species in 53 genera Digestive system of ruminants The primary difference between ruminants and nonruminants is that ruminants' stomachs have four compartments: rumen—primary site of microbial fermentation reticulum omasum—receives chewed cud, and absorbs volatile fatty acids abomasum—true stomach The first two chambers are the rumen and the reticulum. These two compartments make up the fermentation vat and are the major site of microbial activity. Fermentation is crucial to digestion because it breaks down complex carbohydrates, such as cellulose, and enables the animal to use them. Microbes function best in a warm, moist, anaerobic environment with a temperature range of 37.7 to 42.2 °C (100 to 108 °F) and a pH between 6.0 and 6.4. Without the help of microbes, ruminants would not be able to use nutrients from forages. The food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud or bolus. The cud is then regurgitated and chewed to completely mix it with saliva and to break down the particle size. Smaller particle size allows for increased nutrient absorption. Fiber, especially cellulose and hemicellulose, is primarily broken down in these chambers by microbes (mostly bacteria, as well as some protozoa, fungi, and yeast) into the three volatile fatty acids (VFAs): acetic acid, propionic acid, and butyric acid. Protein and nonstructural carbohydrate (pectin, sugars, and starches) are also fermented. Saliva is very important because it provides liquid for the microbial population, recirculates nitrogen and minerals, and acts as a buffer for the rumen pH. The type of feed the animal consumes affects the amount of saliva that is produced. Though the rumen and reticulum have different names, they have very similar tissue layers and textures, making it difficult to visually separate them. They also perform similar tasks. Together, these chambers are called the reticulorumen. The degraded digesta, which is now in the lower liquid part of the reticulorumen, then passes into the next chamber, the omasum. This chamber controls what is able to pass into the abomasum. It keeps the particle size as small as possible in order to pass into the abomasum. The omasum also absorbs volatile fatty acids and ammonia. After this, the digesta is moved to the true stomach, the abomasum. This is the gastric compartment of the ruminant stomach. The abomasum is the direct equivalent of the monogastric stomach, and digesta is digested here in