Effect of extended withdrawal of zilpaterol hydrochloride on performance and carcass traits in finishing beef steers

The objective was to evaluate the effects of an extended withdrawal period after feeding the beta-adrenergic agonist zilpaterol hydrochloride (ZH) for 20 d at the end of the feeding period. Three hundred eighty-four crossbred beef steers were blocked by BW and randomly allocated into 64 pens (6 steers/pen). Pens were assigned to treatments in a 2 x 4 factorial arrangement in a randomized complete block design. Main effects were the addition of 0 (control) or 8.3 mg/kg of ZH (DM basis) to the finishing diet for 20 d before estimated average slaughter date and paired withdrawal periods of 3, 10, 17, or 24 d before slaughter. Individual BW were measured initially, 1 d before ZH feeding, and 1 d before slaughter. The ZH feeding period was initiated so that control cattle in the 3-d withdrawal group would be expected to average 65% USDA Choice Quality grade and have 1.27 cm of 12th-rib fat based on visual appraisal. Carcass data were collected at slaughter. For the 3-d withdrawal steers, 2 steers from each pen were selected to determine visceral organ and total offal mass at slaughter. The ZH x withdrawal day interaction was not significant (P > 0.10) for the majority of variables. There was no difference (P > 0.12) due to ZH feeding for final BW, carcass-adjusted final BW, or ADG. However, DMI was decreased (P = 0.02) and G: F increased (P = 0.01) in steers fed ZH vs. control steers. As day after withdrawal of ZH increased, there was a linear increase (P < 0.001) in final BW and carcass-adjusted final BW, but a linear decrease (P < 0.001) in ADG over the finishing period and over the ZH plus withdrawal period. Overall, HCW was 380 and 369 kg (P < 0.001) for ZH and control steers, respectively. However, the difference between ZH and control was 14, 17, 5, and 6 kg with 3, 10, 17, and 24 d withdrawal, respectively (ZH x withdrawal day, P = 0.09). Feeding ZH increased dressing percentage (65.8 vs. 64.6%; P < 0.001) and LM area (94.8 vs. 89.7 cm(2); P < 0.001), and decreased calculated yield grade (2.69 vs. 2.91; P = 0.03) and percentage of cattle grading USDA Choice (31.1 vs. 42.3%; P = 0.03) compared with controls. Small intestinal mass (g/kg of empty BW) was greater (P = 0.03) for steers fed ZH compared with controls. There were no other differences (P > 0.11) in mass of body components, expressed in kilograms or as a fraction of empty BW. In this experiment, improvements in animal performance and HCW due to feeding ZH were generally maintained when withdrawal was extended through 10 d.