Pictured Above: GETTING IT RIGHT. Terminating cover crops isn’t just about using the right herbicide, but applying the product at the right time and understanding the environmental conditions during application, says retired University of Illinois agronomist Mike Plumer.

While choosing the right cover crop, or mix of covers, for a farm can be a challenge, there’s also an art to terminating covers that don’t winterkill with the right products at the right time.

Taught to stay out of fields when they’re too wet, no-tillers can face a real dilemma during heavy spring rains. Last May, much of the southern Midwest had 20 or more inches of rain, with parts of Illinois seeing 12-13 inches alone last April.

Letting cover crops grow too much and reach maturity can cause issues, even if a no-tiller’s stated goal is building biomass, says retired University of Illinois Extension agronomist Mike Plumer.

“You need to be able to terminate when you want to terminate,” Plumer said during the 25th annual National No-Tillage Conference in St. Louis earlier this year. “But rather than having a hard core plan on how you’re going to manage your cover crops, try to be a little bit flexible.”

Target Vegetative Stage

One key to improving termination results is to understand how cover crops grow and uptake the herbicide.

Ideally, no-tillers should target termination for when cover crops are in the vegetative stage and actively growing, so the systemic herbicides are taken up efficiently. For example, this means killing grasses before they joint, and legumes when they’ve reached three or four trifoliates and the stems haven’t elongated and become woody yet.

Another example is cereal rye. “You kill cereal rye at boot-top high, you’d be amazed in 30 days, there’s hardly any residue at all,” Plumer says. “But if you kill cereal rye at shoulder high, you’ve got some decent cover for your fields next year.”

This timing for vegetative stage also affects the amount of nutrients that might be released once they die.

“That plant in a vegetative stage normally has higher levels of nitrogen (N) and lower levels of lignin and cellulose in it, so a cover crop can break down quick and the N and the nutrients come out really quickly,” he says. “As soon as the plant goes reproductive, we develop cellulose and lignin and there’s not that many nutrients to be released right away. You get a quick shot and probably slow release for a couple of years.”

Once the early reproductive stage arrives, plants change quite a bit in how they grow and function, Plumer notes. Once a grass joints, it stops growing deep roots and will proliferate the surface to help hold stands.

“Normally we’re after deep roots on grasses, so if it gets to the first joint and you want deep roots, you might as well kill it. We’re done,” he says.

There are some later stages of plant growth where there can be even more problems with control, such as grasses getting into the boot stage, or the bud stage in legumes.

Mini Spray Rig Brings Flexibility


FINE FLEXIBILITY. Springerton, Ill., no-tiller Ralph “Junior” Upton’s John Deere Gator has foam markers and fully floating axles that help him terminate cover crops on time in wet field conditions.

When it’s time to terminate covers and the weather isn’t cooperating, Ralph “Junior” Upton rolls out his special rig.

It’s a John Deere Gator enabled with GPS, foam markers and fully floating axles. It can run on standing water and lets Upton spray cover crops any day of the week he wants in spring.

“I’ve known that rig to spray 3,000 acres in a season in order to get it done when they want to get it done,” says Mike Plumer.

In situations where the sun is out and it’s not too muddy to run on, but the soil is too soft and sloppy for a sprayer, plants are still growing and this rig does the job.

Plumer saw many situations last year where having a Plan B on cover crop termination would have benefitted growers.

“You may want to think about last spring. What I ran into with cover croppers was that they wanted to grow cover and grow tonnage. They were wanting to change their soil in one year, so they let everything grow,” Plumer recalls.

“On an average year, we can kind of get away with that. The thing I normally worry about is drying the soil out too much. Last year when the plant was reaching toward maturity, we got a wet period and wet soils, and they let the covers go because they couldn’t get across the field.

“The plants hit maturity, and what happens then? It’s not growing. It’s not transpiring, so what is it? It’s a mulch and it stays wet, wet, wet. You can’t dry it out.”

If no-tillers find themselves in this situation, Plumer says they may want to consider a different spraying plan in future years, such as spraying part of the farm earlier, and then picking certain fields to let the cover grow, rather than letting thousands of acres get that big.

“That plant is trying to reproduce so every bit of energy it uptakes is sent to that bud, boot or head and develop it,” he says. “If you spray a systemic herbicide on it, and maybe it’s not as high a rate as you need it to be, it’s going to move into that reproductive area and not kill the plant because none of it’s going to the rest of the plant.”

At this point, Plumer suggests no-tillers consider switching to a contact herbicide to burn the vegetation down. When a regrowth occurs, the plant is now very easy to control with a systemic or contact herbicide, he notes.

If a plant hasn’t reached maturity, more than one application of a contact herbicide may be needed

Know Your Variety

Variety Gramoxone 48oz Liberty 32 oz PowerMax 40 oz PowerMax Sharpen PowerMax Corvus
Gulf 90 15 98 95 99
King 50 15 98 98 95
KB Royal 70 10 98 95 95
Marshall 45 10 98 98 95
Bounty 80 10 99 98 98
KILLING RATE. Controlling annual ryegrass at the boot stage isn’t that difficult with the right product, says retired University of Illinois agronomist and cover crop expert Mike Plumer. This experiment shows numerous varieties of ryegrass sprayed with different herbicides at the boot stage and the termination success rate.

