Importance of yeast viability for reducing the effects of ruminal acidosis in beef heifers during and following an imposed acidosis challenge

The study was aimed at determining the importance of yeast (Saccharomyces cerevisiae) viability for reducing the severity of ruminal acidosis in cattle during and following an imposed acidosis challenge. Six ruminally cannulated beef heifers were used in a replicated 3×3 Latin square design and fed a diet consisting of 400g/kg barley silage, 100g/kg chopped grass hay, and 500g/kg barley grain based concentrate (dry matter basis). Treatments were: (1) control (no yeast); (2) active dried yeast (ADY; 4g providing 1010 colony forming units/g; AB Vista, UK); and (3) killed dried yeast (KDY; 4g autoclaved ADY). The periods consisted of 2 weeks of adaptation (day 1–14), week 3 of baseline measurements (day 15–21) and week 4 of acidosis challenge (day 22–28). The challenge model involved restricting consumption of the TMR to 0.5 of ad libitum intake for 24h (day −1; prechallenge) followed by adding barley grain (amount equivalent to 0.25 of DMI) directly to the rumen prior to feeding the TMR (day 0; challenge day). Data were collected from 0 to 24h on the challenge day, 25–48h post grain challenge (day 1 postchallenge) and during a 5-day recovery period. No treatment effects were observed on mean (P=0.40), nadir (P=0.37) and maximum (P=0.29) ruminal pH on the challenge day. Similarly, no treatment differences were observed for ruminal lactate (P=0.46) and total VFA concentrations (P=0.15) on the challenge day. However, proportion of ruminal propionate was increased (P=0.01) while caproate reduced (P=0.01) with ADY. The duration of time that pH<5.8 (P=0.26) and 5.6 (P=0.32) was similar for all treatments. No treatment effects were observed on DMI on the challenge day (P=0.95); and day 1 postchallenge (P=0.30). In conclusion, contrary to our hypothesis, yeast supplementation did not influence ruminal pH during a severe acidosis challenge as the efficacy of both viable and killed yeast was reduced at low ruminal pH. Lack of significant effects observed in the present study might also be due to variability associated with the animal responses to an acute acidosis challenge.