Digestion of carbohydrate in the reticulo-rumen

• The diet of ruminants contains considerable quantities of cellulose, hemicellulose, starch & water-soluble carbohydrates mostly in the form of fructans, pentosans, xylans.
• The breakdown of carbohydrates in the rumen may be divided into two stages.

A. Digestion of complex carbohydrates into simple sugars which is accomplished by extracellular microbial enzymes or enzymes produced by microbes found in the reticulo rumen action of which is outside the microbes that produce them.

• Cellulose is one major carbohydrate consumed by ruminants which could be decomposed by one or more -1,3- glucosidases to cellobiose which is then converted either to glucose, or through the action of phosphorylase to glucose -1- phosphate.
• Starch and dextrines are 1st converted by amylases to maltose and isomaltose and then by maltases, maltose phosphorylases or 1,6-glucosidases to glucose or glucose-1- phosphate.
• Fructans are hydrolyzed by enzymes attacking 2, 1 and 2,6 linkages to give fructose, which may also be produced together with glucose by the digestion of sucrose.
• Pentoses are the major product of hemicellulose breakdown which is brought by enzymes attacking -1,4 linkage to give xylose and uronic acid.

B. The 2nd stage in rumen degradation is that the simple sugars produced in the 1st stage of carbohydrate digestion in the rumen are rarely detectable in the rumen liquor as they are immediately taken up and metabolized by intracellular enzymes of micro-organisms intra-cellularly. For this second stage, the pathway involved is similar in many respects to those followed in the metabolism of carbohydrates by the animal itself.

• Pyruvate forms intermediate product. Pyruvate production is the 1st stage both under anaerobic or aerobic condition from glucose.
• Pyruvate under aerobic condition oxidize to CO2 and H20 with further production of energy.
• Under anaerobic environment of the rumen the pyruvate is oxidized in a variety of ways to result different volatile fatty acids (VFA) is cell of microorganisms. The main acids produced in the rumen are acetic, propionic and butyric acids, and gasses CO2 and CH4 (methane). These are main end products of metabolism of carbohydrates in the rumen.
• Of all the gasses produced in the rumen methane account for 30 to 40%, CO2 account for 40%, hydrogen for 5%, and the oxygen and nitrogen variable based on the content in the ingested air. Most of gases are removed by eructation.
• Much of the VFA (>90%) is absorbed in foregut rumen, reticulum and omasal walls. Some may pass through the abomasum and be absorbed in the small intestine (about 80% VFA is absorbed in the rumen wall).
• After the VFA absorbed by diffusion through the rumen wall they go to the portal blood circulation then to the liver.
• Acetic acid is the major end product of carbohydrate digestion, in the ruminants. Acetic acid is used by a various tissue as a source of energy and synthesis of fatty acids.
• Propionic acid passes across the rumen wall where a little is converted to lactate, the remainder carried to the liver where it is changed into glucose which may be changed to liver glycogen, or to fatty acids for triglyceride synthesis.
• Butyric acid (butyrate ) is changed in its passage across the rumen wall or metabolized by rumen epithelium to – hydroxybutyric acid (BHBA) and passes into the portal blood then to the liver.
• The remaining small amount of butyrate that may appear in the portal blood will be removed entirely by the liver, leaving negligible amount in the arterial blood.