Higher systemic progesterone in the immediate post-conception period is associated with an increase in embryonic growth rate, interferon-tau production and pregnancy rate in cattle. The objective of this study was to examine the effect of increasing progesterone concentration on Day 3 on subsequent embryo survival and development. Oestrus (Day 0) was synchronised in beef-cross heifers (n=210) and approximately two-thirds of the heifers were inseminated with semen from a proven sire, while the remainder were not inseminated. In order to produce animals with divergent progesterone concentrations, half of the animals received a progesterone-releasing intravaginal device (PRID) on Day 3 of the oestrous cycle, which was left in situ until slaughter. The four treatment groups were: (i) pregnant, high progesterone; (ii) pregnant, normal progesterone; (iii) non-pregnant, high progesterone; and (iv) non-pregnant, normal progesterone. Animals were blood-sampled twice daily from Days 0 to 8 and once daily thereafter until slaughter on Days 5, 7, 13 or 16, corresponding to the 16-cell stage, the blastocyst stage, the beginning of elongation and the day of maternal recognition of pregnancy, respectively. Embryos were recovered by flushing the tract with phosphate-buffered saline and characterised by stage of development and, in the case of Days 13 and 16, measured. Data were analysed by mixed models ANOVA, Chi-square analysis and Student's t-test where appropriate. Insertion of a PRID on Day 3 increased (P<0.05) progesterone concentrations from Day 3.5 onwards. There was no difference between treatments in the proportion of embryos at the expected stage of development on Days 5 or 7 (P>0.05). While not significantly different, the proportion of viable embryos recovered was numerically greater in the high progesterone group on both Day 13 (58 v. 43%) and Day 16 (90 v. 50%). Elevation of progesterone significantly increased embryonic length on Day 13 (2.24+/-0.51 mm v. 1.15+/-0.16 mm, P=0.034) and Day 16 (14.06+/-1.18 cm v. 5.97+/-1.18 cm, P=0.012). In conclusion, insertion of a PRID on Day 3 of the oestrous cycle increased serum progesterone concentrations on subsequent days, which, while having no phenotypic effect on embryonic development on Days 5 or 7, was associated with an increase in embryonic size on Days 13 and 16.

The aim of the present study was to examine the effect of embryo source (in vivo v. in vitro) and the progesterone environment into which it was transferred on Day 7 on embryo survival and size on Day 13. Day 7 blastocysts were produced either in vivo using superovulation, artificial insemination and non-surgical embryo recovery or in vitro using in vitro maturation, fertilisation and culture. In order to produce animals with divergent progesterone concentrations, following synchronisation recipients were either superovulated (High progesterone; n = 10) or not (Control progesterone; n = 10). Ten blastocysts, produced either in vivo or in vitro, were transferred to each recipient on Day 7. Both groups were killed on Day 13. The mean progesterone concentration from Day 7 to Day 13 (the period when the embryos were in the uterus) in the High and Control progesterone recipients was 36.32 +/- 1.28 and 10.30 +/- 0.51 ng mL(-1), respectively. Of the in vivo embryos transferred, the overall recovery rate at Day 13 was 64%, which was higher (P < 0.001) than that of 20% for the in vitro embryos transferred. The mean area of embryos recovered from High progesterone recipients was 3.86 +/- 0.45 mm(2) (n = 28) compared with 1.66 +/- 0.38 mm(2) (n = 24) for embryos recovered from Control progesterone recipients (P < 0.001). Similarly, the origin of the embryo used for transfer affected embryo size on Day 13. In summary, the recovery rate of blastocysts was higher for in vivo- than in vitro-derived embryos. Blastocyst size was approximately 2.3-fold greater in recipients with high compared with normal progesterone. The present study lends strong support to the hypothesis that an earlier rise in progesterone after conception stimulates blastocyst growth and the development of competent embryos.