Ruminant Digestion

Abstract

This chapter reviews the ruminant digestion with a special emphasis on the mechanical factors, gastro-intestinal tract structure, and nutrient digestibility. Ruminants possess large compartmental gastro-intestinal tract viz. rumen, reticulum, omasum, abomasum, and intestine, which favors handling large amounts of fibrous plant materials. Among the four-rumen compartments, abomasum occupies large space in newborn ruminants; however, the growth rate of the rumen and reticulum will be faster compared to abomasum as the age advances. In adult ruminants, the rumen harbors vast range of microbes enabling microbial fermentation of ingesta before exposing to gastric juices of abomasum. Ruminant digestion involves mechanical processing of feed stuff. Among various mechanical factors, rumination aids in complete digestion of feed stuff and include regurgitation, remastication, reinsalivation, and redeglutition. The rumen microbiota, consisting of bacteria, protozoa, fungi, and archea degrade the ingested fiber-based diets and aids in nutrient fermentation. The fermentation of complex carbohydrates produces short-chain fatty acids (acetate, propionate, and butyrate), isoacids (valeric, isovaleric, isobutyric, and 2-methylbutyric acids), and gases such as CO₂, CH₄, and H₂. About 70% of the ruminant animal’s energy supply will be met by the produced volatile fatty acids. High fiber diet induces the production of acetate while the starch and sugars yield propionate as end product. Milk fat synthesis requires acetate and hence, low fiber diets lead to milk fat depression. Similarly, propionate contributes to most of the energy required for weight gain and lactose production. Rumen pH is an important factor to be considered; low pH suppresses the growth of certain bacteria sensitive to pH-causing rumen dysfunction and subacute rumen acidosis. The protein metabolism in ruminants depends upon the ability of rumen microbes utilizing ammonia. More than 80% of the rumen bacteria utilizes ammonia as nitrogen source for growth and yields microbial protein. For every 1 kg organic matter digested, the microbial yield ranges from 90 to 230 g, which is sufficient for growth and production to certain extent. Fat digestion in ruminants is unique in that the ruminal bacteria split the fatty acids and sugars from glycerol backbone through lipolysis. The metabolism of lipids by rumen microbes involves a four-stepped process viz. hydrolysis of esterified fatty acids, biohydrogenation of unsaturated fatty acids, lipid biosynthesis in the rumen, and metabolism of phytal to phytanic acid. Further, incomplete biohydrogenation generally produces conjugated linoleic acids (CLA), which are proven to benefit human health.

Graphical Abstract