RELATIONSHIP BETWEEN FERMENTATION ACID PRODUCTION IN THE RUMEN AND THE REQUIREMENT FOR PHYSICALLY EFFECTIVE FIBER

The content of ruminally fermented OM in the diet affects the fiber re quirement of dairy cattle. Physically effective fiber is the fraction of feed that stimulates chewing activity. Chewing, in turn, stimulates saliva secretion. Bicarbonate and phosphate buffers in saliva neutral ize acids produced by fermentation of OM in the rumen. The balance bet ween the production of fermentation acid and buffer secretion is a maj or determinant of ruminal pH. Low ruminal pH may decrease DMI, fiber d igestibility, and microbial yield and thus decrease milk production an d increase feed costs. Diets should be formulated to maintain adequate mean ruminal pH, and variation in ruminal pH should be minimized by f eeding management. The fraction of OM that is fermented in the rumen v aries greatly among diets. This variation affects the amount of fermen tation acids produced and directly affects the amount of physically ef fective fiber that is required to maintain adequate ruminal pH. Acid p roduction in the rumen is due primarily to fermentation of carbohydrat es, which represent over 65% of the DM in diets of daily cows and have the most variable ruminal degradation across diets. The nonfiber carb ohydrate content of the diet is often used as a proxy for ruminal ferm entability, but this measure is inadequate. Ruminal fermentation of bo th nonfiber carbohydrate and fiber is extremely variable, and this var iability is not related to the nonfiber carbohydrate content of the di et. The interaction of ruminally fermented carbohydrate and physically effective fiber must be considered when diets for dairy cattle are ev aluated and formulated.