Soybeans are high in protein as well as alfalfa and most commercial food plot seeds. Any legume, soybeans alfalfa clover will be high in protein and deer usually love them. The advantage to some leafier products such as brassica or turnips which are often mixed with soybeans is that after the first freeze they release lots of sugar stored throughout the year and deer love it. They will have a decent amount of green forage after most things are gone.