Sidebone, a condition that occurs when the cartilages on either side of a horse's distal phalanx (coffin bone) ossify, or harden into bone, has puzzled researchers for more than a century. It has a high heritability, particularly in Swedish cold-blooded horses, so most breeders exclude horses with sidebone from their programs to prevent perpetuating the potentially performance-limiting condition.

For 70 years, Swedish trotter and Ardennes breeders have bred selectively to reduce sidebone incidence in the country's horse population. But has it worked? Ove Wattle, DVM, PhD, a senior lecturer in equine medicine at the Swedish University of Agricultural Sciences, and Ulf Hedenström, DVM, lecturer at National Equine Education Centre Wången, conducted a study to find out, and Wattle presented their results at the 2013 International Equine Conference on Laminitis and Diseases of the Foot, held Nov. 1-3 in West Palm Beach, Fla.

Swedish cold-blooded trotters are a slightly heavier version of the typical Standardbred. Their lengthy racing careers span anywhere from three to 15 years, during which time they train three to five times a week on hard surfaces. Ardennes are medium-sized draft horses.

In Wattle's study he looked at distal phalanx radiographs from 58 Ardennes and 229 cold-blooded trotters with average ages of 3 and 4, respectively; 74% of them had raced. He also measured their body and hoof sizes and noted the degree of sidebone on a scale of 0-5, with five being most severe. After at least four years had passed, he re-examined 63 of the original study horses twice