Annual Changes in Day-length,Temperature, and Circulating Reproductive Hormones in Thoroughbred Stallions and Geldings

Changes in follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin,immunoreactive(ir)-inhibin, testosterone, estradiol-17β, and insulin-like growth factor(IGF)-I in Thoroughbred stallions along with changes in prolactin secretion in geldingswere studied. The correlations of day-length with changes in the concentrations of thesehormones were also studied. Five stallions and thirteen geldings were employed to drawblood samples in monthly basis and radioimmunoassay was performed to measure thesehormones. All hormones showed a seasonal pattern, the levels being highest during thebreeding season and lowest during the winter months. Most of the hormones were at theirhighest concentration during the month of April, the mid of spring in northern hemisphere.The concentration of circulating IGF-I also demonstrated seasonality, the peak lying onthe month of April. The plasma concentration of prolactin also increased during thebreeding season. This phenomenon was similar both in stallions and geldings althoughgeldings had lower concentration than that of stallions. The changes in concentration ofprolactin in stallions and geldings correlated more towards the day-length than towardsthe temperature. These results clearly indicate the seasonality of pituitary and gonadalhormones of Thoroughbred stallions, the activity being highest during the month of Apriland May of the breeding season.