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How to kill cover cropsHow to kill cover crops

Terminating cover crops is key to successful management.

January 17, 2014

5 Min Read

Cover crops provide a wide variety of benefits, from capturing root-zone N to improving soil tilth. But cover crops that interfere with your cash crop can do more harm than good, which is why terminating them is so important.

The easiest cover crops in the upper Midwest are probably oats, tillage radish and/or Austrian winter peas, which die naturally in winter weather, notes Florian Chirra, Ohio State University Extension educator in Bryan, Ohio. Chirra is one of three Extension agents in the Great Lakes Cover Crop Initiative, an EPA-funded program to demonstrate, evaluate and promote cover crops in the watersheds that feed Lake Michigan, Lake Huron and Lake Erie.

Planted after wheat harvest or into standing soybeans or corn, oats can anchor nutrients left behind near the surface by the previous cash crop. The oats return most of those captured nutrients to the soil as their own foliage, stems and roots break down after winterkill. Oat root systems also help build soil organic matter and improve tilth, Chirra says.

For more dramatic impact on tight soils’ tilth, tillage radish is known for its long, sturdy roots taproots that dig deep in the soil to loosen hardpans and capture N.

Austrian winter peas, clover and other legumes can fix N in the soil. They are often planted in a blend with other cover crops to provide a boost to the cover, or in dense enough stands to add some “free” N to the field. Legumes are often paired with a grass to capture the fixed N and protect it from leaching.


Rye challenges

For cover crops that overwinter, termination can be a challenge. In fact, fear of a runaway stand concerns many growers..

Cereal rye – a tall grain – can grow several feet tall in just a few weeks if spring conditions are conducive. Annual ryegrass, a shorter plant that is actually no relation to cereal rye, has a hardy root system and a reputation for being tough to kill.

 Both are susceptible to glyphosate, notes Jamie Scott, a cover-crop seed dealer who plants cover crops on nearly all of the 2,000 acres of corn, soybeans and wheat he farms near Pierceton, Ind. However, he warns, early spring is a tough time to work with Roundup.

“Glyphosate is made for 70-to-80-degree weather,” Scott says. “We’re using it at 50 degrees. We have to learn how to help it out.”

The first step is to make sure the cover crop is actively growing. The plant needs to pump nutrients through its system to translocate the herbicide. Avoid applying glyphosate when nighttime temperatures fall below freezing.

Spray in the morning or right after lunch to give the glyphosate several hours to translocate through the plants, Scott advises. “We’ll do one or two loads after noon, but not much more than that – and nothing in the evening,” he says of his own operation. “It takes four or five hours of sunlight to translocate the glyphosate.”

Pushing glyphosate to its limits of temperature and timing means there’s no room to skimp on carrier, Scott adds. He recommends 10 to 12 gallons of water, conditioned if necessary to avoid tying the herbicide up with the ions in hard water.

“Keep it simple that first year,” Scott explains. “That’s normally where we get into trouble. Someone wants to add 28% or some product somebody sells them that’s supposed to help. It may or may not be the problem, but it makes it hard to troubleshoot.”

The Oregon Ryegrass Commission points out that annual ryegrass is more difficult to control after the first node has developed. That’s a reminder that a springtime stand of annual ryegrass is made up of mature plants, not seedlings. With its well-established root system fighting for life, annual ryegrass requires excellent foliar coverage with a full rate of herbicide for control. Medium-sized droplets, moderate spray pressure and careful application help give glyphosate the edge it needs.

Not every mix works with glyphosate, though. The Oregon Ryegrass Commission website says that tank mixing atrazine or mesotrione with glyphosate can significantly reduce burndown.


Taller cereal rye

Waiting for the relatively warm, sunny weather that boosts glyphosate’s activity is just one factor in determining when to terminate a cover crop. Timing decisions can also manage variables like soil moisture.

Mike and Mark Trausch of Edon, Ohio, held off on terminating their cereal rye after the late rains this spring, using the fast-growing foliage to pump water out of the root zone.

“This year, we’re trying to get a little height on it to suck up some moisture,” says Mike. “We want to get at least 2 feet out of it – before it starts to head out. Then we’ll spray it and retain that moisture for later. When it goes down, it will hold onto that moisture for a long time.”

That’s a big difference from the brothers’ strategy during last year’s drought. “Last year, we burned our rye off early,” Trausch explains. “It turned out to be a good thing, dry as it was.”

The Trausches spray their cereal rye about three days before no-tilling into the standing vegetation, allowing them to cut off competition between the cover crop and the cash crop while still being able to drill into the dense foliage while it’s green and flexible.

Managing cover crops takes practice and some finesse, but a little planning and a stiff dose of well-applied herbicide keeps them in check.

“The only way to get through that learning curve,” says Jamie Scott in Indiana, “is to try something.”

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