Alfalfa, a plant of the pea family grown primarily for forage, especially as hay. It is one of the most useful and widely grown hay crops in the world.
Because of its high protein content, alfalfa is used as a food for almost all farm animals as hay, as silage, or as a temporary pasture crop. Because of its nitrogen-fixing properties, it is used in crop rotation to improve soil for other crops. When planted in combination with grasses, it helps prevent soil erosion
Source: Alfalfa Seed Commission - http://www.alfalfaseedab.com/