Abstract
Seventy-two Simmental bull calves were assigned randomly either to a zeranol implanted (I) or nonimplanted (NI) control group. Within 3 d after birth, bulls assigned to the I treatment were implanted with 36 mg zeranol and reimplanted every 84 d until slaughter. Calves were weaned at 7.2 mo, preconditioned for 2 wk, and assigned within treatment to slaughter ages of 12.0, 13.8, 15.7 and 17.4 mo. Blood samples were taken bimonthly from 8.3 to 16.6 mo of age. Weight gains, feed:gain ratios and slaughter and carcass weights were similar (P>.10) for I and NI bulls for the total feeding period. Weight gains for NI bulls were larger (P<.05) from 7.7 to 9.5 mo and from 11.3 to 12.2 mo of age, whereas gains for I bulls were greater from 12.2 to 14.1 mo. Serum estradiol-17β (E2) and testosterone (T) concentrations were higher (P<.05) for NI bulls at 8.3, 9.2, 10.2, 11.1, 12.1 and 13.0 mo. However, at 13.9 and 14.8 mo, E2 concentrations were higher (P<.10) for I bulls. A similar, but nonsignificant (P>.10), trend was noted for T concentrations of I bulls at 13.9 and 14.8 mo. Rapid increases in E2 concentrations were observed from 8.3 to 9.2 mo in NI bulls, but from 12.1 to 13.9 mo in I bulls. Similarly, T concentrations increased rapidly from 8.3 to 10.2 mo in NI bulls, but from 12.1 to 14.8 mo in I bulls. Peak concentrations of T and E2 for NI bulls were observed near 11 and 13 mo, respectively, whereas peak T and E2 levels were delayed in I bulls until near 14 mo. Periods of greater weight gains for both NI and I bulls corresponded to rapidly increasing E2 and T concentrations. Serum thyroxine concentrations were lower (P<.05) for I bulls at 13.9 mo but similar to NI bulls at all other ages. Serum insulin concentrations were higher (P<.05) for I bulls than for NI bulls at 8.3, 10.2, 11.1 and 12.1 mo. Regardless of treatment, insulin concentrations increased with increasing age. Carcasses from I bulls were fatter (P<.05) and tended (P = .09) to have smaller ribeye areas/100 kg carcass. Fat thicknesses for both treatments increased (P<.05) from 13.8 to 15.7 mo. Skeletal maturity scores, 9-10-11th rib bone percentages and femur measurements indicated an increased skeletal maturation in I bulls. Hormonal, growth, developmental and carcass trait differences observed between NI and I bulls in our study indicate that zeranol implantation of prepubertal bulls delays the onset of puberty and does not elicit the same responses reported in steers. Copyright © 1986. American Society of Animal Science . Copyright 1986 by American Society of Animal Science