Rumen metabolic environment of dairy cows varying in susceptibility of subacute ruminal acidosis modifies in vitro bacterial composition and fermentation

The rumen metabolic environment from either SARA susceptible (S) or unsusceptible (U) cows, was hypothesized to alter the fermentative capacity and bacterial community composition. This was evaluated in vitro through exposure for a period of 24 h of the microbial inoculum of three donor cows to sterile ruminal supernatant obtained from a subset of 15 out of a group of 38 early lactating dairy cows which were either SARA S (n = 8) or U (n = 7) cows at pH 5.8 and 6.8. Compared to exposure to sterile supernatant from S cows, microbiota exposed to sterile supernatant from U cows produced more total volatile fatty acids, irrespective of the in vitro pH (6.8 or 5.8). Specifically, branched-chain volatile fatty acids (BCVFA), such as isobutyrate and total BCVFA are elevated/tended to be elevated in incubations with the sterile supernatant from U cows. Net lactate accumulation in incubations with U cows' supernatant at pH 6.8 was higher than with S cows' supernatant. Nevertheless, a lower relative abundance of Firmicutes, uncultured Ruminococcaceae, Lachnospiraceae_UCG-008, and Anaerovoracaceae Mogibacterium, as well as a higher relative abundance of Prevotella was observed after incubation with sterile supernatant from SARA S cows as compared with SARA U cows.