Why do cows have three stomachs?

Cows are true ruminants, which means they have four stomachs, the first of which is the rumen. When a cow takes a bite of grass, it chews it briefly, mixing it with a large amount of saliva. The grass then passes to the rumen, which is a large pouch. The rumen does not produce digestive juices. Instead, it is a fermentation chamber that contains millions of bacteria. These microbes produce digestive enzymes that break down the cellulose in the plants. When a cow “chews its cud,” it returns a small lump, or bolus, of food from the rumen to the mouth, where it is thoroughly chewed. When the cow swallows the bolus for the second time, it is finer and settles at the bottom of the rumen. The rumen contracts, forcing some of this well-chewed food into the second stomach, or reticulum. From there it passes to the omasum (third stomach), where water is extracted. It then enters the true, or fourth, stomach, the abomasum, where gastric juices (containing hydrochloric acid) are added to the food. This kills and disintegrates the microbes from the rumen, making the nutrients in the microbes available for later digestion and absorption.