LONG-TERM TREATMENT WITH THE GNRH AGONIST BUSERELIN IN BULLS AND ITS INFLUENCE UPON SEMEN CHARACTERISTICS AND CONCENTRATIONS OF LH, TESTOSTERONE AND ESTRADIOL-17-BETA

The influence of a continuous treatment with the GnRH agonist Busereli n upon semen characteristics as well as concentrations of LH, testoste rone and oestradiol-17beta was examined in the bull. The drug was admi nistered subcutaneously as a slow-release implant (2,8 mg Buserelin). Two bulls received two implants (5.6 mg), two bulls were injected with four implants (11.2 mg) and one bull was sham-treated. Series of six blood samples were collected from the jugular vein in hourly intervals seven days prior to and 1, 7, 35, and 93 days after GnRH implantation . Hormone concentrations were determined in blood plasma by enzyme-imm unoassays. Ejaculates were obtained twice weekly from four weeks befor e until 13 weeks after drug implantation. Five ejaculates each from th e period before treatment as well from the early (week 1 - 5) and late (week 9 - 13) treatment period were used to determine the effects of GnRH administration on sperm motility (percentage of progressively mot ile spermatozoa, average velocity of progressively motile spermatozoa) , sperm concentration and sperm morphology. LH concentrations were inc reased in two bulls on the first day after drug implantation. Thereaft er, LH was decreasing in all animals. In three out of four treated bul ls, testosterone and oestradiol-17beta were significantly higher on da ys 7 and 35 after GnRH implantation. There was no apparent difference in hormone secretion of bulls receiving either two or four implants. T he sham-treated bull exhibited no change of hormone levels and semen c haracteristics. GnRH treatment had no effect on the semen parameters m otility, concentration and morphology throughout the experimental peri od.