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Long-term obesity predisposes horses to health issues, including insulin dysregulation, metabolic syndrome, laminitis, overheating, even cardiovascular changes. For broodmares, excessive body condition has long been recognized as problematic. One recent study described specific ways that overconditioning may negatively affect reproductive efficiency of broodmares.*

“Although widely known that excess body weight or condition is detrimental to health and fertility, obesity continues to plague the industry and appears to be worsening with time,” said Catherine Whitehouse, M.S., a Kentucky Equine Research nutrition advisor.

Among the negative consequences of obesity in broodmares is the increased likelihood of anovulatory hemorrhagic follicles. Under normal circumstances, a follicle containing an egg develops on an ovary. When mature, the follicle ruptures, releasing the egg into the oviduct, where fertilization occurs if the mare is bred. When the follicle does not mature properly or release the egg in a timely manner, such as in the case of anovulatory hemorrhagic follicles, the time between ovulations lengthens.

Anovulatory follicles and extended periods between estrus make it difficult to predict when the mare will ovulate again. Without being able to pinpoint ovulation, successfully breeding a mare becomes challenging, especially when using advanced reproductive techniques, such as artificial insemination and embryo transfer.

Obesity has also been cited for poor oocyte quality and altered embryonic development. In a recent study, embryos were collected from broodmares kept in both moderate and excessive body condition.* The embryos were then transferred to recipient mares. Scientists found the embryos flushed from overweight mares were more likely to succumb to early embryonic death.

According to the same study, overweight broodmares may have one advantage over their slimmer peers. Researchers found they have an extended period of annual cyclicity, which is the length of time from first ovulation in spring to last ovulation in the autumn. Extended cyclicity allows for more breeding opportunities, which can be helpful in settling subfertile mares.

Despite this potential benefit, veterinarians and nutritionists recommend broodmares maintain a body condition score of 5 or 6 at all times, though occasional mares may fall outside of this range.

“Regular assessment of the mare’s weight and body condition is an important aspect of nutritional management and allows for timely diet modifications,” advised Whitehouse.

In terms of specific recommendations, Whitehouse said:

  • Avoid overfeeding broodmares during early gestation by feeding lower intake, nutrient-dense feeds such as a ration balancer to ensure optimal nutrient intakes without supplying unnecessary calories;
  • Do not underfeed broodmares during the last trimester when fetal growth and development is greatest and in preparation for the high demands of lactation; and
  • Consider supplementing broodmares with marine-derived omega-3 fatty acids to support reproductive health, particularly in problem or aged broodmares.

*D’Fonseca, N.M.M., C.M.E. Gibson, I. Hummel, D.A. van Doorn, E. Roelfsema, T.A.E. Stout, J. van den Broek, and M. de Ruijter-Villani. 2021. Overfeeding extends the period of annual cyclicity but increases the risk of early embryonic death in Shetland pony mares. Animals (Basel) 11(2):361.

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