Cover crops grown in fields where manure has been applied can help keep nutrients in soil. (Photo: Joseph L. Murphy/Iowa Soybean Association)

Cover crops, manure to boost water quality

August 13, 2020

While there’s no simple formula to protect Iowa’s water quality, new research reveals that cover crops grown in fields where swine manure has been applied can help keep more nutrients in the soil where they’re needed most.

“The studies we’ve conducted so far look very promising,” said Scott Nelson, director of agronomy for the Iowa Soybean Association’s (ISA) Research Center for Farming Innovation (RCFI).

Manure management is an area of emphasis for RCFI, which integrates ISA’s On-Farm Network and environmental programs and services to better serve Iowa’s 38,000 soybean farmers.

Earlier this year, Nelson and the RCFI team compared cover crops versus no-cover crops in a manured system at two sites in northwest and north-central Iowa to study nitrogen (N) sequestration.

“Manure is a valuable resource for farming, but there are public concerns around excessive nutrient loss in manured cropping systems,” Nelson said. “We want to help farmers continuously improve their manure management.”

50 pounds of N

In mid-May 2020, Nelson and his colleagues collected 12-inch soil samples at sites in northwest and north-central Iowa to estimate the effect of cover crops (including cereal rye in these trials) on nitrogen sequestration.

“Despite having low above-ground biomass, the cover crop at the site in northwest Iowa sequestered 50 pounds of N per acre, compared to the no-cover-crop comparison,” said Nelson, noting that the cereal rye was approximately 6 inches tall. “This means there were 50 fewer pounds of N that were vulnerable to loss at this site.”

Researchers observed interesting results at trials in north-central Iowa, as well, where the fields also had low above-ground cover crop biomass.

For most of the replicated trials, the differences in N sequestration from cover crops were small, except for replication 5. It showed tremendous N sequestration from the cover crop, measuring more than three times the sequestration rate, compared to the other four replications.

Why the difference?

“Manure applications aren’t always even,” Nelson noted. “In replication 5, it’s likely that there was a better stand of cover crops, and there was probably more manure applied there. These results indicate a large advantage for reducing N loss in this field.”

More studies planned

RCFI has an aggressive plan to expand this research to 15 sites across Iowa starting this fall. Funding for the project is expected to come from a USDA grant and other sources. Iowa State University (ISU) will collaborate with ISA on this project, along with Iowa Select Farms. “We like to engage farmers and industry partners in our research,” Nelson said.

This will build on previous research conducted by ISU.

“Their small-plot studies of cover crops and manure showed favorable results for sequestering N,” Nelson said. “We want to expand to more field trials, since farmers are interested in that data.”

The RCFI’s fall 2020 field trials will focus on areas of Iowa with concentrated livestock feeding operations. The goal? Deliver the very best farmer-led research combining agronomic, conservation and analytics tailored for soybean farmers.

“While manure is a regulated nutrient, and farmers are conscientious in managing manure properly, we see room for continuous improvement in this cropping system,” Nelson said. “The trials we’ve conducted so far with cover crops indicate that farmers can manage nutrients even more effectively with cover crops. Now we want to see if we can get a yield advantage with cover crops, too.”

Get involved

If you’re interested in partnering with the RCFI on these cover crop/manure management trials, contact Nelson at 515-334-1055, or Financial compensation is paid for the use of your plots.