Impact of dietary supplementation of rumen native microbes on ruminal microbiome, ruminal fermentation, and total-tract digestibility in Holstein cows.

Dietary microbial additives often benefit production performance in dairy cows. The objectives were to determine the effects of microbial additives on rumen microbial diversity, total-tract digestibility, incorporation of dietary N into milk N, and serum metabolites. One-hundred and 17 cows at 61 d postpartum (31-87 d) were blocked by parity group and pretreatment ECM yield. Within block, cows were randomly assigned to receive 100 g of corn meal containing no microbial additive (CON; n = 40), 100 g of corn meal containing 5 g of a mixture of Clostridium beijerinckii and Pichia kudriavzevii (G1; 4 × 10⁷ cfu of C. beijerinckii and 1 × 10⁹ cfu of P. kudriavzevii; n = 38), or 100 g of corn meal containing 5 g of a mixture of C. beijerinckii, P. kudriavzevii, Butyrivibrio fibrisolvens, and Ruminococcus bovis (G2; 4 × 10⁷ cfu of C. beijerinckii, 1 × 10⁹ cfu of P. kudriavzevii, 1 × 10⁸ cfu of B. fibrisolvens, and 1 × 10⁸ cfu of R. bovis; n = 39). Treatments were top-dressed for 140 d. Total-tract digestibility and rumen microbial composition were evaluated on d 61 and 124 of the experiment. Orthogonal contrasts compared the effect of microbial additive (MA) and the type of microbial additive (TMA). On experimental d 61, supplementing MA did not influence rumen microbial diversity, although cows fed G2 had a less diverse microbiome than those fed G1. On d 124, both MA and TMA reduced evenness, which resulted in lesser microbial α-diversity in cows supplemented with MA than those in CON. Minor differences were observed in rumen microbial β-diversity, except for multiparous cows on d 124 in which treatment tended to affect diversity because 16 amplicon sequence variant (ASV) groups representing phylogenetically related ASV differed among treatments. Two bacteria supplemented in G2, also native to the rumen, were identified, B. fibrisolvens and R. bovis, but treatment did not affect their abundance. Cows fed MA had increased total short-chain fatty acid concentration and partition of dietary N into milk N on d 61, but not on d 124. On d 61, feeding MA increased milk N secretion by 10 g/d (187 g/d vs. 193 g/d vs. 201 g/d), and incorporation of dietary N into milk N by 1.7 percentage units (CON = 30.8% vs. G1 = 31.2% vs. G2 = 33.7%). An interaction between TMA and parity affected the estimated microbial N production on d 124 because of the differences between G1 and G2 in primiparous cows (G1 = 302 g/d vs. G2 = 264 g/d). Type of MA affected digestibility on d 124, but the effect differed with parity. Feeding G1 increased digestibility of DM, OM, and CP in primiparous cows by 2.7, 2.8, and 4.7 percentage units, but not in multiparous cows. Collectively, supplementing diets of dairy cows with MA resulted in minor effects on the composition of the ruminal microbiota, total-tract digestibility, and use of dietary N. Present findings might not fully explain differences in production performance previously observed with the same MA added to the diet of cows.