Pregnancy rates and corpus luteum–related factors affecting pregnancy establishment in bovine recipients synchronized for fixed-time embryo transfer

The objective was to investigate the influence of corpora lutea physical and functional characteristics on pregnancy rates in bovine recipients synchronized for fixed-time embryo transfer (FTET). Crossbred (Bos taurus taurus × Bos taurus indicus) nonlactating cows and heifers (n = 259) were treated with the following protocol: 2 mg estradiol benzoate (EB) plus an intravaginal progesterone device (CIDR 1.9 g progesterone; Day 0); 400 IU equine chorionic gonadotropin (eCG; Day 5); prostaglandin F2α (PGF2α) and CIDR withdrawal (Day 8); and 1 mg EB (Day 9). Ovarian ultrasonography and blood sample collections were performed on Day 17. Of the 259 cattle initially treated, 197 (76.1%) were suitable recipients; they received a single, fresh, quality grade 1 or 2 in vivo–derived (n = 90) or in vitro–produced (n = 87) embryo on Day 17. Pregnancy rates (23 d after embryo transfer) were higher for in vivo–derived embryos than for in vitro–produced embryos (58.8% vs. 31.0%, respectively; P < 0.001). Mean (±SD) plasma progesterone (P4) concentration was higher in cattle that became pregnant than that in nonpregnant cattle (5.2 ± 5.0 vs. 3.8 ± 2.4 ng/mL; P = 0.02). Mean pixel values (71.8 ± 1.3 vs. 71.2 ± 1.1) and pixel heterogeneity (14.8 ± 0.3 vs. 14.5 ± 0.5) were similar between pregnant and nonpregnant recipients (P > 0.10). No significant relationship was detected between pregnancy outcome and plasma P4, corpus luteum area, or corpus luteum echotexture. Embryo type, however, affected the odds of pregnancy. In conclusion, corpus luteum–related traits were poor predictors of pregnancy in recipients. The type of embryo, however, was a major factor affecting pregnancy outcome.