Supplementing yeast culture to beef heifers consuming a forage-based diet

This experiment evaluated the effects of supplementing yeast culture (YC; Saccharomyces cerevisiae) on in situ ruminal degradability, rumen fermentation and microbiota responses of heifers consuming a forage-based diet. Twelve ruminally-cannulated Angus-influenced heifers were ranked by body weight (BW; 180 ± 4 kg) and assigned to 4 groups of 3 heifers each. Groups were enrolled in a replicated 3 × 3 Latin square design containing 3 periods of 21 d and 14-d washout intervals. Groups were assigned to receive no YC (CON), or 1.5 g of YC/100 kg of heifer BW (YC1.5) or 3.0 g of YC/100 kg of heifer BW (YC3.0). During each period, heifers (n = 12/treatment) were housed in individual pens and offered water and bermudagrass hay (Cynodon dactylon) for ad libitum consumption, in addition to 650 g/d (as-fed basis) of a corn-based concentrate. The YC was mixed daily with the concentrate during each period (d 0 to 21). Ruminal in situ disappearance of forage dry matter (DM) and neutral detergent fiber (NDF) were evaluated by incubating polyester bags with 4 g of dietary hay on d 17 for 96 h. Rumen fluid samples were collected on d 0, 8, and 16, in a manner that the final collection (d 16) did not interfere with in situ procedures on d 17. Apparent total-tract digestibility (aTTD) of nutrients was calculated using fecal samples collected every 12-h from d 17 to 21. Data were analyzed using orthogonal contrasts (linear and quadratic) with heifer as the experimental unit. Feed intake was not affected (P ≥ 0.14) by treatments. Inclusion of YC linearly increased (P ≤ 0.04) propionate and iso-butyrate concentrations in the rumen fluid, and tended (P ≤ 0.09) to increase acetate and total volatile fatty acid (VFA) concentrations. Inclusion of YC linearly decreased (P = 0.03) the relative abundance of the bacterial genus Succiniclasticum, and linearly increased (P = 0.04) genera Shannon diversity index in the rumen fluid. Inclusion of YC linearly increased (P ≤ 0.05) ruminal disappearance rate of hay DM and NDF, but did not affect (P ≥ 0.40) degradability of DM and NDF. Inclusion of YC linearly increased (P ≤ 0.05) aTTD of starch and NDF. Inclusion of YC linearly increased (P = 0.04) heifer average daily gain and gain:feed (G:F). Collectively, supplementing 1.5 or 3.0 g of YC/100 kg of BW linearly enhanced utilization of dietary nutrients and production of ruminal VFA in beef heifers consuming a forage-based diet.