How many stomachs does a cow have?
A cow has four stomach compartments (rumen, reticulum, omasum, and abomasum). The rumen of an adult cow can contain up to 40 gallons of rumen contents (about 85% water) containing 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 monogastric animals cannot utilize effectively.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET