In a postemergence herbicide program, the application of the herbicide can easily be delayed by weather, time constraints, equipment availability, and other such causes. However, if weeds are not controlled early enough, there is potential for yield loss from early season weed competition. This is true even when herbicides provide high levels of efficacy.

It has been observed that a large number of corn and soybean fields in Wisconsin are currently being managed with postemergence programs. Thus, the potential for significant yield loss due to early-season weed competition exists. This became more evident after an initial survey of grower’s fields was taken in 2008. In this survey, weeds were controlled at an average height of 2 inches taller than the recommended critical heights for weed removal (4-inch weed height in corn and 6-inch weed height in soybean).

In order to better understand the potential for yield loss from early season weed competition in Wisconsin corn and soybean fields managed with postemergence herbicide programs, in-field surveys of weed populations were conducted in 2008 and 2009. Individual field data from these surveys was used to estimate yield loss using a computer program called WeedSOFT®. A possible solution in soybean was also evaluated using on-farm trials comparing a single pass glyphosate program with a half rate of a preemergence broadleaf herbicide followed by glyphosate.