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Cover Crop Lessons Proven by Research
Cover crops have been proving their merit for 30 years at the University of Missouri’s Bradford Research Center. Nitrogen contribution to the subsequent crop was the focus of the early research on cover crops. But over the years, the ongoing studies have looked at the benefits through a broader lens.
From the early studies to more recent field trials, Tim Reinbott, assistant director of the University’s Agricultural Experiment Station, has seen firsthand how cover crops impact no-till cropping systems. Of all their benefits, the improvements they make to soil take center stage.
“Often, when farmers are first considering whether or not to plant cover crops, they look for the economic benefits,” he says. “They may be hesitant to plant cover crops if the potential yield increase in the subsequent crop doesn’t pay for the cost of the cover crop seed. But if you’re building up your soil, can you even put a value on that?”
Stopping soil from eroding in water runoff, for instance, was one of the early benefits Reinbott found from growing cover crops. The elimination of soil erosion occurs not only because of the cover crop biomass on top of the soil and the roots anchoring the soil in place, but also because of the improvements in the structure of the soil.
“After three to four years of growing cover crops, we’ve seen significant improvements in aggregate stability and in populations of soil microbes,” he says. “Of course, erosion was reduced and water infiltration was improved.”
Improving soil’s water-holding capacity is becoming of greater importance, he says, as climate change brings increasing severity in weather events.
“Last year, we got 40 inches of rain in just four weather events,” says Reinbott. “It’s important for farmers to hold the rainfall on their farms and not have it run off to end up polluting rivers, streams, and oceans.”
Mellow soil results from the improved structure. This helps to increase the rooting depth of cash crops. “The roots of the cash crop will follow the old root channels of the terminated cover crop,” says Reinbott.
With deeper roots, the cash crop can access moisture stored at deeper levels in the soil. The deeper roots are also better able to extract from the soil nutrients stored at deeper levels, such as potassium and phosphorus.
Reinbott has seen yields of cash crops following cover crops increase over yields of cash crops following cash crops. “With soybeans following a cover crop, we continually get a yield increase of 5 bushels to the acre,” he says. “It doesn’t matter if the previous cover crop was a legume or a cereal.”
However, yield increases of soybeans following hairy vetch have been higher. After one wet growing season, soybean yield following vetch was 9 bushels to the acre higher than that of soybeans following corn. The soybean yield following a cover of hairy vetch and rye was 14 bushels to the acre higher than the control.
“Corn responds differently,” says Reinbott. “When we’ve planted corn into a cereal cover crop, we’ve actually lost 25% of the corn yield, as compared with the yield of the control. Because of its high carbon content, the residue of the cereal cover crop ties up nitrogen as it decomposes. Corn seems to be sensitive to this process.”
However, planting corn into a legume cover crop gives a different picture. “When we plant corn into a hairy vetch cover crop, we expect to get a slight yield increase in the corn,” he says.
One field trial showed corn yield following a cover crop of hairy vetch was 14 bushels per acre higher than that of corn following soybeans. In the same year, corn following a cover crop of triticale and vetch yielded 11 bushels per acre higher than the yield of the control corn.
In Reinbott’s earlier work looking at cover crops’ potential to fix nitrogen (N) in the soil, he found their N contributions to the subsequent crops to be significant. “We found that hairy vetch could pretty well supply all the N needed by grain sorghum if we wait to terminate the hairy vetch until about the end of May,” he says. “For corn, the vetch could supply about half the necessary N.”
A mix of hairy vetch and cereal rye gives advantages of both a legume and a cereal cover crop.
Cover Crop Preferences
The years of evaluating cover crops have shaped some preferences for Reinbott. In legumes, crimson clover tends to stand out, in his experience, for its well-rounded benefits. It’s best suited to a climate even warmer than Missouri’s, though.
“Crimson clover is a little bit out of its comfort zone here,” he says. “But the bumblebees love it, and we like it because it blooms early – in mid-April. If you let it mature to full bloom, it has the advantage of being able to reseed itself.” Recent developments of cold-hardy varieties of crimson clover could extend the regions to which it might adapt.
Hairy vetch is a hardy and a dependable producer of large amounts of biomass, with strong N contributions to subsequent crops. A drawback to vetch is its tendency to spread naturally and potentially become invasive.
Cereal rye has the same drawback as hairy vetch, with its potential to go to seed and cause problems by regrowing like a weed in subsequent crops.
“The advantage of rye is that if you plant it in the fall, it’ll get going like gangbusters the following spring,” says Reinbott.
“Winter wheat also works great as a cereal cover crop,” he says. “If you don’t want to spend a lot of money on cover crop seed, go with winter wheat.”
Other cover crops that have worked well for Reinbott are Austrian winter peas, radish, and triticale.
Reinbott’s seed costs for cover crops have typically run from $10 to $12 an acre for cereal covers and from $15 to $30 an acre for legumes.
After years of proving cover crops’ merit, Reinbott is seeing farmers’ interest in covers take root in the region. “This spring here in this area, I saw more corn and soybeans being planted into standing cover crops than I have ever seen before,” he says.
GET THE MOST FROM COVERS
To get the most biomass possible from a cover crop, Reinbott suggests waiting till the last minute to terminate it. Yields of subsequent crops will benefit.
He terminates the cover right before planting the cash crop or, in some cases, a few days after planting.
Take care when planting corn into a cover crop.
“Corn is not competitive with an existing stand of biomass,” he says. “Seed germination and seedling emergence can suffer. You’ve got to have a good planter and make sure it’s set correctly in order to have good stands of corn.”