Effect of inducing original and accessory corpora lutea with GnRH or Hcg on fertility in recipient dairy heifers and cows

Abstract

We have developed a series of studies aimed to characterize the effect of human chorionic gonadotropin (hCG) and gonadotropin releasing-hormone (GnRH) on luteal activity and its effects on pregnancy outcomes in nulliparous dairy heifers and dairy cows submitted to embryo transfer (ET). The first objective was to determine the effect of hCG, or GnRH administered on the last day of a 5d-CIDR Synch program (d0) with or without an additional hCG 5 days later (d5) to create an accessory corpus luteum (CL) on the luteal area and P4 during the diestrus in dairy heifers. The second objective was to determine the effect of hCG or GnRH on P/ET and pregnancy losses in recipient dairy heifers receiving in vivo or in vitro produced (IVP) embryos. The third objective was to determine the effects of inducing an accessory CL on the day of ET fertility outcomes in nulliparous, primiparous, and multiparous. The main hypothesis was that heifers receiving hCG on the last day of a 5d-CIDR synch protocol would have greater P4 during metestrus and diestrus compared with GnRH-heifers, and the recipients with an accessory CL induced by hCG have greater P4, as well as greater P/ET than those receiving GnRH. Heifers receiving hCG on d0 had a larger luteal area and greater serum P4 on d5 than those receiving d0- GnRH. Heifers that received hCG on d 0 and 5 had greater P4 on d7 than the heifers in the other treatments. The recipient heifers receiving both d0-GnRH and d5-hCG had greater d60 P/ET with fresh embryos compared to heifers treated only with d0-GnRH. Conversely, heifers with only d0-GnRH treatment and those receiving hCG on d0 and 5 treatment had greater d60 P/ET than only d0-hCG transferred with frozen-fresh embryos. Heifers in d0-hCG also had lower d60 P/ET than those treated with only d0-GnRH and GnRH followed by injection of hCG using IVP embryos. The overall pregnancy loss tended to be greater on d0-hCG compared to GnRH followed by hCG. When accessory CL was induced on d of ET, parity and embryo stage (blastocyst or expanded blastocyst) were factors that impacted the fertility outcomes. Nulliparous with accessory CL, (hCG + GnRH combined), tended to have greater overall P/ET than controls. Additionally, GnRH-nulliparous receiving vitrified IVP embryos tended and had greater P/ET and calving/ET than controls, respectively. Primiparous receiving expanded blastocyst fresh IVP embryos with GnRH-accessory CL had greater P/ET and calving/ET than primiparous hCG- cows. Moreover, hCG decreased calving/ET in primiparous with expanded blastocyst fresh embryos than controls. Yet, the overall calving/ET was similar between treated groups. In conclusion, effects of accessory CL induction with hCG, or GnRH on P/ET, pregnancy loss, and calving/ET were distinct according to interaction of ovulatory inducer and embryo produced type, and stage of embryo development and recipient parity. Moreover, hCG had a greater luteotropic effect than GnRH, but had detrimental effects on P/ET when used for original or accessory CL inductions according to embryo characteristics and parity.