Another issue coming to light with cover crop termination has been differences in varieties of cover crops sold on the market.

Plumer says there were some major issues with suppliers last year where the contents weren’t exactly what were stated on the bag tag — including blended varieties that could have varying maturities and susceptibilities to herbicides.

“When I started working with ryegrass back in the 1990s, the first trial I ever did, they gave me a seed sample to do herbicide trials on and we couldn’t kill it,” he recalls. “What I didn’t realize, until we got really serious about it, was there were six different varieties with six different maturities in that bag.”

He recalls spraying it at the right stage and killing it, until a week later, when a second variety would emerge and he’d have to kill that one. Then 2 weeks later another variety would come up. They had to spray that ryegrass plot four or five times before it was all completely terminated.

“We were thinking, ‘Man this herbicide doesn’t work.’ Then we had a patch of control that we watched, and we found out there were six different varieties and six different maturities when they shot out heads.

“Know your seed dealers. Know that you’ve got a real variety, and it’s only one, not blended.”

Other Issues to Watch

  • Seeding Rates. Too high a seeding rate in cover crops can definitely affect control, Plumer notes, as dense stands might prevent the herbicide from getting down to lower vegetation.

“You may have to plan on multiple applications, burning down most of it and coming back 2 weeks later to finish up,” he says. “If you can’t get the herbicide to the plant in adequate quantity, you’re not going to get control.”

TAKING OFF. In this field, Gramoxone and Bicep were sprayed to control annual ryegrass but it wasn’t the right herbicide at the right time and control was poor, says cover crop expert Mike Plumer. The ryegrass soaked up the nitrogen and tied it up, likely causing some lost yield even if sidedressing was done. “If they had used Gramoxone and Bicep and followed it 2 weeks later with glyphosate, it would have been clean. But they didn’t,” Plumer says.

  • Herbicide Carryover. Another potential problem depends on the specific herbicide. As a general rule, post-applied herbicides have a higher potential to cause issues — especially if they have more residual qualities.

“We’ve got corn herbicides that you can apply in April that you can still plant most of your crops in August or early September. But if you move that corn herbicide to May 30, it’s going to kill half of your cover crop,” he says. “It’s the same herbicide, but we just need an extra 30-40 days to break it down.”

A no-tiller who plans to seed brassicas behind Scepter or Pursuit, for example, may need to wait as long as 2 years, regardless of how much rain occurs, Plumer says. But atrazine is highly water-soluble and most no-tillers should be able to use the full rate and seed most cover crops they want in August.

“We’ve used full rate of atrazine by the end of May and can plant many cover crops around Aug. 15 and not had a problem in much of the Midwest, if we get good rainfall. But if it’s a long-lasting residual, I wouldn’t plant the cover crop without a soil bioassay,” he says.

  • Saturated Soils. During a very rainy period, no-tillers would do well to pay attention to what’s happening in their fields ahead of termination, especially if a systemic herbicide like glyphosate is used, Plumer cautions.

Wet soil inhibits a systemic herbicide uptake, as the products only work if a plant is actively growing and transpiring, moving the chemical through the vascular system. If the plant isn’t growing, systemic herbicide control can significantly be reduced.

“Cold and cloudy weather really shuts down systemic herbicides. In cold and cloudy weather on ryegrass, our control gets down to the 20-30% range. It needs higher temperatures to increase control.

“You may want to switch to a contact residual or wait for warmer weather or drier weather.”

Tips for Controlling Specific Covers

Annual Ryegrass

The primary way no-tillers kill annual ryegrass is with a systemic herbicide such as glyphosate.

These herbicides are temperature sensitive and plants must be actively growing for the product to be taken up into plants.

“Normally when you spray, you’re going to want ryegrass to get 8-10 inches in height, so you know everything is germinated and up,” Mike Plumer says. “It should be sprayed on a bright sunny day, with daytime temperatures of at least 55-60 F and nighttime low temperatures above 40.”

The herbicide must also have enough time to translocate into the plant. This is crucial, says the retired University of Illinois Extension agronomist.

“Glyphosate requires 4 hours before sunset to translocate at a minimum. It takes 3 hours for it to get into the plant and get into the site to translocate. And you’ve got to have a minimum of 1 hour of translocation time, if it’s sunny,” he says. “If it’s cool and cloudy, longer than that.”

Plumer doesn’t advise spraying ryegrass past 2 p.m. in cold weather, and advises waiting 2 days after freezing weather to spray, and to make sure there are no triazines or Calisto in the mix.

“Typically we’re saying in the spring, if you’re killing it early, by 2 p.m. you’re done spraying glyphosate on ryegrass.”

Plumer notes that ryegrass is also easy to kill at flowering, but no-tillers assume the risk of terminating at that time because the plants are also setting viable seeds.

