Cover crops can be tricky to kill

Cover crops can be important tools to improve soils, but there’s no single control solution

Cover crops can do a lot for the soil, but keeping them from becoming covering weeds requires the right herbicides applied at the right rate — to kill them at the right time.

What cover crop is used, too, should be appropriate for the job the farmer wants to accomplish.

Why it matters: Cover crops are a great way to improve soil health, but the diversity of species and growing conditions means there’s no single way to terminate. Knowing what herbicides work and what crop will be planted in the subsequent growing season is critical to good cover crop management.

Heavy clay requires early burn-down

“I like to spray early,” says Henry Denotter, a grain farmer from the Kingsville area who incorporates cover crops into his rotation schedule.

Denotter says one of the most significant issues with growing cover crops in his Brookston Clay soil is getting the ground uniformly dry and warm before spring planting. He’s tried applying herbicide to green cover early in the fall as well as later in spring. The former, he says, was the most effective way to ensure good soil conditions at planting time.

Denotter also says they had particular success following oats and buckwheat, usually applied at a light 10 pounds per acre, with just glyphosate at burn-down. Since glyphosate doesn’t leave a residue, he is not restricted in what he can plant afterward.

“If you leave oats and it goes to seed, it’s a real challenge to get the ground dried out in spring,” he says.

Right herbicide critical

Kris McNaughton, research associate at the University of Guelph’s Ridgetown Campus, says early burn-down conducted in either autumn or early spring is an effective way to reduce issues during planting season for most green cover. However, she says the desired level of residue in some cover crops are more easily attained than others, provided the right herbicide or herbicide mixture is used.

In field demonstrations during the University of Guelph’s Diagnostic Days event in Ridgetown, McNaughton showcased the effectiveness of multiple herbicide formulations on different cover crops. One such crop was cereal ryegrass, which can be easily killed with a mid-rate dose of Roundup (about 1.3 litres per acre). Because it’s easy to kill, she says growers can more easily determine how much residue they want, based in part on what their planter can handle.

On the other hand, McNaughton says annual ryegrass is much more challenging and believes it should be avoided. Its perennial nature makes it difficult to achieve near-complete control, even with high rates of diverse herbicide mixtures. Consequently, resistance issues are a much more significant concern.

“Use a high rate of Roundup, about two litres per acre, plus a Group One herbicide like Assure or Select,” says McNaughton regarding annual ryegrass. “Kill it early. Doing it in the fall is best but then there’s no biomass, so you have to ask yourself what you’re really doing it for. You can use it for other jobs, but not as overwinter cover.”

For red clover, McNaughton says a natural tolerance to glyphosate, something common among legumes, means it should be approached with tank mixes containing Banvel, 2 4-D or other Group Four herbicides. Eragon can also be used, though is less preferable in her experience.

These formulations should all be applied when clover is small.

For red clover, McNaughton says a natural tolerance to glyphosate, something common among legumes, means it should be approached with tank mixes containing Banvel, 2 4-D or other Group Four herbicides.
photo: Nnorozoff/iStock/Getty Images

Terminating cover-crop mixtures more difficult

When it comes to multi-species cover crop mixes, McNaughton says a prescription for herbicide termination is generally not possible.

“You really need to know what you’re going to plant,” she says. “Roundup should definitely be put on at the two-litre rate. You need something that kills but doesn’t leave a residue. Maybe put in a Group One to kill grasses, plus a Group Four.”

“You should be watching for escapes anyway.”

Adam Ireland, a dairy and grain grower from Teeswater, plants a multi-species cover crop after wheat, and terminates it in the fall to help take care of perennial weeds. However, he also tries to leave some green cover in the field until spring “as a test.”

“(My) herbicide is Roundup with something added depending on what weed species is of concern. The timing is usually late fall to allow maximum growth of the cover crop,” Ireland wrotes in an email.

“When using annual rye in corn, I’ll leave that until spring for termination and use Roundup as well as some kind of an active to manage resistant weeds such as Fleabane. This year it was Optill. If we have escapes, we usually catch it with the in-crop application. I try and time the in-crop a little earlier if I know something was missed.

McNaughton added that covering plants that survive into the second year are much harder to kill, so sometimes a second herbicide application is required. Tillage is also an option and may be the only viable control left to farmers looking to follow cover with vegetable crops susceptible to herbicide residues.

Use peer networks

Despite potential drawbacks and differences between single and multi-species mixtures, McNaughton reiterated that cover crops can be a valuable part of crop rotations.

“Make sure you kill it early. Make sure you’re using the right herbicide at the right rate,” she said. “Talk to the people in your area who are using cover crops. They can give you tips. You don’t need to do it alone.”

About the author

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Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.

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