Wisconsin corn and soybean growers and their advisors understand that weeds need to be controlled before the critical period of weed removal, which is the time when early-season weeds begin to compete with crops and cause yield loss. Despite this knowledge, the potential for yield loss from weed competition exists because of cropping systems that rely on postemergence herbicide programs. In particular, the adoption of glyphosate-resistant corn and soybean allows weeds to be controlled exclusively with postemergence glyphosate applications. If glyphosate is applied to these crops before the critical period of weed removal, full yield potential can be achieved. However, if glyphosate applications are delayed, yield losses will occur. The potential for such yield losses is significant in Wisconsin because over 90% of soybeans are glyphosateresistant and estimates of glyphosate-resistant corn may exceed 70%. Of course, the potential for yield losses associated with postemergence herbicide programs can also occur in conventional or LibertyLink crops. The yield loss is a function of the timing of weed management, not the herbicide or genetic trait of the crop.
Soil Science Extension
University of Wisconsin Madison