Asynchronous Embryo Transfer in Ewes

The aim of the present study was to evaluate the effects of spatially and/or temporally asynchronous transfer of in vivo embryos at different stages in ewes during the breeding season. Four experiments were carried out. In Experiment 1, 207 blastocysts that had been frozen and thawed were transferred into the oviducts of 43 day two recipients, the oviducts of 23 day six recipients, and the uteri of 141 day six recipients, respectively. In Experiment 2, 287 fresh blastocysts were transferred into the oviducts of 26 day six recipients and the uteri of 261 day six recipients, respectively. In Experiment 3, 104 2-cell stage embryos were transferred into the oviducts of 80 day two recipients and the uteri of 24 day two recipients, respectively. In Experiment 4, 175 8-cell stage embryos were transferred into the oviducts of 160 day three recipients and the uteri of 15 day three recipients, respectively. The results showed that spatially asynchronous but temporally synchronous uterine transfers of fresh cleavage-stage embryos or oviductal transfers of fresh blastocysts did not compromise pregnancy and lambing. Furthermore, the oviductal transfers of frozen blastocysts significantly increased the pregnancy rate compared with uterine transfers of frozen blastocysts. Additionally, it is worth noting that the oviduct may be the most suitable microenvironment for frozen embryos in ewes. More studies should be conducted to assess the effects of asynchronous embryo transfer in ewes in a large sample size.