The effect of hCG or GnRH administration on pregnancy rates in Holstein heifers when used to induce ovulation as part of a 5-day Co-Synch plus Progesterone-Releasing Intravaginal Device protocol

The aim of this study was to investigate how administering human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH) effects pregnancy rates in heifers when used to stimulate ovulation as part of a 5-day Co-Synch + Progesterone-Releasing Intravaginal Device (PRID) protocol. In this study, Holstein heifers (n=242) were randomly divided into 4 groups. In Group GPG (GnRH-PG-GnRH, n= 60), GnRH was administered on day 0, and a PRID was inserted. After the PRID was removed on day 5, prostaglandin F2 alpha (PGF(2a)) was administered, and 72 h later fixed-time artificial insemination was performed with sexed semen, and GnRH was administrated. Group PG (NoGnRH-PG-GnRH, n= 59) was given the same protocol as Group GPG, except no GnRH was administered on day 0. Group GPH (GnRH-PG-hCG, n= 62) differed from Group GPG in that hCG was administered at artificial insemination. Group PH (NoGnRH-PG-hCG, n= 61) did not receive GnRH treatment on day 0, but like Group GPH, hCG was administered to induce ovulation. Based on progesterone levels measured 10 days prior to the application of the protocol and on day 0 of treatment, it was determined that all of the heifers included in the study were cyclic. Pregnancy rates on day 30 were 48.3%, 54.2%, 53.2% and 45.9% in Groups GPG, PG, GPH and PH, respectively. Pregnancy rates on day 60 were 45%, 50.8%, 50% and 44.3% in Groups GPG, PG, GPH and PH, respectively. Furthermore, gestational losses in these groups were 6.8%, 6.2%, 6.1% and 3.6%, respectively (P>0.05). In conclusion, in the present study, similar pregnancy rates (P>0.05) were achieved in all groups. Furthermore, it was determined that similar pregnancy rates were also achieved with no GnRH treatment on day 0 and with administration of hCG or GnRH for induction of ovulation (P>0.05).