Horse embryo diameter is influenced by the embryonic age but not by the type of semen used to inseminate donor mares.

The research investigates whether the type of semen (frozen or fresh) used to impregnate mares has an effect on the diameter of the recovered embryos, along with an analysis of how embryonic age and size influences the likelihood of successful pregnancy. The study concluded that embryonic age and the age of the donor mare have a significant effect on embryo diameter, but the type of semen used for insemination doesn’t.

Study objectives and design

• The study aimed to understand the impact of the type of semen (frozen-thawed vs. cooled-transported) on the size of horse embryos. Moreover, it looked to establish the connection between embryonic age, embryo size, and subsequent pregnancy outcomes.
• A total of 179 embryos were retrieved from mares that had been inseminated with various combinations of semen type and insemination timing (Pre or Post ovulation).
• A general linear model of variance was utilized to understand the influence of various factors (embryonic age, semen type, donor mare age, number of ovulations, and embryos per flush) on the embryo size.
• The proportions and viability of small embryos (less than 300 μm) across all groups were compared with respective larger embryos using Fisher’s exact test.

Results and interpretation

• The study found that the age of the embryo (P < 0.001) and the age of the donor mare (P = 0.07), but no other factor, had a noticeable impact on the diameter of the embryo. This indicates that as the embryo or the donor mare is older, the embryos tend to be larger in diameter.
• The proportion of small embryos varied across groups, with Group 1 (inseminated with frozen semen post-ovulation 8 days previously) having a significantly higher proportion (42.9%) compared to the other groups.
• The study, however, found no significant difference in pregnancy status among recipient mares receiving small embryos from different groups 35 days post-transfer. The results suggest that no matter the type of semen used, the likelihood of a successful pregnancy doesn’t show a significant difference.

Conclusions

• Therefore, the study concluded that the type of semen used, whether it was cooled-transported or frozen-thawed, doesn’t influence the diameter of the embryos. What matters more are the embryonic age and the age of the donor mare.