Date of Award


Degree Name

MS in Agriculture - Animal Science


Animal Science


Robert J. Delmore, Jr.


The effect of feeding Zilpaterol hydrochloride (ZH) with a 3 or 10 d withdrawal (WD) period to calf-fed Holstein steers (N=2993) on carcass characteristics and tenderness were evaluated in a feed lot experiment. Cattle were fed 0 or 8.3 mg/kg of ZH for the final 20 d of the feeding period, each treatment level was assigned a WD period of either 3 or 10 d. Treatment groups consisted of Control 3 d WD (C3) and 10 d WD (C10), and ZH fed 3 day WD (Z3) and 10 d WD (Z10). Cattle were slaughtered at a commercial facility, carcasses chilled for at least 40 hours and carcass characteristics evaluated by trained personnel. Loins (n=60) were randomly selected from each treatment group for Warner Bratzler Shear (WBS) analysis. Rib-eye area (REA) increased 3.8 cm2 for ZH fed 3 day WD cattle (P<0.01) when compared to control, and 5.4 cm2 for ZH fed 10 day withdrawal cattle (P<0.01) when compared to control. There was no significant difference in REA between ZH fed 3 and 10 d WD periods (P>0.05). A trend was observed for ZH fed 10 d WD cattle to have an increased hot carcass weight when compared to control (P=0.0589), while there was no significant difference for cattle fed ZH with a 3 day WD (P=0.3763) comparatively. There was no difference in ZH fed cattle when compared to control on; kidney pelvic and heart fat %, adjusted preliminary yield grade, calculated yield grade, marbling score, or lean and bone maturity (P>0.05). There was an increase in WBS for ZH fed cattle when compared to control for Choice 7 d and 14 d aged steaks for both WD periods. Choice Z10 steaks aged 21 d showed an increase in WBS (P<0.05) while the Z3 had no effect. Select Z3 7 d aged steaks had higher WBS when compared to control while Z10 had no effect. Oppositely, the Select Z10 14 d aged steaks had increased WBS while the Z3 had no effect. There was no difference in Select 21 d aged steaks. There was no difference in WBS between the Z3 and Z10 for any of the aging periods. Feeding Zilpaterol hydrochloride for 20 d increased carcass leanness while having little effect on carcass fat of calf-fed Holstein steers. There was no difference observed between 3 d and 10 d WD period. Zilpaterol hydrochloride treatment decreased steak tenderness, although as aging progressed there little to no difference between steaks from ZH fed and control cattle.