Asynchronous embryo transfer as a tool to understand embryo–uterine interaction in cattle: is a large conceptus a good thing?

The aim was to examine the effect of embryo–uterine synchrony on conceptus elongation and pregnancy rate in cattle. In Study 1, crossbred beef heifers each received 10 Day-7 in vitro-produced blastocysts on either Day 5, 7 or 9 after oestrus. A proportion of Day 5 recipients were supplemented with progesterone, via a progesterone-releasing intravaginal device from Days 3–5 plus either 750 IU equine chorionic gonadotrophin or 3000 IU human chorionic gonadotrophin on Day 3. At embryo age Day 14, all heifers were slaughtered and the uterus was flushed. Fewer recipients yielded conceptuses (P < 0.05) and fewer conceptuses were recovered (P < 0.05) following transfer on Day 5 compared with Day 7 or 9. Supplementation with progesterone resulted in short cycles in approximately 50% of recipients. Mean conceptus length was greater (P < 0.05) following transfer to an advanced uterus. In Study 2, overall pregnancy rate following the fresh transfer of a single in vitro-produced blastocyst was 43.5% (2065/4749). Transfer of a Day 7 embryo to a synchronous Day-7 uterus resulted in a pregnancy rate of 47.3%. Transfer to a Day-5 (40.8%) or a Day-8 (41.3%) uterus moderately impacted pregnancy rate (P < 0.01) while transfer to a uterus 2 days in advance (Day-9, 24.4%) or 3 days behind (Day-4, 27.0%) reduced (P < 0.001) pregnancy rate compared with synchronous transfers. In conclusion, transfer of an embryo into an advanced uterus results in an acceleration of conceptus development, but does not result in greater pregnancy rates.