Cow ‒ Stomach

The process by which ruminants regurgitate previously consumed feed and chew it again is known as rumination. The ruminant stomach is divided into four compartments: the rumen, reticulum, omasum, and abomasum. The ability of ruminants to digest plant fibers relies on fermentation, a process that occurs in the first three compartments of their stomach. The rumen and reticulum, the first two compartments, together hold approximately 150 liters and are among the most densely populated microbial environments on earth, with up to 10 billion bacteria per milliliter. Here, volatile fatty acids produced by fermentation are absorbed. The third compartment, the omasum, absorbs water and serves as a filter, allowing only feed particles smaller than 1 mm to pass to the final compartment, the abomasum. The abomasum is the "true" stomach, where the animal's own enzymes and acids chemically break down the ingested feed.