If a Chayah is permitted to eat, why does the Torah add that a Beheimah may eat as well? You are obligated to feed your Behemos!
Rashi (Ta'anis 6a): The Torah is comparing Beheimah to Chayah, in that, as long as the Chayah is able to find any particular species in the field, one is permitted to feed one's Behemah in the house. 1
Rashi (in Shabbos, 68a): The Pasuk also teaches us that the laws of Shemitah extend to animal food.
Ramban: But once it is no longer available in the field, one is obligated to clear it out of the house and declare it Hefker, a procedure that the Chachamim call 'Bi'ur Shevi'is' (See Ramban, who elaborates at length).