The four parts of a cow's stomach are the rumen, reticulum, omasum and abomasum. The first three chambers break down the food into pieces small enough for the fourth chamber to digest. These four chambers allow cattle to digest tough foods that many other animals can't process.
The rumen is the largest chamber and begins the digestion process with enzymes. During this process, some food is regurgitated to be re-chewed as cud. The reticulum works with the rumen to further soften grasses and filter out indigestible objects. The omasum breaks down food into smaller pieces and expels excess water. The abomasum is the true stomach and does the digestion and distribution of nutrients before waste is sent to the intestinal tract.