Association between the severity of subacute ruminal acidosis and the microbiome of the rumen, and the metabolome of the rumen, blood, and milk in primiparous cows.

Cows have been shown to develop different severities of SARA when fed the same diet. In the present study, we used 24 Holstein dairy cows to investigate the association between SARA severity and the ruminal microbiome as well as the metabolomes of the rumen, blood, and milk from 21 d before parturition until 70 DIM. The dietary concentrate was increased from 32% to 60% (DM basis) during the first week of lactation. Based on the severity of SARA, 3 groups were compared: high, moderate, and low severity (HI, MD, and LO). Relative to parturition, the rumen microbiome and metabolome were analyzed on d -21, 14, 28, and 56; blood metabolome was analyzed on d -21, -7, 1, 3, 7, 10, 14, 21, 28, 42, 56, and 70, and milk metabolome was analyzed on d 1, 3, 7, 14, 21, 56, and 70. Statistical analysis of the metabolome was performed with MetaboAnalyst v. 6.0; and statistical analyses for microbial abundance data were conducted in R using Maaslin2 package. Rumen microbial abundance profile was not associated with the SARA severity, but several amylolytic acid tolerant genera (i.e., Acidobacteria and Lactobacillus) were found only within the HI SARA group. In addition, the transition to the lactation diet decreased fibrolytic bacteria and increased amylolytic and proteolytic bacteria. In the ruminal metabolome, HI SARA was associated with greater levels of nucleotides, nitrogen compounds, and pyrimidine derivatives as well as enrichment of metabolic pathways for nitrogen and AA metabolism. In the blood metabolome, HI SARA was associated with greater levels of cholesterol esters, phosphatidylcholines, and enrichment of metabolic pathways for steroid and bile acid biosynthesis. In the milk metabolome, MD SARA was associated with greater levels of triglycerides as well as enrichment of metabolic pathways for fatty acid biosynthesis. The change from pre- to postpartum increased ruminal triglycerides, amino acids, carbohydrates and their derivatives in the rumen, and increased sugars and lipid derivatives in the blood. Milk showed a decrease mainly in triglycerides with increased DIM compared with first day of lactation. Overall, findings from this study reveal differential metabolomic response of dairy cows when fed the same diet. The greatest differences in the cow metabolome among SARA severity groups were observed either around calving or at peak of lactation, likely due to increased nutrient metabolism. High severity of SARA was associated with enriched metabolic pathways for nitrogen and AA metabolism in the rumen, as well as greater steroid biosynthesis, cholesterol esters and phosphatidylcholines in blood. Moderate SARA severity was associated with enrichment of metabolic pathways for fatty acid biosynthesis and triglycerides in milk.