Effects of dietary changes and yeast culture (Saccharomyces cerevisiae) on rumen microbial fermentation of Holstein heifers.

The effects of a dietary challenge to induce digestive upsets and supplementation with yeast culture on rumen microbial fermentation were studied using 12 Holstein heifers (277 +/- 28 kg of BW) fitted with a ruminal cannula, in a crossover design with 2 periods of 5 wk. In each period, after 3 wk of adaptation to a 100% forage diet, the dietary challenge consisted of increasing the amount of grain at a rate of 2.5 kg/d (as-fed basis) over a period of 4 d, until a 10:90 forage:concentrate diet was reached, and then it was maintained for 10 d. Between periods, animals were fed again the 100% forage diet without any treatment for 1 wk as a wash-out period. Treatments started the first day of each period, and they were a control diet (CL) or the same diet with addition of yeast culture (YC, Diamond V XPCLS). Digestive upsets were determined by visual observation of bloat or by a reduction in feed intake (as-fed basis) of 50% or more compared with intake on the previous day. Feed intake was determined daily at 24-h intervals during the adaptation period and daily at 2, 6, and 12 h postfeeding during the dietary challenge. Ruminal liquid samples were collected daily during the dietary challenge to determine ruminal pH at 0, 3, 6, and 12 h postfeeding, and total and individual VFA, lactic acid, ammonia-N, and rumen fluid viscosity at 0 and 6 h postfeeding. The 16s rRNA gene copies of Streptococcus bovis and Megasphaera elsdenii were determined by quantitative PCR. Foam height and strength of the rumen fluid were also determined the day after the digestive upset to evaluate potential foam production. A total of 20 cases (83.3%) of digestive upsets were recorded in both periods during the dietary challenge, all diagnosed due to a reduction in feed intake. Rumen fermentation profile at 0 h on the digestive upset day was characterized by low ruminal pH, which remained under 6.0 for 18 h, accompanied by elevated total VFA concentration and, in some cases, by elevated lactate concentration. Addition of YC during the dietary challenge did not affect the incidence (10 cases per treatment) or time (7.00 +/- 0.62 d) to digestive upset. However, YC reduced (P < 0.05) the foam strength on the day after digestive upset, suggesting potential benefits of reducing the risk of developing bloat. The proposed dietary challenge model was successful in causing a digestive upset as indicated by reduced feed intake, but the YC addition had no significant impact on rumen fermentation.