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

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 jug 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/ml. 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.