How to Time Cover Crop Termination and Get an Effective Burndown

10 min readFeb 12, 2020

Published Jul 1, 2019

With May 2018 to April 2019 being the wettest 12-month period on record, according to NOAA, many farmers across the nation were forced to delay planting. In fact, for the first time on record, less than half of corn was planted by May 19, says the USDA. Even by June 2, “both corn and soybean planting were proceeding at a record slow pace.”

For those with cover crops, these wet conditions likely affected their termination plans, causing some to debate whether they should terminate before or after planting.

Penn State Extension Educator and Agronomist Heidi Reed says she saw several farmers in York County, Pa., planting into living cover crops, commonly known as planting green, because they were forced to. In fact, some found planting green was better because the cover crops helped keep their fields drier, making it easier for them to drive on their fields and plant than if they would’ve killed the cover crop sooner.

But there are many factors in determining whether you should terminate your cover crops before or after planting. In the next section are several questions you should ask yourself in when deciding on the timing of your cover crop termination.

Termination Timing: Before vs. After Planting

What are your cash and cover crop species?

One of the first factors to consider in deciding whether to terminate cover crops before or after planting is the cash crop that will follow.

While soybeans perform well when planted green, corn requires finer attention to management, says Reed, who was part of a team of Penn State researchers who studied planting green on five locations across Pennsylvania over three years. Their research on planting green was recently published in the Agronomy Journal.

For starters, it’s preferable to roll the cover crop when planting corn green, as that will help sunlight reach the young germinating corn plants and warm the soil evenly across the field for uniform emergence. She notes that it doesn’t matter whether you roll then plant or vice versa. You could also roll and plant at the same time, if you have the equipment to do so.

Slugs may also be a bigger problem for corn planted green, as Penn State researchers did see more slug damage, but Reed notes that it only occurred in 2 out of 12 site-years.

Growers also need to consider the cover crop species they’re planting into. While radishes winterkill in Pennsylvania, researchers believe they may have contributed to higher slug populations. “We think they were providing better habitat and forage in the fall for the slugs,” Reed says, adding that this based solely on observation and that they don’t have data to support this yet.

Penn State Weed Management Extension Specialist John Wallace adds that he doesn’t know any growers planting green into annual ryegrass because allowing it to get too big will make it more difficult to control. For this reason, it’s best to terminate it before planting.

Farmers also need to consider whether their equipment can handle living green cover crops. For example, Penn State researchers discovered that crimson clover wasn’t easy to plant into because of its mature, thick roots. Reed says hairy vetch can also be difficult because it’s a vine that can wrap around equipment.

Despite these challenges, Reed knows it is possible to have success planting into these species. But for those who have never planted green before, it would be better to first try planting green into a grass, like cereal rye or triticale.

What’s your termination zone?

You should also take your cover crop termination zone (see map below) into consideration when deciding whether to terminate before or after planting.

These zones, developed by the NRCS, were created to develop the following cover crop termination guidelines for non-irrigated cropland.

For those in…

Zone 1 — Terminate the cover crops as soon as practical prior to planting early spring crops, but for covers ahead of late spring or fall-seeded crops, they should be terminated at least 35 days ahead of planting.

Zone 2 — Same guidelines for early spring seeded crops as Zone 1, but for late spring- or fall seeded crops, terminate covers at least 15 days prior to planting.

Zone 3 — Terminate cover crops at or before planting the cash crop.

Zone 4 — Terminate cover crops at or within 5 days after planting, but before crop emergence.

The NRCS adds that for farmers in no-till systems, cover crop termination can be delayed up to 7 days from the above guidelines, but prior to crop emergence for all zones and systems.

What’s the weather forecast?

While the Zone map lays out specific guidelines for termination, the NRCS notes that farmers should make adjustments based on the weather.

If the season is drier than normal, the NRCS suggests terminating cover crops earlier to conserve moisture, while a wetter-than-normal spring may warrant a later termination to help soak up excess soil moisture and improve seedbed conditions.

Unfortunately, there’s no tool available to help a farmer determine if the weather is too wet or too dry to make changes to their termination strategy. Reed says it’s a judgment call only the farmer can make, because they know their fields and soil types better than anyone.

With that said, she recommends erring on the side of caution. If the forecast is calling for it to be dry and there’s an opportunity to get in the field and spray, Reed suggests going ahead and terminating it.

Best Management Practices for Planting Green

If you’re forced to plant green because weather conditions didn’t allow you to terminate your cover crop in time, there are some best management strategies you should try to follow.

At Penn State’s Russell E. Larson Agricultural Research Center, soybeans were planted green into cereal rye that was being rolled in the same pass. Photo by Heidi Reed.

The first is having your equipment set up properly. As explained in “ Planting Green 101: Penn State Research Summary,” planting green doesn’t usually require special attachments, but having row cleaners can help. If you have a roller-crimper, researchers suggest rolling mature and/or high biomass cover crops to minimize shading of the cash crop.

Reed adds that planters should be well-calibrated, with sharp coulters and gauge wheels that are adjusted properly.

And while farmer should always get out of the cab and check their planter’s performance, it’s especially important to do so when planting green.

“You can get a thick cover crop mat that can push up the gauge wheels and make you plant shallower than intended,” Reed says, adding that it’s a good idea to check multiple times that you’re reaching that 2-inch seeding depth for corn.

