Plenty of animals eat grains, but grains aren't available year-round. Grain, and seeds in general, are more nutrient-dense than the stalks and leaves of the pla

Plenty of animals eat grains, but grains aren't available year-round. Grain, and seeds in general, are more nutrient-dense than the stalks and leaves of the plant they come from, but are only available for a short period. Animals which subsist on them must either be highly mobile, to reach many different types of plants which flower and produce seed at different times of the year, or they must be able to store the seeds until they become available again. A cow eats the grain along with the rest of the grass, but if it tried to live on seeds alone it would be a pretty thin cow.