| In cattle, there are 3 time frames when early pregnancy losses may occur; before maternal recognition of pregnancy (MRP), during MRP (~d 15 to 18) and after MRP up to pregnancy diagnosis around d 30. Losses before MRP may be attributed to problems in fertilization, gamete transport and early embryonic death; mainly related to follicle/oocyte quality. Embryonic mortality during MRP may be related to insufficient production of interferon t by the developing embryo and consequent activation of PGF2a induced regression of the CL. Finally, losses occurring after MRP to the first accurate pregnancy diagnosis around d 30 may reflect the inability of the uterus to sustain the embryo. A powerful approach to begin to partition these losses would be to use AI and ET in the same experiment. In this manner, losses related to follicular/oocyte maturity and fertilization/gamete transport would be present in females that were AI but minimized in cows that received a healthy embryo via ET. Postpartum beef cows (n = 327) were used to compare the effect of preovulatory estradiol concentration (Pre-E2) and length of proestrus on pregnancy rate achieved with timed AI (AI) vs. embryo transfer (ET). Ovulation was pre-synchronized with the 5-d CO-Synch + CIDR protocol and follicle ablation was performed 6.5 d later (d -7 of experiment). Cows received 50 mg PGF2a on either d -3 (3 d proestrus; long proestrus treatment; LPE), or on d -1.5 (1.5 d proestrus; short proestrus treatment; SPE) and 100 mug GnRH on d 0. Ultrasonography was performed on d -7 and 0 to confirm ovulation to pre-synchronization and ovulatory follicle diameter, 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, 10, 20, 25 and 35 were analyzed for progesterone (P4) to confirm presence of CL on d -3, its regression and formation and lifespan of the new CL, respectively. Cows in the LPE and SPE treatments were either AI on d 0 or ET on d 7, resulting in 4 distinct combinations (LPE-AI, n=77; LPE-ET, n=67; SPE-AI, n=98; SPE-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 P4 on d -3, 0, 7 and 10 did not differ among treatments. As expected, Pre-E2 on d -3 (4.5 +/- 0.1 pg/ml) was similar between treatments and greater (P < 0.05) in the LPE on d -1.5 and 0 (7.6 +/- 0.2; 10.1 +/- 0.3 pg/ml, respectively) compared to SPE (5.1 +/- 0.2; 9.2 +/- 0.2 pg/ml, respectively). Pregnancy rate was greater (P < 0.05) in the LPE treatment and AI group (LPE-AI, 69.9%; LPE-ET, 55.7%; SPE-AI, 51.2%; SPE-ET, 43.7%) compared to the SPE treatment and ET group, however no interaction of treatment x breeding technique was detected for pregnancy rate. Reduced Pre-E2 was a primary outcome of shortened proestrus which resulted in similar reductions in AI (26.8%) and ET (21.5%) pregnancy rate; relative to that achieved in the LPE treatment. Based upon the similarity of these responses, it is concluded that the primary impact of shortened proestrus and deficient Pre-E2 is to impair the ability of the uterus to sustain the embryo during early gestation. |
