The protein a cow gets comes from two origins: feed protein and microbial protein. Depending on its degradability, a portion of the rumen nutritional protein (RDP) is broken down by certain microorganisms that use nitrogen (and sometimes other elements), carbon chains, hydrogen, oxygen and energy to produce their own protein.

The digestive passage from the primary stomach to the intestine will carry this protein as a component of microbial cells to be digested in the intestine. The non-degradable protein in the rumen will pass intact to the intestine to be digested there as well. There is usually a small fraction (heat damaged protein) that cannot be digested and passes through the digestive system intact, ending up in the compost.