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Effects of Two Dietary Niacin Concentrations on Ruminal Fluid Free Niacin Concentration, and of Supplemental Niacin and Source of Inoculum on In Vitro Microbial Growth, Fermentative Activity and Nicotinamide Adenine Dinucleotide Pool Size^1,2,

University of Wisconsin, Madison 53706

Abstract

Diets containing high (barley) or low (oats) concentrations of niacin were fed to two groups of two dairy cows for 15 d. On d 1, 5, 10 and 15, samples of ruminal contents were collected twice daily, 4 to 5.5 h after feeding. Ruminal fluid from cows fed the barley diet tended to have higher free niacin concentrations, particularly on d 1, 5 and 10. Irrespective of dietary niacin intake, niacin concentrations in ruminal fluid collected on d 10 and d 15 were approximately three to four times higher than those determined earlier (.12, .16, .40 and .51 µ/ml on d 1, 5, 10 and 15, respectively). Dietary niacin level did not influence (P>.05) bacterial nicotinamide adenine di-nucleotide (NAD) to dry weight or protein ratios. On d 15, ruminal contents collected 4 h after the first daily feeding were used to inoculate 6-h in vitro incubations supplemented with 0, 1 or 10 µg niacin/ml of incubation contents. Niacin supplementation did not increase (P>.05) protein and total or individual volatile fatty acid production. Independent of the diet the donor animal received, inocula apparently contained sufficient niacin to satisfy microbial requirements. Bacterial NAD pool size did not increase (P>.05) after incubation, and was unaffected by niacin supplementation or source of inoculum. Results of this experiment indicated that niacin synthesis in the rumen compensated for low niacin intake and that supplemental niacin did not improve in vitro growth of ruminal microorganisms when inocula were obtained from cows fed high (barley) or low (oats) niacin diets.

¹ Paper no. 866 from the Dept. of Meat and Anim. Sci. Research supported by the College of Agr. and Life Sci., Univ. of Wisconsin-Madison, Hatch grant no. 2629 and the Tunisia Agr. Technol. Transfer project.

² We thank Professor N. J. Benevenga for his helpful advice and Dr. G. Broderick for automated ammonia and total amino acid analyses.

³ Present address: E.S.A., Mateur, Tunisia.