Respray requests on custom-applied, Glyphosate-resistant soybeans in Illinois: how many and why?

Authors: Schutte, Brian; HAGER, AARON - University Of Illinois; Davis, Adam

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2010
Publication Date: 11/1/2010
Citation: Schutte, B.J., Hager, A.G., Davis, A.S. 2010. Respray Requests on Custom-applied, Glyphosate-Resistant Soybeans in Illinois: How Many and Why? Weed Technology. 24(4):590-598.

Interpretive Summary: Approximately 45% of herbicides applied to soybean hectares annually in Illinois are administered by contracted application services (hereafter referred to as ‘custom applicators’). Typically, custom applicators and farmers develop an herbicide program that applicators implement and farmers evaluate. If an herbicide application fails to sufficiently control the targeted weed community, farmers can request and receive a reapplication. Despite the implications of these reapplication requests on the development and spread of herbicide-resistant weeds, their 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: 1) determine the relative frequencies of requests for reapplications of the herbicide ‘glyphosate’, and 2) identify weed community factors associated with glyphosate reapplication requests. A literature review was then used to project the impacts of request-driving weed community factors on crop yield. Glyphosate reapplications were requested for 14% of surveyed fields in 2005 (n = 43) and 2006 (n = 90). In 2005, reapplication requests were most highly associated with both weed communities visible from roadsides and incidences of custom application failure (i.e., ‘skips’). In 2006, glyphosate reapplication requests were associated with weed communities identified by walking through fields. Contrary to 2005, requests in 2006 were concentrated in those fields with low weed population densities. Weed communities capable of causing substantial soybean yield loss were present in both reapplication-requested and nonrequested fields. Although this investigation indicated that custom applicators can take actions to reduce reapplication 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.

Technical Abstract: When an herbicide application fails to control a targeted weed community, farmers may try to eliminate surviving weeds with a repeat application of the herbicide. Despite the implications of repeat applications on the development and spread of herbicide-resistant weeds, their frequencies and causal factors are poorly understood in agronomic settings. A two-part survey of glyphosate-resistant soybean fields and custom application services was conducted in Illinois during 2005 and 2006 to: 1) determine the relative frequencies of requests for reapplications of postemergence glyphosate, and 2) identify weed community factors associated with glyphosate reapplication requests. A meta-analysis was then utilized to project the impacts of weed community characteristics driving respray requests on crop yield. Glyphosate reapplications were requested for 14% of surveyed fields in both 2005 (n = 43) and 2006 (n = 90). In 2005, reapplication requests were most 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 reapplication request by more than 9-fold, and, for road-side visible weed communities, population densities were, on average, 2.5 times greater in reapplication-requested fields compared with nonrequested fields. In 2006, reapplication 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. Weed communities capable of causing substantial soybean yield loss were present in both reapplication-requested and non-requested fields in 2005 but in only non-requested fields in 2006. Although this investigation indicated that custom applicators can take actions to reduce reapplication 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.