Winter annual cover crops are a natural fit for the short growing season in the Northern Plains. The two most winter-hardy species for our region are cereal rye (cool season grass) and winter camelina (cool season broadleaf).
There are many reasons to use winter annuals as a cover crop, including but not limited to: (1) erosion control when a cash crop is not growing or does not have a closed canopy, (2) weed suppression primarily through competition and residue cover, (3) diversity to the system to break disease cycles and improve soil conditions, (4) nutrient capture after harvest and (5) moisture management.
Establishing a cover crop either in-season by interseeding or post-harvest on a short season crop provides enough time for decent establishment prior to frost. The benefit of a winter annual is not only the fall growth but also the rapid growth in the spring. However, the rapid growth in the spring needs to be actively managed to avoid issues with over-drying of the seed bed and also achieving desired biomass of the cover crop prior to planting a cash crop. Knowing when to terminate a cover crop is key to successful inclusion in any system.
Here's what we are currently trying at North Dakota State University. Cereal rye and winter camelina can be interseeded in corn any time after five leaf stage (coinciding with side-dress). We typically interseed cereal rye at a 30-40 pound per acre rate and winter camelina at a 2-6 pound per acre rate. Winter camelina is not currently sold by most seed companies, so as we multiply seed in the industry, including winter camelina will become more of a feasible option.
In the meantime, avoid purchasing spring camelina (it's not the same thing as winter camelina). Radish could be used at a 2 pound per acre rate with cereal rye to get the benefits of a broadleaf and taproot in the fall, but not the spring since radish will not typically overwinter.
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If cereal rye is being flown on into a corn crop, bump up the rates to reflect seed getting caught up in the corn plants and not reaching the ground - in this case rates between 50-70 pounds per acre are often used while still using 2 pounds per acre radish to keep costs minimal. Remember, these are all ballpark seeding rates as a starting point. Seeding rates and timing of seeding on your farm should be determined based on comfort level and on-farm goals.
With selecting a winter annual cover crop, always keep in mind the next crop in rotation. At NDSU, we are primarily using winter annuals prior to soybean as a cash crop. Cereal rye should not be used ahead of wheat, and termination 10-14 days in advance of planting corn is required. Keep in mind these tips as you include cover crops in rotation.
For more information on cover crops and upcoming field days, visit the NDSU Soil Health webpage (ndsu.edu/soilhealth) or follow me on Twitter (@NDSUsoilhealth).
