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The Digestive System The main parts of the cow’s digestive system are illustrated in the diagram below. Ingested feed is chewed, mixed with saliva and formed into a bolus before it passes down the esophagus into the reticulum and rumen, the first two rectum dorsal sac of rumen sections of the four compartment anus esophagus ruminant stomach. Later, boluses (cuds) duodenum of feed are regurgitated and further broken down by chewing. Whenever omasum cecum feed is chewed, saliva is secreted from reticulum salivary glands in the mouth. Over the abomasum course of a day, a lactating cow will spiral colon ventral sac of rumen secrete more than 200 litres of saliva small intestine which has three main functions: • it lubricates feed as it moves down the esophagus; The main parts of the cow’s • it provides the liquid flow which carries feed particles into digestive system. the omasum, and; • it contains buffers which prevent rumen pH from falling too low as microbes produce acids from feed fermentation. STARCH GRANULE BACTERIA PROTOZOA Electron photomicrograph of bacteria and protozoa digesting Barley starch granules. On entering the rumen, an extremely varied population of bacteria, protozoa and fungi attach themselves to feed and begin the breakdown process. Enzymes are secreted onto the feed and into the fluid contents of the rumen. Distinct microbial populations digest specific types of feed. For example, starch digesters are distict from fibre digesters. BACTERIA The inner lining of the rumen is not unlike pile carpet, having innumerable small, flat projections called papillae. These serve two primary functions. They vastly increase the area for absorption of nutrients and they also provide attachment sites for additional, specific populations of bacteria. For example, the bacteria responsible for digesting urea are mainly found attached to papillae. continued... The Digestive System (continued) Continual mixing of rumen contents is essential to efficient fermentation. The muscular walls of the rumen and reticulum produce waves of contraction travelling their combined lengths at about half-minute intervals. This process, in addition to mixing the rumen contents, facilitates both regurgitation for further cud-chewing and eructation, which releases gases produced by fermentation (mainly hydrogen and methane). Under some conditions (e.g., grain overload) these contractions may stop, resulting in rumen stasis and placing the animal at serious risk of bloating. After the feed has been sufficiently chewed and broken down by microbial action, digesta enter the omasum. Flow into this third segment of the stomach is regulated by a small opening called the reticulo-omasal orifice which prevents large particles from leaving the rumen. The omasum is a muscular organ which is thought to have two main functions. The first is the extraction of water from the digesta, yielding a product for further digestion which has a significantly higher proportion of dry matter. Secondly, the omasum serves as a pump, propelling digesta from the rumen and reticulum into the fourth segment of the stomach, the abomasum. The ruminant abomasum is analogous to the true stomach of monogastric animals such as humans, pigs, dogs and cats. The digestion of proteins, fats and oils is initiated by acids and enzymes secreted by the tissue lining the abomasum. As these initial stages progress, the digesta pass to the duodenum, the first segment of the small intestine. Here, bile from the liver and gall bladder along with digestive enzymes from the pancreas are added. Digestion and absorption of its products progress as the digesta pass through the two lower segments of the small intestine, the jejunum and the ileum. Digestion is almost complete by the time the unabsorbed digesta reach the large intestine. One of the main functions of this part of the system is the absorption of water and minerals. In addition, some further breakdown is carried out here by a permanent population of microbes (bacteria and protozoa) with some of the products being absorbed into the blood. Food material which has escaped both enzymatic and microbial digestion is excreted. source: Dairy Production Primer - Feeds & Feeding Management ©2004 Western Dairy Science Inc.