Effects of sorting prior to feeding Zilpaterol hydrochloride and effects of different implant strategies on feedlot performance and carcass characteristics of steers and heifers

Two studies were conducted to evaluate sorting and feeding Zilpaterol hydrochloride (ZH) on feedlot performance and carcass characteristics. Cross-bred beef steers were used in a randomized block design finishing trial. In Exp. 1, (initial BW 342 ± 10 kg, n = 1,000) 5 treatments included an unsorted non-ZH fed negative control (CON), unsorted ZH fed positive control (+CON); and three treatments where the heaviest 20% within the pen were sorted and marketed 28 d early and the remaining 80% were fed ZH. The 20% were identified at the beginning (EARLY), 100 d from slaughter (MIDDLE), or 50 d from slaughter (LATE). Because of sorting, remaining steers in sorted treatments were fed 14 d longer than -CON and +CON. Average days on feed for control treatments were 165 d and 173 d for the EARLY, MIDDLE, and LATE treatments. In Exp. 2 (initial BW 376 ± 29 kg, n = 1,400) the 4 treatments included CON, +CON, an early weight sort fed ZH (1-SORT) with heaviest 20% identified at d 1, and sorted 50 d from harvest and marketed 14 d prior to –CON and +CON, with the remaining 80% of the pen fed 7 d longer than –CON and +CON, and a four-way sort 50 d from harvest fed ZH (4-SORT) with steers sorted into a HEAVY, MID-HEAVY, MIDLIGHT, and LIGHT group marketed -14 d, 0 d, +7 d, and +28 d from –CON and +CON, respectively. Average days on feed for control treatments were 154 d, 157 d for the 1SORT, and 159 d for 4-SORT. Steers were fed Zilmax at 8.3 mg/kg DM for 20 d followed by a 3 d withdrawal. In Exp. 1, steers fed +CON had 13 kg greater HCW than CON. Steers sorted EARLY, MIDDLE, and LATE had 28, 25, and 24 kg heavier (P < 0.01) HCW than -CON, respectively. Carcass weight SD was greater (P = 0.01) for +CON than -CON, but not different (P = 0.17) between -CON and ZH sorted treatments.