PS5-5. Comparison of Melengestrol Acetate versus CIDR in Follicular Dynamics of Angus Heifers Using a Modified ReBreed21 Resynchronization Program.

Abstract The objective of this study was to evaluate follicular dynamics in pubertal heifers using a modified ReBreed21 resynchronization program with melengestrol acetate (MGA) or a controlled internal drug release (CIDR) compared to an untreated control. Angus heifers (n = 59) 14-16 months of age on a random day of the estrous cycle were synchronized with a standard 7-day Co-Synch+CIDR protocol (GnRH+CIDR insertion on d-10; CIDR removal+PGF2α on d-3; and GnRH on d0). No AI was performed on d0 to ensure return to estrus and avoid bias from early pregnancy loss. On d14, heifers were randomly assigned to three treatments: Control (n = 19; no further treatment); RB22 (n = 20; CIDR [1.38 g progesterone] inserted on d14, removed on d19, followed by GnRH [0.1 mg gonadorelin i.m.] on d22); or RB22MGA (n = 20; MGA fed at 0.5 mg/head/day from d14 to 18, followed by GnRH [0.1 mg gonadorelin i.m.] on d22). Transrectal ultrasonography was performed from d17 to 25 to monitor follicular dynamics. Ovulation was defined as the day the largest growing follicle (>9 mm) disappeared between consecutive scans. Data were analyzed using ANOVA and chi-square tests in SAS (P ≤ 0.05 significance), with post-hoc orthogonal contrasts (C1: Control vs. RB22 + RB22MGA; C2: RB22 vs. RB22MGA). Levene's test compared the variance in ovulation day among groups. Dominant follicle diameter on d17 was averaged 10.06 ± 0.69, 8.76 ± 0.51, and 10.79 ± 0.61 mm for Control, RB22, and RB22MGA, respectively (C1=0.72; C2=0.01). On d19, diameters were not different (P = 0.16), averaging 10.21±0.58, 10.01 ± 0.52, and 11.39 ± 0.55 mm for Control, RB22, and RB22MGA respectively (C1=0.49; C2=0.07). On d22, diameters were 10.70 ± 0.68, 9.69 ± 0.48, and 13.02 ± 0.50 mm for Control, RB22, and RB22MGA respectively (P < 0.01; C1=0.40; C2 < 0.01). There was a tendency (P = 0.07) for a difference in pre-ovulatory follicle size at ovulation among groups (Control [11.30±0.53], RB22 [10.91±0.57], RB22MGA [12.61±0.51 mm]; using further post hoc contrast, a larger follicle was observed for RB22MGA vs. RB22 (C2=0.03). The average day of ovulation was earlier in Control (21.27±0.38 d) than in RB22 (22.94±0.37 d) and RB22MGA (23.20±0.33 d; P < 0.01, C1 < 0.01), with no difference between RB22 and RB22MGA (C2=0.59). The variance in the distribution of the day of ovulation varied among groups (Control variance: 3.92, RB22: 1.26, RB22MGA: 1.64; P < 0.01, C1 < 0.01, C2=0.60). In RB22MGA, all heifers (100% [20/20]) ovulated by day 25, compared to 80% of RB22 (16/20), and 79% of Control (15/19) (P = 0.16). The use of MGA resulted in follicular dynamics similar to those observed with CIDR in this modified ReBreed22 program. Further studies with larger sample sizes are needed to investigate the fertility outcomes, on-farm application, and overall reproductive performance of RB22MGA on Bos Taurus heifers.