PSI-2 - Follicular dynamics 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, multiparous 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, wherein each cow underwent all three treatments in three replicates.  In 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 PG administered on d14. In 7&7G) cows received PG, GnRH, and a CIDR on d0; PG on d7; CIDR removal and PG on d14. In 7&7Max cows received PG, GnRH, and a CIDR on d0; PG and GnRH on d7, and PG on d14. In all 3 replicates the largest follicle, corpus luteum size and presence were measured daily by ultrasonography (US) from d0 until ovulation. Cows were observed every 6 hours for standing estrus and US was used to monitor for ovulation in the same interval. Sixty-six hours after P4 removal, GnRH was administered to all cows.  Statistical analysis was performed using the MIXED and GLIMMIX procedure of SAS 9.4. There was no significant difference between treatment of standing estrus hour (7&7: 50.07±1.84hrs; 7&7G: 49.68hrs±1.94; 7&7Max: 49.11hrs ±1.98; p=0.79) or ovulation hour (7&7: 74.62hrs ±1.98; 7&7G: 76.56hrs±2.31 n=25; 7&7Max: 76.0hrs ±1.24; p=0.67). Additionally, there was no significant difference in onset of estrus between cows with (48.30±2.73 n=61) and without CL presence on d14 (50.06±1.56 n=20; p=0.46). Cows with a CL present on d14 tended to have a delayed time of ovulation (76.66hrs±1.21 n=61; p=0.07) compared to cows without a CL present on d14 (72.50hrs±2.20 n=20). Cows that did not respond to PGF on d7 had delayed onset of estrus (51.09hrs ±1.29 n=68; p=0.02) compared to cows that did respond to PGF on d7 (44.84hrs ±1.61 n=19). Delayed ovulation was also observed in cows that did not respond to PGF on d7 (76.97hrs ±1.26; p=0.05) compared to cows that did respond to PGF on d7 (71.47hrs ±1.89). 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 may reduce the synchronization success rate for FTAI.