Studies analyze long-term benefits of cover crops at planting, termination

Producers in Louisiana have used cover crops for years to help protect their soil, and now two LSU AgCenter researchers are studying the practice to precisely understand the benefits of the practice.

AgCenter soil scientist Brenda Tubaña is studying the cultural management practices of cover crops, focusing on the planting date and fertilization. Agronomist and weed scientist Josh Copes is researching cover crop effects on row crop production with a concentration on cover crop termination dates.

Anecdotal information on cover crops is plentiful, Tubaña said, but the scientists want to put hard numbers on their long-term contributions to the main crops.

“What we are doing now is understanding what is really happening,” Tubaña said. “What are the specific benefits, and what are the compounding effects that would lead to improved soil health and quality and then improved crop productivity?”

Tubaña is studying nutrient recovery in cover crops. Cover crops pick up nutrients from the soil during the fallow periods and incorporate them into their biomass. When producers terminate the cover crops, the biomass can decompose and release nutrients back into the soil.

“What if we try to improve biomass accumulation of cover crops to begin with because that will become the main source of nutrients,” Tubaña said. “It’s like a storage of nutrients that would have otherwise been left in the field and lost via runoff or leaching.”

In her research, Tubaña plants a combination of legumes and plants from the brassica family, hairy vetch, tillage radish and crimson clover. The studies compare fertilized and nonfertilized plots planted in September, October and November. Soybeans and corn planted in rotation are the commercial crops in the study.

“What we want at the end of the day is to improve the biomass that contains all the nutrients so that when we burn them down in spring prior to planting annual crops, the composition of biomass will result in the release of nutrients that will be used by the main crops,” Tubaña said.

The earlier-planted cover crops accumulate more biomass, Tubaña said, and soil tests show this leads to an increase in nutrients.

While Tubaña is focused on the planting date and establishment of cover crops, Copes is studying the effect of termination timing on the main crop. The study includes a rotation of corn and soybeans, a rotation of soybeans and cotton, and a rotation of cotton and corn.

“We look at terminating the crop either six weeks prior to anticipated planting, or four weeks, or two weeks, or at planting,” Copes said. “The at-planting treatment either got a nematicide or it did not. I’m trying to see how the nematode population could be affected by allowing the cover crop to grow longer.”

So far, Copes has not seen a positive or negative effect on nematode populations.

Also, Copes has found that termination timing of cover crops matters, but a late termination is not a calamity for a crop.

“You have to try to get it done in a timely manner — four to six weeks out,” he said. “But if you couldn’t for some reason, don’t freak out. It’s not really hurting us as bad as we once thought because we’re all running seed treatments on our seeds.”

Another study focuses on cover crop species and includes two cereals — cereal rye and black oats — a legume, hairy vetch and combinations of these. They are all terminated six weeks before the target crop is planted.

Cover crops suppress winter weeds, and the evidence is seen in the fields, Copes said. In his research, Copes wants to see how cover crops affect the main crop, and that answer will likely take years of research.

“Really, with cover crops, I have not seen any negative impacts from growing them,” Copes said.

Short-term findings show that the main crop’s yield doesn’t benefit a great deal from cover crops.

“They aren’t getting a bump from the cover crop, but they are doing a lot of other things: protecting from the soil erosion, building soil health,” Copes said. “The main thing is, in my opinion, we’ve got to keep the soil on our fields instead of losing it.”

The choice of a cover crop depends on the producer’s needs. The “jury is still out whether there is a better cover crop for each crop,” he said.

Like Tubaña, Copes is also planning to analyze the effects of cover crops on soil for long-term studies.

“It’s hard to measure changes in a few years,” he said. “It takes a little time.”

Tubaña agrees that the benefits of cover crops will likely be found in long-term studies.

“It is not going to be seen on a short-term adoption of this practice,” Tubaña said. “It will take a while. Cover crops have a compounding effect, bringing in benefits. That does not happen overnight.”

This story is featured in the Louisiana Soybean and Grain Research and Promotion Board 2020 Report.

Nov vs Sep planting dateJPG

Two months of growth can have a large effect on the biomass produced by cover crops. The plot on the left, photographed at the AgCenter Central Research Station in February 2020, was planted in November, and the plot on the right, which was photographed the same month, was planted in September planting date. Photo by Daniel Forestieri
9/16/2020 4:31:47 PM
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