185 Effect of time of equine chorionic gonadotrophin administration in a fixed-time artificial insemination protocol on ovulation, and corpus luteum area and blood flow of heifers

Anticipating two days in the use of equine chorionic gonadotrophin (eCG) in anoestrus suckled beef cows under a fixed-time artificial insemination (FTAI) program has been shown, in our previous experiments, to hasten the ovulation of a greater follicle, developing a larger corpus luteum (CL) and greater serum progesterone (P4) levels, and therefore generating greater pregnancy rates. However, we have questioned whether similar results on the CL formation could be replicated in cycling heifers. Thus, the present study aimed to assess the effect of eCG on Day 5 or Day 7 of the FTAI protocol in cycling heifers on spontaneous ovulation, and CL area and % blood flow up to Day 7 after ovulation. Holstein heifers (n = 30) were synchronised and randomly selected for eCG treatment (400 IU) on Day 5 (T5, n = 10) or Day 7 (T7, n = 11) of the FTAI protocol, and without the eCG (control group; CG, n = 9). All heifers received the same FTAI protocol (Day 0, 2 mg I.M. of oestradiol benzoate and an intravaginal progesterone releasing device [IVD, 750 mg]; Day 7, IVD was removed together with 150 mg I.M. of D-cloprostenol). No hormone was used to induce ovulation. The heifers were not inseminated. Daily ovarian ultrasound (Sonoscope S9) was performed from IVD removal up to the ovulation (disappearance of preovulatory follicle). From Day 1 after ovulation (Day 0 = ovulation), the CLs were scanned every 48 h up to Day 7 to determine the area (mm²) and % blood flow (% of the coloured CL area). The ANOVA for repeated-measures and chi-squared and Fisher tests were used to analyse the data by the Statistix (V.10.0) software. Concerning ovulation, the groups (T5 [7/10], T7 [7/11], and CG [2/9]) had similar (P > 0.05) rates (70, 63.3, and 77.8%, respectively). Therefore, the heifers who did not ovulate (up to 96 h after IVD removal) were removed from the other analyses. The ovulation dispersion was similar (P > 0.05) among groups, where Day 11 of the FTAI protocol had a greater (P < 0.05) percentage of spontaneous ovulation (76%) compared with Day 10 (24%). There was a tendency (P = 0.07) to differ among groups (T5 = 226.5 ± 16.1, T7 = 172.8 ± 16.1, and CG = 188.0 ± 16.1 mm²) in the CL area. As expected, the CL area increased (P < 0.05) daily, from 87.6 ± 11.0 (Day 1) to 387.32 ± 11.0 mm² (Day 7). However, there was no effect (P > 0.05) for the interaction between treatment and day. Meanwhile, T5 had the greatest (P < 0.05) CL % blood flow compared with T7 and CG (48.2 ± 2.1, 38.0 ± 2.1, and 40.2 ± 2.1%, respectively). Also, the CL % blood flow increased (P < 0.05) from 26.4 ± 1.9 (Day 1) to 53.1 ± 1.9% (Day 7). However, there was no effect (P > 0.05) for the interaction between treatment and day. In conclusion, the eCG two days before the withdrawal of IVD did not change the moment of spontaneous ovulation, tended to develop a major area of CL, and increased the CL blood flow. Therefore, further studies are required to determine the effect of eCG on CL formation concerning histological differences, as well as field trials to prove the possible benefits of anticipating the eCG in FTAI protocols.