We evaluated the effect of Bovamine ( Lactobacillus acidophilus strain NP51 and Probionibacterium freudenreichii strain NP24) feeding on the digestive system microbiome of dairy cattle during late lactation (average DIM = 202). To unveil the underlying mechanisms, we examined Bovamine effects on ruminal total volatile fatty acid (VFA) concentration and the Firmicutes : Bacteroides. Dairy cows were randomized to treatment groups that were either fed the Bovamine or the lactose carrier top-dressed on the total mixed ration (TMR) for six weeks. Feeding Bovamine favorably modified the digestive system microbiome as determined by the bacterial tag encoded FLX amplicon pyrosequencing (bTEFAP). Fecal Firmicutes : Bacteroides was decreased, suggesting decreased shedding of potential pathogens. Conversly, ruminal Firmicutes : Bacteroides was increased indicating increased energy harvest. As a result, concentrations of the total VFA were increased significantly in the rumen. Also, natural killer cells (NK) frequency was increased under stimulation with mitogen concanavalin (ConA) in vitro, suggesting improved cattle immunity. Results advocate that feeding Bovamine to lactating dairy cows favorably alters their digestive system microbiome, improves the dietary energy availability, and health of cattle with significant implications to transition and early lactation dairy cows. A future study will be conducted to determine possible improvement in feed efficiency of mid-lactation dairy cows