How to kill a cover crop

Many Iowa farmers planted cover crops for the first time last fall. Whether this first time will lead to a “love ’em or hate ’em” experience depends largely on successful termination this spring. If you planted oats or another cover likely to winterkill, then you might be in the clear, especially given the bitter cold this winter. For other cover crops, work must be done to ensure success.

How to kill a cover crop

Many Iowa farmers planted cover crops for the first time last fall. Whether this first time will lead to a “love ’em or hate ’em” experience depends largely on successful termination this spring. If you planted oats or another cover likely to winterkill, then you might be in the clear, especially given the bitter cold this winter. For other cover crops, work must be done to ensure success.

Factors for success

The window for successful cover crop termination can be short, depending on weather and field conditions. Having equipment prepared and scouting fields ahead of time are critical to success. Scout fields by mid- or late March. Before corn planting, expect to be in the fields by early April, depending on weather, the cover crop used and the method to kill it.

Scouting is critical. Growth stage and height of the cover crop are critical to successful termination. Plants need to have “greened up” to be ready for a chemical termination. Annual ryegrass varieties are ready to be killed when the lawn is to be mowed, as compared to winter rye, which is sometimes green under the snow. Cover crop species that are bolting or growing reproductive structures will be harder to kill. Taller and more robust plants may take a greater rate of herbicide or a second tillage pass. To minimize the chances of a yield hit to corn, the rule of thumb is to terminate winter rye near 6 to 8 inches. There is a wider window prior to soybean planting.

Herbicides and tillage are the typical termination methods for cover crops. Grazing cover crops in the fall or spring is an option for reducing the stand to make it more manageable, but if returning with a chemical, be sure to leave enough surface area for the herbicide.

Chemical burndown or control. Farmers choosing a single species of cover crops (either a grass or a broadleaf plant) will be successful using a selective graminicide or broadleaf herbicide at label rates. Those with cover crop mixes will need to use a nonselective herbicide, such as glyphosate or glufosinate. While it is possible to combine selective herbicides to terminate a cover crop mix, it is not recommended. Also, check the pH of mix water and add citric acid to adjust pH as needed.

Glyphosate is most commonly used in Iowa and will effectively kill overwintering cereal grain cover crops, such as winter rye — the most common cover crop used in Iowa today. Follow planting restrictions for herbicides to ensure a healthy cash crop establishment following termination; for example, 2,4-D has a 14-day planting restriction for soybeans.

Experienced cover crop farmer Steve Berger of Wellman, says, “We like the rye to be brown and completely dead before planting corn. The aboveground growth will die and disappear over time, but the roots will last longer below the surface and hold soil.” Berger uses glyphosate to terminate the cover crops in his no-till system, and then plants directly into the dead mulch layer.

“You’ve got to be ready to kill cover crops when the time is right,” says Berger. He uses an ATV and a pull-type sprayer to ensure he can get into fields to terminate the cover crops, even if conditions are too wet to get in with a tractor.

Using tillage. Farmers who routinely use spring tillage may find cover crops fit most seamlessly into their operation since they are already planning to be out pulling steel in the spring. Still, there are factors to consider.

Shallow disking at 2 to 4 inches should be enough to kill most cover crops, says Tom Yucus, a north-central Illinois farmer who’s been using cover crops for more than 10 years. In his organic operation, spring tillage is a regular part of weed management. “At first I found when I got a good stand of cover crops, the disk would just skip across the soil surface. I had to get a heavier disk, which does a better job of getting into the ground,” Yucus says.

If using a combination of tillage and chemicals, one primary tillage pass can knock down a lot of the biomass, and any escapes can be later controlled with a herbicide. Be sure to balance steel vs. herbicides to avoiding overusing either.

Killing covers without tillage or chemicals. A few Iowa farmers are finding luck with nonchemical, non-tillage termination. The most well-established route here is the Roller Crimper developed by the Rodale Institute. The implement rolls the cover crop flat and crimps the stem to kill it. This method needs a level field, or the crimper will miss crops in dips.

For soybeans following winter rye, some Iowa farmers are finding minimal yield impacts, even when killing rye in the same planting pass of soybeans. Arlyn Kauffman of Weldon in southern Iowa had great results planting soybeans into a standing cover crop in 2013. The cereal rye, mature by the time Kauffman was ready to plant soybeans, was easily bent over with his 40-foot Henniker air drill and tractor. The few rye plants left standing after planting were rolled with a Brillion cultipacker. This method of delayed cover crop termination paid off, with these soybeans yielding 10 bushels per acre more than other beans on his farm.

Other considerations

Once the cover crop is terminated, there are a few other factors to consider to make cover crops a successful tool in your management tool box. Hitting the right planting date for cash crops is critical.

For corn following a winter rye cover crop terminated with glyphosate, experienced cover crop users in Iowa suggest a 10- to 14-day window; pushing it too close can be a recipe for disaster due to potential tie up of nitrogen and moisture. During this time, scout the field to ensure a successful termination.

Planting into a cover crop mulch may also require some adjustments to your planter equipment settings. Planters with a shoe seed opener or fingered row cleaners may drag the mulch, and therefore a disk opener is preferred. Overall, adding weight to the planter may be needed to ensure the equipment penetrates the mulch layer.

The most important thing to learn when using cover crops is to manage nitrogen, says Berger. Live and decaying cover crops are going to compete for nitrogen, and your corn can be left wanting without careful planning. Berger adds some N during corn planting and later at sidedressing.

“It’s not about using more nitrogen,” he says, “but getting timely applications.”

Larsen is a communications and ag policy associate with PFI in Ames.

Key Points

Successfully terminating a cover crop takes planning.

Timing is key and depends on cover crop type and how you plan to kill it.

Timing also depends on whether corn or soybeans are to be planted as next crop.

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TIMING: Some cover crops, like radishes, turnips and oats, are easy to terminate because they winterkill. If you’re going to plant corn into winter rye, however, it’s harder to kill, and the cover crop needs to be terminated 14 to 21 days before planting corn.

This article published in the March, 2014 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

Cover Crops

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