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A b s t r a c tWide use of artificial insemination necessitates a deeper understanding of how hormones and immune system influence on semen production in sires. With this regard, the role of follicle stimulating hormone and luteinizing hormone in spermatogenesis is under consideration. Blood testosterone and estradiol levels are related and depend on a testosterone-estradiol binding globulin function. We first examined seasonal effects of blood estradiol levels as a reproduction marker in Holstein bull sires and showed the relationship between the blood estradiol concertation, semen fertilizing ability, semen volume, and semen concentration which, in turn, impact on the results of artificial insemination. Estradiol level in Holstein sires aged 30±6 months (n = 18) was assayed using Immuno-FA-E ELISA kit and a Uniplan equipment (ZAO Pikon, Russia). The effectiveness of artificial insemination was tested in 214 cows. We showed that in the bulls the blood estradiol level varied significantly depending on a season (Р < 0.001). In spring, the lowest (0.100 nmol/l) estradiol level detected in 78 % of the bulls was mostly characteristic, and only in 17 % of the bulls estradiol was beyond 0.200 nmol/l (Р < 0.001). At autumn, blood estradiol concentration increased in 94 % bulls (Р < 0.001). When estradiol level rises two times and more, a 31 % decrease in semen volume per ejaculation is observed (i.e. 3.4 ml vs 4.6 ml) which results in about 50 % decrease in semen dose number (112 vs 171). When low blood estradiol on the daу of semen collecting, a 12-17 % success rate occurred in cows after a single insemination (Р < 0.05), and 17-29 % heifers became pregnant to first insemination (Р < 0.001). Thus the blood estradiol in bulls additionally indicates a fertilizing ability of the semen and can be used to improve effectiveness of artificial insemination technique.

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