The Effect of Preovulatory Concentration of Estradiol and Length of Proestrus on Pregnancy Rate to Timed-AI and Embryo Transfer in Beef Cows
Postpartum beef cows (n = 327) were used to investigate the effect of preovulatory estradiol concentration (Pre-E2) and length of proestrus on timed-AI (TAI) and embryo transfer (ET) pregnancy rates. Ovulation was pre-synchronized with the 5-d CO-Synch+CIDR, follicle aspiration performed 6.5 d later (d -7 of experiment) and cows received 50 mg PGF2α on either d -3 (normal estradiol treatment; HiE), or on d -1.5 (deficient estradiol treatment; LoE). All cows received 100 μg GnRH on d 0, creating a proestrus of either 3 or 1.5 d. Ultrasonography was performed on d -7, 0 and 7 to confirm ovulation to pre-synchronization, ovulatory follicle diameter and corpus luteum (CL) formation after GnRH, respectively. Blood samples on d -3, -1.5 and 0 were used to determine Pre-E2. Blood samples on d -3, d 0 and d 7 were analyzed for progesterone to confirm presence of CL, its regression and formation of new CL, respectively. Cows in the HiE and LoE treatments were either TAI on d 0 or ET on d 7, resulting in 4 distinct combinations (HiE-AI, n=77; HiE-ET, n=67; LoE-AI, n=98; LoE-ET, n=85) in a 2 x 2 factorial arrangement. Pregnancy diagnosis was performed on d 35. Ovulatory follicle diameter (11.9 ± 0.1 mm) and progesterone on d -3 (3.8 ± 0.1 ng/ml), d 0 (0.4 ± 0.0 ng/ml) and d 7 (2.6 ± 0.1 ng/ml) did not differ among treatments. As expected, Pre-E2 on d -3 was similar between HiE (4.5 ± 0.1 pg/ml) and LoE (4.5 ± 0.1 pg/ml). In contrast, Pre-E2 was greater (P < 0.05) in the HiE on d -1.5 and 0 (7.6 ± 0.2; 10.1 ± 0.3 pg/ml, respectively) compared to LoE (5.1 ± 0.2; 9.2 ± 0.2 pg/ml, respectively). Pregnancy rate was greater (P < 0.05) in the HiE treatment and AI group (HiE-AI, 68.8%; HiE-ET, 52.2%; LoE-AI, 46.9%; LoE-ET, 37.6%) compared to the LoE treatment and ET group, however no interaction treatment (HiE/LoE) and breeding technique (AI/ET) was detected for pregnancy rate. Reduced Pre-E2 was the primary outcome of shortened proestrus which resulted in similar reductions in TAI (31.8%) and ET (28.0%) pregnancy rate; relative to that achieved in the Hi-E treatment. Based upon the similarity of these responses, it is concluded that the primary impact of deficient Pre-E2 is to impair the ability of the uterus to sustain the embryo during early gestation.
Preovulatory Estradiol, TAI, ET