Our lamb is raised on 100% grass... actually, take that back, there was a lot of alfalfa and other plants that they ate too, but 100% of they're feed comes from our pasture, which in the farming world is called "grass-fed" for some reason :)
Fun fact, sheep will eat different plants depending on they're nutritional needs, so the bigger the variety of plants they can choose from the healthier (and tastier) they will be!