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 se ason of mature stallions. the mechanism which 'delays' puberty beyond the f irst potential breeding opportunity ha snot been described. Four pre- and f our postpubertal colts were monitored from November for 13 (group 1, 6 to 1 9 months of age) and 9 months (group 2, 18 to 27 months of age). Fortnightl y, 15 flood samples wee collected at hourly intervals from each colt to det ermine concentrations of prolactin (PRL), LH, FSH and testosterone (T). Tes ticular 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 concen trations of all four hormones were observed in both groups. Prolactin conce ntrations were correlated with changes in day length (r=0.88) in both group s, but a biphasic profile was noted in group 1. Moulting of the winter coat was coincident with seasonally increased PRL concentrations (P<0.001). Sea sonal changes in FSH, LH and T concentrations were similarly timed between groups and levels were maximal in mid April,mid March and mid May, respecti vely However, overall mean concentrations of LH and T in group 1 animals we re only 0.33 and 0.22 of group 2 values (P<0.001). Suppression of gonadal a ctivity during the colts' first 'quiescent' breeding seasonal was associate d with low concentrations of circulating LH. Appropriately timed changes in plasma PRL, FSH and T concentrations and pelage indicated that the photope riodic mechanism was functional in the pre-pubertal colt but pituitary LH r elease may be blocked by immaturity or active suppression.