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United States Department of Agriculture

Agricultural Research Service

Title: Relationship Between Ruminal Fibrolytic Populations, Ruminal Vfa Concentra-Tions, and Milkfat in Cows Fed Diets Containing Corn Silage and Alfalfa Silage at Two Levels of Fiber

item Weimer, Paul
item Mertens, David

Submitted to: Journal Dairy Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: June 23, 1997
Publication Date: N/A

Technical Abstract: Although ruminal microbes produce VFA used by the animal for energy and milk production, few direct correlations have been made between animal performance, VFA, and populations of individual microbial species. The purpose of this study was to relate ruminal VFA concentrations and milk production to specific populations of fibrolytic bacteria. Four cows in mid dlactation were used in a balanced 4X4 Latin square design with factorial arrangement of treatments (2 fiber sources and 2 fiber levels). Diets containing alfalfa silage or corn silage with either 24 or 32% aNDF (NDF assayed following amylase treatment) were fed at 12 h intervals during 4- week periods. After dietary adaptation (23 d) ruminal contents were collected at 12 time points over a 4-day period and analyzed for pH, VFAs, and populations of three species of fibrolytic bacteria. Milk production and composition also were measured. Ruminal VFA were nearly constant with time for each cow-diet combination, but varied greatly among combinations. Milk fat and acetate/propionate ratio (A/P) were strongly correlated (r2=0.73), particularly in samples having acetate concentrations > 100 mM (r2=0.88). Poor correlation was observed between pH just prior to feeding and 3 h-postfeeding (r2=0.14). Significant effects (p<0.05) of both cow and diet on A/P, total VFA, and pH were observed. The alfalfa-based diet generally gave higher A/P and higher milk fat levels than did the corn- based diet at equivalent aNDF levels. Ruminal samples having low A/P ratios had elevated levels of Fibrobacter succinogenes, whose main fermentation product is succinate, a major propionate precursor. The data indicate that diet-induced changes in milk fat concentration in individual cows are associated with changes in microbial populations and VFA ratios.

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