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Implanting Young Bulls with Zeranol from Birth to Four Slaughter Ages: III. Growth Performance and Endocrine Aspects^1,2,

Kansas State University, Manhattan 66506

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.

¹ Contribution No. 86-122-J, Dept. of Anim. Sci. and Ind., Kansas Agr. Exp. Sta., Manhattan 66506.

² The authors are grateful to Ecco Ranch, Buffalo, KS, and D. M. Hallford, Dept. of Anim. and Range Sci., New Mexico State Univ., Las Cruces, for their cooperation in this research.

³ Present address: Dept. of Anim. and Range Sci., Montana State Univ., Bozeman 59717.

⁴ Present address: Coop. Ext. Serv., Washington State Univ., Pullman 99164-6230.