Which crop is grown between the main crops to avoid soil erosion?

Cover crops are being used more frequently in between seasonal cash crops to help reduce erosion and improve soil tilth. The species planted can be any of a number of different grasses, broadleaves, and/or legumes. Some farmers are even using a "cocktail" blend of as many as 17 different species in their cover crop. Sometimes they are referred to as green manure crops, but these are usually only one species. They are almost always killed in some way before they can produce seed, since it is the roots and vegetative matter that the farmer wants.