tailieunhanh - Quantitative aspects of ruminant digestion and metabolism - Phần 7

Rumen fermentation is an exergonic process that converts feedstuffs to short-chain volatile fatty acids (VFA), methane, ammonia and occasionally lactic acid. Some of the free energy is used to drive microbial growth, but heat is also evolved. The efficiency of rumen microbial growth can have a profound effect on animal performance | 9 Microbial Energetics . Russell1 and . Strobel2 Agricultural Research Service USDA and Department of Microbiology Cornell University Ithaca NY 148531 USA 2Department of Animal Sciences University of Kentucky Lexington KY 40546-0215 USA Introduction Rumen fermentation is an exergonic process that converts feedstuffs to short-chain volatile fatty acids VFA methane ammonia and occasionally lactic acid. Some of the free energy is used to drive microbial growth but heat is also evolved. The efficiency of rumen microbial growth can have a profound effect on animal performance and organic acids produced during microbial fermentations are an important source of energy for the host animal. Microbial protein is an important amino acid supply for the animal and it is now apparent that the yield of microbial protein can vary significantly Nocek and Russell 1988 . A diverse and complex microbial population that includes bacteria protozoa and fungi inhabits the rumen Orpin and Joblin 1989 Stewart and Bryant 1989 Williams and Coleman 1989 . Given the observation that the density of protozoa in omasal contents was less than 10 of that in the rumen it appears that protozoa contribute little microbial protein to the animal Weller and Pilgrim 1974 Leng 1982 . Protozoa are involved in the turnover of bacterial protein Leng and Nolan 1984 and regulation of starch fermentation engulf-ment of starch grains but defaunation studies have indicated that protozoa are not required for a normal rumen fermentation Abou Akada and El-Shazly 1964 Eadie and Gill 1971 . The role of the fungi is less clear Bauchop 1979 . When animals were fed highly lignified fibre fungi accounted for approximately 8 of the microbial mass Citron et al. 1987 but their numbers were much lower in animals fed diets rich in concentrates Fonty et al. Mandatory disclaimer Proprietary or brand names are necessary to report factually on available data however the USDA neither guarantees nor warrants the standard of the