When it comes to herbicide interaction with cover crops, Nathan Johanning, University of Illinois extension educator, says “there are a lot of unknowns.” Herbicide labels are not updated with cover crop guidelines, so cover-crop farmers follow grain usage guidelines.

Johanning advises farmers to be aware of late-season herbicide applications in soybeans. Carryover can be an issue when residual herbicides are applied within two months of planting cover crops. However, rainfall following residual herbicide applications and before planting cover crops can “dilute the herbicide”, Johanning explains, leaving less residue to impact cover crops.

BE ADVISED: Nathan Johanning, University of Illinois extension educator, says cover-crop species react to herbicides differently and there’s variability between different herbicide modes of action. (Photo: Shalamar Armstrong)

“The worst scenario is late-spring planting, followed by late-season herbicide applications and dry spells,” he says. “There’s not enough moisture to process the residual herbicide.”

The potential challenges don’t end there.

Johanning says cover-crop species react to herbicides differently and there’s variability between different herbicide modes of action. “Brassicas are very sensitive to residual herbicides,” he notes. “PPO-inhibitor herbicides, like Flexstar and Cobra, have the potential to have lingering effects in cover crops.”  Cover crop species with smaller seeds, such as cereal rye, are also at a higher risk from PPO-inhibitor herbicide injury.

In dry years, Johanning says HPPD inhibitors and atrazine may also cause carry-over concerns. For this season, rain events throughout the summer may help protect cover crops from herbicide interactions. “I’m optimistic this year, especially in the south,” he says. “Rain really helps with diluting those (herbicides) down.”

For this season, be mindful of herbicide modes of action that potentially cause carryover issues and sensitive cover crop species. Check herbicide labels for rotational restrictions. For example, Flexstar restricts planting small grains, such as rye, for four months after application.

Still not sure if your herbicide program will impact cover crops? Johanning recommends reaching out to the Midwest Cover Crops Council at www.mccc.msu.edu or your local University of Illinois Extension office for more information.