AbstractsMedical & Health Science

Fertility in dairy cows has decreased worldwide, also in Finland. The most common reason for treatments of dairy cattle is various fertility disorders (18 % of all animals in 2013). In practice, decreased fertility warrants use of hormonal estrus synchronization protocols to control follicular waves and luteal regression to achieve acceptable pregnancy rates, without unwanted side effects such as short estrous cycles. In the earlier studies of our research group, short estrous cycles were noted in some cyclic dairy heifers and cows when estrus and ovulation were induced with agonistic analogues of prostaglandin F2α (PGF2α) and GnRH treatments administered 24 h apart. Possible causes for such induced short estrous cycles were further elucidated in the four experiments described in this thesis. Estrus and ovulation were induced with PGF2α and GnRH given 24 h apart during early (Day 7 after ovulation) or late (Day 14 after ovulation) diestrus. Also the effect of gonadorelin doses of 0.1 mg or 0.5 mg given 24 h after PGF2α on the occurrence of short estrous cycles, and on the preovulatory release of LH during 6 h following the gonadorelin administration was investigated. The effect of the time interval between PGF2α and GnRH (0 vs. 24 h) given during early diestrus on the occurrence of short estrous cycles was investigated in cyclic dairy heifers and cows. The expression of endometrial receptors of oxytocin, estrogen-α and progesterone as well as enzymes 20α-hydroxysteroid-dehydrogenase and cyclo-oxygenase-II, on Days 2 and 5 after ovulation was analyzed with real-time quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry. The occurrence of induced short estrous cycles was significantly increased with simultaneous administration of PG and GnRH, but was neither related to the size of the preovulatory follicle nor to the GnRH-induced preovulatory release of LH. Also the basal postovulatory release of LH on Days 1, 3 and 5 after ovulation was similar for induced short and normal length estrous cycles. Lower basal LH concentration after ovulation coincided with higher progesterone concentration. The size of the preovulatory follicle was significantly larger when PG and GnRH were given 24 h apart during early diestrus in comparison with late diestrus, but the occurrence of short estrous cycles was similar. The size of the preovulatory follicle in cows did not correlate with the preovulatory secretion of estradiol-17β. The endometrial expressions of receptors and enzymes analyzed were similar for short and normal length estrous cycles. The results described should be taken into account in estrus synchronization protocols utilizing sequential treatments with PG and GnRH. Lypsylehmien tiinehtyvyys on heikentynyt usean vuosikymmenen ajan kaikkialla maailmassa, myös Suomessa, jossa lehmien poikimaväli on pidentynyt noin kuukaudella viimeisten parinkymmenen vuoden aikana. Toisaalta hedelmällisyyshäiriöt ovat tavallisin eläinlääkärien antamien hoitojen syy lypsylehmillä (vuonna 2013 hoidettiin…