Effects of Adding Live Yeast or Yeast Derivative on Dry Matter Disappearance of High-Forage Diet in Batch Culture

The objective of this study was to screen for the effects of live yeast (LY) or yeast derivative (YD) on DM disappearance (DMD), fermentation characteristics and microbial profiles in batch culture. The study was a 2 × 7 factorial arrangement including low (5.8) and high media pH (6.5); treatments were: control (no additives), three LY (LY1, LY2, LY3, 8×10⁶ cfu/bottle for each LY), two YD (YD4, YD5, 30 mg/bottle for each YD), and monensin (positive control; 0.17 mg monensin/bottle). Substrate was a high-forage diet containing 60% barley silage and 40% concentrate (DM basis). Inoculum was obtained from two ruminally fistulated beef heifers fed the same diets to the substrate. Substrate (0.75 g) ground through a 1-mm sieve was weighed into a filter bag and incubated for 24 h in a gas-tight culture bottle in three replications by each combination of treatments. The culture was repeated at different day. Data were analyzed using mixed procedure of SAS with a model that includes fixed effects of yeast, pH level and their interactions, and the random effect of day. There was no interaction of yeast products with media pH on DMD. Increased media pH from 5.8 to 6.5 improved (P < 0.01) DMD (averaged 47 vs. 54%). Supplementation of LY1 (48.2%) or YD5 (47.8%) had greater (P < 0.05) DMD than control (45.6%) or monensin (46.1%) at pH 5.8; and at pH 6.5, the DMD was greater (P < 0.05) with all 5 yeasts (averaged 54.8%) than control (52.8%) or monensin (52.7%). An interaction between pH and treatment was noticed (P < 0.02) for total VFA concentration (mM) which was greater (P < 0.05) with LY1, LY3, YD4 and YD5 (averaged 62.5) than control (57.0) and monensin (57.8) at pH 5.8. Ratio of acetate to propionate was less with monensin at either pH 5.8 (1.89) or pH 6.5 (2.22), whereas no differences were found between yeast and control. Overall, microbial profiles that were measured based on copy number of 16s rRNA gene (log₁₀) were not affected by yeast supplementation except that the copy number of Fibrobacter succinogenes was less (P < 0.01) with monensin (7.16) than yeasts (7.95). These results suggest that in vitro DMD of high-forage diet varied with source of yeast and media pH. The improved DMD at pH 5.8 with LY1 and YD5 and at pH 6.5 with all five yeast products would be beneficial to high-forage fed cattle.