high levels of weed control, the glyphosate-only system did allow nearly a 66% reduction in the total amount of herbicide active ingredient applied compared with the system relying on conventional preemergence and postemergence herbicides.