If an herbicide application fails to control a targeted weed community sufficiently, farmers may try to eliminate surviving weeds with a follow-up application (hereafter “respray”). Despite the implications of resprays on the spread of herbicide-resistant weeds, respray frequencies and causal factors are poorly understood. A two-part survey of glyphosate-resistant soybean fields and custom application services was conducted in Illinois during 2005 and 2006 to determine the relative frequency of respray requests for postemergence glyphosate, and to identify weed community factors associated with glyphosate respray requests. A meta-analysis was then utilized to project the impacts of weed community factors driving respray requests on crop yield. Glyphosate resprays were requested for 14% of surveyed fields in both 2005 (n = 43) and 2006 (n = 90). In 2005, respray requests were highly associated with both population densities of weed communities visible from roadsides and incidences of skips (i.e., rectangular areas of escaped weeds indicating custom application failure). A skip increased the odds of respray request by more than ninefold, and population densities of weed communities visible from roadsides were, on average, 2.5 times greater in respray-requested fields compared with nonrequested fields. In 2006, respray requests were associated with population densities of weed communities identified by walking through fields. Contrary to 2005, requests in 2006 were concentrated in those fields with low weed population densities. Prior to resprays, weed communities capable of causing substantial soybean yield loss were present in both respray-requested and nonrequested fields in 2005 but in only nonrequested fields in 2006. Although this investigation indicated that custom applicators can take actions to reduce respray requests (i.e., avoiding skips), farmers and custom applicators should be prepared to implement additional weed control after postemergence glyphosate applications because damaging weed communities may remain.
Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr.