The Effect of GnRH vs No-GnRH at the Initiation of a 7-d CIDR Synchronization Treatment on AI Pregnancy Rates in Postpartum Lactating Angus and Bos indicus x Bos taurus Cows1,2

Bos indicus x Bos taurus (n=251) and Angus (n=323) cows received a progesterone containing device called an Eazi-BreedTM CIDR® for 7 d with a prostaglandinF2a (PGF2a) injection at CIDR removal. At CIDR insertion half the cows received a GnRH injection and half did not. Upon CIDR removal, half the cows in both GnRH and No-GnRH groups were equally divided into two AI treatment groups: 1) injection of ECP 24 h after CIDR removal with AI 8 to 10 h after a detected estrus and any cow not exhibiting estrus was timed-AI 48 h after ECP (GnRH/ECP; No-GnRH/ECP treatments); or 2) estrous detection for 72 h with AI 8 to 10 h after a detected estrus and any cow not exhibiting estrus by 72 h was timed-AI plus GnRH at 80 h (GnRH/No-ECP; No-GNRH/No-ECP treatments). When data were pooled for cows that did or did not receive GnRH at CIDR insertion, overall pregnancy rates were greater (P<0.05) for the GnRH (56.3%) compared with the No-GnRH (42.4%) treatment for Bos indicus x Bos taurus cows; whereas, overall pregnancy rates were similar between Angus cows that received GnRH (60.6%) compared with No-GnRH (60.8%). Overall pregnancy rates were similar between the four treatments for both the Bos indicus x Bos taurus and Angus cows. Administration of GnRH at insertion of a 7-d CIDR increased overall AI pregnancy rate in postpartum lactating Bos taurus x Bos indicus cows compared with cows that did not receive GnRH at CIDR insertion; whereas, no increase in overall pregnancy rate was observed in Angus cows. INTRODUCTION The “Select Sync” estrous synchronization system, which consists of a GnRH injection followed 7 d later with a prostaglandin F2a (PGF2a) injection with 5 d of estrous detection, has been used in Bos indicus x Bos taurus cows with limited success. The primary reason for the poor pregnancy rates appears to be the result of a decreased estrous response. Injecting an estrogen-like compound after removal of a 7-d progesterone-releasing device known as the Eazi-BreedTM CIDR® has been shown to be effective in inducing estrus and ovulation without affecting fertility at the subsequent AI. Previous research in our lab with Bos indicus x Bos taurus cows reported that administration of estradiol benzoate at the initiation of a 7-d CIDR synchronization treatment increased pregnancy rates by greater than 10% compared with no estradiol benzoate at CIDR insertion. Estradiol benzoate results in the death of most large ovarian follicles resulting in growth and ovulation of a new and more fertile follicle after CIDR removal. However, it is unclear if a similar increase in pregnancy rates will be observed when GnRH is administered at CIDR insertion in Bos indicus x Bos taurus cows.