Thoroughbred Growth and Future Racing Performance

The success of a Thoroughbred racehorse is determined by a multitude of factors, many of which are impossible to evaluate. Thoroughbreds are commonly selected for racing ability by pedigree and conformation analysis. There have been few studies associating growth and body size with racing performance (Pagan, 1998a; Cain, 2006; Smith et al., 2006). The genetic make-up, or genotype, contributes to a horse’s racing ability by influencing physical characteristics including conformation, lung capacity, and growth potential, and mental attributes including the “will to win.” Environmental factors such as nutrition, exercise, and conditioning are difficult to quantify, and management during growth and development as well as trainer variation all influence the horse’s potential for success. Many methods of anatomical or pre-competitive performance assessment are employed by Thoroughbred breeders, trainers, bloodstock agents, and owners. However, these methods are subjective and there have been no established and fail-proof methods for predicting potential success. A large retrospective study conducted by Cecil Seaman of Thoroughbred Analysts using body measurements taken at Thoroughbred sales concluded that horses in the “underweight” to “ideal weight” (±2% optimum weight) range earned significantly more than those in the “overweight” to “obese” categories (Pagan, 1998a; Cain, 2006). However, body weights in this study were estimated and not actually measured. A study of 260 Thoroughbreds born in 1981 and 1982 described associations between yearling body measurements and racing performance (Smith et al., 2006). It was reported that hip height, body length, and heart girth were positively correlated with win percentage (number of wins/number of starts). There were no significant correlations between any body measurement and lifetime earnings; however, the study reported a trend linking taller fillies and greater earnings. Horses that won or placed in a stakes race tended to be taller as yearlings. Mature size and the pattern of growth are influenced by the genetic make-up of the horse and the environment, but it is inconclusive if growth plays a role in the success of a racehorse. It is known that heavier foals have a greater incidence of osteochondrosis (Pagan, 1998b). Foals that developed hock and stifle OCD tended to be heavier at birth and fast-growing between 3 and 8 months. Furthermore, it is reported that heavier but not taller yearlings command higher prices at public auction (Pagan et al., 2005). The purpose of this study in Thoroughbreds was to make objective measurements of a horse’s growth to determine if certain characteristics affect the odds of its success as a race horse.