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

Area of Science:

• Veterinary Reproduction
• Embryology
• Equine Science

Background:

• Equine embryos exhibit high tolerance for negative uterine asynchrony.
• Positive uterine asynchrony exceeding 2 days significantly increases early embryonic loss (EEL).
• Small Day 8 embryos (<300 μm) show a higher incidence of EEL, but the cause remains unclear.

Purpose of the Study:

• To investigate if small equine embryo viability is affected by recipient uterine stage.
• To differentiate between intrinsic embryo quality and uterine asynchrony as causes for EEL in small embryos.

Main Methods:

• Logistic regression analysis compared pregnancy rates (PR) and EEL for small (<300 μm) and larger Day 8 equine embryos.
• Embryos were transferred to recipients at different uterine stages (Days 4-5, 6-7, and 8-9).
• A total of 62 small and 215 larger embryos were analyzed.

Main Results:

• Overall EEL was significantly higher for small embryos (21.2%) compared to larger ones (7.1%).
• Recipient uterine stage did not influence PR or EEL for either embryo size group.
• EEL for small embryos was 20.8% (Day 4-5), 18.7% (Day 6-7), and 25.0% (Day 8-9) recipients.

Conclusions:

• Day 8 equine embryos with diameters <300 μm are at a higher risk of EEL.
• This increased EEL risk is attributed to factors other than positive uterine asynchrony.
• Findings suggest intrinsic quality, not recipient synchrony, is the primary driver of EEL in small equine embryos.