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Weight Changes in Racehorses: Season and Sex DifferencesBy Kentucky Equine Research Staff · September 5, 2017

Trainers expect body weight changes in Thoroughbreds as fitness ebbs and flows through conditioning and racing cycles, but few would anticipate fluctuations in weight based on season or sex. A recent study suggests that body weights of racehorses vary from season to season, and differences in energy metabolism might exist between sexes*.

Seasonal weight changes have been investigated in some equine populations. Because of forage availability, wild horses experience seasonal weight fluctuations, gaining weight as grass greens in the spring and grows through fall, and losing weight as grass dies off. Further, Kentucky Equine Research recorded body weight and condition changes in mares and foals based on foaling month, season, and gender**.  While this knowledge is well documented, few studies have explored seasonal weight fluctuations of stabled performance horses with consistent access to energy sources, like racehorses.

The researchers used over 630,000 body weights collected at Japanese racecourses from 2002 to 2014. Of these weights, 63% were taken from males, either intact or gelded, and 37% from females. Weights were recorded within 80 minutes of each race start using electronic scales.

Researchers found that the body weight of stallions and geldings peaked in autumn and winter and dipped to their lowest in summer. The body weight of mares, on the other hand, crested in autumn and plunged to their lowest point in spring. Seasonal fluctuations in weight loss amounted to 15.4 lb (7 kg), 17.6 lb (8 kg), and 13.2 lb (6 kg) for stallions, geldings, and mares, respectively.

Many of the horses were tracked from year to year, so weight changes over the course of a racing career could be calculated. Differences in body weight included stallions from 461 kg to 493 kg (1,016 lb to 1,089 lb), geldings from 458 kg to 484 kg (1,010 lb to 1,067 lb), and mares from 442 kg  to 472 kg (974 lb to 1,041 lb).

Potential explanations for seasonal weight changes include environmental factors such as temperature and photoperiod. Regardless of their causes, weight changes may affect performance, soundness, and longevity. By understanding natural variances in weight, racehorse trainers can manipulate diets to increase or decrease body weight at certain times of the year. For example, a heavy-boned, four-year-old stallion with a robust appetite may need to have dietary energy reduced slightly in autumn or winter to offset the natural tendency for stallions to be heavy in these seasons.

*Takahashi, Y., and T. Takahashi. 2017. Seasonal fluctuations in body weight during growth of Thoroughbred racehorses during their athletic career. BMC Veterinary Research, 13:257.

**Pagan, J.D., C.G. Brown-Douglas, and S. Caddel. 2006. Body weight and condition of Kentucky Thoroughbred mares and their foals as influenced by month of foaling, season, and gender. In: Advances in Equine Nutrition, Vol. IV, p. 137-145. Nottingham University Press, Nottingham, U.K.