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 LSU AgCenter Macon Ridge Research Station on Jan. 21.
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 said 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 said. Tillage radishes, a brand-name cover crop, reach full ground cover one month after planting.
Some growers use cover crops to maximize organic matter and nitrogen. Those goals are sometimes opposite ends of the same equation, Lofton said. Radishes, for example, have high nitrogen 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, Lofton said.
Cover crops need to be terminated at least four to six weeks before planting summer field crops. Applying 2,4-D will usually wipe them out, but not if radishes have begun flowering, Lofton said. Mechanical removal is the only option then.
Now is a good time to start thinking about “burning down” radishes, Lofton said. 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 said. “If you wait and have to plow your field to remove the cover crop, that defeats your purpose.”
Live cover crop roots keep microbes active, so nitrogen isn’t immobilized in spring. Grass cover crops that are left too long and become straw, though, can tie up nitrogen, said AgCenter soil specialist Beatrix Haggard.
Cover crops such as winter peas, clover and vetch create nitrogen, while radishes are nutrient scavengers. They do not make nutrients, but pull them from the soil.
“The nutrients have to be there first,” Haggard said. “If you planted corn, it used up a lot of the nitrogen. Or if you want to plant radishes on poor soil, you need fertilizer.”
Mike Lindsey, Natural Resources Conservation Service soil scientist, showed attendees how poorly-managed, light-colored soil quickly disintegrated in a jar of water. Cover crops provide important benefits, he said, including erosion control and better nutrient management.
AgCenter agronomist Wink Alison said 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 nitrogen that would otherwise be released into perennial grasses, he said.
AgCenter weed scientist Daniel Stephenson said cover crops can help control weeds by offering a physical barrier that restricts sunlight to the soil.
Some cover crops also contain allelochemicals that inhibit plant growth, which can be both good and bad. Stephenson said 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, Stephenson said, which means they won’t hit the soil. Valor and other common herbicides are not effective after crop emergence.
AgCenter plant pathologist Trey Price said 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 said. “That could kill the cover crop, or it could cause the cover crop to serve as inoculum for the field crop.”
AgCenter entomologist Sebe Brown said it is critical to kill the “green bridge.” 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, Brown said. 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, Brown said. Rolled rye, for example, can reduce thrips in cotton. However, neonicotinoid seed treatments remain the best defense against insects.
NRCS agronomist Chris Coreil said financial incentives to try out cover crops are available. Interested farmers should contact their NRCS field office.