Ruminant animals, such as the dairy cow, have four stomach compartments called the rumen, reticulum, omasum, and abomasum. The rumen is the largest and can hold up to 40 gallons of digesta and contains billions of bacteria, protozoa, fungi, and other organisms. The bacteria can digest fiber allowing the dairy cow to consume grass, forages, and fibrous by-product feeds that humans or monogastric animals cannot do effectively.
The abomasum is the stomach compartment similar to the human stomach.
Dave Fischer, Extension Dairy Educator, University of Illinois – Illini DairyNET