The earliest ruminant feed is some plant raw materials rich in microorganisms. Ruminants digest food by ruminating, and the process of ruminating needs the help of microorganisms. Therefore, the earliest ruminant feed is some plant materials rich in microorganisms, such as grass and leaves. These plant materials are decomposed by microorganisms in the stomach of ruminants, and then deeply digested during rumination, thus providing nutrients needed by ruminants.