Comparative effects of zilpaterol hydrochloride and ractopamine hydrochloride on growth performance, carcass characteristics, and longissimus tenderness of feedlot steers fed barley-based diets

A randomized complete block design experiment was conducted with 4,114 beef steers in a commercial feedlot in western Canada that were fed barley-based diets to evaluate the effects of ractopamine hydrochloride (Optaflexx) and zilpaterol hydrochloride (Zilmax) on health, performance, carcass traits, and LM tenderness. Treatments consisted of feeding ractopamine hydrochloride at 30 mg/kg (100% DM basis) for 32 d at the end of the feeding period or zilpaterol hydrochloride at 8.3 mg/kg (100% DM basis) for 20 d at the end of the feeding period, followed by the required minimum 4-d meat withdrawal period before slaughter (range 6 to 10 d). No differences (P > 0.05) were detected in mortality, number of railers (animals shipped to slaughter before the rest of their pen), ADG, G:F, or DMI between steers fed zilpaterol hydrochloride or ractopamine hydrochloride. Steers fed zilpaterol hydrochloride had heavier HCW (8.4 kg, P < 0.0001), greater percentage of overweight carcasses (2.1 percentage units, P = 0.006), greater DP (1.0 percentage unit, P = 0.0001), greater percentage of YG 1 carcasses (7.0 percentage units, P = 0.01), lower percentage of YG 3 carcasses (4.8 percentage units, P = 0.02), lower percentage of prime carcasses (0.32 percentage units, P = 0.05), and a lower percentage of Canadian AAA carcasses (5.6 percentage units, P = 0.04) compared with steers fed ractopamine hydrochloride. No differences in LM tenderness (P > 0.05) were detected between steers fed zilpaterol hydrochloride or ractopamine hydrochloride. Based on the marketing grid on which these feedlot steers were sold, the additional economic value of zilpaterol hydrochloride at 8.3 mg/kg to ractopamine hydrochloride at 30 mg/kg was $30.36 (Canadian) per steer. Feeding zilpaterol hydrochloride for 20 d to finishing steers resulted in leaner carcasses and greater carcass weight without compromising tenderness compared with feeding ractopamine hydrochloride at 30 mg/kg.