Impact of sorting before feeding zilpaterol hydrochloride on feedlot performance and carcass characteristics of yearling steers

Two studies evaluated sorting and feeding zilpaterol hydrochloride (ZH) on feedlot performance and carcass characteristics in randomized block-designed finishing trials. In Exp. 1 (initial BW 342 ± 10 kg, = 1,000), 5 treatments included an unsorted non-ZH fed negative control (-CON), an unsorted ZH fed positive control (+CON), and 3 treatments in which 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, the remaining steers in sorted treatments were fed 14 d longer than -CON and +CON. Average days on feed for control treatments were 165 and 173 d for the EARLY, MIDDLE, and LATE treatments. In Exp. 2 (initial BW 376 ± 29 kg, = 1,400), 4 treatments included -CON; +CON; an early weight sort fed ZH (1-SORT) with the heaviest 20% identified at d 1 and sorted 50 d from harvest and marketed 14 d before -CON and +CON, with the remaining 80% of the pen fed 7 d longer than -CON and +CON; and a 4-way sort 50 d from harvest fed ZH (4-SORT) with steers sorted into HEAVY, MID-HEAVY, MID-LIGHT, and LIGHT groups marketed -14, 0, +7, and +28 d from -CON and +CON, respectively. Average days on feed for control treatments were 154 and 157 d for the 1-SORT 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 (P 0.10) between +CON and -CON, whereas carcass weight SD of 4-SORT was reduced (P < 0.01) compared with that of -CON and +CON. Steers fed ZH had a greater percentage of carcasses over 454 kg than steers fed -CON (P < 0.01). Although not statistically different (P = 0.27), the percentage of carcasses over 454 kg was reduced by 28% for 4-SORT compared with +CON. Feeding ZH increases carcass weight, but sorting reduces variation, allowing further increases in carcass weight while minimizing overweight carcasses.