Certified Crop Advisor Perceptions of Giant Ragweed Distribution, Herbicide-Resistance, and Management in the Corn Belt.

Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern of spread in crop fields and the reasons for it are lacking. To address this gap, we conducted a web-based survey of Certified Crop Advisors in the U.S. Corn Belt and Ontario, Canada.  Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice.  Responses were mapped and correlation analyses were conducted among the responses to determine key factors associated with giant ragweed populations.  Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported herbicide resistance to ALS-inhibitors, glyphosate, and/or to both herbicides.  Giant ragweed was reported to appear in crop fields most recently along the periphery of the east-central Corn Belt.  Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in non-crop edge habitats, prolonged emergence period, and presence of the seed-burying common earthworm in crop fields.  Results suggest that giant ragweed is increasing in crop fields outward from the east-central Corn Belt, based on the pre-existence of populations in non-crop habitats and a favorable habitat provided by similar crop rotation, tillage and herbicide practices across much of the Corn Belt.  Managing giant ragweed in non-crop areas could reduce giant ragweed migration from non-crop habitats into crop fields and slow its spread.  Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations and select against late-emerging and herbicide-resistant giant ragweed genotypes.