Influence of zeranol and breed on growth, composition of gain, and plasma hormone concentrations
1991
Williams, J.E. | Ireland, S.J. | Mollett, T.A. | Hancock, D.L. | Bever, E.E. | Hannah, S.
Seven Angus and six Brangus steers averaging 225 and 245 kg, respectively, were assigned randomly to zeranol (36 mg) implant (I) and no implant (NI) treatments. Steers had ad libitum access to a corn silage diet plus .68 kg of a soybean meal-based supplement fed daily. Steers were bled via jugular catheters on d 0, 28, 56, and 84 at 15-min intervals for 4 h before and 4 h after feeding. Concentrations of growth hormone (GH), insulin (INS), triiodothyronine (T3), thyroxine (T4), and glucose were determined. Whole-body protein and fat contents were monitored. A breed X I interaction (for d 56 to 84 and d 0 to 84) was observed for ADG (P < .05 and P < .07, respectively), feed conversion (P < .05 and P < .07, respectively), and protein deposition (for d 0 to 29 and d 0 to 84; P < .07 and P < .05, respectively). These interactions were attributed to a greater response to I by Angus than by Brangus steers. A feeding X period interaction (P < .10) was observed for mean GH concentration, and INS, T4, and T3 concentrations were higher (P < .05) during the 4-h postfeeding period than during the 4-h prefeeding period. The implant increased (P < .08) mean GH concentration but did not alter the frequency, duration, or amplitude of plasma GH peaks. Steers that were implanted had lower (P < .05) plasma T3. Brangus steers had lower (P < .05) plasma glucose, T3, and T4 concentrations than Angus steers. Results indicate that growth factors beyond those measured are responsible for the anabolic response to zeranol.
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