Impact of luteal blood perfusion and expression of estrus on pregnancy rates of Bos taurus embryo recipients exposed to fixed-time embryo transfer.

This study evaluated the impact of luteal blood perfusion (BP) and expression of estrus on pregnancy rates of Bos taurus embryo recipients exposed to fixed-timed embryo transfer (FTET) using a gonadotropin-releasing hormone (GnRH)- and progesterone-based ovulation synchronization protocol. Postpartum beef cows (n = 746) were exposed to a GnRH/progesterone-based ovulation synchronization protocol. Luteal morphometry and BP were assessed using color Doppler ultrasonography 9 days after CIDR removal concurrently with FTET. Embryo recipients were retrospectively categorized according to CL area [small (< 2.5 cm²), medium (2.5-3.5 cm²), or large (> 3.5 cm²)] and BP [low (≤ 30 %), medium (>30 and ≤ 40 %), or high (> 40 % of luteal area with BP signals)]. Luteal area classification did not impact pregnancy rates (P = 0.50); however, cows with high luteal BP had greater (P < 0.01) pregnancy rates compared with low BP cows and tended to have greater pregnancy rates compared with medium BP cows (P = 0.07). Embryo recipients that expressed behavioral estrus had larger (P < 0.01) and more vascularized CL (P = 0.02), as well as greater (P = 0.02) pregnancy rates compared with females that did not express estrus. Nevertheless, no interaction between expression of behavioral estrus and luteal BP classification was observed (P = 0.41). In summary, luteal BP at FTET and expression of behavioral estrus were positively associated with pregnancy rates in Bos taurus embryo recipients exposed to progesterone- and GnRH-based protocols.