Cover crop has been the buzz word in conservation in recent years, especially due to promotion of the Iowa Nutrient Reduction Strategy. In Iowa, cover crops are planted in late summer or early fall to provide a living cover after the cash crop has been harvested. Depending on which cover crop species is planted, they have the potential to reduce soil erosion, limit nitrogen leaching, suppress weeds, increase soil organic matter, reduce compaction, and improve overall soil quality. Many cover crops can have the added benefit of providing forage for livestock.
When choosing a cover crop species to plant, there are many factors to consider.
Goals – What do you want to achieve by planting a cover crop? Reduce erosion? Scavenge nitrogen? Provide forage for livestock? Your goals will help determine which cover crop or mix to choose. Click here to open a chart showing performance rating for different cover crop species from the SARE book Managing Cover Crops Profitably, 3rd edition.
Growing season and timing – In Northeast Iowa, we have a fairly short window for planting cover crops, depending on seeding method. Cover crops can be divided into winter hardy species and winter kill species. Winter hardy species survive the winter and will need to be terminated in the spring. Winter kill species are sensitive to cold and usually die naturally without any additional termination needed. In order to get much benefit from winter kill species, they need to be planted early enough to have a sufficient amount of growth before winter. They can be planted on silage ground or through aerial seeding (using a helicopter or plane) or high-boy seeding into a standing crop. In most years, winter kill species do not get enough growth to provide much benefit in NE Iowa. According to NRCS Tech. Note 38, non-winter hardy cover crops need to be seeded by September 9. Winter hardy cover crops need to be seeded by October 21.
Seeding rate/method – The seeding rate will be affected by the seeding method and the goals for the cover crop. The rate for broadcast seeding cover crops is least 1.2 x the rate for drilled cover crops. For example, cereal rye should be drilled at a minimum rate of 55 lbs./acre (approximately 1 bushel) of PLS (pure live seed). The broadcast rate should be at least 66 lbs./acre PLS. Many sources suggest broadcasting at 1.5 x the rate of drilled cover crops. If you plan to graze or chop your cover crop for forage, you will likely want to double your planting rate. NRCS Technical Note 38 (see above) also has some suggested seeding rates and seeding mixes.
Herbicide carryover – When planting a cover crop, it is important to evaluate the potential for herbicide persistence. Herbicide carryover could prevent successful establishment of the cover crop. It is important to follow label restrictions, especially if you plan to use the cover crop as a forage source. The following articles have information about different herbicides and potential issues with cover crops. Make sure to read your herbicide label before planting a cover crop.
Herbicide Use May Restrict Grazing Options for Cover Crops – Iowa State University Extension
Herbicide Persistence and Rotation to Cover Crops – Penn State University
Herbicide Rotation Restrictions in Forage and Cover Cropping Systems – University of Wisconsin-Extension
Termination – If you plant a winter-hardy cover crop, you’ll need to terminate it in the spring. Termination should be done as late as possible to maximize plant growth while allowing time for sufficient kill to minimize risk of impacting the yield of the following cash crop. Termination methods include crimpers, mowing, tillage, and herbicides compatible with the following crop. Cover crops should be killed 1-2 weeks before planting corn and within a few days (before or after) planting soybeans. You may want to use a starter fertilizer, especially for corn, to help microbes decompose the organic matter and provide nutrients before the cover crop nutrients can be released into the soil for plant uptake.