421 Award Talk: Estrus improved pregnancy success but not embryo growth or uterine attachment.

Abstract Preovulatory estradiol concentrations and expression of estrus have been associated with greater pregnancy success in beef cattle following AI, thus an experiment was designed to determine if supplemental estradiol (E2) at GnRH-induced ovulation would improve pregnancy success in beef cows. Crossbred beef cows (n=62) were synchronized utilizing the 7-day CO-Synch + CIDR protocol. Two days post-CIDR removal (d0), cows were allocated to one of three treatment groups: 1) Estrus Expression (ESTRUS), 2) No Estrus + induced ovulation with gonadotropin releasing hormone (GnRH; GNRH), 3) No Estrus + induced ovulation with GnRH and 0.1 mg estradiol 17β (GNRH+E2). All cows were artificially inseminated on d0 with semen from a single sire. Cows were ovariohysterectomized at harvest on d20, d26, and d32. Overall, an embryo was recovered from 63% of animals. ESTRUS animals tended (P= 0.07) to have greater pregnancy success compared to GNRH animals (79.3% vs 51.9%, respectively); GNRH+E2 animals were intermediate (55.6%). To determine attachment to the uterus, circulating concentrations of pregnancy associated glycoproteins (PAGs) were evaluated. Crown-Rump length of recovered embryo was impacted by day of recovery with embryos recovered on day 32 being longer (P< 0.01) than embryos recovered on day 26 (14.36±0.39 mm vs 7.33±0.35 mm). Crown-Rump length; however, was not impacted by treatment (P = 0.85) or treatment by day (P= 0.19). Embryo weight was also impacted by day with embryos recovered on day 32 being heavier (P< 0.01) than embryos recovered on day 26 (0.69±0.10 g vs 0.27±0.27 g). In addition, treatment tended (P= 0.07) to impact embryo weight with embryos recovered from GNRH animals weighing more (P=0.03) than embryos recovered from GNRH+E2 animals (0.70±0.11 g vs 0.29±0.13 g, respectively). Embryos recovered from ESTRUS animals were intermediate (0.45±0.10g). Concentrations of PAGs were impacted by embryo recovery (P< 0.001) and increased as days post AI increased (P< 0.001; 0±0.19, 1.06±0.18, and 1.94±0.19 for days 20, 26, and 32, respectively). Concentrations of PAGs also tended (P= 0.09) to be impacted by a treatment by day interaction. On day 20 PAG concentrations were greater in GNRH (P< 0.01) animals compared to Estrus animals, but there was no difference among treatments on other days. ESTRUS improved pregnancy success, but embryos recovered from GNRH animals were heavier and associated with increased PAG concentrations indicating that embryos that survived when estrus was not exhibited may have accelerated growth.