Weed scientists at Ohio State University completed a survey of Certified Crop Advisors across the North Central region to determine the relative abundance of giant ragweed, and the factors influencing its spread. The results of this survey are summarized in a Powerpoint file posted to the giant ragweed section of the OSU Weed Management website http://agcrops.osu.edu/specialists/weeds. Emilie Regnier, Ph.D., lead investigator on the survey, also provided the following abbreviated summary of the findings.

Giant ragweed is one of a relatively few native plant species that is a major weed of grain crops in North America. We conducted a web-based survey of Certified Crop Advisors in the Corn Belt to determine the distribution of giant ragweed and gain insights into possible factors associated with its spread. The questionnaire asked participants to provide their perceptions and county-level estimates of giant ragweed related to its first occurrence as a problematic weed in crop fields, the proportion of crop acres infested, and habitats where found. 

Based on the survey responses, giant ragweed was reported to appear in crop fields 20 years ago or longer in western Ohio, most of Indiana, northern Illinois, southern Wisconsin, southeastern Minnesota, and eastern Iowa. In most counties outside this area of the Corn Belt, giant ragweed was reported to appear in crop fields more recently, and in some counties, only in the past five years (e.g., northern Wisconsin). Nearly all respondents indicated that giant ragweed was already present in non-crop edge habitats such as riverbanks and fencerows before it appeared in crop fields. 

Although giant ragweed is considered a riparian species, the survey results indicated that it is well established throughout the Corn Belt in both riparian and upland edge habitats. Giant ragweed was listed as the most difficult weed to manage in counties located in Indiana, Wisconsin, Minnesota, Iowa and Nebraska. Most of these counties were located near the upper Mississippi River where Wisconsin, Minnesota, Iowa, and Illinois meet.

Counties reporting giant ragweed present in 60 percent or more of crop fields were located in the same region but also southward along the Mississippi River to Tennessee, west of the Mississippi along the Iowa-Missouri border and into eastern Nebraska, and also along the Missouri River in Missouri. East of the Mississippi, counties with giant ragweed present in 60 percent or more of crop acres were located in northwest Illinois, most of Indiana, and west-central Ohio. 

The timing of giant ragweed emergence varied across the region with giant ragweed emerging earlier and for a longer period of time in the east-central region of the Corn Belt (i.e., Ohio and Indiana) than in other areas.  Difficulty of managing giant ragweed was associated with its presence in waterways, and with an earlier and longer emergence period. 

Reduced use of conventional tillage in corn and soybean fields was associated with increased difficulty of managing giant ragweed. Based on these results, it appears that giant ragweed first became a problem weed in the east-central region of the Corn Belt and is now becoming established in crop fields in areas outside of that region, especially toward the North and West. It is likely that giant ragweed spreads initially through a variety of non-crop edge habitats and then becomes established in areas adjacent to crop fields such as waterways and fencerows, and from there it can quickly get established in crop fields. 

Giant ragweed emergence characteristics and reduced tillage both play a role in the development of giant ragweed as a problem weed in crop fields. Late-emerging giant ragweed genotypes that create the most problems for soybean growers are prevalent in Ohio, likely due in part to a combination of reduced tillage and earlier crop planting dates over the past several decades.