I was recently perusing the regular e-newsletter sent out by the cover crop company Green Cover Seed and noticed an excellent article from Kansas farmer, salesman and educator Dale Strickler that is worth sharing.

Strickler, who’s joining the company as a salesperson and cover crop specialist, warns of the perils of herbicide carryover and the damage it can cause to your cover crop stands. Doing a soil bioassay is the best way to make sure you’re safe, he says.

“I can’t tell you how many times I have carefully put together what I thought was the perfect cover crop mixture for a customer and place the order,” he writes, “only to have him call back an hour later and ask me if it is a problem if he applied some residual herbicide to the previous crop.”

Whether it’s a problem, Strickland says, depends on soil texture, soil organic matter, soil moisture and soil temperatures. Clay soils have less carryover than sandy soils, higher organic matter levels have less carryover, and more soil moisture and warmer soils also have less carryover. Higher pH levels can mean more carryover with most herbicides, Strickland says.

Additionally, the rate of residual and length of time since it was applied can be a factor, too, as well as what species is in the cover crop, he notes.

While rotational intervals listed on herbicide labels can be useful, Strickland notes you’re not going to find information for every cover crop you may be considering.

“Similar crops listed on the label can be a rough guide to the reaction of related cover crops,” Strickler writes. “For example, canola is similar to other brassicas like radishes and turnips, wheat would be similar to rye or barley, alfalfa would be similar to other small seeded legumes, soybeans would be similar to other large seeded legumes.”

But there are no promises. Strickler says the only way to make sure cover crops will be safe is to do a bioassay — far enough in advance of cover-crop planting so that species selections can be made.

To do this, dig some surface soil from the herbicide-treated area to be planted, as well as from a similar, untreated area like an adjacent waterway or road, he says.

Try to pick the soil situation most likely to have carryover, such as a high-pH soil area. Put each soil sample in a pot where they can be taken care of and observed, and plant a selection of cover crop species in them.

Grow the plants out and compare plants in the treated soil to the untreated soil. If the plants in each pot grow at a similar rate and are both free of herbicide symptoms, then you are probably going to have success, Strickler says.

If some of the treated area has symptoms like yellowing, purpling, or stunting, then you will need to observe which species are growing well and which ones aren’t, and select which cover species are most likely to succeed.

“There is no reason to plant cover crops that will fail because of herbicide carryover from a previous crop,” Strickland concludes.

No-Till Farmer also published an article about herbicide carryover issues and how to avoid them, which you can read here.