172 Effect of period of insertion of a progesterone-releasing device and pro-oestrus length on follicular and luteal characteristics and pregnancy rates to fixed-time AI in Bos indicus heifers

An experiment was designed to evaluate the effect of the length of insertion of a progesterone (P4)-releasing device and the length of pro-oestrus on follicular and luteal characteristics and pregnancy rates to AI (P/AI) in Bos indicus heifers treated with oestradiol/P4-based treatments. Bos indicus beef heifers (n=374), 22-26 months of age, with a corpus luteum (CL) or at least one follicle ≥8mm in diameter and with a body condition score between 2.5 and 3.5 (1-to-5 scale) were synchronised using three treatments for fixed-time AI (FTAI). On Day 0, all heifers received 2mg of oestradiol benzoate (Sincrodiol, Ourofino) and an intravaginal device with 1g of P4 (Sincrogest, Ourofino). The P4 device was removed on Day 6 in heifers in the J-Synch 6 group (n=120) and on Day 7 in heifers in the J-Synch 7 group (n=105) and conventional group (n=165). All heifers received 500μg of cloprostenol (Sincrocio, Ourofino) and 300IU of equine chorionic gonadotrophin (SincroeCG 6000UI, Ourofino) at the time of P4 device removal. Furthermore, heifers in the conventional treatment group received 0.5mg of oestradiol cypionate (SincroCP, Ourofino) at the same time. In addition, all heifers were tail-painted for oestrus detection (CeloTest, Biotay). Heifers that had lost ≥50% of the tail paint by 70-74h (J-Synch groups) or 48-52h (conventional group) after device removal were FTAI at that time. Heifers not showing oestrus by 70-74h (J-Synch groups) or 48-52h (conventional group) received 10μg of gonadotrophin-releasing hormone (Sincroforte, Ourofino) at the same time and were FTAI 8h later. All heifers were also examined using ultrasonography (Mindray DP50 Vet) every 12h from the time of device removal to determine the time of ovulation, 6 days after ovulation to determine the diameter of the CL, and 28 days after FTAI for P/AI determination. Data were analysed using the MLGM procedure (InfoStat) for normal data families (follicular dynamics) and binary data family (P/AI). The results are shown in Table 1. The diameter of the dominant preovulatory follicle and the CL did not differ among groups (P>0.12). However, the interval from device removal to ovulation was longer in heifers in the J-Synch groups than in heifers in the conventional group (P