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

ABSTRACT 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