Enzymes are secreted that digest and break down feed. In a new-born calf, the rumen and reticulum are not developed. This is why no forage is fed to young calves. During the nursing process, milk bypasses the rumen. As a calf consumes calf starter and forage, the rumen develops and bacterial fermentation starts.
Many complicated reactions occur in the rumen.
The rumen microbes convert feed to products the animal can use directly. The microbes also grow and multiply, and the ruminant animal digests them. Proteins are partially broken down to ammonia. The rumen microbes use the ammonia to live and grow. Some protein is not broken down in the rumen, but escapes it and is digested in the abomasum and intestine.