J. Anim Sci.
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Published online first on January 16, 2009
J. Anim Sci. 1910. doi:10.2527/jas.2008-1071
© 2009 American Society of Animal Science
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Effects of pre-harvest withdrawal period on response of feedlot heifers to zilpaterol hydrochloride supplementation: growth performance and carcass characteristics

J. C. Robles-Estrada*, A. A. Arrizon*, A. Barreras*, J. F. Calderon*, F. Figueroa-Saavedra*, N. Torrentera*, A. Plascencia* and R. A. Zinn{dagger}

* Universidad Autónoma de Baja California, México 21100 {dagger} Department of Animal Science, University of California, Davis 95616

aplas_99{at}yahoo.com

Abstract

Sixty-four crossbred heifers (451 ± 23 kg) were used in a 42-d feeding trial (4 pens per treatment in a randomized complete block design) to evaluate the influence of pre-harvest zilpaterol hydrochloride (β-agonist) withdrawal period on growth performance and carcass characteristics. Heifers were fed a diet based on steam-flaked corn (2.13 Mcal NEm/kg). Treatments were: 1) control, no zilpaterol supplementation; 2) zilpaterol supplementation for 30 d, drug withdrawn from the diet 3 d pre-slaughter (ZIL-3); 3) zilpaterol supplementation for 30 d, drug withdrawn 6 d pre-slaughter (ZIL-6), and 4) zilpaterol supplementation for 30 d, drug withdrawn 12 d pre-slaughter (ZIL-12). Zilpaterol was supplemented at the rate of 0.15 mg/kg BW daily. Intake of DM averaged 9.2 ± 0.26 kg/d and was not affected (P ≥ 0.36) by treatment. Compared with control heifers, ZIL-3 increased (P < 0.01) carcass-adjusted ADG (59%), G:F (57%), apparent dietary NEm (31%), and decreased observed/expected DMI (25%). Treatment with ZIL-3 did not affect marbling score (P = 0.49) or backfat thickness (P = 0.17), but, compared with control group, increased HCW (3.6%, P = 0.03), carcass dressing percentage (3.2%, P = 0.02), LM area (6.3%, P = 0.05), and reduced trimmed fat (31%, P = 0.03). Prolonging the period of zilpaterol withdrawal pre-harvest tended to decrease carcass-adjusted ADG (linear, P = 0.11), G:F (linear, P = 0.08), apparent dietary NEm (linear, P = 0.11) and carcass dressing percentage (linear, P = 0.11). We conclude that growth performance and carcass yield responses to zilpaterol supplementation are negatively impacted by prolonging the period of zilpaterol withdrawal beyond 3 d (the required minimum withdrawal period according to label). Drug withdrawal period may be a relevant factor in explaining variation in performance response to zilpaterol supplementation in commercial feedlots.

Key Words: β-agonist • beef cattle • carcass characteristics • performance • zilpaterol






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