Cow ‒ Stomach

The process by which ruminants regurgitate previously consumed feed and masticate it a second time is called rumination. The ruminant stomach has four compartments (rumen, reticulum, omasum and abomasum). What allows ruminants to digest plant fibers is the process called fermentation that takes place in the first three compartments of their stomach. The rumen and reticulum are first two. With the capacity around 150 liters they are also the largest and some of the densest microbial habitats in the world (up to 10 billion bacteria per 1 ml). Volatile fatty acids formed by fermentation are absorbed there. The third compartment omasum absorbs water and serves as a filter since it allows particles of feed not larger than 1 mm to pass to the last compartment called abomasum. The abomasum is the true stomach where the animal’s own enzymes and acids chemically break down the ingested feed.