Growth and Physiological Responses to Heat Stress and Zilpaterol Hydrochloride in Angus and Brahman Steers

Abstract The response to zilpaterol hydrochloride supplementation by two breeds of beef steers under controlled heat stress conditions were evaluated for impacts on growth, carcass merit, and physiological stress indicators. In two separate studies, Brahman and Angus steers were placed in environmental chambers and housed for 21 d under 1 of 2 environmental conditions 1) heat stress (THI = 73 to 85) or thermoneutral (THI = 68). In a 2 x 2 factorial, steers also received 8.38 mg· kg-1· d-1 on 88% DM of zilpaterol hydrochloride (ZH) or soybean meal carrier only for the 21-d period. Feed intake, respiratory rate, and rectal temperatures were evaluated daily. Total weight gain was recorded and used to calculate average daily gain (ADG) and gain-to-feed ratio (G:F). Brahman steers were harvested at the market weight (489.31 ± 97.7 kg) and Angus steers on d ~22 after treatment (307.26 ± 6.23 kg). In both studies, body weight gain and ADG did not differ between environments or supplements. Brahman steers exposed to heat stress exhibited greater (P < 0.05) respiratory rate than thermoneutral steers, regardless of supplement. An environment x day interactions (P < 0.04) was observed for rectal temperature. On d 8, unsupplemented heat-stressed Brahman steers had higher (P < 0.05) rectal temperatures than ZH-supplemented thermoneutral steers. On d 11, ZH-supplemented heat-stressed Brahman steers had higher (P < 0.05) rectal temperatures than unsupplemented thermoneutral steers. Angus steers exposed to heat stress exhibited greater (P < 0.05) respiration rates throughout the experiment than thermoneutral steers. On d 8, unsupplemented heat-stressed Angus steers exhibited greater (P < 0.05) respiration rates than ZH-supplemented heat-stressed steers. On d 8 and 19, heat-stressed Angus steers exhibited greater (P < 0.05) rectal temperatures than thermoneutral steers. Angus steers exhibited an environment x supplement x day interaction (P < 0.05) for rectal temperature, with a decrease (P < 0.05) rectal temperature in ZH-supplemented steers. Carcass merit traits did not differ between environments in Brahman steers. Cold carcass weights were lighter (P < 0.05) in heat-stressed Angus steers compared with thermoneutral steers. ZH-supplemented Angus (but not Brahman) steers exhibited decreased (P < 0.05) respiration rates and rectal temperatures. Together these findings indicate that Brahman steers were more resistant to heat stress than Angus steers but were less efficient, with ~10% less G:F. Based upon the data collected, supplementation with zilpaterol hydrochloride did not negatively impact production in either breed.