PSIV-7 Follicular dynamics and endocrine environment using variations of the 7&7 Co-Synch Protocol in beef cows.

Abstract The objective of this study was to evaluate the follicular dynamics and endocrine environment of the 7&7 protocol (Andersen et. al., 2021, Anim Reprod Sci, 235) and its variations (Palcheff et al. 2023, J.Anim.Sci., V.101). Dry, Angus-crossed beef cows (n=29) were randomly assigned to one of the three treatments (7&7, 7&7G, and 7&7Max) in a Latin square design. On 7&7, cows received Prostaglandin F2alpha (PGF; 25 mg dinoprost tromethamine, Zoetis, NJ, IM) and an intravaginal progesterone (P4) implant (CIDR®, 1.38 mg, Zoetis, NJ) on day (d) 0; gonadotropin-releasing hormone (GnRH; 100 µg gonadorelin acetate, Zoetis, NJ) on d7; P4 removal and PGF administered on d14. On 7&7G, cows received PGF, GnRH, and a CIDR on d0; PGF on d7; CIDR removal, and PGF on d14. On 7&7Max, cows received PGF, GnRH, and a CIDR on d0; PGF and GnRH on d7, and PGF on d14. In all 3 periods the largest follicle (LF), corpus luteum (CL) size, and presence were measured daily by ultrasonography (US) from d0 until ovulation. Cows were observed every 6 hours for standing estrus, using an estrus detection aid (Estrotect™, Rockway INC, WI). Ovulation was monitored by US until 84h after P4 removal. Sixty-six hours after P4 removal, GnRH was administered to all cows. Blood samples were collected on days 0, 7, 14, 17. Radioimmunoassay (RIA, MP Biomedicals, Irvine, CA) was used to measure P4 concentration of those days. Statistical analysis was performed using the GLIMMIX procedure of SAS 9.4. There was no difference in onset of estrus hour among treatments (7&7: 50.07±1.84 h; 7&7G: 49.68±1.94 h; 7&7Max: 49.11 h±1.98 h; p=0.81) or ovulation hour (7&7: 74.62±1.98 h; 7&7G: 76.56±2.31 h; 7&7Max: 76.0±1.24 h; n = 25; p=0.77). Cows that did not undergo luteolysis after d7 had delayed onset of estrus (51.09 ± 1.29 h, n = 68; p = 0.03) compared to cows that underwent luteolysis after d7 (44.84±1.61 h, n=19). Delayed ovulation was also observed in cows that did not respond to PGF on d7 (76.97 ± 1.26 h; p = 0.04) compared to cows that responded (71.47±1.89 h). When the onset of estrus occurred before hour 54 after P4 removal, cows had decreased circulating P4 on d14 (0.73±0.13ng/ml; n=57) compared to cows that the onset of estrus occurred at or after h 54 after P4 removal (1.07±0.21ng/ml; n=30; p=0.03). In conclusion, while the current protocol recommends fixed-time artificial insemination (FTAI) at 66 hours after P4 removal, cows exhibiting variable responses to protocol injections, have earlier estrus expression, as well as the age of the CL on day 14 may reduce the synchronization success rate for FTAI.