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Manure, Cover Crops Fit Well Together

Soil Health Summit Farmers Learn that Cover Crops Help Take Up Excess Nitrogen

In Melissa Wilson’s home state of Minnesota, cover crops aren’t an easy practice to adopt. The growing season is short – just 1,500 to 2,700 growing degree days, compared to 3,000 to 3,900 GDDs in Iowa.

“It’s not all rainbows and unicorns,” says Wilson, assistant professor in the Department of water, soil and climate at the University of Minnesota. “Cover crops can sometimes be difficult.”

And that is one of the challenges with using animal manure in conjunction with cover crops.

Wilson, who focuses on manure management and nutrient cycling at the University, explains that manure feeds the soil, but ammonium and nitrates in manure can run off or leach. Fall application of manure tends to drive nutrient loss. She was one of the featured speakers at the Soil Health Summit January 15-16 in St. Louis.

In theory, the combination of manure and cover crops can serve double-duty. Manure is a great nutrient, while cover crops can take up nutrients and keep them from leaching or being lost to mineralization.

Research at Iowa State University shows that, when 100 pounds of swine manure were applied into a rye cover crop, nearly 60 pounds of nitrogen were retained in the cover crop, as opposed to manure without a cover crop. Manure application into standing cover crops can hold onto the nitrogen and keep it from leaching.

“Where there was manure but no cover crop, there’s a lot of nitrate in the soil. The cover crop took a lot of nitrate up,” Wilson says.

Research by Wilson and colleagues in Minnesota shows that cereal rye took up an average of 45 pounds of nitrogen in on-farm tests where manure is applied. “Anytime you take up 45 pounds of N, you’re not putting it into water. I think that’s a win,” she adds.

That’s important to growers who apply manure. And the best news is that corn yields following these manure applications in cover crops are not affected.

“Some years, yield is increased; some it is decreased. In any given year, you can see a yield hit or a yield bump,” she says.

Growers typically want to get cover crops planted as quickly as possible to get as much biomass as possible. Therefore, producers need to apply manure into standing cover crops. One concern with doing that is with damaging the cover crop, Wilson explains.

On-farm research in a variety of crop conditions in Minnesota indicates that knife or sweep-type manure applicators do a good job of applying manure without damaging the cover crop.

The concept of slurry seeding – or putting seed in with liquid manure and applying the mix, is surprisingly effective. Tank agitation is necessary, as heavier seeds tend to sink and lighter seeds float.

“Our findings show that manure decreases germination. Manure has high ammonia values and that’s what burns plants,” she says. “It reduces germination when you seed.”

However, daikon radish seed applied with the manure actually has more than twice the biomass – 3.1 tons per acre, vs. 1.2 tons per acre in fields with drilled radish seed. “What doesn’t kill it makes it stronger,” Wilson says. “What lives, does really well.”

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