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Manure Research Update: Injecting liquid manure into a cover crop

liquid manure cover crop Minnesota

By: Melissa Wilson, Extension manure management specialist, and Manuel Sabbagh, graduate student

Key Points:

  • Injecting cover crops with liquid manure can work but using low-disturbance injection is important.
  • When it comes to corn yield, manure application timing in the fall appears to be more critical than if a cover crop was planted or not.
  • When corn followed corn (sweet corn or silage), fall manure tended to increase yield compared to spring-applied fertilizer. This was especially the case when manure was applied when soil temperatures were at or below 50°F.
  • On the other hand, when corn followed soybean, manure and fertilizer resulted in similar yield.

What we did:

Our goal was two-fold. First, we wanted to test different cover crop planting times or techniques to get cover crops established more consistently. Second, we wanted to see if a cover crop can help retain the nutrients provided by the manure to the soil, even if the manure was injected a little earlier in the fall than we usually suggest. Typically, we recommend injecting manure when soil temperatures cool down to 50°F or lower.

We worked with three different cropping rotations:
  • Continuous silage corn in Morris, MN - A winter rye/annual ryegrass mix was either interseeded at the 4th leaf collar stage (V4), overseeded at the kernel dent stage (R5), or drilled after harvest in mid-September. Then liquid dairy manure was injected into each cover crop planting combination in early or late October - before and after soil temperatures had cooled to or below 50°F.
  • Soybean followed by corn in Morris and Waseca, MN - Winter rye was overseeded near the time of leaf drop in September or it was drilled after harvest. Since the soybeans weren’t harvested until mid-October, we only injected swine manure in late fall once the soil temperature was below 50°F.
  • Sweet corn followed by corn in Waseca, MN - Winter rye, oats, or a winter rye/oat/radish mix was drilled after sweet corn harvest in mid-August. Similar to the continuous corn rotation, liquid swine manure was injected in late September or late October.
For each system, we also had plots without manure (preplant-applied fertilizer was used for the following crop), without cover crops, or without either. We planned to apply 40 pounds of nitrogen fertilizer in the spring, so the manure was applied to meet the remaining nitrogen needs of the crop. Fertilizer in the non-manured plots was applied at a similar nitrogen rate as the manure. Corn was planted in the following spring and we collected yield data in fall 2020 to see how it was affected by our treatments.

What did we find?

The first year of this two-year study has been completed. We found that injecting manure with a low-disturbance system is key. The percent ground cover was reduced after application but the plots that had winter rye recovered nicely by spring. In fact, they had more cover crop biomass than the non-manured plots! We also found that the earlier seeded cover crops tended to have more biomass in the fall than cover crops drilled after silage corn or soybean.

In the continuous silage corn rotation, manure application increased yield compared to spring fertilizer, regardless of whether it was applied early or late. On the other hand, in the sweet corn-corn rotation, yield was highest with the late manure application followed by spring fertilizer, while early manure significantly reduced yield. It should be noted that “early manure” following sweet corn was applied about two weeks before “early manure” following silage corn due to wet weather conditions. That two weeks of time likely made a difference in soil temperature. In the soybean-corn rotation, there were no yield differences between manure and fertilizer plots.

Cover crops only affected corn yield in the sweet-corn rotation. The rye or rye/oat/radish mix reduced yield by about 15 bushels per acre on average. The oat cover crop resulted in similar yield to the plots with no cover crops. Cover crops in the other rotations did not affect corn yield.

What’s next?

We still have soil and plant samples from the 2020 growing season to analyze for nutrient content. This will help us understand the nutrient availability from the manure versus the fertilizer and how it was impacted by cover crops. This past fall we also set up a new set of research plots to repeat the experiment. This year’s weather is turning out quite different from the 2019-2020 winter, so it should be interesting to see what the next cropping season brings us!


This work is supported by the Natural Resources Conservation Service, U.S. Department of Agriculture, under grant number 2020-68008-31410, and the Minnesota Corn and Research Promotion Council.

Learn more about this study by viewing Melissa's visual, interactive StoryMap on this study at:


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