73 Comparison of GnRH Administration day in the 7 & 7 Synch Versus 8 & 6 Synch Protocol for Fixed-Timed Artificial Insemination in Beef Heifers

Abstract The study was conducted to identify effects of a decreased interval progesterone on the ovulatory follicle before fixed-timed artificial insemination (FTAI) by altering day gonadotropin-releasing hormone agonist (GnRH;00µg gonadorelin; Cystorelin) administration. Evaluation 7 & Synch and altered estrous synchronization protocols done using estrus expression (EE) FTAI pregnancy rates (PR). Using frozen conventional Angus semen (Select Sires, Inc. Plain City, OH), Angus-cross heifers (n = 68) from University Arkansas beef cattle research unit near Fayetteville were stratified into two treatment groups cyclicity status (CYC) age. Cyclicity determined reproductive tract score (RTS) taken one week prior beginning synchronization. Scores ranged 1-5, where RTS 1-3 considered pre- or peripubertal while scores 4 5 cycling. Heifers with 2 culled Each used controlled internal drug releasing device (CIDR; Eazi-breed; 1.38g progesterone) inserted intravaginally –14 d simultaneous administration prostaglandin F2α analogue (PGF; 500 µg cloprostenol sodium; Synchsure). In first (7 7), GnRH administered –7 d. second (8 6), –6 Both treatments then PGF CIDR removal 0d, 72 ±h GnRH. Estrus recorded at time an Estrotect patch (Rockway Inc., Spring Valley, WI). as 50% more color rubbed off. On contrary, less than off, not. Pregnancy via ultrasound 58 PR analyzed GLIMMIX procedure SAS 9.4 (SAS Institute Cary, NC) CYC fixed effect for EE; EE PR. Statistical differences α ≤ 0.05 tendencies observed between 0.06-0.1. There (P 0.001) tendency affect 0.088). no 0.892) 0.708). conclusion, in synch ± hours did not PR; however, will continue investigate adjusting length injection evaluate effects.