Reproductive performance of lactating dairy cows after inducing ovulation using hCG in a five-day progesterone-based fixed-time AI protocol

This study compares the effects of inducing ovulation using human chorionic gonadotropin (hCG) or gonadotropin releasing hormone (GnRH) at the end of a 5-day progesterone(P4)-based protocol for fixed-time artificial insemination (FTAI) in lactating dairy cows on ovarian dynamics and fertility. A P4 intravaginal device (PRID) was fitted for five days and GnRH administered upon PRID insertion and a double dose (24 h apart) of prostaglandin F2α upon PRID removal. Cows received either GnRH (GnRH group; n = 98), 1000 IU hCG (hCG-1 group; n = 97), or 3000 IU hCG (hCG-2 group; n = 104) 36 h after PRID removal and were inseminated 50-56 h after PRID removal. Based on Tukey-Kramer tests, cows in hCG-1 and hCG-2 showed a larger follicle diameter at AI than cows in GnRH (P < 0.05). HCG-2 treatment increased corpus luteum (CL) size on Day 7 post-AI compared with the GnRH and hCG-1 treatments (P < 0.05). Based on odds ratios, the likelihood of ovulation failure decreased with increasing follicle diameter at AI (P = 0.002). Cows in hCG-1 or hCG-2 that did not become pregnant were more likely to return to estrus than cows in GnRH (P < 0.01). A larger CL size post-AI was related to an increased conception rate at FTAI (P = 0.003). Cows given hCG-2 treatment showed a 1.9-fold cumulative pregnancy rate after two rounds of AI compared to cows receiving GnRH. Our results indicate that hCG treatment hCG treatment used to induce ovulation at the end of a short protocol for FTAI improves follicular/luteal dynamics compared to GnRH treatment. Of the two hCG treatments tested, better results were obtained with the 3000 IU dose.