Manure is a valuable resource for improving soil health and crop nutrition, but nutrient loss is an ongoing issue with manure application. Cover crops can help keep nutrients where they belong and also aid in taking up nutrients farmers are trying to keep in place.

Brian Dougherty, field agricultural engineer, Iowa State Extension, discussed new research focused on how cover crops can enhance manure application.

“We think about trying to keep nitrogen in the system,” said Dougherty, explaining how manure and cover crops work together. “There are several different pathways by which nitrogen is lost. Runoff is a small component but can occur if organic nitrogen attaches to soil particles. Leaching is the main concern, and it’s what we’re targeting with cover crops.”

Losing nitrogen through volatilization is another issue. Soil nitrate production from native soil organic matter occurs in early spring, but the crop requires nitrogen later in the growing season.

“We have periods where there is a higher risk of losing nitrogen from the system,” said Dougherty. “What we’re trying to do with cover crops is plug some of those leaks and fill in gaps when we otherwise wouldn’t have anything growing to take up nitrogen from the soil system.”

Cover crops can provide additional opportunities to get manure out to fields. “If you can establish a cover crop in fall, it might help with trafficability,” said Dougherty. “You could wait until spring and apply manure on fall cover crop fields – get out in the field when you otherwise wouldn’t be able to.”

Another strategy to get manure out earlier in the season is by adding a small grain to the crop rotation and following with a cover crop.

Manure can help with nitrogen availability. “There’s a lot of research to back up the fact that cover crops and manure both benefit soil,” said Dougherty. “They can increase organic matter, nutrient cycling, infiltration, water holding capacity and aggregate stability.” Cover crops also provide erosion control, weed control, opportunities for grazing and double cropping.

Grass and brassica cover crops have proven to be highly effective in reducing nitrate leaching. “If you can get [a cover crop] established, even if you only have a few inches of growth in spring, it still helps reduce that nitrate leaching,” said Dougherty. “Those roots may be a foot deep or deeper even with a couple inches of top growth.”

Dougherty referenced a study in which manure was injected after cover crop seeding. There was no tile drainage data for the farm, so researchers measured nitrate levels in the soil profile in spring. “In the cover crop system, there was about 78 pounds less nitrate in the soil profile in spring compared to the no cover treatment,” he said. “There was no difference in silage or grain yields.”

In another research project, a rye cover crop was seeded with a drill in autumn after harvest and manure application. Rye was seeded at 80 lbs./acre, and the cover crop was terminated in spring with glyphosate about two weeks prior to corn or soybean planting. The crop reduced cumulative nitrogen loss over three years by about 35%.

In a cover crop/injection study, when manure was injected on a rye cover crop, streaking was noticeable. “There was much more cover crop growth directly over manure injection bands,” said Dougherty. “Those areas were sampled separately.”

The cover crop was taking up nutrients, but it was difficult to determine when and how nutrients were released. Dougherty said it’s almost impossible to know precisely because multiple factors influence nutrient take up.

“Temperature and moisture are the big wild cards,” said Dougherty. “It’s different every year. Soil fertility and species mix are important, and tillage or no-till makes a difference.”

The biological activity of the soil is another aspect and is influenced by how long cover crops have been used. “Carbon-to-nitrogen ratio [C:N] is a big factor. The longer cover crops grow, the higher the C:N will be and slower the breakdown.”

If a farmer intends to plant green with corn, nitrogen should be applied at planting. “There was a lot of talk for several years about what was going on with yield drag with corn and cover crops,” he said. “There was speculation that maybe there’s disease pressure or the cover crop was inhibiting the corn seed. The more people are playing around with this, the more they’re finding it’s mostly a nitrogen issue. You need some nitrogen with the plant early in the season when the cover crop might be tying up nitrogen.”

An early-terminated cover crop breaks down much faster than a later terminated one that has more biomass. Lush, green cover crops have a lower C:N ratio and break down faster. However, a thicker, heavier cover crop may insulate the soil and speed up microbial activity.

When tillage is added to the system, the cover crop is chopped up and incorporated and it will break down faster. With no-till, the total nitrogen doesn’t change but release is slower.

TJ Kartes of Saddle Butte Ag explained cover crop seeding can be accomplished by aerial application, high clearance machines, a no-till drill, vertical tillage with a seeder or broadcast followed by vertical tillage, early interseeding corn or frost-seeded prior to spring manure application.

“For interseeding and frost seeding we’ve used annual ryegrass because annual rye is a cousin to corn and gets along with corn,” said Kartes. “Cereal rye tries to reproduce. After corn silage or an early harvested crop, cocktail mixes work well. They all take up nitrogen and phosphorus and prevent it from going downstream, which is the goal of keeping nutrients in place.”

Winter grains are a good starting point for using cover crops, and Kartes said every farmer should be able to use winter rye and winter triticale in a cover crop system. “They work with most herbicide programs,” he said. “They overwinter and scavenge nitrogen in spring and fall, they’re easy to terminate and have great weed suppression.”

Brassicas reduce soil compaction and help scavenge nitrogen and phosphorus but are more susceptible to herbicides. Legumes are a source of rhizobia and potentially provide extra feed and increase soil microbial activity, but herbicides can be an issue.

Kartes urged farmers to continually experiment with cover crop combinations. “If we do the same thing over and over, something is going to happen that isn’t going to work,” he said, adding that he uses mixes for autumn planting.

“I like to put a spring grain with oats,” he said. “I’ve seen a great benefit of 20 pounds of oats with 50 pounds of cereal rye and 50 pounds of triticale. In spring, we’ve seen a little bit better growth from winter grains. I think there’s a synergy effect.”

by Sally Colby