During the sales season, veterinary surgeons and bloodstock agents spend many thousands of hours examining yearlings in detail to assess their potential as athletes. One of the main factors considered is the conformation of the animal. Although many trainers and their vets will have their own ‘hunch’ over which types of conformation are most likely to produce lameness and injury, there has been relatively little scientific work looking at this subject. Only 2 large studies have considered the effect of conformation on soundness. The first, published in Equine Veterinary Journal in 2006, was carried out by bloodstock veterinary surgeon Peter Calver. The study took place over a 7-year period between 1993 and 1999 and included examination of over 4,000 yearlings. Racing success of these individuals was judged by the number of starts and their eventual BHA rating. Interestingly despite huge variation in conformation only 7% of these horses failed to race and these 7% were by no means the most ‘crooked’. Of all of the remaining conformational defects, very few were proven statistically to have an impact on racing. A marked grade of toeing out (Figure 1) was linked to reduced performance.
In the second study by Wayne McIlwraith and his group at the University of Colorado, also published in Equine Veterinary Journal in 2004, the conformation of 115 horses owned by a single owner/breeder in France, was logged over their development. Rather than rating race success, the rate of various injuries was recorded for each of the conformational defects. As with Peter Calver’s study surprisingly few conditions jumped out as being inextricably linked to lameness or injury. Some abnormalities, for instance a degree of carpal valgus, (deviating out through the knee) actually seemed to have a protective effect. Offset knees had a higher incidence of front fetlock joint effusion and other problems but was not linked to knee problems or to the development of splints, which would be most veterinary surgeons; experience.
Sometimes it helps to turn the situation round and begin with the disease entity rather than the conformational defect. For instance the development of subchondral bone pain in the front fetlock joint in the racehorse is linked over and over again to horses showing short upright pasterns as a conformational defect. What confounds the scientific studies is that many other horses will have short upright pasterns and not develop front fetlock problems. This dilutes the impact of this conformational trait despite the fact that in a group of horses suffering from subchondral bone disease, short upright pasterns would be over-represented, ie above the average number encountered. Similarly with fractures of the hock and the development of curb, sickle hocks are massively over-represented but again this does not mean that every horse with sickle hocks will develop a hock fracture or throw a curb. These associations are swamped by the huge variability of the racehorse to cope with a conformational defect.
It may be surprising that both scientific studies showed a relatively small impact of conformation on specific injury or race success, but this does make sense. If conformational defects had a very dramatic impact on soundness and ability on the racecourse, over the period of time racehorses have been bred, presumably these conformational defects would have become less and less common. The fact that they are still well distributed throughout the population must mean that horses which carry these conformational traits can and do win good races. In the Thoroughbred, race success alone determines whether an animal goes forward to breed, and presumably enough horses carrying these conformational defects do achieve race success to maintain the frequency of the defects within the population.