The majority of pregnancy loss in cattle occurs during the first 2 to 3 weeks of pregnancy. This loss can be studied by the transfer of in vivo- and in vitro-produced embryos. The objective of this study was to examine the relationship between recipient serum progesterone levels both at the time of embryo transfer and at conceptus recovery on conceptus development from in vivo- or in vitro-produced embryos. Embryos were produced in vivo by superovulation of Holstein cows (IVO; n = 17) or in vitro with either serum-containing (IVPS; n = 27) or serum-restricted medium (IVPSR; n = 34). Single grade-1 blastocysts from each embryo production system were transferred into heifers at Day 7. Conceptuses were recovered at Day 17 of gestation (number recovered/number transferred: IVO, 11/17; IVPS, 16/27; IVPSR, 18/34) and classified as complete, degenerated, or no conceptus. Recipient serum progesterone concentrations were determined by radioimmunoassay and compared with conceptus development outcomes. Sex of conceptus was determined by PCR using a Y-chromosome specific probe. Data were analysed using Fisher's exact test or ANOVA and Duncan's multiple range test. Compared with the IVO group, in vitro-produced embryos had more (P = 0.055) degenerated conceptuses (IVO, 0%; IVPS, 18.5%; IVPSR, 20.6%). There were no differences (P > 0.05) in serum progesterone concentrations in recipients assigned to different treatments at Day 7. There was also no effect (P > 0.05) of treatment on progesterone levels in recipients with either male or female conceptuses at the time of transfer. Interestingly, heifers in the in vitro treatment groups had lower (P < 0.01) progesterone concentrations at Day 7 when no conceptus was recovered at Day 17 (IVPS, 2.1 ± 0.4 ng mL^–1; IVPSR, 2.7 ± 0.4 ng mL^–1; Least squares means ± standard error of the mean) compared with the IVO group (4.5 ± 0.6 ng mL^–1). There was no difference in progesterone concentration between treatment groups for heifers with shorter conceptuses (≤194 mm). However, when longer (>194 mm) conceptuses were recovered, heifers with in vitro produced embryos had lower (P < 0.05) progesterone levels at Day 7 compared with those with in vivo produced embryos (IVPS, 2.2 ± 0.6 ng mL^–1; IVPSR, 2.3 ± 0.5 ng mL^–1; IVO, 3.9 ± 0.6 ng mL^–1). In summary, serum progesterone concentrations in recipients at the time of transfer of in vivo- or in vitro-produced embryos were associated with conceptus development at Day 17 of gestation.

Research supported by NC State University GAANN Biotechnology Fellowship (C. V. Barnwell) and the College of Veterinary Medicine.