Abstract #T452

# T452
Flow of microbial crude protein out of the rumen when dairy cattle are supplemented with 2-hydroxy-4-methylthio-butanoic acid (HMTBa).
C. J. R. Jenkins1, S. C. Fernando1, C. L. Anderson3, N. D. Aluthge2, E. Castillo-Lopez4,1, H. A. Tucker5, G. I. Zanton5, D. Hostetler*6, P. J. Kononoff1, 1Department of Animal Science, University of Nebraska-Lincoln, Lincoln, NE, 2Food Science and Technology Department, University of Nebraska-Lincoln, Lincoln, NE, 3School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE, 4Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California, Ensenada, México, 5Novus International Inc, St. Charles, MO, 6The School of Veterinary Medicine and Biomedical Sciences, University of Nebraska-Lincoln, Lincoln, NE.

Four multiparous, lactating Holstein cows (average DIM 169.5 ± 20.5 d), fitted with ruminal and duodenal cannulas, were used in a 4 × 4 Latin square with a 2 × 2 factorial arrangement of treatments to investigate the effects of 2-hydroxy-4-methylthio-butanoic acid (HMTBa; Alimet, Novus International, St. Charles, MO) when fed with diets deficient or in excess of metabolizable protein (MP) on milk production and composition, rumen microbial activity and protein flow out of the rumen, and rumen microbial community composition. Cows were housed in individual tiestalls and fed diets designated as “Low MP” or “High MP,” which were top dressed once daily with 250 g of a corn carrier or 250 g of a corn carrier containing 10% Alimet yielding 25 g of Alimet/hd/d .No interactions were observed between HMTBa and level of dietary MP, with the exception of ruminal acetate to propionate ratio (P = 0.04). Milk yield was not affected by treatment and averaged 23.8 ± 2.06 kg. There was a tendency (P = 0.06) for increased milk protein percent in cows receiving Low MP diets, averaging 3.30 ± 0.09% and 3.21 ± 0.09% for Low MP and High MP, respectively. The DM, OM, NDF, total-tract digestibilities were greater (P ≤ 0.03) in cows consuming the Low MP diet. Rumen pH was lower (P = 0.05) in cows consuming High MP diets as well as in those consuming HMTBa (P < 0.01). Rumen ammonia concentrations tended to be greater (P = 0.06) in cows consuming HMTBa. Rumen VFA concentrations were greater (P = 0.02) in cows consuming HMTBa. Duodenal DM flow, N flow, and bacterial N flow did not differ between treatments (P ≥ 0.15). The microbial community structure of cows receiving HMTBa was affected at the phylum level, as the relative abundance of Fibrobacteres was increased (P = 0.04). Several correlations (P ≤ 0.05) between microorganisms and metadata were observed, including animal, HMTBa supplementation, level of dietary MP, DMI, digestibility, rumen ammonia, microbial N flow, and milk production. Results suggest that HMTBa affects rumen microbial activity, irrespective of level of MP. Consequently, further investigation is warranted into the mechanism of these effects in the rumen.

Key Words: 2-hydroxy-4-methylthio-butanoic acid, microbial protein, rumen