Reed also recommends those planting corn green increase their seeding rates about 5%. She says that in their research they saw lower populations in the planted green treatment vs. corn planted in cover crops that were burned down earlier. They predict this is due to interference caused by the cover crop residue, or cooler soil temperatures.

She adds that corn also needs sufficient nitrogen at planting, especially if it’s planted into a cover crop that contains a grass like rye or triticale, which have high carbon-to-nitrogen (C:N) ratios and can keep nitrogen tied up from the corn. According to the Penn State Research Summary, (C:N) ratios above 25:1 typically result in immobilized nitrogen, and they found that rye had a ratio of 44:1 at the time of planting corn green into it.

So how much nitrogen do you need? Penn State recommends applying at least one-third to half of your total nitrogen for the season at planting, with the rest applied during sidedressing.

For more information on how to adjust your nutrient management program with cover crops, check out the article “How Cover Crops Affect Your Fertilizer Strategy.”

Once the crop has been planted, you need to be diligent in your scouting to prevent insect issues, Reed says, adding that you also need to make sure your cover crops have been terminated properly.

“The last thing you want is to go back in July when you’re spraying your post and realize that your rye was never actually killed and it’s been a weed for a month and a half,” she says. “Make sure to scout and stay on top of that.”

Effectively Terminating Thick Cover Crop Mixes

Whether you’re terminating your cover crops before or after planting, achieving a complete kill of your cover crops depends on the right herbicides applied under the right conditions.

John Wallace says that warm, sunny days when the plants are actively growing is the best time to do a burndown, as it helps increase the activity of the herbicides. As for the right herbicide to use, it depends on the species in the field and their sensitivities to a product.

Glyphosate is the most common herbicide used, Wallace says, as it’s not only effective in burning down grass species like cereal rye, it will also help control winter annual weeds that may be in the field. In an article written by Wallace and Penn State Weed Science Extension Associate Dwight Lingenfelter, they say that glyphosate applied on sunny days above 55 degrees Fahrenheit at .75–1.5 pound acid equivalent (ae) — which would be 22 fluid ounces of Roundup — per acre with appropriate adjuvants should provide good control of most grasses.

With that said, winter wheat may be more difficult to terminate, so Wallace and Lingenfelter recommend making an application with higher glyphosate rates if terminating wheat under more challenging weather conditions.

If there are legumes in the mix, using a plant growth regulator like dicamba or 2,4-D ester will help control it, but you’ll need to consider planting interval restrictions. In the article, Wallace and Lingenfelter, say that either one of these needs to be applied 7–14 days before corn planting.

But if you want to — or have to — plant green, Wallace says that these products can be applied after corn is planted but before emergence, however, injury can occur, particularly if seed furrow closure is an issue. If you’re planting green and planting conditions are optimal, it would make more sense to burndown the cover crop with glyphosate and 2,4-D just after planting.

For brassicas that don’t winterkill, the Sustainable Agriculture Research and Education (SARE) offers different recommendations based on the cover crop species.

SARE recommends applying at least 1 quart of glyphosate per acre on rapeseed, as it can be difficult to terminate, and notes it may require multiple applications.

As for radishes, mustards and turnips, they can be terminated with multiple applications of glyphosate, glyphosate with a pint of 2,4-D per acre, or with a full rate of paraquat.

Bigger Biomass Considerations

If you decide to plant green, you may be wondering whether the bigger biomass warrants a higher burndown rate. Wallace says he hasn’t seen any general guidelines on this, explaining that it has more to do with cover crop species, the growth stage and weather conditions at the time of termination. If you have winter wheat, you may need a higher rate than if you had cereal rye.

For tougher to control cover crops, Wallace says you may need 1–1.5 pounds of glyphosate ae per acre. The actual rate will depend on the product you’re using, as the different brands are formulated differently.

One thing to keep in mind if you terminate late and have a lot of biomass is that the cover crop may prevent residual herbicides that are included in the tank with burndown products from reaching the soil profile and becoming activated. The bigger the cover crop, the more likely this is to be an issue, Wallace says.

“If you kill it earlier and apply the residual product at planting, the cover crop residue has already broken down a little bit, and you have less biomass you’re planting into, so you’re probably going to have fewer issues with activating those residual herbicides,” he explains, noting that weed scientists don’t have a good idea right now how much cover crops tie up the residual herbicides.

Preparations Begin in the Fall

While you can’t control the weather, you can be prepared for it. That starts by making plans the fall before and having a back-up plan in place if what you intended to do doesn’t work out.

This is especially true if you plan on planting green the following spring. Reed says you can set yourself up for success with this by lowering your cover crop seeding rate. For example, she recommends cutting the rate for rye in half (i.e., from 120 pounds per acre to 60), and if you plan to apply fertilizer or manure to the cover crop, cut it back even more. Seeding rates for other species should be reduced as well, although legumes probably don’t need to be cut a full 50%.

“By letting the cover crop grow so long in the spring, you accumulate a lot of biomass, and if you’ve got a heavy seeding rate out there, it’s going to be really tough to be successful planting green,” she explains.

Wallace recommends looking at planting green as an adaptive management practice.

“In some years based on the weather, it’s going to make sense to kill that cover crop earlier, in other years there might be an opportunity to let that cover crop get bigger,” he says. “If we can make sure we’re using the best management practices that can help us down the road as far as keeping herbicides effective, planting green can be an important integrated weed management tool.

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