Use of Prevotella bryantii 25A and a commercial probiotic during subacute acidosis challenge in midlactation dairy cows.

The objective of this study was to determine the efficacy of Prevotella bryantii 25A as a probiotic during a subacute ruminal acidosis (SARA) challenge using a commercial probiotic as a positive control. Six multiparous ruminally fistulated cows (BW=685 ± 65 kg; (mean ± SD) in the mid-phase of lactation (70 to 148 DIM) received the following treatments in a replicated 3×3 Latin square design: (1) total mixed ration (TMR; control, CON), (2) TMR + 2g/head per day of a probiotic combination of Enterococcus faecium and Saccharomyces cerevisiae (EFSC), or (3) TMR + Prevotella bryantii 25A. The Latin square consisted of 3 wk of adaptation to the respective treatments during which the animals were fed ad libitum once per day a conventional early-lactation TMR and 1.5 kg of hay. The adaptation was followed by 4 d of SARA (no hay) and 10d of rest (adaptation diet without probiotics). Dry matter intake and milk production were depressed during SARA (22.0 and 31.8 kg/d, respectively) compared with adaptation (24.4 and 34.0 kg/d, respectively) and did not recover during rest (22.3 and 30.7 kg/d, respectively). During SARA, P. bryantii 25A had no effect on rumen pH, whereas EFSC reduced the percentage of time with pH <6.0 (71%) compared with CON (85%) and increased maximum pH. The EFSC treatment tended to increase mean pH over 24h (5.65) compared with CON (5.45). Proportion of time with pH <5.6 tended to be lower with EFSC (46%) than with CON (62%). Populations of bacteria considered to be the most important cellulose digesters in the rumen (Ruminococcus flavefaciens, Ruminococcus albus, and Fibrobacter succinogenes) were also monitored during these treatments using culture-independent real-time PCR methods. The population of R. flavefaciens was similar between the 2 feeding phases, whereas F. succinogenes and R. albus were lower during SARA compared with rest. In light of the present study, P. bryantii 25A did not prove to be an effective preventative for SARA. The role of EFSC in regulating rumen pH was confirmed, with a possible effect of maintaining R. flavefaciens populations during SARA.