Hager, Owen and Sandell all rank giant ragweed as the second-hardest weed to control for corn.

“Giant ragweed is a top concern in Nebraska cornfields, due to pockets of glyphosate resistance that have been found in the eastern part of the state,” Sandell says. “We advise farmers to take care of giant ragweed prior to planting with an effective spring burndown.”

In Iowa, pockets of giant ragweed populations have developed resistance to glyphosate, Owen says. “Giant ragweed becomes very difficult to control with most postemergence products, with the exception of 2,4-D and other auxinic herbicides,” he says. “Post HPPD-inhibitor herbicides and triazines will do a nice job if the giant ragweeds are small. If you plan to use glyphosate, do not skimp on the rate and apply sooner rather than later.”

Hager says, in Illinois, giant ragweed is another example of a weed species that has evolved. “Its emergence extends well into June or even to early July, so it’s difficult to control with just one application,” he says. “There are now pockets of giant ragweed in the state with resistance to ALS-inhibitor herbicides, but we have not yet documented instances of giant ragweed that are resistant to glyphosate here as in other states.”