Overfeeding Extends the Period of Annual Cyclicity but Increases the Risk of Early Embryonic Death in Shetland Pony Mares.
Authors: D'Fonseca Nicky M M, Gibson Charlotte M E, Hummel Iris, van Doorn David A, Roelfsema Ellen, Stout Tom A E, van den Broek Jan, de Ruijter-Villani Marta
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary Obesity in mares is known to impair reproductive function, yet the specific mechanisms by which excessive nutrition compromises fertility remain incompletely understood. Researchers conducted a three-year controlled study in which Shetland pony mares were fed at 200% of their net energy requirements, comparing reproductive outcomes—including cyclicity duration, follicular abnormalities, and embryo viability—against maintenance-fed controls. Overfeeding extended the breeding season by prolonging the period of annual cyclicity across all three years, but this apparent reproductive advantage came at a significant cost: embryos from obese mares demonstrated substantially elevated rates of early embryonic death when transferred into recipient mares, despite showing no visible morphological defects. The findings suggest that maternal obesity creates an intrauterine or systemic metabolic environment hostile to embryo survival, even when embryos themselves appear structurally sound at day 7. For practitioners managing breeding programmes, this underscores that achieving and maintaining appropriate body condition is essential for maximising both conception and carrying capacity; feeding mares to achieve weight gain may paradoxically reduce overall reproductive success by compromising embryonic viability rather than enhancing it.
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Practical Takeaways
- •Avoid overfeeding breeding mares—excessive energy intake increases embryonic loss risk even when outward fertility markers appear normal
- •Extended cyclicity in overfed mares may appear beneficial but masks underlying reproductive dysfunction and compromised embryo quality
- •Monitor body condition strictly in breeding programs; obesity-related embryonic death may not be detected without embryo transfer studies, so maintain mares at appropriate weight for their type
Key Findings
- •High-energy feeding (200% NE requirements) extended the period of annual cyclicity in Shetland pony mares across all three study years compared to maintenance-fed controls
- •Embryos recovered from overfeeding mares were significantly more likely to succumb to early embryonic death following transfer
- •Overfeeding mares became obese but showed no consistent differences in hemorrhagic anovulatory follicle incidence compared to control mares
- •No differences in embryo morphometric parameters were apparent between embryos from high-energy versus control mares despite differences in embryo viability