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* Intervet Inc., Millsboro, DE 19966
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Department of Animal Science, Oklahoma State University, Stillwater,OK 74078
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# AgriResearch Center, Canyon, TX 79015
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Johnson Research, Parma, ID 83660
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HMS Veterinary Development, Inc, Tulare, CA 93274
¶ Division of Agriculture, West Texas A&M University, Canyon, TX 79016
jayden.montgomery{at}targacept.com
Abstract
Experiments were conducted at three U.S. locations (CA, ID, and TX) to determine the effects of dietary zilpaterol hydrochloride (ZILMAX, Intervet Inc., Millsboro, DE) and duration of zilpaterol feeding on performance and carcass merit of finishing steers and heifers. At each site, 160 steers and 160 heifers were stratified within sex by initial BW (study d -1) and assigned randomly within BW strata to 1 of 4 treatments in a randomized complete block design (4 blocks/treatment for each sex). The 4 treatments were arranged in a 2 (no zilpaterol vs. zilpaterol) x 2 (20 or 40 d duration of zilpaterol feeding) factorial. When included in the diet, zilpaterol was supplemented at 8.3 mg/kg DM. Each pen consisted of 10 animals. Each animal was individually weighed unshrunk on d 1, 21 or 41, and 66 of the experiment. Following d 66, cattle were slaughtered and carcass data collected. Feeding zilpaterol increased (P < 0.01) final BW of steers and heifers by 11.6 and 6.7 kg, respectively. In addition, feeding zilpaterol hydrochloride increased (P 
0.001) ADG 36% and 18%, and increased (P < 0.001) G:F 28% and 21% for steers and heifers, respectively. For heifers, DMI was decreased (P < 0.001) 6.2% when zilpaterol was fed, whereas in steers DMI tended (P = 0.09) to be decreased. For steers and heifers, feeding zilpaterol increased (P < 0.001) HCW 16.4 and 12.1 kg, dressing percentage 1.5 percentage units for each sex, and LM area 8.23 and 6.37 cm2, respectively. Twelfth-rib fat (P 
0.12) and KPH (P 0.70) were not affected by feeding zilpaterol to either steers or heifers. Feeding zilpaterol decreased (i.e., improved; P = 0.02) calculated yield grade of both steer and heifer carcasses. Marbling score (P = 0.002) and quality grade (P = 0.002) were decreased when zilpaterol hydrochloride was fed to steers, and the decrease in marbling score and quality grade tended to be greater when zilpaterol was fed for 40 compared with 20 d (zilpaterol x duration interaction, P = 0.07). For heifers, marbling score tended (P = 0.07) to be decreased and quality grade was decreased (P = 0.05) when zilpaterol hydrochloride was fed. In general, it appears from this data that zilpaterol hydrochloride fed for 20 to 40 d at the end of the finishing period enhances growth performance and carcass leanness for both steers and heifers.
Key Words: beef cattle • β-adrenergic agonists • carcass characteristics • finishing performance • zilpaterol
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