Exposure to progesterone previous to the protocol of ovulation synchronization increases the follicular diameter and the fertility of suckled Bos indicus cows.

The objective was to evaluate the effect of injectable progesterone previous to the timed artificial insemination (TAI) protocol on follicular growth, ovulation and pregnancy rate of suckled Bos indicus cows. In experiment 1, 10 days before the beginning to TAI protocol (D-10), 431 suckled-anestrus Nelore cows (249 multiparous and 182 primiparous), were allocated to one of three treatments groups (control, P4i and P4iGnRH). At this moment, cows in the P4i and P4iGnRH group received 150 mg of injectable progesterone intramuscularly (Sincrogest injetável, Ouro Fino, Brazil). On Day 0 (D0), all cows were synchronized using an estradiol/progesterone-based TAI protocol. Simultaneously, in the P4iGnRH group, cows received 10 μg of Busereline intramuscularly (Sincroforte, Ouro Fino, Brazil). Ultrasound exams were performed to evaluate the diameter of the largest follicle (D0, D8 and D10), ovulation rate and diameter of the corpus luteum (D24). In experiment 2, 681 suckled Nelore cows (356 multiparous and 325 primiparous) were synchronized using an estradiol/progesterone-based TAI protocol and received treatments similar to experiment 1. TAI was performed 48 h after removal of the progesterone (P4) device. Pregnancy diagnosis was 30 d after TAI. In experiment 3, blood samples were collected to evaluated the progesterone concentration for 168 h after administration of injectable progesterone intramuscularly. Statistical analyses were performed by GLIMMIX procedure of SAS. In experiment 1, the diameter of the largest follicle (LF) on D10 (P = 0.21), follicular growth rate (P = 0.34) and ovulation rate (P = 0.62) were similar among experimental groups. However, there was difference among groups for the LF on D0 [Control (10.9 ± 0.2 mm), P4i (12.7 ± 0.3 mm) and P4iGnRH (12.6 ± 0.3 mm); P = 0.001], LF on D8 [Control (9.7 ± 0.2 mm), P4i (10.4 ± 0.2 mm) and P4iGnRH (9.9 ± 0.2 mm); P = 0.05], presence of the CL on D8 [Control 0% (0/136), P4i 0% (0/140)and P4iGnRH 26.4% (38/144); P = 0.001], diameter of the CL on D24 [Control (19.7 ± 0.4 mm), P4i (20.1 ± 0.4 mm) and P4iGnRH (18.5 ± 0.4 mm); P = 0.001] and pregnancy rate [Control 35.0% (78/223), P4i 45.9% (105/229) and P4iGnRH 40.6% (93/229); P = 0.01]. The circulating concentration of P4 remained above 1.5 ng/mL until 168 h after the P4i treatment. In conclusion, the injectable progesterone previous to the TAI protocol increased diameter of the LF on D0 and D8 without interfering on the ovulation rate. Furthermore, such exposure increases the pregnancy rate in suckled Nelore cows.