Small day 8 equine embryos cannot be rescued by a less advanced recipient mare uterus.

This research focused on whether smaller equine embryos can be successfully developed in the uterus of a less advanced recipient mare, with the results indicating that this is not a viable solution for reducing early embryonic loss (EEL). The study found that neither successful pregnancy rates nor EEL rates were influenced by the stage of the recipient mare’s uterus at the time of embryo transfer.

Research Method and Variables

• The study examined whether smaller equine embryos, which suffer increased EEL, could survive better if transferred into recipient mares with a less advanced uterine stage.
• The variable being tested was the stage of the recipient mare’s uterus (Days 4-5, 6-7, and 8-9) at the time of the embryo transfer.
• The researchers compared the chances of successful pregnancy and EEL for two sets of Day 8 horse embryos of varying sizes – 62 small embryos (<300 μm in diameter) and 215 larger ones – using logistic regression.
• The viability of smaller embryos (those smaller than 300 μm in diameter) is often less, but it was uncertain whether this was due to embryo quality or inadvertent positive asynchrony (when the recipient’s uterus is more advanced than the embryo).

Results and Findings

• The research found that the EEL was higher (21.2%; P < 0.05) for small embryos than for larger embryos (7.1%).
• However, the data indicated that neither pregnancy rate nor EEL rate was affected by the recipient mare’s uterine stage at the time of transfer (P > 0.1).
• The EEL for small embryos transferred into Day 4-5, 6-7, and 8-9 recipients showed rates of 20.8%, 18.7%, and 25.0% respectively.

Conclusion of the Research

• The research concluded that smaller Day 8 embryos (those with a diameter of less than 300 μm) are at an increased risk of EEL, but the reason does not relate to inadvertent positive asynchrony with the recipient mare’s uterus.
• Overall, the viability of smaller equine embryos cannot be improved by transferring them into the uterus of a less advanced recipient mare.