Many fields have been treated with postemergence herbicides within the past two weeks, often during hot, humid conditions. These conditions frequently lead to significant crop response, triggering questions about the likelihood of the crop yield being affected by the herbicide. The potential for a yield response is dependent upon many factors including the type of herbicide, timing of the application, additives used in the spray solution, environmental conditions before and after application, and other stress factors that can affect the crop. Interactions among these factors make it difficult to predict the impact of a herbicide on a potential crop yield.

Soybean response to postemergence herbicide.

There is growing evidence that postemergence herbicides are more likely to cause small yield losses (less than 5 percent) than previously thought. It is important to note that current research has shown this yield response can occur with any herbicide, not only those typically associated with early-season injury. Chris Boerboom at the University of Wisconsin evaluated the effect of Cobra, Galaxy, Pinnacle, and Pursuit on soybean growth and yield in several experiments during the 1996 growing season. Early-season injury ratings from labeled herbicide rates averaged 16 percent, ranging from 2-45 percent. Yields from the labeled rates averaged 4 percent less than nontreated soybean, with yield responses ranging from 11 percent less to 6 percent more. Importantly, there was little correlation between early-season injury ratings and yield response.

How important is this potential yield impact? In the University of Wisconsin research, similar yield losses were observed with all herbicides. The losses were typically small, and the benefit of weed control outweighed the yield losses. The potential for injury can be minimized by applying herbicides in a timely fashion while weeds and crops are small.

Herbicides have a greater likelihood of causing significant damage if the crop is under stress from other factors (diseases, hail, drought, etc.). If applications must be made during conditions that favor crop responses, the likelihood of early-season injury can be reduced by selecting products that are less likely to injure foliage and adjusting herbicide rates and spray additives. However, these adjustments may not necessarily reduce any impact on crop yields.