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Varying cover crop mixture, seeding rates may improve outcomes

cool-season cover crops with a shovel in the middle to show how tall they grow
This largely cool-season cover crop mixture planted near Vermillion during a wet year absorbed moisture and cycle nutrients while preventing soil erosion. A team of South Dakota State University precision agriculture faculty and SDSU Extension specialists, led by assistant professor Ali Mirzakhani Nafchi, will demonstrate how varying the cover crop mixture and seeding rate within a field can improve outcomes. The three-year precision cover cropping project is funded through a U.S. Department of Agriculture Natural Resources Conservation Service Conservation Innovation grant. (Photo courtesy of Sara Bauder)

A precise approach to selecting and planting cover crops that considers variability within a field will produce better results for farmers, according to South Dakota State University assistant professor Ali Mirzakhani Nafchi. He works on precision agriculture research through the agronomy, horticulture and plant science and agricultural and biosystems engineering departments.

 “Currently, we plant a cover crop seed mixture at a uniform, flat rate across the field, but when we have tremendous variability within a field, this is not logical,” said Nafchi, who is also the SDSU Extension precision agriculture specialist. Varying cover crop mixtures and seeding rates based on the unique soil conditions and topography within a field will help maximize the economic and environmental benefits of this conservation management practice.

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Assistant professor Ali Mirzakhani Nafchi leads a team of 10 faculty members and SDSU Extension specialists demonstrating how a precision cover cropping system can improve outcomes.

Nafchi leads a team of 10 faculty members and SDSU Extension specialists demonstrating how a precision cover cropping system can improve outcomes. The team has expertise in agronomy, soil science, entomology, crop diseases, water management and agricultural economics.

The research is funded by a three-year Conservation Innovation Grant from the U.S. Department of Agriculture’s Natural Resources Conservation Service with in-kind support from the university, SDSU Extension and participating producers. Nafchi, who previously worked with cover crops in vegetables and row crops, such as corn and soybeans, at Cornell Cooperative Extension in Rochester, New York, reported 14 producers have signed up for the project.

The researchers will divide each cover crop field into zones based on the historical yield data including the producers’ knowledge about the field, soil characteristics and conditions, as well as disease and pest challenges. Based on the data, they will then determine the cover crop seed mixtures and the seeding rates for each zone.

Extension and NRCS personnel will use the results to help those already planting cover crops improve their results through a precision cover cropping system and to encourage more farmers to use cover crops to increase sustainability and profitability.

Considering soil characteristics

Cover crops improve soil health, which then contributes to better yields, Nafchi explained. Two key factors in increasing yields are water infiltration rate—how quickly the soil absorbs water—and water-holding capacity, which helps keep water and nutrients from running into adjacent lakes and streams. These factors are directly related to the organic matter in the soil.

Mapping the field will give the researchers data on the variability within the field. For example, measuring the soil’s ability to conduct electricity, known as electrical conductivity, helps quantify soil structure, Nafchi said. “When you have clay soil, the particles are closer together, so the (electrical conductivity) number will be higher. Sandy soil have air spaces, or voids, among the particles and less electrolytes, meaning nutrients wash away more easily, so electrical conductivity will be lower.”

To be able to identify drainage problems, the researchers will take shallow and deep electrical conductivity readings.

Making precise choices

Daikon radish in the ground among crop residue
Incorporating tubers, such as these radishes, into the cover crop mixture can help break up compacted soil and add organic matter that will improve the soil’s water-holding capacity.  

Certain cover crops can improve the soil porosity and the void ratio, which are associated with water-holding capacity, Nafchi explained. Incorporating tubers, such as tillage radishes, into the cover crop mixture can help break up compacted soil and add organic matter that will also improve water-holding capacity.

The slope of a field can also be a factor in adjusting the cover crop planting to get a uniform stand, Nafchi continued. “On a north-facing slope, plants germinate later because of the cooler soil temperatures. If we compensate for the lag by choosing the right cover crop and a higher seeding rate, we will have uniform germination.”

Specific cover crops can also help reduce pressure from soil-borne diseases and pests. For instance, cereal rye and tillage radishes can help reduce soybean cyst nematode populations.

Combining these factors, along with producer input, will help the research team determine the best cover crop mixture and optimum seeding rate for each field zone, Nafchi said, emphasizing that “precision ag means using the right input in the right amount at the right place at the right time and in the right manner to help farmers increase profitability and sustainability.”