Effect of implanting bull calves with testosterone propionate, dihydrotestosterone propionate or oestradiol-17 beta prepubertally on the pituitary-testicular axis and on postpubertal social and sexual behaviour.

Twelve non-implanted crossbred bull calves served as controls and 30 crossbred bull calves (10/treatment) were implanted for 82 days, beginning at 34 days of age, to determine the influence of testosterone propionate (TP), dihydrotestosterone propionate (DHTP) and oestradiol-17 beta (E2) on prepubertal and pubertal pituitary-testicular function and on postpubertal social and sexual behaviour. Compared with control bulls, concentrations of serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and inhibin concentrations were suppressed (P less than 0.01) in all implanted bulls. Testosterone (T) concentration increased (P less than 0.001) in TP-implanted, but decreased (P less than 0.01) in DHTP and E2 bulls during the implant period. LH response to gonadotrophin-releasing hormone (GnRH) challenge during the implant period (2.5 months of age) was less (P less than 0.01) in TP, E2 and DHTP bulls than in controls. A small but significant T response to GnRH occurred in control bulls at 2.5 months of age. LH and T responses to GnRH challenge at 7 months of age (100 days after implant removal) was similar (P greater than 0.20) in control and implanted bulls. Steroid implants administered prepubertally had no effect (P greater than 0.10) on postpubertal social and sexual behaviours, including number of flehmen responses, abortive mounts, services and competitive order score. Body weight did not differ (P greater than 0.10) between treatment groups, but testis size was reduced (P less than 0.01) during the implant period and up to 10 months of age in treated bulls compared with controls. Testes remained smaller in E2-treated bulls up to the end of the study (23 months of age), but daily sperm production and epididymal weight did not differ (P greater than 0.10) between treatment groups at slaughter. Control bulls reached puberty earlier (P less than 0.01; 270 +/- 11 days of age) than did TP (302 +/- 11 days), DHTP (309 +/- 11 days) or E2 (327 +/- 11 days) bulls. Although puberty was delayed in all implant groups, there was no difference in scrotal circumference at puberty (average 28.4 +/- 0.4 cm) between treatment groups. Our findings indicate that TP, DHTP and E2 implants administered prepubertally result in acute suppression of serum LH, FSH and inhibin during the implant period and in post-implant suppression of testis size and delayed puberty in bulls. The lack of treatment effect on behaviour suggests that steroidal programming of sexual behaviour occurs before 1 month of age in bulls.