“I worked with one farmer who said, ‘Wow, that’s neat because that way I don’t have to seed ryegrass next fall.’

“We don’t recommend that. Residue at that stage of growth is high lignin. It’s not going to decompose for a couple of years. Ryegrass at 12 inches high is going to disappear in 30 days.”

If no-tillers are in a bind with cold, wet and cloudy weather, they can burn the field down and start over, getting up to 70% control with Gramoxone and coming back with a standard rate of glyphosate or another application of Gramoxone.

No-tillers must also pay close attention to the mixing of glyphosate. Plumer offered these tips:

  • Fill the tank with water all the way up, minus the room for the herbicide.
  • Treat it with ammonium sulfate at the full rate of ammonium sulfate. Agitate for 5 minutes.
  • Throw in some acidifier to take the pH to a 5.2 and let it agitate for a couple more minutes.
  • Place glyphosate in the tank.

The herbicide should be sprayed at 1.25-1.5 pounds per acre, and if these steps are followed “it’s pretty rare to not get excellent control,” Plumer says. “For an acidifier I use citric acid, and I’ve got farmers that use everything imaginable — vinegar, even industrial grade sulfuric acid.”

No-tillers may want to watch how much dew is left on leaves as a cue to their starting time.

“I would spray as soon as you get some of the dew off the leaves, because when you’re spraying, the droplet with the surfactant that you’re using, the Roundup will run it off the leaf, so if you can get the leaf just a little bit dry, if you don’t have too much dew, 9 or 10 o’clock is fine. I’ve sprayed it as early as 7 when I didn’t have much dew.”

Afternoon spraying can be a little trickier. Plumer did a study on how late in the afternoon he could spray annual ryegrass and concluded from 3-6 p.m. he loses 10% of control every hour later he sprays.

“After 6 o’clock I’m down to 20-30% because we don’t have time to translocate the glyphosate,” he says.

The amount of glyphosate truly needed to kill ryegrass depends on the variety, size and weather conditions, but Plumer doesn’t recommend no-tillers cut rates below 1.25 pounds an acre — at least until they know how their system works.

Rapeseed (Canola)

Smaller plants can be controlled with an application of dicamba and glyphosate, or Gramoxone applied with residuals, Plumer says.

Larger plants may need an application of chlorimuron, Pursuit, Scepter or other herbicides that brassicas are sensitive to, or a corn herbicide. But Plumer warns some of those products are hot enough on brassicas that you may not be able to seed them again for 2 years.

“There’s enough soil residual in them to kill the rape,” he says.

Plumer typically recommends the dwarf Essex variety because it’s highly allelopathic and has good activity on plant diseases, but he notes some rape varieties are being developed for increased effects and that could affect termination efforts.

Fields with rapeseed standing 6-7 feet tall likely didn’t have the dwarf Essex variety planted. He recalled visiting one farmer who said he couldn’t get rape terminated, and Plumer discovered it was apparently a cross between rape and kale at full maturity

For radishes, Plumer says Gramoxone and a residual such as the active ingredient chlorimuron, or Synchrony or Authority can be used to control seedlings coming up, or glyphosate and 2,4-D or dicamba will kill small plants.

Radishes will winterkill at 15 F, but turnips, kale or rapeseed may not, he notes.


Hairy or woolypod vetches aren’t all that difficult to kill, even though some no-tillers think it is, Plumer says.

Vetch is highly sensitive to 2,4-D and most varieties will die after an application of 1 quart per acre of the herbicide. But glyphosate isn’t very effective on vetches.

“Vetch tolerates glyphosate. I’ve got farmers that actually spray their vetch, when it’s a foot high, with glyphosate to control weeds in the field to make a pure stand of vetch,” Plumer says. “The vetch will stand there and quit growing for 4 or 5 days. It’s not recommended. It’s a long ways off any label, but I’ve known people who do it.”


There are many different varieties of clover being used as a cover crop and Plumer has found red clover is harder to terminate than crimson, balansa or alsike varieties.

Normally, many farmers use Gramoxone, a corn herbicide, 2,4-D, dicamba or glyphosate with dicamba and 2,4-D followed a week later with a typical normal corn herbicide, but red clover dies slowly.

“With red clover you’ll think it’s never ever going to die. It may take 3 weeks,” he says. “Crimson clover will be dead in 5 days.”

Cereal Rye

Plumer says he doesn’t know of many no-tillers having trouble terminating cereal rye. Gramoxone, plus a residual herbicide or glyphosate, applied at 0.75 pounds per acre normally works.

For those planning to use a roller-crimper, crimping cereal rye at yellow pollination will normally provide 90-95% control, he says.

“If you crimp it before the boot stage, you’re only going to get 50-60% control, so you will then have to crimp it three or four times,” Plumer says. “So crimping will work, you’ve just got to do it at the right stage of growth. You’ve got to break the stem but not cut it, so the moisture is wicked out of it and the plant dries out. But if you cut it off, you get poor control.”