[The effect of the season and sexual stress on the concentration of testosterone and estradiol-17beta in the seminal plasma of stallions].

Semen from three stallions was collected weekly for six months (December through May) to determine semen parameters and hormones (testosterone and estradiol-17 beta) in seminal plasma. Once a month three ejaculates were collected at intervals of one hour and examined accordingly. Testosterone and estradiol-17 beta were also determined in peripheral blood plasma (V. jugularis) collected twice a week. Semen parameters (volume, gel-free volume and sperm concentration) were clearly influenced by season. The testosterone concentration in peripheral blood plasma was lowest during December whereas in seminal plasma testosterone concentration gradually increased throughout the experimental period. Testosterone concentrations in seminal plasma were only one tenth of those in peripheral blood. Estradiol-17 beta in blood plasma was highest in April and May whereas no significant differences occurred in seminal plasma. Average monthly concentrations of estradiol-17 beta in blood and seminal plasma were in the range of 27.4 pg/ml to 45.3 pg/ml. Collection of three successive ejaculations led to a significant decrease of ejaculate volume as well as sperm concentration. Ejaculation frequency did not influence testosterone in seminal plasma whereas the concentration of estradiol-17 beta decreased significantly in the second and third ejaculate. The results indicate that estradiol-17 beta in the stallion's semen may be bound to spermatozoa or is accumulated in seminal plasma.