In the present study, a commercially available anabolic implant containing trenbolone acetate and oestradiol-17p was investigated in intact and castrated bulls. Measurements of growth rate, serum hormones and metabolites in both phenotypes, and testicular development and function in bulls, were obtained. The experimental objectives were realized in a 2 x 2 factorially designed experiment utilizing eight Angus-sired crossbred bulls and eight similar steers castrated at 7 months of age. Half of each group were implanted with 140 mg trenbolone acetate plus 28 mg oestradiol-17|3. Body weights were monitored every 7 days and plasma samples were obtained at 28-day intervals from 9 to 15 months of age. In the implanted groups, mean concentrations of plasma trenbolone and oestradiol-17p were 380 ng/1 and 31 ng/1, respectively. Trenbolone concentrations were undetectable in both non-implanted groups, and plasma oestradiol-17|3 was less than half that in implanted animals. While plasma GH concentrations were stratified according to implant treatment, they did not differ among the groups. Cortisol levels showed phenotype x implant interaction; implanted steers and non-implanted bulls had lower concentrations than non-implanted steers and implanted bulls. The implant resulted in no significant changes in gross testicular measures in the bulls but did affect the testosterone response to a GnRH challenge and basal plasma concentrations of testosterone were reduced. Plasma urea concentrations were reduced by the implant, but sex phenotype had no effect. Over the 175-day trial, implantation did not significantly improve the growth rate of bulls, except during the first third of the experiment. In contrast, implantation improved the growth rate of steers over the entire experiment, although implanted steers still grew more slowly than either implanted or non-implanted bulls. It can be concluded that the endocrine status of the young bull is set for a fast growth rate, making improvements via an anabolic implant marginal, except in the immature animal.
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