Mare Obesity and Endocrine Function on Foal Weight

Horsemen have traditionally believed that birthweight of a foal was determined, at least in part, to the body condition of the mare. Foals born to Thoroughbred mares are heavier now than in past years, and researchers recently set out to discover if the one-two punch of obesity and endocrine dysfunction affects birthweight.

British researchers used 66 mares in the study* and began tracking them 40 days after breeding. Weight and blood samples were collected at that time and collected every 60 days during pregnancy. Serum/plasma insulin, leptin and triglyceride concentrations, and foal birthweight were also recorded. Leptin is a hormone manufactured by adipose cells that helps to regulate energy balance by inhibiting hunger. A decreased sensitivity to leptin occurs with obesity, ending up in the body’s inability to flip off the hunger switch. Additionally, relationships between hormone and triglyceride concentration with body condition score, stage of pregnancy, and birthweight were established.

The anecdotal notion that body condition of the mare influenced birthweight of the foal proved true in this study: obese mares had heavier foals than nonobese mares. Results of the study also indicate that significant hyperinsulinemia was not noted in this study population. Leptin concentrations, however, increased in early and late pregnancy and was associated with decreased foal birthweight.

“Feeding broodmares need not be tricky or complicated,” said Catherine Whitehouse, M.S., a nutrition specialist at Kentucky Equine Research (KER). “Like other horses, they should be fed a balanced diet, one with good-quality forage as the foundation, and kept in moderate to moderately fleshy body condition.”

Knowing how individual mares gain, maintain, or lose weight offers considerable insight into how they are managed throughout pregnancy. “As an example, if you know that your mare is a champion milker and because of it will lose weight during lactation, then it’s appropriate that she enter the end of her gestation heavier than a mare that is a far easier keeper during lactation,” advised Whitehouse. This specialized knowledge of how individual mares respond to physiological stressors comes with mindful management, year in and year out.

Learn more about feeding broodmares and young horses by downloading a copy of Nutritional Management of Horses on a Breeding Farm, a 48-page guide written by the nutritionists at KER.

*Smith, S., C.M. Carr, C. Dunnett, and N.J. Menzies-Gow. 2016. The effect of mare obesity and endocrine function on foal birthweight in Thoroughbreds. Equine Veterinary Journal. In press.