PSXI-27 Supplementing Bovacillus to feedlot cattle finished under heat stress conditions

Abstract This experiment compared performance, physiological, and carcass responses of finishing cattle receiving a Bacillus-based probiotic under heat stress conditions. Angus-influenced steers (n = 113) were purchased from an auction yard and assigned to the experiment [arrival body weight (BW) = 345 ± 2.7 kg]. On d 0, steers were ranked by initial BW and allocated to 1 of 20 drylot pens (5 or 6 steers/pen). Pens were randomly assigned to receive a finishing diet containing BOV (3 gּ animal-1ּ d-1 Bovacillus) or no additive (CON) until slaughter. Diets contained monensin but not tylosin. Steer BW was recorded on d 0, 28, 56, 84, 112, and at the time of shipping, which was considered final BW. Initial and final BW were used in average daily gain (ADG) calculation. Blood samples were collected immediately after BW assessment on d 0, 28, 56, 112 of the experiment as well as 8 d prior to slaughter. Plasma was analyzed for concentrations of alanine aminotransferase, aspartate transferase, alkaline phosphatase, gamma-glutamyl transferase, bilirubin, and blood urea nitrogen. Real-time temperature and humidity were recorded every hour for the duration of the trial for temperature-humidity index (THI) assessment. Steers were shipped to a packing facility (Tyson; Amarillo, TX) in 3 groups according to BW, which contained an equal number of pens from both treatments. Upon slaughter, hot carcass weight, backfat thickness, Longissimus muscle area, USDA marbling score, USDA yield and quality, and liver abscess score were recorded. The THI during the first and second thirds of the experimental period (79.8 and 78.1, respectively; SEM = 0.22) were substantially above the heat stress threshold (THI = 72). No treatment differences were recorded for BW parameters, ADG, or gain to feed ratio (P ≥ 0.21), despite a 10 kg increase in final BW and a 4 kg increase in hot carcass weight in BOV steers. Dry matter intake did not differ among treatments (P = 0.48). No treatment differences were recorded for carcass quality response, although incidence of liver abscess was reduced in BOV compared with CON groups (7.3 vs 19.0%; P = 0.04). A treatment × day interaction was detected (P = 0.02) for plasma concentration of GGT, which tended (P = 0.08) to be greater for CON vs. BOV steers on d 56. No other treatment differences were detected for plasma metabolites (P ≥ 0.69). Collectively, BOV supplementation mitigated the incidence of liver abscesses in finishing cattle, whereas improvements to steer performance were only numerical.