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Changes in rumen fermentation, bacterial community, and predicted functional pathway in Holstein cows with and without subacute ruminal acidosis during the periparturient period.

We investigated changes in rumen fermentation, the fluid bacterial community, and predicted functional pathway profiles in Holstein cows with and those without subacute ruminal acidosis (SARA) during the periparturient period. Eighteen multiparous Holstein cows categorized in the SARA (n = 9) or non-SARA (n = 9) groups depending on whether they developed SARA during the 2 wk after parturition. Reticulo-ruminal pH was measured continuously throughout the study. Rumen fluid and blood samples were collected 3 wk before and 2 and 6 wk after parturition, with an additional blood sample collected during the 0 and 4 wk after parturition. The 7-d mean reticulo-ruminal pH was significantly decreased in both groups postpartum compared with prepartum, with a greater and longer lasting change in the SARA group. The postpartum total volatile fatty acid concentration was significantly higher in the non-SARA than in SARA cows. Rumen bacterial richness and diversity were not affected by the periparturient period, but the non-SARA group had a significantly higher Simpson diversity index postpartum. No significant change was observed in the composition of major bacterial phyla or genera during the periparturient period. Among 17 operational taxonomic units (OTU) shared by all cows, the relative abundances of OTU36 (genus Pseudobutyrivibrio) in the SARA group and OTU4 (genus Prevotella) in the non-SARA group decreased significantly during the postpartum period compared with prepartum. In contrast, non-SARA cows had more diverse core bacterial OTU community including 23 additional OTU than those with SARA including 3 additional OTU (40 vs. 20 OTU). The OTU36 was significantly correlated with the proportion of i-valeric acid (r = -0.353), and OTU4 was significantly correlated with the 7-d mean reticulo-ruminal pH (r = 0.432) and the proportions of acetic acid (r = 0.353) and propionic acid (r = -0.380). Prepartum, OTU2 (genus Prevotella) was the only OTU that differed significantly between the 2 groups. Changes in predicted functional pathway were primarily identified during the postpartum period, with 2 pathways upregulated at 3 wk prepartum and 24 upregulated and 34 downregulated at 6 wk postpartum in the SARA group. Our results suggest that SARA incidence is not strictly related to changes in rumen fermentation or fluid bacterial community structure, but the predicted functional pathways of bacterial communities showed late responses to a postpartum decrease in reticulo-ruminal pH. Therefore, postpartum predicted functional pathway analysis may indicate the underlying mechanisms causing SARA in Holstein cows during the periparturient period.