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Impact of Niacin and Length of Incubation on Protein Synthesis, Soluble to Total Protein Ratio and Fermentative Activity of Ruminal Microorganisms^1,2,

University of Wisconsin, Madison 53706

Abstract

The influence of niacin supplementation on growth of ruminal microorganisms was investigated in a series of in vitro incubations. In Exp. 1, inocula were taken from a ruminal-cannulated steer fed corn silage and soybean meal-urea-mineral supplement. Protein and ammonia concentrations in incubation contents were inversely related and indicated net cell growth in the first 6 h and progressive lysis during the next 18 h. Protein production within 6 h was not affected by niacin supplementation and was .24, .22 and .24 mg/ml for cultures supplemented with 0, 1 and 100 µg niacin/ml of incubation contents, respectively. In Exp. 2, microbial soluble protein and total protein concentrations were determined. Results showed that niacin had no effect on cell growth. Growth curves were characterized by a growth phase with little cell lysis in the first 3 h, a stationary phase over the next 6 h, and a subsequent death phase in which ammonia concentrations increased. Net protein production within the first 3 h was unaffected by niacin supplementation and was .47, .48 and .45 mg/ml for cultures supplemented with 0, 1 and 100 µg niacin/ml, respectively. Cell pellets of ruminal microorganisms were used as inocula in Exp. 3. Net increases in microbial protein concentration after 6 h were .88 mg/ml for the control and .85 and .87 mg/ml for 1 and 100 µg niacin/ml, respectively. Growth rates of actively growing, mixed populations of ruminal microorganisms were not affected by final added niacin concentrations of 1 and 100 µg niacin/m). The lack of microbial growth responses to niacin in the three experiments suggested that the control incubations had sufficient endogenous niacin to meet the needs of the microbes.

¹ Paper no. 864 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.