Rumen
The first and largest chamber of a cow's stomach, where microbes ferment plant matter.
Also: paunch, first stomach
The rumen is a large fermentation vat on the left side of the abdomen, holding well over a hundred litres in an adult cow. It is full of bacteria, protozoa and fungi that break down cellulose and other tough plant fibres the cow could not digest on its own, producing fatty acids that supply most of her energy, and gases including methane, which she belches out. Food passes back and forth between the rumen and the reticulum, is brought up to be chewed again as cud, then moves on to the omasum and the abomasum, the true stomach. A calf's rumen develops as it begins to eat solid feed.