Evaluation of active dried yeast in the diets of feedlot steers. II. Effects on rumen pH and liver health of feedlot steers.

The objective of this trial was to determine the benefits of supplementing active dried yeast (ADY; 3 × 1010 CFU/d of Saccharomyces cerevisiae) in diets of growing and finishing steers on ruminal pH and liver health, and evaluate the relationship of these variables with performance traits. Growing beef steers (n = 120) were blocked by weight (i.e., heavy and light) and allocated to 1 of 4 pens in an automated feed intake monitoring system. Steers were fed either control (CON; no ADY) or ADY supplemented in 4 sequential diets: grower diet from days 0 to 70, 2 step up diets (STEP1 and STEP2) for 7 d each, and finishing diet from days 85 to 164. Indwelling rumen boli were administered to monitor rumen pH during days 56 to 106 during the dietary transition. An exchange of pen assignment, within block, occurred on day 70 resulting in 4 final treatment (TRT) assignments: steers fed CON before and after the exchange (CC; n = 30), steers fed CON before and ADY after the exchange (CY; n = 30), steers fed ADY before and CON after the exchange (YC; n = 30), and steers fed ADY (YY; n = 30). Ruminal parameters were analyzed as a randomized complete block design with repeated measures of day, diet and TRT as fixed effects, and block as random effects, using 2 approaches: preliminary analysis of the means or drift analysis (DA; units change from basal values over time). Ruminal pH duration (DUR) below 6.0 (P = 0.05) and 5.8 (P = 0.05) was greater for CY steers than CC steers. Acidosis bout prevalence (pH < 5.6 for 180 consecutive minutes; P < 0.01) and bout DUR (P = 0.05) were greater for CY than other TRT groups. The DA indicated that the ruminal pH variables range, variance, and amplitude of steers in the YC group drifted further from basal pH values than CY and YY steers during the dietary transition (P ≤ 0.02), indicating that removing ADY during the dietary transition was not favorable, but including ADY may reduce ruminal fluctuation. Steers with fewer days experiencing bouts (DEB) had numerically greater ADG (P = 0.11) and tended to have greater G:F (P = 0.06). Liver abscess severity negatively affected ADG (P = 0.04). However, liver abscess severity was not affected by DEB (P = 0.90). There is evidence to suggest that the addition of the specific ADY strain in the diets of beef cattle during the dietary transition may aid in ruminal stabilization, but our study did not find evidence that acidosis bouts were related to abscess prevalence or severity.