Luteal Function and Pregnancy Rate in Bos taurus Heifers Treated With GnRH Prior to Timed Artificial Insemination.

The study aimed to evaluate the effects of GnRH given 34 h after progesterone (P4) intravaginal device (IVD) removal in a timed artificial insemination (TAI) protocol. In Experiment 1, 17 Hereford and Braford heifers (control, n = 7; GnRH34; n = 10) received 2 mg estradiol benzoate (EB) and a 1 g P4 IVD on Day 0 (D0). On D8, IVDs were removed and 150 μg d-cloprostenol (PGF) and 1 mg estradiol cypionate (EC) were administered. In the GnRH34 group, animals received 25 μg lecirelin 34 h after the IVD removal (D9). TAI was performed in both groups on D10 (48 h after IVD removal). Follicular dynamics were evaluated from D8 until ovulation; blood samples were collected 7 and 12 days after TAI. In Experiment 2, to evaluate the effect of GnRH34 on pregnancy rates (control, n = 187; GnRH34; n = 203), heifers were subjected to the TAI protocols described in Experiment 1. No significant differences were observed in preovulatory follicle (POF) diameter on D10, follicular growth and ovulation rates until 72 h and P4 concentrations on days 7 and 12 after TAI (p > 0.05). Pregnancy rates (PR) were similar between groups (Control = 50.2%; GnRH34 = 50.2%; p = 0.65). A significant effect of body weight (BW) and body condition score (BCS) on PR was observed for control (p< 0.05), but not for GnRH34. In conclusion, GnRH 34 h after the IVD removal in beef heifers previously treated with EC had no effect on luteal function and fertility.