GROWTH RESPONSE OF BEEF CATTLE AT PASTURE TO ZERANOL OR PROGESTERONE-ESTRADIOL IMPLANTS

Abstract

Forty-eight field trials were conducted in 1980 and 1981 on 38 commercial beef herds to determine the effectiveness of zeranol (36 mg/head of the commercial implant Ralgro®) to promote growth in suckling steer and heifer calves and of zeranol and progesterone-estradiol (Synovex-S®) as growth promotants in yearling steers at pasture. In each trial, approximately even numbers of animals were randomly alloted to a control (nonimplant) or an implanted group. Zeranol and progesterone-estradiol groups were implanted once at initial weighing. Animals were re-weighed when the trials were terminated at various times in the fall. Steer calves implanted with zeranol gained 4.2 and 5.0% more (P < 0.01) than controls for 1980 (0.98 vs. 0.94 kg/day) and 1981 (1.06 vs. 1.01 kg/day) trials, respectively. Zeranol-implanted heifer calves gained 3.5% more (P < 0.05) in 1980 trials (0.88 vs. 0.85 kg/day) and 7.4% more (P < 0.01) in 1981 trials (1.01 vs. 0.94 kg/day) than nonimplanted heifer calves. In trials where steer and heifer calves were pastured together, steers gained 3.4 and 5.0% more (P < 0.01) than heifer calves in 1980 and 1981, respectively. The trial × sex and sex × implant interactions were not significant. Both progesterone-estradiol- and zeranol-implanted yearling steers grew faster (P < 0.01) than control steers by 21.3 and 18.7% (0.91, 0.89 and 0.75 kg/day, respectively) over all 1980 trials, and by 13.0 and 11.7% (0.87, 0.86 and 0.77 kg/day, respectively) over all 1981 trials. Gains in yearling steers implanted with progesterone-estradiol were not different (P > 0.05) than gains in those implanted with zeranol. Key words: Beef cattle, zeranol, progesterone-estradiol, pasture