Seasonal changes in reproductive and pelage status during the initial 'quiescent' and first 'active' breeding seasons of the peripubertal pony colt

Pony colts (similar to 12 m) are generally infertile during the breeding season of mature stallions. the mechanism which 'delays' puberty beyond the first potential breeding opportunity ha snot been described. Four pre- and four postpubertal colts were monitored from November for 13 (group 1, 6 to 19 months of age) and 9 months (group 2, 18 to 27 months of age). Fortnightly, 15 flood samples wee collected at hourly intervals from each colt to determine concentrations of prolactin (PRL), LH, FSH and testosterone (T). Testicular biopsies were collected monthly to evaluate spermatogenic activity. Puberty occurred between 17 and 19 months, and at the onset of the 2-year-old breeding season, all colts were fertile. Seasonal changes in the concentrations of all four hormones were observed in both groups. Prolactin concentrations were correlated with changes in day length (r=0.88) in both groups, but a biphasic profile was noted in group 1. Moulting of the winter coat was coincident with seasonally increased PRL concentrations (P<0.001). Seasonal changes in FSH, LH and T concentrations were similarly timed between groups and levels were maximal in mid April,mid March and mid May, respectively However, overall mean concentrations of LH and T in group 1 animals were only 0.33 and 0.22 of group 2 values (P<0.001). Suppression of gonadal activity during the colts' first 'quiescent' breeding seasonal was associated with low concentrations of circulating LH. Appropriately timed changes in plasma PRL, FSH and T concentrations and pelage indicated that the photoperiodic mechanism was functional in the pre-pubertal colt but pituitary LH release may be blocked by immaturity or active suppression.