Evaluation of the Effects of Live Yeast on Rumen Parameters and In Situ Digestibilities in Beef Cattle Fed Growing and Finishing Diets

Live yeast (LY) supplementation to ruminants has shown to increase nutrient digestibility and improve rumen environment by increasing pH in dairy cows. Few studies have determined the impact of LY in growing cattle receiving high-concentrate diets. Two studies were designed to evaluate effects of LY (Saccharomyces cerevisiae) on in vitro gas production (IVGP) fermentation dynamics, rumen parameters, and in-situ digestibility of dry matter (DMD) and neutral detergent fiber digestibility (NDFD) during three feeding phases [grower (GRW), transition (TRANS), finisher (FIN)]. In the first study, eight ruminally-cannulated cattle were used in a randomized complete block design over 55 d to determine effects of top-dressed yeast at 0g/d (CON), 2.5 g/d (LY1), 5 g/d (LY2), 10 g/d (LY3). During the GRW diet, TRT altered DMD and NDFD (P ≤ 0.05) and tended to affect rate of degradation (kd) and acetate-to-propionate ratio (P ≤ 0.10). During the TRANS diet, TRT affected total gas production (TGP), protozoa count (PC), DMD, and NDFD (P ≤ 0.05). Throughout the FIN diet, TRT affected kd, volatile fatty acid concentration, PC, DMD, NDFD (P ≤ 0.05), and tended to impact CH4 and pH (P ≤ 0.10). We conclude that LY affected rumen parameters and digestibility, but dose-response varied by diet phase. The second study was similarly designed, instead with twenty ruminally-cannulated steers supplemented 45 d with LY at inclusions: CON (0g/d), 5 g/d (LY1), 10 g/d (LY2), 15 g/d (LY3). During GRW phase, TRT altered TGP of nonfiber carbohydrates (NFC) and kd of fiber carbohydrate (FC; P ≤ 0.05). LY2 had the most TGP and fastest kd. TRT also influenced DMD and NDFD (P ≤ 0.05) with LY2 providing greatest digestibility. For TRANS, TRT tended to affect NFC kd (P = 0.078) and influenced pH and DMD (P ≤ 0.05) where LY2 yielded highest pH, fastest kd, and greatest DMD. For FIN, TRT affected TGP and kd of the NFC pool, FC kd, CH4, PC, DMD, and NDFD (P ≤ 0.05). TRT response varied during the FIN phase. Overall, no constant dose-response pattern was observed; however, supplementation with LY affected IGVP, rumen parameters, and digestibility consistently with LY2 providing greatest benefit