Source: Louisiana State University AgCenter

Cover crops can improve soil health, but it is important to make careful decisions when choosing and growing them. Farmers learned about the benefits of cover crops and how to manage them during a workshop at the Louisiana AgCenter Macon Ridge Research Station on Jan. 21. 

Seeding, Terminating Covers

There are a variety of cover crops, including grasses, legumes and brassicas, which are planted in winter and can be used for different purposes. AgCenter agronomist Josh Lofton says many farmers want to cover their soil and keep it in place, which necessitates early growth.

Clover offers good cover, but not until February, when heavy rains that move soil have already passed. Cereals, such as forage oats, and radishes offer similar coverage and grow faster, Lofton says. 

Some growers use cover crops to maximize organic matter and nitrogen (N), but Lofton says those goals are sometimes opposite ends of the same equation. Radishes, for example, have high N content, but can’t build much organic matter because their leaves decompose quickly.

The earlier cover crops are planted, the more biomass they will produce.

Cover crops need to be terminated at least 4-6 weeks before planting summer field crops. Applying 2,4-D will usually wipe them out, but not if radishes have begun flowering, Lofton says. Mechanical removal is the only option then.

He adds that now is a good time to start thinking about “burning down” radishes. Rye, clover and vetch can stay a little longer. Planning is crucial, however.

“If you’re planting corn or early soybeans, think about that when you’re planning,” Lofton says. “If you wait and have to plow your field to remove the cover crop, that defeats your purpose.”

Sequestering Nutrients

Live cover crop roots keep microbes active, so N isn’t immobilized in spring. Grass cover crops that are left too long and become straw, though, can tie up N, says AgCenter soil specialist Beatrix Haggard. 

Cover crops such as winter peas, clover and vetch create N, while radishes are nutrient scavengers. They do not make nutrients, but pull them from the soil.

“The nutrients have to be there first,” Haggard says. “If you planted corn, it used up a lot of the N. Or if you want to plant radishes on poor soil, you need fertilizer.”

Benefitting Soil Structure, Livestock 

Mike Lindsey, NRCS soil scientist, showed attendees how poorly managed, light-colored soil quickly disintegrated in a jar of water. Cover crops provide important benefits, he says, including erosion control and better nutrient management.

AgCenter agronomist Wink Alison says ryegrass and clover can be used in pastures not only to cover the ground but also to extend the livestock grazing season. Clover is high in minerals, which is good for dairy producers. Radishes may also help collect leftover N that would otherwise be released into perennial grasses, he says.

Managing Pests and Disease

Cover crops can also help control weeds by offering a physical barrier that restricts sunlight to the soil, says Daniel Stephenson.

Some cover crops also contain allelochemicals that inhibit plant growth, which can be both good and bad. The AgCenter weed scientist says wild radishes — not the same as other radishes — inhibit growth in pitted morningglory, but also corn and cotton.

Residual herbicides can be intercepted by cover crops, which means they won’t hit the soil. Valor and other common herbicides are not effective after crop emergence.

Trey Price, AgCenter plant pathologist, says cover crops retain soil moisture and lower soil temperatures at planting time, which are ideal conditions for seedling diseases. They also increase surface debris, where pathogens can survive.

“Many foliar pathogens overwinter in plant debris, and others stay in the soil, so cover crops will come in contact with them and they could become hosts for diseases,” Price says. “That could kill the cover crop, or it could cause the cover crop to serve as inoculum for the field crop.”

Sebe Brown says it's critical to kill the "green bridge." The AgCenter entomologist explains that in order to break the life cycle of insects before planting field crops, cover crops should be chemically burned down several weeks before planting.

The bean leaf beetle and grape colaspis, which are soybean pests, love legumes, he says. The sugarcane aphid, which attacks grain sorghum, is a grass feeder. Certain cover crops could cause a buildup of those insects.

But cover crops may also help with pest management, he adds. Rolled rye, for example, can reduce thrips in cotton. However, neonicotinoid seed treatments remain the best defense against insects.