118 Effects of an Active Dry Yeast Supplement on Ruminal pH, Feeding Performance, and Carcass Characteristics of Feedlot Steers.

Active dry Yeast (ADY) has been suggested to moderate ruminal acid load in growing beef cattle diets potentially enhancing feeding performance and carcass characteristics. The objective of this study was to determine the effects of adding ADY (1.5 g/d; 6 × 1010CFU/d) to the diets of feedlot steers on ruminal pH, feeding performance, and carcass characteristics. Sixty British-cross steers were blocked by weight (Heavy; HVY(n=2) or Light; LT (n=2)) across four pens in a two treatment by two period parallel design where steer was the experimental unit. Treatments were control basal diet (CON) or basal diet plus ADY. Period 1 (P1) consisted of 70 d of a grower diet (NEg of 0.95 Mcal/kg) and period 2 (P2) consisted of a 14 d step up to a finishing diet (NEg of 1.22 Mcal/kg) fed for 56 d. A stratified subset of 30 steers were equipped with rumen pH measuring boli spanning the diet transition (50 d). Mean ruminal pH did not differ between CON and ADY (6.28 vs. 6.21, respectively; P= 0.21) but tended to be different between P1 and P2 (6.26 vs. 6.24, respectively; P= 0.12). There was no significant effect of TRT or period on area under the curve (AUC) of pH 5.4, average time below pH 5.4, or frequency of crossing pH 5.4 threshold. However, cumulative variation of AUC revealed a TRT x Period interaction (P< 0.001) increasing significantly for CON steers from P1 to P2 but not for ADY steers (3.89 to 109.35 h vs. 1.04 to 17.76 h, respectively). There was no significant effect of TRT on final BW, DMI, ADG, F:G ratio, RFI, or carcass characteristics (HCW, 12th rib fat thickness, REA, Yield Grade, or Quality Grade). Liver abscess occurrence was 26.3% and tended to be affected by TRT in which frequency was greater for CON than for ADY (34.48 vs. 17.9 %, respectively; P= 0.15). Severity of abscess was not affected by TRT (P= 0.46) but CON steers had a greater frequency of A+ liver scores than ADY steers (14.92 vs. 7.14 %, respectively). At the current ADY feeding rate, feedlot steers had similar growth performance and carcass characteristics across treatments. However, we observed a significant decrease in the variability of AUC in the sub-acute acidosis domain and a tendency for fewer liver abscesses in ADY fed steers compared to CON fed steers.