The rumen is the first compartment of a cow's four-part stomach.
A cow swallows forage into the rumen, a large fermentation chamber where microbes break down tough plant material such as cellulose. The cow later regurgitates partially digested material as cud, chews it again, and swallows it for further processing. This process is called rumination.
The four compartments are the rumen, reticulum, omasum, and abomasum. The rumen is the largest and is often described as the main fermentation vat. The reticulum works closely with it, while the omasum absorbs water and some nutrients. The abomasum is the glandular compartment most similar to the simple stomach of non-ruminant mammals.
Cattle are ruminants, as are sheep, goats, and many deer. The rumen does not independently digest grass with its own enzymes; its microbial community performs the crucial fermentation. That distinction explains why changes in feed can affect both digestion and animal health.