The digestive system of ruminants can also manufacture many essential nutrients. Drawings of the four compartmentalized stomachs of the ruminant are shown in Figure 1, indicating the changes from youth to maturity. This is the largest of the four compartments representing about 80 percent of the total stomach area. The rumen is often called a fermentation vat because it contains a large number of microorganisms bacteria and protozoa, which supply enzymes that break down fiber and other parts of the feed. The cellulose is converted to volatile fatty acids that are absorbed through the rumen wall and provide up to 75 percent of the total energy needs of the animal.
Protein is produced by the microorganisms from the nitrogen in the feed.
The microorganisms also manufacture all the B vitamins along with Vitamin K. 3 A unique feature of the rumen is that it is located in the beginning of the digestive system. This position makes it possible for the host animal to take advantage of all the nutrients produced by the microorganisms as well as the organisms themselves further on down in the digestive tract. This is the second stomach, sometimes called the ''honeycomb'' due to the structure of its wall and is located just below the entrance of the esophagus into the stomach. Act ually the reticulum is a part of the rumen, separated only by a partial wall. It consists of hanging layers of tissue similar to the curtains on a stage.