Protected, or undegradable, protein is a new important method of improving the metabolism of energy in ruminant feeds. Dietary protein and nonprotein nitrogen substance are degraded to a variable extent by ruminant microorqanisms. When soluble nitrogen sources are fed, high levels of ammonia are absorbed; since, not all of the ammonia produced in the rumen is incorporated into microbial protein. Thus, research workers have studied the effect of by-passing the rumen with certain critical amino acids and high quality proteins to avoid wasteful ammonia production by administering them posterior to the rumen or treating them in such a manner that they will not be attacked easily by rumen microbes.