PS5-4. The Effect of Oxytocin Use on Luteal and Follicular Dynamics in a Modified ReBreed21 Protocol for Postpartum Angus Cows After Timed-AI.

Abstract The goal was to evaluate the effect of oxytocin on follicular and luteal dynamics of postpartum Angus cows (n = 95) in a modified ReBreed21. Cows were synchronized with a standard 7-day Co-Synch+CIDR protocol (Timed-AI d0). A subgroup of 15 cows was not AI to allow a negative control without bias of early pregnancy losses. All cows were fed with melengestrol acetate (MGA) at 0.5 mg/head/day from d14 to d18 and randomized into two groups on d19: Rebreed22 + oxytocin (RB22Oxt, n = 40, 50IU of oxytocin i.m.) or ReBreed22 (RB22, n = 40, 2.5mL of Saline i.m.). On d22 corpus luteum (CL) blood perfusion was evaluated using ColorDoppler ultrasound. Cows with an absent CL or less than 25% of the total area with blood perfusion were inseminated and given gonadorelin (100 mcg i.m.). Ovarian scans were performed on d17, 19, and 22 to measure follicle and luteal dynamics and on d32 for pregnancy diagnosis (embryo heartbeat). Blood samples were collected on days 17, 22, and 25 for pregnancy-associated glycoproteins (PAGs) ELISA. On d22, cows with PAGs one-fold greater than negative control (virgin heifer serum), one-fold greater from d17 (within animal), and a positive increase to d25 were called pregnant. Non-AI cows were randomized to receive oxytocin (nAI_RB22Oxt, n = 8) or not (nAI_RB22, n = 7) and underwent the same follicular assessment. Data were analyzed using ANOVA and generalized linear models in SAS (Tukey post-hoc for subgroups). No differences were observed between RB22 and RB22Oxt for first-service (d0 AI) pregnancy per AI on d25 PAGs (70% [28/40] vs. 65% [26/40]; P = 0.58) or d32 (60% [24/40] vs. 65% [26/40]; P = 0.65), pregnancy loss from d25 to d32 (14% [4/28] vs. 0% [0/26]; P = 0.11), Doppler false positives (14% [4/28] vs. 10% [3/29]; P = 0.65), or nonpregnant rate on d22 (30% [12/40] vs. 27.5% [11/40]; P = 0.82). Including non-AI cows, differences for ovarian dynamics were on d19 (P = 0.02) for dominant follicle (RB22Oxta [0.6±0.07cm3], RB22ab [0.4±0.04cm3], nAI_RB22ab [0.4±0.1cm3], nAI_RB22Oxtb [0.4±0.1cm3]), and on d19 for CL volume (P = 0.04, RB22a [2.5±0.3cm3], RB22Oxtab [1.9±0.3cm3], nAI_RB22Oxtab [1.5±0.4cm3], nAI_RB22b [1.2±0.4cm3]. There was a tendency (P = 0.07) for greater P/AI at second service (d22 AI) for RB22Oxt (90.9%, 10/11) vs. RB22 50% (6/12). For cows inseminated on d22, 9% (1/11) vs. 33% (4/12), showed an increase in PAGs from d17 to d22, RB22Oxt vs. RB22, respectively (P = 0.20), and P/AI was 78% (14/18) for cows with increased PAGs and 40% (2/5), P = 0.14. Overall pregnancy at the end of a 22-day breeding season tended (P = 0.09) to be greater for RB22Oxt (90%, 36/40) than for RB22 (75%, 30/40). Taken together, the addition of oxytocin to the ReBreed21 showed similar ovarian outcomes and tended to increase overall pregnancy rates at the end of a 22-day restricted Timed-AI breeding season.