WORTHINGTON — As corn and soybean fields reach maturity and the crop canopy starts to open, a prime window also opens for seeding a cover crop.

A cover crop can be interseeded in the fall into standing corn and soybeans via the air or with ground equipment such as a high-clearance seeder. Cover crops can also be seeded after corn or soybean harvest, which allows use of a drill for more accurate seed placement and seed-to-soil contact.

Waiting to seed a cover crop until corn or soybeans are harvested for grain, however, may leave little, if any, time for the cover crop to establish and grow in the fall. Regardless, adequate moisture and seed-to-soil contact will aid in successful establishment.

How important is seeding timing in the fall?

Cereal rye is the most commonly planted cover crop for a number of reasons. It is relatively inexpensive, easy to establish, and seed is typically widely available. A cereal rye cover crop can also provide soil health and water quality benefits by reducing erosion potential, increasing soil organic matter, improving soil structure, and by scavenging nitrogen that might otherwise leach through the soil profile.

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Cereal rye can also tolerate many of the herbicides used in a traditional corn/soybean system, even though it can be easily terminated with timely application of an appropriate herbicide. When allowed to overwinter and produce adequate biomass in the spring, cereal rye may also help suppress early-emerging weeds.

Although cereal rye can be seeded from late July into November in Minnesota, research conducted at the University of Minnesota Southwest Research and Outreach Center in Lamberton shows that fall seeding date can impact spring biomass production. Cover crop biomass production is an important indicator of the potential for the cover crop to provide soil health and water quality benefits, as well as weed suppression.

In this trial, cereal rye was seeded at 60 pounds per acre with a Penn State Interseeder at four seeding dates (Sept. 6, 17 and 26, and Oct. 8). In the spring, plots were terminated at three different dates: May 4, 12 (at planting) and 21 with an herbicide (Cornerstone Plus at 1 quart per acre).

Cover crop biomass production was measured prior to termination, and statistical analysis was conducted on the results. Preliminary results show that within a seeding date, cereal rye biomass increased as termination was delayed (Figure 1). Biomass production was generally similar across the September seeding dates at each termination timing, although the Sept. 26 date produced significantly less biomass than the earlier seeding dates at the first termination timing. In comparison, cereal rye seeded Oct. 8 never really took off in the spring, and biomass production was similar across termination dates.

When seeded in October, biomass production, regardless of termination timing, was similar to or less than what was produced by cereal rye seeded in September and terminated in early May.

These preliminary results show that spring growth of cereal rye was optimized when the cover crop was seeded by mid-September. Care must be taken in interpreting this data, as this was the first year of a trial conducted at one location. The initial results, however, highlight the importance of seeding cereal rye as soon as possible in the fall to glean the most benefit from the cover crop in the spring. This trial is being repeated for the 2020-21 season.

The Minnesota Cover Crop Recipe sites, Post Corn, Going to Soybean: Use Cereal Rye (https://bit.ly/3n9CjoX) and Post Soybean, Going to Corn: Use Oats (https://bit.ly/3cHkZTp) describe easy entry points for cover crops in a corn/soybean rotation. For more information on which cover crops are good candidates for seeding in the fall, check out the Midwest Cover Crop Council Cover Crop Decision Tool (mccc.msu.edu/covercroptool). The tool also includes information and guidance on suggested seeding rates, planting depth, etc.

All of these resources are available on the Midwest Cover Crop Council website (mccc.msu.edu). The U of MN Extension Cover Crop website (z.umn.edu/cover-crops) is also an excellent resource for cover crop research and information.