Planting Fall Covers? Know Your Herbicide Carryover Risks.

It’s fall cover crop seeding time, and with large swaths of the Midwest facing droughty soils, herbicide carryover injury could be a real threat to many growers this year. 

A lot of factors contribute to the risk of your soil residual herbicides burning your cover crops, as Virginia Tech researchers noted in this 2019 study. Some are out of your control, such as dry weather, soil type and pH levels. 

Halex GT and atrazine carryover injury to hairy vetch. (Photo credit: Michael Flessner, Virginia Tech)

But knowing your summer herbicide selections, herbicide use rates, plantback restrictions and the sensitivities of your cover crop species can help you avoid a cover crop stand failure this year. 

Cover Crop Sensitivity

In addition to the Virginia Tech study, researchers from the University of Missouri and the University of Arkansas have also examined the risks the most common row crop herbicides pose to a broad range of cover crops. 

The good news is that many of the most common fall-seeded covers fared pretty well. For example, small grains such as cereal rye, wheat and barley tolerated soil residual herbicide exposures, with rates of visible injury staying under 20% in the Virginia study. 

Cereal rye, a popular cover crop choice for its weed-suppressing abilities, proved particularly tough. Only a handful of the 27 herbicides tested by the Virginia scientists bothered it, including pyroxasulfone (Zidua), prosulfuron (Peak), isoxaflutole (Balance), fluometuron (Cotoran), and cloransulam-methyl (FirstRate). 

Broadleaf cover crops are a more herbicide-sensitive crowd, however. Top of the diva list are legumes such as Austrian winterpea and crimson clover, as well as rapeseed and radish species. 

Don’t despair if you do see some damage this year – visible herbicide injury didn’t always translate into poor cover crop stands. While the Missouri and Arkansas scientists did see stunting and smaller stands for some species, the Virginia scientists didn’t actually track any difference in final biomass production, regardless of injury symptoms. 

After cutting, collecting and weighing plant biomass from every tested cover crop, Virginia Tech researchers ultimately found no significant reductions in biomass, regardless of herbicide injury. (Photo credit: Michael Flessner, Virginia Tech)

The Common Culprits

Some common herbicide carryover culprits emerge from these studies. Among soybean herbicides, fomesafen (Reflex, Flexstar) was quite tough on radish species and many kinds of legume cover crops, as were chlorimuron (Classic) and imazethapyr (Pursuit). 

On the corn side, lingering atrazine residues often took a toll on many types of cover crop species, and mesotrione (Callisto) occasionally caused trouble for both legumes and mustard species. 

There are resources available to help you examine your spring and summer herbicide program and check for potential carryover issues. You can check herbicide plantback restrictions on your labels and use these WSSA charts explaining the half-lives of common herbicides to help figure out what you can safely plant this summer and fall. 

For more information on using cover crops to suppress weeds, see this GROW webpage and follow these News Page stories

Text by Emily Unglesbee, GROW