If you are considering planting a cover crop after grain harvest, consider whether the planting might also serve as fall forage for livestock. Cover crops planted after harvesting a major cash crop can serve multiple benefits. Not only can they help prevent soil erosion, provide organic matter, and scavenge nitrogen, but an additional benefit can come from using cover crops as forage for livestock grazing. Nutrient content of these forages is generally quite high, meeting or exceeding the nutritional needs of dry or lactating cows and growing calves. With good ground moisture conditions, forage production can be high enough to support a significant amount of grazing.
A variety of species and mixes can be used for cover crops based on multiple goals. For example, forage turnips and radishes (brassicas) can produce a large amount of biomass for grazing and also scavenge nitrogen and help prevent soil compaction. Comprehensive information about a wide variety of crops that may be used as cover crops in the northern plains is provided on the USDA-ARS Northern Great Plains Research Laboratory website. A guide to cover crop selection for the Midwest has been developed by the Midwest Cover Crop Council. This latter guide was developed for areas east of South Dakota, but may also be useful in the eastern part of the state.
Both the tops and bulbs of brassicas are nutritious and will be eaten by livestock, although their use may be affected by weather conditions. Brassicas often contain 15 – 25% crude protein and 75 – 85% TDN. The tops are highest in protein while the bulbs are highly digestible. Brassica forages can be very high in moisture and low in fiber so that it can be difficult for livestock to consume enough dry matter. When grazing radishes or turnips, additional forages may be needed to be available either in the forage mix itself, or as dry hay, to help livestock consume enough nutrients. Usually no more than about two-thirds of the diet should be brassicas. Adding grass-type forages like millets, sorghum-sudan, oats and annual rye to mixes can help provide dry matter for grazing. Legumes such as field peas and lentils will both fix nitrogen and provide high quality forage for grazing.
A survey of more than 700 producers growing cover crops in the upper Midwest identified establishment as the greatest challenge to cover crop use. Soil moisture is critical to germination and initial growth of cover crops following grain crop harvest. Shading provided by the crop canopy minimizes soil surface evaporation. Canopy removal at harvest accelerates moisture loss. Planning and preparation that facilitates seeding immediately following harvest will enhance successful cover crop establishment.