Though it does have a relatively high amount of protein for plant material, most of alfalfa hay (and all hay, for that matter) is still cellulose, which people

Though it does have a relatively high amount of protein for plant material, most of alfalfa hay (and all hay, for that matter) is still cellulose, which people cannot digest and cows can (thanks to their symbiotic gut bacteria). Feeding grain to cattle is pretty inefficient, because people can eat grain directly, but hay not so much.