Cover-crop use has been increasing — at least in the past few years.

The use has increased due to properties such as mitigating soil erosion and improving soil health as well as how those cover crops affect nitrogen availability and fate in corn-production systems. That has been a research focus of Shalamar Armstrong, an assistant professor of agronomy at Purdue University. Armstrong recently shared his research findings regarding cover crops and nitrogen with farmers at the UW Discovery Farms Conference in Wisconsin Dells, Wisconsin.

The Mississippi River-Gulf of Mexico Watershed Nutrient Task Force – also known as the Hypoxia Task Force – created an action plan about 10 years ago. The task force’s primary mission is to reduce the amount of nutrients contributing to hypoxia in the Gulf of Mexico. Hypoxia is a condition in which oxygen supply is inadequate. Referred to as the “dead zone” in the gulf, hypoxia kills fish and other marine life.

Among the task force’s goals has been to reduce – by 2025 – nitrogen runoff by 15 percent to help reduce the level of hypoxia. Another goal has been to quantify nutrient-load reductions.

Armstrong’s research may provide a potential solution to the hypoxia problem. Purdue University is partnered with the task force to help support state-level strategies and actions to curb water pollution.

Before implementing any solutions, nutrient-management strategies must be addressed in order to help reduce nitrogen-runoff levels. In corn production that means addressing both spring and fall applications of nitrogen, including manure.

Cover crops have been selected as one of most effective adaptive-management practices, Armstrong said.

Once cover crops become established on a given field, farmers begin to see nitrogen being conserved. Cover crops interact with the inorganic-nitrogen pool, which can be comprised of residual nitrogen from a previous corn crop, nitrogen from manure being released and becoming available in solution, and nitrogen being mineralized in organic-matter. Cover crops are able to absorb and interact with that nitrogen, which reduces leaching and denitrification.

“Farmers are happy to see that cover crops take up nitrogen, but they also want to know when it’s released,” Armstrong said.

The Purdue University agronomy professor shared findings from field-plot tests conducted between 2014 and 2017. The tests involved strip-tilled corn and no-till soybeans.

He evaluated two nutrient-loss strategies. The first involved moving nitrogen application from fall to spring, which accounted for a 10 percent reduction in nitrogen loss.

The second strategy involved shifting nitrogen application from fall to spring and also using cover crops to further reduce nitrogen loss by as much as 30 percent.

Armstrong implemented five treatments to observe the effect of nitrogen-application timing and cover crops.

  • control-no fertilizer and no cover crop
  • spring split-application of nitrogen – 20 percent fall-applied diammonium phosphate and 80 percent anhydrous ammonium
  • spring split-application of nitrogen – 20 percent fall-applied diammonium phosphate, 80 percent anhydrous ammonium and cover crops
  • fall split-application of nitrogen – 70 percent fall-applied diammonium phosphate and anhydrous ammonium and 30 percent sidedress – anhydrous ammonium
  • fall split-application of nitrogen – 70 percent fall –applied diammonium phosphate and anhydrous ammonium and 30 percent sidedress – anhydrous ammonium and cover crops

The fall anhydrous ammonia was strip-tilled into a stand of 92 percent cereal rye and 8 percent Daikon radish. Total nitrogen rate for all of the plots was 200 pounds per acre.

In the tests, Armstrong found that cover crops interacted with 30 percent of the equivalent mass of nitrogen-fertilizer applied. The cover crops reduced nitrogen loading – via tile drainage – by 42 percent to 50 percent, despite timing of nitrogen application.

There was no difference in soybean yield for the cereal rye-radish mix compared to the control treatment. But the cereal rye and radish mix significantly reduced corn yield compared to the control treatment.

To address the yield lag in corn, Armstrong designed another study testing what he calls “the double rotation,” a rotation of cover crops and cash crops together.

The idea is to plant lesser carbon-to-nitrogen-ratio cover crops before corn and greater carbon-to-nitrogen-ratio cover crops before soybeans. The study results indicated that a combination of radish, rape, oats, hairy vetch and a small amount of cereal rye improved corn yield compared to cereal rye alone in both continuous corn and in soybean-corn rotations. Contact sarmstro@purdue.edu for more information.

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Lynn Grooms writes about the diversity of agriculture, including the industry’s newest ideas, research and technologies as a staff reporter for Agri-View based in Wisconsin. Email lgrooms@madison.com to contact her.