Cover crops and manure are tools that help farmers keep soil in place and provide nutrients for row crops. University of Minnesota Extension Associate Professor Melissa Wilson has spent three growing seasons looking at the best ways to use these tools together. There have been two basic findings: the earlier the cover crop is planted, the more biomass it will produce, and with fall placement of manure, later is better than early.Her work, funded by the Minnesota Corn Research & Promotion Council, focused on timing of liquid manure placement, along with a variety of cover crops mixes. The combination of the two practices was examined in several rotations: sweet corn followed by field corn and soybeans followed by corn. As controls, her team also included plots where no cover crops were planted and where fertilizer was used instead of manure. The project utilized plots both on experiment station land and working farms.
Wilson said her team had more success with cover crops following sweet corn, because they were able to drill the cover crop early in August. Compared to drilling the seed in late summer, broadcasting between the rows of standing crops didn’t produce as much biomass. Wilson attributes this to less soil-to-seed contact.
“Drilling the seed is always a better scenario,” Wilson said. “When you compare drilling versus broadcast at the same time, drilling will always win. If you would have to wait until October to drill seed, then in that case, broadcast in late summer or early fall into the standing crop usually wins out. And we did see that trend in our studies.”Comparing the various cover crop seed mixes, using oats only, versus no cover crop, was a statistical dead heat in terms of yields the following year. On the other hand, planting a rye cover crop, or a rye mix, led to reductions in yield, Wilson said. The no cover crop plot saw an average yield of 241 bushels over two site years, versus 236 bushels per acre for oats. A rye-only cover crop saw an average yield of 222 bushels per acre, and the mixture of rye, oats and radish offered an average yield of 221 bushels per acre.Timing of the cover crop planting led to dramatic differences in cover crop performance.“The biomass of the cover crop is what drives weed suppression and other benefits,” Wilson said. “In our sweet corn study, where we were able to drill the cover crop seed early, we were getting anywhere from 1,000 to 1,400 pounds of biomass per acre in the spring. … When we were interseeding into soybean, or waiting until after harvest to drill, we were only seeing 100 to 400 pounds of biomass.”Additionally, timing of nutrient placement had a dramatic effect on cash crop performance.Following sweet corn, researchers were able to do an early swine manure application, in early-mid September. They compared that to a late fall application, when soil temperatures had cooled to below 50 degrees, which is the usual recommendation.Corn grow on plots with early-applied manure saw a yield reduction of 20 bushels per acre, Wilson reported. With late-applied manure, the results were more variable.“In some cases, it improved yield over the commercial fertilizer treatment in the spring by 30 bushels per acre — very impressive,” Wilson said. “But in the second year, it was actually about the same yield as the spring fertilizer. That was in 2021, which was a really dry year, so in that case, we don’t think the manure mineralized as much as it had in the previous year.”The research suggests that cover crops do not provide enough activity in the soil to prevent nitrogen loss when manure is applied early, Wilson said. It also suggests that having living root systems in the ground over the winter can help keep nutrients in place when they are applied in the late fall.
The on-farm plots demonstrated that cover crops and manure can work well together, and they also demonstrated that terminating the cover crop before planting the cash crop leads to better yields than planting into living cover crops.In both cases, the farmers used strip-till cultivation. One farmer tested fall placement of manure versus fall placement of fertilizer, and the other farmer only tested manure. Both farmers tested the timing of terminating the cover crop.Yields were similar regardless of the nutrient source, but they did decline slightly when the cover crop was allowed to stay on the field later into the spring.“From the photos, you could see that the corn was a lot bigger where the rye had been terminated before planting, and the corn was spindly where the rye had gotten really big,” Wilson said.Judging by the demand for Wilson’s presentation — she has given it to farm groups and academic meetings over a dozen times across the northern United States — there is a great interest and need for research into how to succeed with both manure and cover crops.
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