The horse’s bone structure is determined principally by genetics and influenced somewhat by nutrition. Although improper nutrition when a horse is young and growing can damage an inherently well conformed skeletal structure, adequate nutrition can’t do much to improve a structure that is inherently poor.
Likewise, no amount of muscle condition to try to enhance a horse’s ability or appearance can change his basic conformation, although some faults may be disguised somewhat by good muscle development or fat covering. Some leg deformities in foals can be helped with surgery or corrective trimming of hooves while the leg bones are still growing, but for the most part of a horses;s basic structure is present at birth cannot be changed. The skeletal structure must be carefully evaluated when selecting a horse.
The conformation of any animal depends on the shape and character of the vertebral column. The horse’s backbone ties the entire skeleton together, and all other structures of the animal attache to the spine. If you can breed the perfect spine you can breed the perfect horse.