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The “size of the prize” at the time of foaling remains a much-discussed topic among horse breeders. How big should a foal be? What is too big? Will foals that are small lose excess condition at time of weaning or remain small as yearlings? Are larger foals more likely to cause dystocia, potentially putting both mare and foal in harm’s way? Possibly most importantly, do horse owners or breeders have any true control over how large the foal will end up being at the time of foaling?

According to a group of British researchers, birth weight of Thoroughbreds as a whole is slowly increasing, and the underlying cause remains uncertain.

“We know that larger mares produce larger foals, but it is unclear  if the trend for larger foals over the past few years is simply due to broodmare obesity or if an underlying endocrine, or hormonal, dysfunction plays a role,” explained Kathleen Crandell, Ph.D., an equine nutritionist for Kentucky Equine Research (KER).

To better understand this issue, the research team recruited 57 pregnant Thoroughbred mares and followed them throughout gestation. Every 60 days, body condition score (BCS) and various blood parameters that gauged endocrine function were assessed. Key findings included:

  • 55% of mares were overweight during gestation, with a BCS ≥7;
  • Fasting hyperinsulinmenia (high resting insulin levels that are often seen in horses with decreased insulin sensitivity) was noted in only 1.5% of the mares, and no association with insulin levels and BCS were noted;
  • Leptin levels were associated with BCS. Leptin is a hormone produced by fat tissue that purportedly regulates fat storage; and
  • Foal birth weight was positively associated with mare BCS and inversely associated with mare leptin concentrations.

“These data confirm that obese mares have heavier foals and also suggest that mare leptin levels could predict which mares will have heavier foals,” Crandell summarized. In turn, mares with larger foals identified prior to foaling could receive extra attention during parturition to avoid dystocia.

*Smith, S, C.M. Marr, and N.J. Menzies-Gow. 2015. The effect of obesity and endocrine function on foal birthweight in Thoroughbred mares. Clinical Research Abstracts. British Equine Veterinary Association Congress. Equine Veterinary Journal 47(Suppl. 48):2.

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