title: Effects of preslaughter withdrawal period on response of feedlot heifers to zilpaterol hydrochloride supplementation: growth performance and carcass characteristics.
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 preslaughter zilpaterol hydrochloride withdrawal period on growth performance and carcass characteristics. Heifers were fed a diet based on steam-flaked corn (2.13 Mcal of NE(m)/kg). Treatments were 1) control, no zilpaterol supplementation; 2) zilpaterol supplementation for 30 d, drug withdrawn from the diet 3 d preslaughter (ZIL-3); 3) zilpaterol supplementation for 30 d, drug withdrawn 6 d preslaughter (ZIL-6), and 4) zilpaterol supplementation for 30 d, drug withdrawn 12 d preslaughter (ZIL-12). Zilpaterol was supplemented at the rate of 0.15 mg/kg of BW daily. Intake of DM averaged 9.2 +/- 0.26 kg/d and was not affected (P > or = 0.36) by treatment. Compared with control heifers, ZIL-3 increased (P < 0.01) carcass-adjusted ADG (59%), G:F (57%), apparent dietary NE(m) (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 the 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 preslaughter tended to decrease carcass-adjusted ADG (linear, P = 0.11), G:F (linear, P = 0.08), apparent dietary NE(m) (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 affected 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.
authors.affiliation: Universidad Autónoma de Baja California, México 21100.
authors.forename: J C
authors.initials: JC
authors.lastname: Robles-Estrada
authors.forename: A A
authors.initials: AA
authors.lastname: Arrizon
authors.forename: A
authors.initials: A
authors.lastname: Barreras
authors.forename: J F
authors.initials: JF
authors.lastname: Calderon
authors.forename: F
authors.initials: F
authors.lastname: Figueroa-Saavedra
authors.forename: N
authors.initials: N
authors.lastname: Torrentera
authors.forename: A
authors.initials: A
authors.lastname: Plascencia
authors.forename: R A
authors.initials: RA
authors.lastname: Zinn
mesh_terms: D000822:Animal Husbandry / Q000379:methods*; D000818:Animals; D001823:Body Composition / Q000187:drug effects*; D002417:Cattle / Q000254:growth & development*; D019587:Dietary Supplements; D005260:Female; D006128:Growth / Q000187:drug effects*; D011897:Random Allocation; D013997:Time Factors; D014297:Trimethylsilyl Compounds / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D014297:Trimethylsilyl Compounds; C475840:Zilpaterol