Many trainers in the United States start training and/ or riding their horses long before they are considered “mature”. The racing industry is a perfect example of this; as soon as the young horse’s cannon bones are finished growing, the trainers put them in work. Even though the cannon bones are finished developing, that does not mean the rest of the body is. A horse is not fully mature until around six years of age.
A trainer’s belief that horses are ready to be worked under saddle at two years of age stems from the fact that the horse has almost reached his full height and weight by that time. A thoroughbred will reach 84% of his adult height at six months, and by two years it will be 97%. At two years old he will also be 92% of his mature weight. In looking at a two or three-year-old from the outside, he may look like he is fully mature, but unfortunately the physical appearance of a horse is not representative of his internal structures.
The short pasterns are the one of the first large bone structures to reach maturity. By twelve months, the growth plates in the short pasterns have fused together (growth plate fusion is the process of converting the plates into bone), and the long pastern growth plates are fused by fifteen months. The cannon bones are next, with the growth plates fusing by 1.5 years, then the smaller knee bones are mature by age two. At this point, only the bones in the lower half of the legs are finished growing, yet it is common to see horses in training at this age. Riding this early can cause a huge amount of damage to the spine and any other bones that aren’t mature.
The radius, ulna, humerus, and scapula take between 3 and 3.5 years to mature. The hock growth plates also take 3.5 years to fuse, even though the hocks are positioned lower on the body. Hocks are weak points in a horse’s body because they mature so much slower than the rest of the leg. If a horse is ridden too early, the rider risks compromising the cartilage in the hocks (this damage is why hock injections are becoming more and more common in young horses). By age 4, the tibia and femur are mature, but the growth plates in the pelvis do not finish fusing until age 5.
In the spine, there are thirty-two vertebrae, and there are several growth plates on each one. The growth plates will not finish fusing until around age six. The last vertebrae to fully close are the ones at the base of a horse’s neck. Males tend to be six months behind females when it comes to physical maturity, and if the horse is tall, the last fusions will occur even later. A big male warmblood may not be fully mature until his eighth year. Although many trainers believe it is the bones in the legs that must be mature in order to start a horse under saddle, it is actually the spine. There are far more growth plates in the spine than there are in any other single location in the body. The spine is also parallel to the ground, which means any weight on a young horse is pushing the spine toward the ground, creating an arch in the back. It is easy to damage a horse’s spine in this manner, and some horses end up with such weak backs that they must be retired from riding.
All breeds mature at the same rate, which may be a surprise. What matters most is the size of the horse; a 16 HH warmblood will mature faster than an 18 HH warmblood. No matter the breed, a trainer has to be careful not to start riding the horse too soon. Not all of the growth plates need to fuse before a horse is ridden, but the more that are fused, the less risk of injury. Repetitive stress on the bones before the horse is fully mature (jumping and racing are some of the worst activities), the more likely it is that the horse will damage the bones and end up with arthritis long before he should. A general rule is to avoid riding until the horse is four years old. This will be enough time for most of the growth plates in the horse’s body to fuse, although a rider must still be careful not to overwork the horse since the pelvis and spine are not finished growing.
Remember, simply looking at the horse is not enough to determine maturity. It is best to follow the training guidelines researchers have established.
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