Conformation Development in Growing Thoroughbreds

Selection of Thoroughbred horses for racing and breeding is based primarily on equine performance, pedigree, and phenotype (physical characteristics). Although conformation plays a critical role in the evaluation of horses, current methods of analyzing equine conformation are largely subjective and vary according to personal opinion and individual experience.

To develop practical, objective methods of analyzing conformation in Thoroughbreds, researchers at Colorado State University's (CSU) School of Veterinary Medicine designed a longitudinal study to identify the growth-related skeletal changes that occur in horses from weanling to age three. The findings were published in the November 2004 issue of Equine Veterinary Journal.

Lead investigator Wayne McIlwraith, BVSc, PhD, DSc, and his team used a photographic computer method to measure conformation in a population of racing Thoroughbreds foaled between 1992 and 1996. Photographs were taken of the horses as foals, yearlings, 2-year-olds, and 3-year-olds. Using reference points marked on specific anatomical sites, annual conformation measurements were made from the photographs over a four-year period.

McIlwraith, Director of Orthopaedic Research at CSU's School of Veterinary Medicine, noted, "Much has been written about the conformation of horses and its relationship to performance, but there isn't a lot of objective data available. Our study gave us an opportunity to look at a number of horses for several years, and we now have some hard numbers to put behind subjective observations."

The study yielded important information about the skeletal changes that occur in horses as they mature. According to McIlwraith, "Horses achieve increases in height not just by bone growth, but also by becoming more upright in the shoulder."

Data analysis revealed highly significant relationships between long bone length and wither height for all ages from weanling age to the 3-year-old year. "This supports the theory that horses are proportional," said McIlwraith. "Taller horses have longer bone lengths than shorter horses."

The CSU study confirmed previous reports that bone growth in the lower limb stops by 140 to 210 days. This finding suggests that mature height might be predicted by the length of the cannon bone in young horses.

In the front legs, conformation changed progressively from "back at the knee" to slightly "over at the knee" in horses from the time they were weanlings until they reached the age of three years. McIlwraith said this phenomenon should be considered when evaluating weanlings and yearlings.

Evaluation of equine conformation typically includes an assessment of the slope of the scapula (shoulder blade). The researchers found that the angle of the scapula increased significantly with age, becoming more upright. McIlwraith noted that the progressive increase in angle of the scapula should be taken into account when evaluating the conformation of a horse at a young age. "You need to recognize that this can change with maturity," he explained.

About the Author

Rallie McAllister, MD

Rallie McAllister, MD, grew up on a horse farm in Tennessee, and has raised and trained horses all of her life. She now lives in Lexington, Ky., on a horse farm with her husband and three sons. In addition to her practice of emergency and corporate medicine, she is a syndicated columnist (Your Health by Dr. Rallie McAllister), and the author of four health-realted books, including Riding For Life, published by Eclipse Press and available at or by calling 800/582-5604.

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