More to terminating cover crops than often meets the eye

Cover crops have become a hot topic in agricultural circles nationwide because of soil health benefits. In the South, they’re also getting a closer look because of the potential for delaying Palmer amaranth emergence.

In both cases, growers have to make sure the cover crops – whether grasses, legumes or a combination of the two – don’t interfere with the timely planting and emergence of the cash crop, according to the University of Tennessee’s Garret Montgomery.

Timing is critical and the best time to terminate the winter cover can vary depending on which cash crop you’re planting, says Montgomery, a graduate student working with Dr. Larry Steckel at the West Tennessee Research and Education Center at Jackson. Montgomery was one of the speakers at this year’s Milan No-Till Crop Production Field Day.

“With soybeans cover crop termination is one of the biggest factors in how much weed control you can achieve,” said Montgomery. “In soybeans, you can actually kill your cover crop after you plant, still have perfectly good soybeans yields and get added benefit from your weed control program.”

Cover crops – especially with plants like cereal rye – can get big, notes Montgomery, who has been conducting cover crop research for his doctoral dissertation at the University of Tennessee.

Two-pass system

In some cases, terminating these large cover crops may require more than one herbicide application. “It can be extremely hard to do with one pass,” he said. “Think about defoliating really rank cotton; it’s going to take a couple of passes to be effective.”

Apply glyphosate first, plant the cash crop and then apply paraquat or Gramoxone. “That’s been our most effective and consistent program day in and day out for terminating these big cover crops,” says Montgomery.

University of Tennessee researchers recommend growers plant a mixture of grass and legume cover crops for corn. “Maybe with cotton you can get away with just a legume,” says Montgomery. “With soybeans you can get by with just a cereal cover crop. But, as a general rule for getting the most biomass every year, plant a grass and a legume cover crop.”

Montgomery says the two best grasses are wheat and cereal, “and, in Tennessee, we get more biomass out of wheat than cereal rye, and the two most common legumes are hairy vetch or crimson clover.”

Growers also need to think about cover crops the way they do herbicides these days – don’t use the same cover crop in the same way each time they plant cover crops in the fall.

Shorter corn window

In corn, growers should plan to terminate cereal cover crops about 14 days prior to planting; and legume or cereal plus legumes from seven to 10 days prior to planting. Scientists have found little benefit for delaying termination in corn.

“Corn we’re talking about planting early – the end of April or early May,” says Montgomery. “Cover crops aren’t growing that fast then. You wait an extra week, and you really don’t get that much more biomass out of it. Also, corn doesn’t like coming up in a green cover crop quite as much.”

Cotton is slower growing off early in the year and seems to benefit from a legume cover crop. “The little bit of nitrogen from those helps get it up out of the ground,” he says. “Delaying terminating cover crops preceding cotton seems to provide greater weed suppression.”

Soybeans are the most robust growing of the cash crops, and the “most advantageous” crop to plant into a weed control cover crop, according to Montgomery.

No matter which cash crop they’re planting, growers need to keep several things in mind: 1) Planter must be set properly; 2) Maintain good seed-to-soil contact; 3) Make sure the seed are going in the furrow; and 4) Closing the furrow.

Don’t bet the farm

“Practice planting into the cover crop a time or two,” he says. “I wouldn’t do this wall-to-wall on your farm the first year.”

With soybeans, University of Tennessee research indicates termination of the cover crop should occur within 14 days of planting; again, before or after planting the soybeans.

Montgomery and fellow researchers at the University of Tennessee have terminated vetch and wheat/vetch cover crops 14 days and seven days before, at planting and seven and 14 days after to determine the level of weed suppression from the winter vegetation.

“The longer we wait the more weed control we get,” he said. “This is the number of days we get before we have 4-inch pigweed in those plots. You get seven to 10 more days of weed suppression out of a mix of vetch and wheat than you would a vetch cover crop.”

The downside is the vetch fixes nitrogen, and, once pigweed does emerge, it acts just like the crop, takes up the nitrogen, “and it can get away from you in a hurry.”

For more information on the Field Day, visit

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