Preovulatory estradiol and the establishment and maintenance of pregnancy in suckled beef cows

In postpartum beef cows, GnRH-induced ovulation of small dominant follicles decreased pregnancy rates and increased late embryonic/fetal mortality. In Exp. 1, single ovulation reciprocal embryo transfer (ET) was utilized to examine the relationship between preovulatory serum concentrations of estradiol at GnRH-induced ovulation, in donor and recipient cows, and establishment and maintenance of pregnancy. Suckled beef cows (n = 1,164) were administered GnRH on d -9 (GnRH1), prostaglandin F2a (PGF) on d -2, and GnRH2 on d 0 (CO-Synch protocol) either with (donors; n = 810) or without AI (recipients; n = 354). Single embryos (n = 394) or oocytes (n = 45) were recovered from the donor cows (d 7; ET) and all live embryos were transferred into recipients. Serum concentration of estradiol at GnRH2 was positively correlated with follicle size at GnRH2 (r = 0.45, P < 0.01) and progesterone at ET (r = 0.34, P < 0.01). Donor cows with greater estradiol at GnRH2 were more likely to yield an embryo than an unfertilized oocyte (P < 0.01). Donor and recipient cows were retrospectively divided into four groups (low estradiol [< 8.4 pg/ml] or high estradiol [ = 8.4 pg/ml]) based on serum concentration of estradiol at GnRH2. Pregnancy rate at d 27 for low-low (n = 78), low-high (n = 80), high-low (n = 91), and high-high (n = 101) groups (donor-recipient; respectively) was 45a, 65b, 43a, and 61%b respectively (abP < 0.02). Since recipient cows with greater estradiol concentration at GnRH2 had greater pregnancy rates in Exp. 1, the objective of Exp. 2 was to evaluate the effect of estradiol supplementation on pregnancy rate. Ovulation was synchronized in suckled beef cows (n = 600) using the CO-Synch protocol with the insertion of a CIDR (intravaginal progesterone supplement) from d -9 until d -2. Approximately one-half of the cows (n = 297) received an injection of estradiol cypionate 24 h before AI (ECP; 0.5 mg i.m.). Compared with the Control cows, ECP treatment increased (P < 0.01) pregnancy rates of cows induced to ovulate smaller dominant follicles (< 12.2 mm). In conclusion, GnRH-induced ovulation of small dominant follicles was associated with reduced serum estradiol, fertilization rate (donor cows) and pregnancy establishment (recipient cows). Furthermore, ECP supplementation during the preovulatory period increased pregnancy rates in cows induced to ovulate smaller dominant follicles.