The effect of mare obesity and endocrine function on foal birthweight in Thoroughbreds.
Authors: Smith S, Marr C M, Dunnett C, Menzies-Gow N J
Journal: Equine veterinary journal
Summary
# Editorial Summary Rising birthweights in Thoroughbred foals have prompted investigation into whether maternal obesity and metabolic dysfunction play a causal role, prompting Smith and colleagues to track 66 pregnant mares from day 40 through to term, sampling blood every 60 days and measuring body condition score (BCS), weight, insulin, leptin and triglyceride concentrations alongside foal birthweight at delivery. Whilst mare BCS showed a weak positive correlation with foal birthweight (r = 0.13), suggesting heavier mares do produce heavier foals, the endocrine picture proved more nuanced: insulin levels fluctuated naturally across gestation without evidence of pathological hyperinsulinaemia, leptin concentration correlated strongly with obesity status but paradoxically showed an inverse relationship with birthweight (r = -0.64 at mid and late gestation), and triglycerides increased late in pregnancy independent of condition. These findings suggest that simple obesity alone cannot explain the trend towards heavier foals, and that leptin's negative association with birthweight warrants further investigation—possibly indicating that certain metabolic profiles in obese mares may actually restrict foetal growth despite the mare's increased weight. For practitioners managing breeding stock, the data indicate that monitoring leptin alongside traditional condition scoring in the second and third trimesters might help identify mares at risk of delivering lighter foals, though establishing leptin as a predictive biomarker will require larger prospective studies with more consistent body condition distributions.
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Practical Takeaways
- •Monitor broodmare body condition throughout gestation as obesity correlates with increased foal birthweight, which may have implications for foal viability and dystocia risk
- •Leptin concentration in late gestation may serve as a predictive biomarker for foal birthweight, though further validation studies are needed before clinical application
- •Insulin resistance does not appear to be a significant factor in Thoroughbred mares affecting foal birthweight, contrary to some assumptions about obesity-related endocrine dysfunction
Key Findings
- •Mare body condition score correlated weakly but significantly with foal birthweight, with obese mares producing heavier foals (r = 0.13, P<0.001)
- •Leptin concentration at 181-240 days gestation was significantly elevated and inversely correlated with foal birthweight (r = -0.64, P<0.05)
- •Serum insulin concentrations varied by gestational stage but showed no significant hyperinsulinaemia and no association with mare body condition score
- •Triglyceride concentrations peaked at 241-300 days gestation but were not associated with mare body condition score or foal birthweight