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Thoroughbred Growth and Future Racing Performance

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Growth measurements were made on 3,734 Thoroughbreds raised in the U.S. between 1996 and 2002. Racing performance data were collected for these horses and their growth records were retrospectively examined to determine if various growth characteristics could be associated with success as a racehorse. To account for genetic variation, the sire index was collected for each horse to indicate the average racing class of foals sired by a particular stallion.

Of the 3,734 foals, 79% started in a race and 71% of those won at least one race. More colts raced than fillies, and more colts won at least one race. There was no difference in body weight percentile or height percentile as foals, sucklings, and weanlings between horses that were raced versus unraced. Yearlings that raced had significantly lower average weight percentiles than those that did not. Horses that raced as two-year-olds were significantly shorter and lighter than those that did not start as two-year-olds in all age groups. There was no significant difference in body weight percentile or wither height percentile between winners and non-winners in all age groups. Sire index of graded stakes winners was significantly greater in both foals and yearlings than those that did not win a graded stakes race. Horses in the lowest weight quartile at all age groups had significantly more starts than those in the upper weight quartile. In foals, height percentile was the only variable included in the model that significantly predicted the probability of being a stakes winner, with taller foals having a better chance.

Tall but not heavy young growing horses are more likely to become successful athletes. It is recommended that young growing be weighed and measured to ensure the skeleton maintains a steady rate of growth while preventing the animal from becoming too heavy.

Brown-Douglas, C.G., J.D. Pagan, and A.J. Stromberg. 2009. Thoroughbred growth and future racing performance. In: Advances in Equine Nutrition IV. Nottingham University Press, U.K., p. 231-245.

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