Weed scientists generally don’t agree on the exact ranking of soybean’s worst weeds, but they mostly agree on which weeds belong in the top four or five that Midwestern soybean growers should be concerned about. Common waterhemp, giant ragweed, marestail (also commonly known as horseweed) and common lambsquarters are listed among the top problematic weeds for soybeans, according to weed experts in Illinois, Iowa, Kentucky and Minnesota. Other yield-robbing culprits in these states include common ragweed, giant foxtail, velvetleaf and annual morningglory.

“In soybeans, common waterhemp can be a huge problem, because of the limited number of postemergence herbicides that will work well on it,” says Aaron Hager, University of Illinois Extension weed scientist. “If you’re planting conventional soybeans, the only postemergence herbicide options for it would be diphenylethers [for example, Flexstar and Cobra]. If you plant LibertyLink soybeans, you can use glufosinate [Liberty], and if you plant Roundup Ready soybeans, you can use glyphosate. However, if you have a weed resistant to both glyphosate and PPO inhibitors, then there’s nothing else you can use postemergence to effectively control it in glyphosate-resistant soybean varieties.”

As a result, Hager recommends applying a preemergence herbicide with soil residual activity to control common waterhemp in soybeans and then scouting to determine if a postemergence application or inter-row cultivation is needed later. He adds that for corn, there are more products that can be used in a postemergence tankmix that will control common waterhemp, compared to the number of products available for use in soybeans.

Although the top-five worst weeds in Iowa are identical for corn and soybeans, the herbicide choices available are not, notes Mike Owen, Iowa State University (ISU) Extension weed specialist. “You can use different herbicides to control these weeds in corn than you can in soybeans,” he says. “Probably the biggest difference is that you can use 2,4-D and dicamba for corn, but not for soybeans.”

Another commonality among the top worst weeds in Iowa is their resistance to herbicides. “In particular, giant ragweed and common waterhemp populations represent significant resistance to glyphosate in Iowa,” Owen says.