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

A randomized complete block design trial was conducted with 3,444 beef heifers housed in 10 pens per treatment and fed barley-based diets to evaluate the effects of ractopamine (Optaflexx, Elanco Animal Health, Greenfield, IN) and zilpaterol (Zilmax, Intervet/Schering-Plough Animal Health, De soto, KS) on health, performance, carcass traits, and longissimus muscle tenderness in a commercial feedlot in western Canada. Treatments consisted of feeding Optaflexx at 20 mg/kg (100% DM basis) for 29 d at the end of the feeding period or Zilmax at 8.3 mg/kg (100% DM basis) for 20 d at the end of the feeding period, followed by a 4-d withdrawal period before slaughter. No differences (P > 0.05) were detected in morbidity, mortality, number of salvaged animals, ADG, feed efficiency, or final BW between heifers fed Zilmax or Optaflexx. Dry matter intake was reduced (P < 0.008) when heifers were fed Zilmax compared with when heifers were fed Optaflexx. Heifers fed Zilmax had heavier (P = 0.0001) HCW (9.7 kg), greater dressing percent (1.66 percentage units), greater percentage of YG 1 carcasses (6 percentage units), a lower percentage of YG 3 carcasses (8 percentage units), and a lower percentage of Canadian AAA carcasses (9 percentage units) compared with heifers fed Optaflexx. No differences (P = 0.98) were detected in tenderness between heifers fed Zilmax or those fed Optaflexx. Based on the grid on which these feedlot heifers were sold, the additional economic value of Zilmax to Optaflexx was $20.35 (Canadian) per heifer. Feeding Zilmax for 20 d to British-influenced finishing heifers resulted in leaner carcasses and greater carcass weight without compromising tenderness when compared with feeding Optaflexx.