The Effects of HMTBa (2-hydroxy-4-methylthio-butanoic acid) Supplementation on Ruminal Microbial Crude Protein Synthesis and Community Structure in Dairy Cattle

Metabolizable protein (MP) is protein that reaches the small intestine and is available for absorption and utilization by the cow. Dairy rations may be limited in the supply of MP essential to meeting the demands of milk synthesis, however as much as half of the MP flowing to the small intestine may be attributed to microbial origins and is referred to as microbial CP (MCP). Experiment 1 utilized a technique in which DNA was used as a microbial marker to estimate the concentration of bacterial CP (BCP) in the solid and liquid portions of rumen digesta. Rumen digesta was sampled and separated into solid and liquid fractions and microbes were isolated from whole ruminal digesta. Targeting bacterial DNA in samples using real-time PCR, in addition to N analysis, allowed for estimates of the concentration of BCP in the solid and liquid fractions to be attained. The concentration of BCP tended to be higher in the solid portion, highlighting the need to consider both particle and liquid associated bacteria when conducting experiments involving the microbial community. Experiment 2 focused on the ruminal effects of a commercial feed additive when fed with diets low or high in MP. The feed additive, 2-hydroxy-4-methylthio-butanoic acid (HMTBa) molecule (Alimet, Novus Internation, St. Charles, MO), a methionine analog, is believed to result in several positive effects on rumen fermentation, including increased MCP yield. Rumen pH was decreased in response to the additive, while rumen VFA and ammonia were increased. The MCP yield was unaffected across treatments. Nutrient digestibility was increased in cows fed the diet low in MP. Rumen bacterial DNA was sequenced and analyzed bioinformatically; the proportion of Fibrobacteres were increased in cows receiving the additive, and a number of associations of the relative abundance of microorganisms with ruminal observations and treatments were observed. Co-advisors: Paul J. Kononoff and Samodha C. Fernand