How many stomachs does a cow have?

Cows do not have four stomachs, as is sometimes thought. They have one stomach divided into four compartments—the rumen, reticulum, omasum, and abomasum. This specialized structure is typical of ruminants, a group of mammals that includes cattle, sheep, and goats, and it allows them to digest fibrous plant material that many other animals cannot.

The largest chamber, the rumen, serves as a fermentation vat where microbes break down cellulose in grasses and other forage. This microbial activity produces nutrients, including volatile fatty acids, that supply much of the cow’s energy. Cows aid this process by regurgitating and rechewing partly digested food, a behavior commonly known as chewing the cud.

Next to the rumen is the reticulum, which works closely with the rumen to mix food. It also helps catch dense or foreign material that may have been swallowed accidentally. Together, the rumen and reticulum are also referred to as the reticulorumen. The third chamber, the omasum, has many folds of tissue, resembling the pages of a book. It absorbs water and additional nutrients, concentrating the food before it passes to the final compartment.

The abomasum, often called the “true stomach,” functions much like the stomach in humans and other nonruminant animals. It produces acid and enzymes that digest proteins and prepare food for absorption in the small intestine. This four-part system makes cows highly efficient at turning grass and other fibrous plants into energy and protein, enabling them to live on diets that would provide little nourishment to humans or other animals with a single-compartment stomach.