Sexual activity influences the secretion of reproductive hormones in the stallion

The aim of this study was to investigate the effect of sexual activity on c oncentrations of reproductive hormones in plasma of stallions. In the first experiment, two groups of stallions were monitored for secretion of lutein izing hormone (LH), testosterone and oestradiol from the beginning until sh ortly after the end of the breeding season. One group of animals were reser ve stallions not used for breeding (group 1, n = 10), the other group consi sted of active breeding sires (group 2, n = 8). Blood samples were withdraw n from March to August at 14-day intervals. In sexually nonactive stallions (group 1), seasonal variations in LH, testosterone and oestradiol occurred and concentrations of these hormones reached a maximum in May (p < 0.05). In the breeding stallions (group 2), no significant changes in the concentr ations of these hormones were found between March and August. Concentration s of LH and testosterone were significantly lower in breeding stallions tha n in reserve stallions at most blood sampling times(p < 0.05). In the reser ve stallions, oestradiol concentrations were significantly higher than in t he breeding stallions in April and in June (p < 0.05). In a second experime nt, the effect of regular sexual activity (semen collection three times per week) on the concentration of LH, testosterone and oestradiol was tested i n a group of breeding stallions after a period of sexual rest for several w eeks. Blood samples were taken once daily starting the day before the first semen collection was performed. Testosterone concentration significantly d ecreased in the first days after semen collection started (p < 0.05), while LH secretion was only transiently decreased and no effects on oestradiol c oncentration were found. In both experiments, semen parameters were within the normal range of fertile stallions. Mo correlations between the sexual d rive of the stallions and concentration of reproductive hormones occurred. It can be concluded that in the stallion the secretion of reproductive horm ones is influenced by sexual activity. Regular semen collection seems to in hibit testosterone release by unknown mechanisms while the effects on LH an d oestradiol secretion are less pronounced.