Cover crops under the microscope

Cover crops were a long way from being “cool” when University of Nebraska Extension educator Gary Lesoing and others at the university began studying the impact of cover crops on soil health, namely soil erosion and plant-available nitrogen.

That was more than 40 years ago, and the research has broadened over time. Lesoing, who recently retired, has learned much about what cover crop species work best for specific soil health goals. He’s also witnessed a slow but steady evolution of farmers adopting cover crops and harvesting the benefits as a result.

“Long-term cover crop growers sometimes realize better yields of cash crops, and sometimes they find they’re able to use fewer chemical inputs,” he says. “Because they’re keeping living roots in the soil, the channels created by the roots help with water infiltration. The cover crops shelter the soil surface, and this, along with some of the glue secreted by roots, helps soil resist erosion. The cover crops also help to suppress weeds and cycle nutrients in the soil.”

Benefits of combining practices

Researchers’ trials have shown that combining cover crops with no-till creates soil health benefits exceeding those created by no-till alone. The two practices combined with applications of manure create the greatest soil health benefits.

In an on-farm project, researchers compared a field in no-till for more than 10 years to an adjoining no-till field growing cover crops for five years. They compared both fields to an adjoining no-till field growing cover crops and receiving manure. The straight no-till field had soil organic matter (SOM) of 3.3%; the no-till/cover crop field had SOM of 4.5%; and the no-till/cover crop field receiving manure had 5% SOM.

Water-infiltration rates followed a similar trajectory. Soil in straight no-till fields infiltrated water at a rate of 1.09 inches per hour. Adding cover crops to no-till increased water infiltration to 2.51 inches per hour. Applying manure to the no-till/cover crop fields further increased water infiltration to 3.33 inches per hour.

Urine and manure deposited by livestock grazing the cover crops similarly benefit soil health. Livestock grazing offers the added economic benefit resulting from the harvesting of livestock feed without losing benefits to soil health.

“A significant percentage of the nutrients in the cover crops grazed by livestock are recycled in the soil,” Lesoing says. “Seventy percent to 80% of the nitrogen, 60% to 85% of phosphorus, and 90% of potassium is excreted in the manure. Over time, [these nutrients] replace fertilizer for crops. Farmers who are most successful with cover crops are those using livestock to harvest some of the cover crops.”

Prime opportunity

A complex crop production system offers prime opportunity to incorporate cover crops, Lesoing says. Including winter wheat in the rotation, for instance, provides a window for establishing cover crops in midsummer, after wheat harvest. This gives a greater chance for the cover crop to get established and provide fall grazing for livestock.

With or without grazing, the most effective use of cover crops results from a planned approach. “You need to have a goal in mind when deciding what species to plant,” he says. “Cover crop species and mixes have different purposes. Some fix nitrogen in the soil. Some address issues with compaction. Some soils with low organic matter need cover crops high in carbon. Some mixes are best for early-season grazing, and others are best for late-season grazing.”

In general, the differing plant families of cover crops each provide different services to the soil. Grasses tend to yield cover crops that are high in residue, providing shelter for the soil surface. Legumes fix nitrogen in the soil. Brassicas counteract compaction with their roots, and their residue contributes to nutrient cycling in the soil system.

“While both the taproots and fibrous roots of different species of brassicas help with compaction, they also help with scavenging excess nitrogen in the soil,” says Lesoing.

Cereal rye popular choice

The most popular cover crop species in Nebraska is cereal rye, notes Lesoing. “Rye is winter-hardy, pesticide-tolerant, and stimulates soil biology, which helps recycle nutrients and build soil structure,” he says.

Reduced seed cost is another benefit of cereal rye as a cover crop. According to a survey conducted in 2022 by Practical Farmers of Iowa with Nebraska farmers, the cost of cereal rye seed came to $10 or less per acre, while the cost of seed for mixed-species cover crops averaged a little more than $20 an acre. However, some multispecies cover crop seed mixes cost considerably more (see “Seed Cost Comparison”).

While cereal rye is cost-competitive to establish and offers multiple benefits to the soil system, when grown as a single-species cover crop, it usually yields less biomass than multispecies cover crops. “Biomass is important because, as it is recycled in the soil, it puts carbon back into the soil system,” says Lesoing. “This process often helps farmers get by on fewer inputs.”

On-farm yield comparisons of various cover crops have shown that cereal rye planted in October and November following corn and soybean harvest yielded 1.45 tons of dry matter per acre in May.

By comparison, the average yield of a five-species cover crop mix following wheat and measured in October amounted to 3.24 tons of dry matter per acre. While yielding less, the rye was comparable to the mixes in total digestible nutrients, with both testing about 61%. However, the rye was higher in crude protein, testing 14.70%, while the mixes averaged 9.71%.

High amounts of biomass typically result from cover crop mixes designed to build soil carbon. Given adequate summer moisture, University of Nebraska trials have shown that high-carbon cover crop mixes have produced nearly 4 tons of dry matter per acre. The biomass yielded nearly 3,500 pounds of carbon per acre and 100 pounds of nitrogen per acre.

One multispecies high-carbon mix researchers evaluated had sorghum sudan, pearl millet, grain sorghum, spring barley, sunn hemp, okra, sunflower, flax, African cabbage, spring oats, and buckwheat.

When the goal is fixing nitrogen, researchers have found legumes provide varying amounts of nitrogen to the soil system. Research indicates hairy vetch typically supplies 90 to 200 pounds of nitrogen per acre. Field pea supplies 90 to 150 pounds; red clover, 70 to 150 pounds; sweet clover, 90 to 170 pounds; and white clover, 80 to 200 pounds. Nitrogen amounts vary with growing conditions and timing of termination.

Regardless of your goal in planting cover crops, Lesoing says it’s important not to give up after just two or three years. “The benefits to soil health are long term,” he says. “It may take four to five years to start seeing changes in the soil and cropping system. Be patient.”