INFLUENCE OF NIACIN SUPPLEMENTATION ON IN-VIVO DIGESTIBILITY AND RUMINAL DIGESTION IN DAIRY-COWS

The effect of niacin supplementation on digestion was investigated usi ng four cows in a crossover design. After peak lactation, cows receive d a diet of corn silage (55%) and concentrates (45%), including a form aldehyde-treated mixture of soybean meal, rapeseed meal, and urea. Thi s diet was either supplemented with niacin (6 g/d) or not supplemented . Total tract apparent digestibility of OM and fiber and ruminal diges tibility of OM were not modified by treatment. Niacin supplementation enhanced the theoretical degradability of ruminal DM in situ (44.7% vs . 40.6%) and concentration of protozoa in ruminal fluid (459 vs. 311 x 10(3)/ml), especially Ophryoscolecidae. The percentage of butyrate in the VFA mixture of ruminal fluid was increased by niacin supply, but acetate and propionate percentages and total VFA concentration did not vary. Ruminal digesta, ruminal pools, and ruminal kinetics were not a ffected by treatment. Duodenal flow of nonmicrobial N tended to increa se with niacin supplementation. Duodenal now of microbial N did not va ry, as measured using a microbial sample, for fluid-associated bacteri a or for a mixture of fluid-associated and particle-associated bacteri a. These results are discussed in relation to the characteristics of t he diet.