Certified crop advisor perceptions of giant ragweed distribution, herbicide-resistance, and management in the corn belt

Giant ragweed, a native annual plant of North America, 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 Corn Belt.  Participants were asked questions regarding giant ragweed, crop rotation, tillage, and herbicide practices for the county in their state with which they were most familiar.  Respondents rated giant ragweed as one of the most difficult weeds to manage in 41% of the surveyed counties, located mainly in Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio.  Although giant ragweed was reported to have been present in non-crop habitats throughout most of the surveyed area for at least 20 yr, it was reported to appear in crop fields more recently in areas peripheral to the east-central Corn Belt.  Fifty-eight percent or more of all respondents reported giant ragweed in corn or soybean fields with resistance to acetolactate synthase (ALS) inhibitors or glyphosate; 44% or more reported giant ragweed with resistance to both ALS inhibitors and glyphosate.  Crop production practices associated with giant ragweed-infested crop acreage and/or herbicide-resistant giant ragweed included minimum tillage, continuous soybean production, and multiple-application herbicide management programs.  Ecological factors associated with giant ragweed-infested acreage included the presence of giant ragweed in non-crop edge habitats, a prolonged emergence period, and the presence of the seed-burying common earthworm in crop fields.  Results suggest that giant ragweed has spread within crop fields into new areas outward from the east-central Corn Belt, based on favorable habitat provided by similar crop rotation, tillage and herbicide application 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.  In areas where giant ragweed is already established in crop fields, changing production practices to include a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce population densities and to select against late-emerging and herbicide-resistant giant ragweed genotypes.