The goat are a group of animals called Ruminants derived from the word Ruminate (chew their cud). Ruminants have special four-compartment stomachs especially designed to digest roughage (food high in fiber) such as grass, hay and silage.
Their stomach has four chambers: 1) the Rumen, 2) the honey-comb like Reticulum, 3) the Omasum, and 4) the Abomasum. The size relationship of the four chambers changes as the animal grows up. The abomasum will proportionally smaller. To understand why this happens, you need to consider the individual function for each chamber and then look at the goat’s diet.
1) The Rumen acts as a big fermentation container. Bacteria and Protozoa in the Rumen supply the enzymes needed to break down the fiber in the goat’s feed. This is similar as to how bacteria can ferment the sugars in grape juice to make wine. The tiny organisms in the rumen also help to build proteins from the feed and manufacture all of the B vitamins needed by the goat. Many nutrients that help provide the goat with energy are also absorbed here. The fermentation process produces heat that helps to keep the goat warm. When roughage is eaten by the adult goat, it is chewed on, soaked with saliva, and then swallowed. This bolus of food is called “the cud”. It goes down into the rumen to be attacked and broken down or digested by the micro-organisms. At regular intervals the cud is brought back up to the goat’s mouth to be chewed on some more and then swallowed again. This entire process is called rumination. If you watch the goat’s neck carefully, you can see him swallow and later regurgitate his cud. The goat will often burp to get rid of the gas produced by all the fermentation going on in his rumen. You can really smell the fermentation process on his breath. If something causes the goat to stop being able to burp up the gases, the gas will build up and bloat or swell up his rumen and he may become very sick with “bloat”.
2) Once the food particles of cud become small enough, they pass to the second compartment or reticulum. Here any foreign objects that may have been accidentally swallowed with the feed settle out in the honeycomb structure of the reticulum’s walls. Another name for the reticulum is the “hardware stomach”.
3) The fermenting particles then pass on to the omasum. The omasum removes the water from them and also absorbs more nutrients called volatile fatty acids that help supply the goat with energy.
4) The particles are then forced into the abomasum or true stomach. Here, the particles are digested by the stomach acid, hydrochloric acid (HCl). This form of digestion is the same as what occurs in our stomachs. The remaining particles are then passed on to the small intestine where most of the nutrients are absorbed by the body and made available to the goat.
When a goat kid is born, its rumen, reticulum and omasum are very tiny and not useful. The goat kid depends on a liquid, milk, not roughage for its feed source. When the kid swallows milk, the milk goes directly to the abomasum through the esophageal groove. Everytime the kid swallows, a flap of skin at the entrance to the rumen folds over to form a grove that bypasses the rumen and sends the milk straight to the abomasum to be digested by stomach acid. As the kid gets older, he starts trying to consume roughage. The rumen becomes active and starts to enlarge. Its population of micro-organisms increases. The reticulum and omasum also respond to the changes in diet by getting bigger. By the time the kid is an adult goat, roughage is his main source of food and his rumen is far larger than his abomasum.