Cover crop are plants that are not typically harvested as a crop but they serve many other purposes. Cover crops are planted after the cash/feed crop is harvested, typically in the fall. The type of cover crop depends on the planting date in the fall. Cereal rye is a popular choice after corn grain because it can be planted as late as early November most years. Another common choice is oats which needs to be planted by the end of September. Oats have a good quick growth in the fall but then die after some hard frosts.
The newest trend in cover crops is to plant more than one type of seed. This increases diversity and takes advantage of the unique properties of different plants. The mixture will depend on what soil characteristic the farmer is trying to improve. Legumes (peas, vetch, clover, sunhemp) will increase the amount of nitrogen in the soil and make it available for the spring crop. Other plants have a deep tap root (daikon or oilseed radish, sunflowers) that can loosen compacted soil and allow the water to soak into the ground and roots to grow. Some plants are very good at making nutrients and trace elements more available in the soil (buckwheat, sunflowers, radishes). A diverse mix of plants will also increase the diversity of the insects and microbes in the soil. Most of these mixtures will need to be planted by late summer which makes them available only after small grain, tobacco, or vegetable harvest. Mixtures should include at least one plant species that survives the winter.
General information for cover crops can be found on this Penn State webpage or at the Sustainable Agriculture Research and Education website Cover Crop Topic Room. Pre-mixed seed is available at many seed companies or you can also design your own mixture at Green Cover Seed.