Effect of the follicular stage and circulating progesterone concentrations at the beginning of an estradiol/progesterone-based synchronization protocol on profile of gonadotropins and follicle dynamics in beef heifers

This study evaluated the effects of the follicular stage (pre-vs. post-deviation) at the beginning of an estradiol (E2)/progesterone (P4)-based synchronization protocol, and of circulating P4 concentrations (higher vs. lower) during the protocol, on profiles of gonadotropins and development of a new follicular wave in Bos indicus heifers. Cycling Nelore heifers (n = 23; 24.0 ± 2.7 mo old) were pre-synchronized to initiate an E2/P4-based protocol (on Day 0) with either a post-deviation 7-d old follicle (F7d) or a pre-deviation 2-d old follicle (F2d) in the absence of a corpus luteum. Heifers were then assigned to a 2 × 2 factorial arrangement (follicular stage × P4 treatment), receiving either a new intravaginal 2 g P4 device (higher P4, HP4) or a 14-d used 1 g P4 device (lower P4, LP4), plus 1.5 mg of E2 benzoate. Ultrasound evaluations were performed on Days −10, −5, −2, and daily from Day 0–7. Blood samples were collected every 12 h from Day 0–5, and daily until Day 7. Statistical analyses were performed by SAS 9.4 (P ≤ 0.05). On Day 0, the diameter of the initial follicle (F0) was larger (10.7 ± 0.4 vs. 6.7 ± 0.2 mm; P < 0.01), the circulating E2 was greater (5.7 ± 0.9 vs. 2.7 ± 0.5 pg/mL; P = 0.01) and FSH concentrations were lower (0.15 ± 0.01 vs. 0.20 ± 0.01 ng/mL; P < 0.01) in F7d than F2d group. Mean P4 concentrations over time were greater in HP4 than in LP4 group (3.1 ± 0.1 vs. 2.0 ± 0.1 ng/mL; P < 0.01). No interaction effects were observed. Circulating FSH concentrations were greater (P = 0.02) in F2d than F7d group at 0 and 12 h, with no further differences, whereas no differences were observed in LH concentrations over time. The time of emergence (P = 0.73) and deviation (P = 0.81) were not affected by the follicular stage, nor the follicular growth until Day 7 (P = 0.41). Regarding P4 treatments, circulating LH was lower (P < 0.01) in HP4 than LP4 group at 12 and 24 h, with no further differences. FSH concentrations were similar (P = 0.89) from 0 to 60 h but increased from 72 to 120 h in HP4 group (P = 0.04). Although the time of emergence did not differ (P = 0.12), in HP4 group, the number of follicles (≥4 mm) was smaller (6.9 ± 0.5 vs. 9.4 ± 0.4; P < 0.01) and the time of deviation was slightly delayed (5.7 ± 0.2 vs. 5.2 ± 0.2 d; P = 0.04), compared to LP4 group. Moreover, although the follicular development by the time of deviation was not affected (P = 0.59), the follicular growth by Day 7 was slower in HP4 group, resulting in a smaller dominant follicle (7.7 ± 0.2 vs. 8.8 ± 0.4 mm; P < 0.01) and a tendency for lower E2 concentrations (1.1 ± 0.2 vs. 1.5 ± 0.3 pg/mL; P = 0.10), compared to LP4 group. In conclusion, the follicular stage at the beginning of an E2/P4-based protocol did not have major effects on gonadotropin secretion, nor on new follicular wave development. In contrast, higher P4 concentrations resulted in fewer subordinate follicles and increased FSH concentrations after emergence, slightly delaying the time of deviation. In addition, despite not affecting the follicle development during the common growth phase, higher P4 concentrations resulted in a smaller dominant follicle with reduced E2 production.