Recent evidence indicates that the progesterone (P4) secretion by corpus luteum (CL) during early diestrus is affected by the size of ovulatory follicle and has a significant impact on embryo development and conception rates. Therefore, strategies to promote the growth of the dominant follicle and/or to stimulate the early development of the CL to increase P4 secretion become an alternative to improve conception rates in the beef cattle industry. Our aim was to study the effect of manipulations of the follicle growth on the diameter of the preovulatory follicle (POF) and subsequent size and function of the CL. Cyclic and non-lactating Nelore cows, pre-synchronized by 2 injections of prostaglandin F2α (PGF) 14 d apart, were manipulated to ovulate large or small follicles according to 3 experiments. In Experiment 1 (Exp. 1; n = 23), animals received a second-use intravaginal P4-releasing device along with an injection of oestradiol benzoate on Day –10 (Day 0 = GnRH injection). Cows were split to receive (large follicle group; LF) or not (small follicle group; SF) a PGF injection on Day –10. Progesterone devices were removed on Day –2.5 in the LF group and on Day –1.5 in the SF group. The PGF was injected at the removal of the P4 device. In Experiment 2 (Exp. 2; n = 38), cows in the LF group had the P4 device removed on Day –2.25 or Day –2, whereas in Experiment 3 (Exp. 3; n = 23), the device (first-use) was removed on Day –1.75 in the LF group and on Day –1.25 in the SF group; the other manipulations were similar to Exp. 1. Data analyses were done only on cows that had a functional CL on Day –10 (P4 > 1 ng mL–1) and that ovulated within 24 and 48 h post-GnRH (Exp. 1, n = 14; Exp. 2, n = 14; Exp. 3, n = 12). The three experiments were successful in inducing POF with different sizes, as indicated by the greater diameter of the POF in the LF group compared with SF in Exp. 1 (12.9 ± 0.5 mm v. 10.7 ± 0.6 mm; P < 0.03), Exp. 2 (14.1 ± 0.6 mm v. 11.7 ± 0.4 mm; P < 0.006), and Exp. 3 (13.8 ± 0.6 mm v. 11.7 ± 0.8 mm; P < 0.06). To evaluate the effect of POF size on size and function of the CL, a factorial analysis was performed by SAS software to test the effect of group, day, and their interaction. For CL volume, an effect of group was detected in Exp. 1 (P < 0.02) and in Exp. 3 (P < 0.06), but not in Exp. 2. The group effect represented greater average CL volume from Day 3 to Day 7 in LF (2.42 ± 0.27 and 2.5 ± 0.39 cm3) than in the SF group (1.39 ± 0.18 and 1.2 ± 0.15 cm3) for Exp. 1 and 3, respectively. For P4 concentrations, a group effect was detected only in Exp. 3 (P < 0.007), as indicated by greater average P4 concentrations from Day 3 to Day 7 in LF (2.31 ± 0.31 ng mL–1) than in the SF group (1.37 ± 0.19 ng mL–1). A day effect was detected in all experiments (P < 0.0001), as indicated by a progressive increase of CL volume and P4 concentrations from Day 3 to Day 7. Manipulation of follicle growth performed in Exp. 3 was the most efficient to modify the function and size of the CL. In conclusion, control of POF size by manipulation of P4 concentrations during growth of the dominant follicle alters the size and function of CL postovulation.
CNPq, FAPESP, Ourofino, and PUSP-P.
Factors which interfere in the results of the Fixed-Time Artificial Insemination in beef cattle