Melissa Wilson

ALEXANDRIA, Minn. – Cover crops can serve more than one purpose.

They can reduce soil erosion and build organic matter. Cover crops can also store nitrogen found in manure to keep that easily-carried-off nutrient in place. This can help prevent runoff, leaching or denitrification.

At the recent Minnesota State Cattlemen’s Association annual meeting, Melissa Wilson, University of Minnesota Extension soil scientist, discussed manure application with or without cover crops.

Just a refresher – there are three “pools” of nitrogen in the soil – organic nitrogen that is bound to carbon, ammonia nitrogen, and nitrate nitrogen. The plant takes up ammonia nitrogen and nitrate nitrogen. The organic nitrogen, as it is broken down, is taken up by plants over two or three years.

Manure provides ammonium – the fast release nitrogen, and organic nitrogen – the slow release type. In addition, manure contains organic matter, as well as other macro and micro nutrients, to help the soil and plants.

“That is why we love manure so much, it’s basically recycling nutrients. But with just about anything – too much of a good thing can be a bad thing,” said Wilson. “You can release too many nutrients, especially nitrogen and phosphorus into our fields and get into our waterways and cause havoc as well.”

Wilson pointed out that the northern areas have fewer growing degree units than areas to the south. Minnesota could have 1,500-2,400 GDUs to Iowa’s 3,000-3,600 GDUs. The challenge in the north is when cover crops are seeded at harvesttime, manure is applied once temperatures get down to 50 degrees. That doesn’t leave much time for cover crops to do their work.

The goal of Wilson’s work is coming up with best management practices for getting manure applied to a growing, living cover crop, and keeping nutrients in place.

A University of Minnesota Liquid Manure and Cover Crops study was conducted in 2016-17 that dealt with applying liquid manure on growing cover crops once temperatures reached 50 degrees.

Winter rye seed was drilled in strips after the harvest of silage or soybeans (from Sept. 25 to Oct. 9 on average). Using the drill allowed for good seed-to-soil contact and better germination. Strips of soil were also left unseeded for comparison.

Four or five weeks later (around Oct. 27 to Nov. 13 on average), manure was injected into the entire field. When deep shanks were used to apply the manure, the cover crop was ripped up more than if knife or sweep injections were used.

“Sweep injections put the manure below the surface without tilling the surface greatly,” she said.

The cover crop was killed in the spring using glyphosate and the biomass was incorporated into the soil.

The researchers felt that incorporating the biomass into the soil helped the rye decay better and release nutrients.

“Some people have seen a tie up in nitrogen if they leave the rye on the surface,” she said.

The amount of nitrogen that was taken up in the cover crop and the amount of soil nitrate in the top 2 feet were both measured. Corn was planted in the spring and harvested in the fall.

“The later the cover crop was planted, the less biomass in the spring,” she said. “What is interesting is there is about 330 pounds/acre of biomass not produced for every week later you planted the cover crop.”

The researchers found that in Southern Minnesota, about 63 pounds of nitrogen were taken up in the cover crops ahead of spring planting. In Central Minnesota, about 32 pounds were taken up.

While the cover crop didn’t hold as much nitrogen in Central Minnesota as in Southern Minnesota, since it didn’t grow as much, it was still important. When a cover crop wasn’t used, soil nitrate levels remained higher than in areas where cover crops were used.

“There’s 123 pounds of nitrate left (in the top 2 feet) with cover crops, vs. where there’s no cover crop is 202 pounds,” she said. “That is on-average a difference of 79 pounds per acre. That’s something that is being prevented from potentially leaching out in those wet springs.”

As far as yields – for corn grain and corn silage, average yields were 197.5 bushels per acre with cover crops, vs. 199.4 bushels per acre without a cover crop.

Wilson said there are other variables affecting yields too, so sometimes the cover crop fields will have higher yields, and sometimes the bare soil will have higher yields. There was no real statistical difference in yields in this study.

“One thing I like to point out is the people who are trying it the first year, if you have a bad year, it might have just been random luck,” she said. “Don’t give up after one bad year.”

Wilson had several take home points for producers who want to try cover crops. These included planting the cover crop as soon as possible to maximize biomass. Interseeding cover crops may help, but if you wait to drill after harvest, you can still get some growth. Drilling the cover crop vs. broadcast should result in better germination.

The manure injection equipment choice matters, with knives and sweeps tending to minimize surface disturbance.

On average, corn yields were not impacted following cover crops and manure – although in any given year there could be a yield increase or decrease.

Most importantly, winter rye conserves manure nitrogen. When more biomass was produced, more nitrogen was conserved.

Wilson hopes to try interseeding cover crops into standing crops. Another possible study involves metering cover crop seed into the manure dragline hose (slurry seeding).

“I’d love to know how different types of equipment impact cover crops, and I’d like to do more with solid manure application,” she said. “I would suspect with a solid manure application, you’re not going to be able to put 10 tons per acre out there on top of a cover crop. You might have to use a rake, but again, finding out what the best management practices are might be good.”