Turnips are a cool season annual that are extremely high in protein and highly digestible to deer.  Unline cereal grains and other forage crops, the fiber content of turnips does not increase with age, keeping it extremely digestible.  Deer seldom eat the tops until they have become maure or experienced a good frost, which often means that turnip leaves are still around in winter and fill a potential nutrition gap for whitetails in that season.