Sections of the different stomach compartments of a ruminant
Across from top left: Rumen, Reticulum, Omasum, Abomasum
Ruminants are a class of animal that digest their food through fermentative digestion. This allows them to digest feeds which are indigestible to monogastrics - like us.
Microbes including bacteria, fungi, protozoa, and even viruses work together to break down the structural components of ingested plant material. These microbes live in the rumen and reticulum of the animal. These compartments are known collectively as the ruminoreticulum and are the first compartment into which food enters.
The major by-products of microbial digestion are Volatile Fatty Acids (VFAs) which are free fatty acids which pass through the wall of the rumen and are used for energy by the animal.
Protein in the feed is used by the microbes for their own reproduction and growth. The animal gets protein from its feed when the microbes pass through the next section of the stomach, the omasum (where water is absorbed), and on into the abomasum. The abomasum secretes acid to kill and digest the microbes which have passed from the ruminoreticulum.