More to the point, though, is the mistaken notion that we have to use food crops for fuel production. In test fields in Minnesota, Tilman and his colleagues have found that the best energy yields actually come from native prairie grasses, not corn or soy. And, said Tilman, "there's a surprising benefit from the mixture of species. Farmers know this from growing pastures. Nobody plants a pasture of a single species. They put out a variety of grasses, legumes...and so on. They do that because that gives them a higher yield."