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An experiment was conducted to determine whether implantation with zeranol would reduce the aggressive behavior and probability of producing dark-cutting carcasses that characterize slaughter bulls. Thirty-six bull calves were allocated to each of four treatments – nonimplanted bulls, steers castrated at an average age of 101 d, bulls implanted with 36 mg zeranol at an average of 101, 170, 263 and 319 d of age and bulls implanted on the latter three dates only. The results reported include observations of weight gains, linear measurements, scrotal circumferences, serum testosterone concentrations, feed consumption, feed efficiency ratios and behavioral traits. Bulls exceeded steers in weight gain during a feedlot test and in body length after 140 d on feed, but neither implantation treatment significantly affected the growth of bulls. Feed consumption and conversion efficiency were lower in steers than in bulls, but were not significantly affected by implantation. Scrotal circumference measurements and serum testosterone concentrations suggested that implantation reduced testicle development during the preweaning and early postweaning periods, but that this effect diminished or disappeared during the later postweaning period. Implantation depressed aggressive and sexual activity between days 111 and 155 of the feedlot test. When evaluated at 13–14 mo of age, both implanted groups reacted to preslaughter mixing of nonfamiliar animals with equivalent head bunting to, but less mounting activity than, nonimplanted bulls. At 15–16 mo of age, however, the implanted bulls exhibited more head bunting than nonimplanted bulls, and the early implanted group exhibited fully equivalent mounting activity to nonimplanted bulls. The dark-cutting carcass condition was associated with mounting activity during preslaughter lairage. Key words: Slaughter bull, zeranol, growth, linear measurements, testicular development, behavior

EFFECTS OF ZERANOL IMPLANTATION ON GROWTH, FEED CONVERSION, TESTICULAR DEVELOPMENT AND BEHAVIORAL TRAITS OF YOUNG BULLS FED FOR SLAUGHTER