Skeletal Maturation in Horses

Do we ask too much too soon?

by Emma Green, Veterinary Physiotherapist

We often hear people in the equine industry referring to their horse as an “early maturing horse” or a “slow maturing horse” when in actual fact there is no such thing. Every horse of every breed does not skeletally mature before the age of six.

Many people believe that there are just a few growth plates in the horses’ body, when in actual fact growth plates are present on the ends of every bone, other than the skull, and in some places there are multiple growth plates (e.g. in the pelvis). The process of bone growth begins when the horse is just a foetus and throughout its early years. The cartilage throughout the ‘pre-formed bone’ (produced at the foetus stage) gradually ossifies and it is this that gives strength and stability to bone.

At the foal stage, thin bands of cartilage remain at the ends of each bone, these are the growth plates. For a bone to be fully developed (six years old, plus) the growth plates will have also fused with the only remaining cartilage being the coating on the outer ends of the bone. The process of growth plate conversion takes place from the distal limb and works up the horse’s body. Therefore the lower the bone, the earlier the growth plates will fuse.

The vertebral column is the last to develop with several growth plates in each of the 32 vertebrae (breeds can vary in vertebrae total). The main body of the vertebrae (centrum) does not fuse until most horses are five and a half years old, with the vertebrae at the base of the neck fusing last.

The vertebral column differs from the limbs because of the increased number of growth plates, which unlike the limbs, are parallel and therefore can struggle with the stress of carrying a load (i.e. saddle and rider). For this reason the vertebral column is more at risk and much easier to damage from inappropriate exercise and weight carrying in the early years.

The triangular sacrum is composed of five vertebrae that fuse together at the fourth or fifth year of the horse’s life. The sacrum becomes a sturdy platform for the horse’s huge pelvis. If the sacrum is stressed early on, the horse can become susceptible to pelvic issues, such as constant pelvic tension, sacroiliac problems and shortening of the hind strides, to name a few.

Bigger breeds, taller horses and longer backed horse’s will all take longer to mature through the skeletal structures, with stallions and geldings taking longer to mature than mares. Therefore a 17hh warmblood gelding will not fully skeletally mature until eight years of age.

So, do we ask too much too soon?

As we have established, horses growth plates do not fully fuse until the horse is approximately six years of age. Therefore if the horse is required to carry the weight of a rider prior to this there is a risk that strain will be put on the spine and other joints.

As pointed out previously the growth plates throughout the limbs are less likely to damage because they are vertical structures, however there is the chance that the growth plates move and the cartilage within the joint can become crushed, causing a lack of lubrication which can then lead to degenerative conditions such as OCD (osteochondritis dissecans). In some cases joint deformity can occur.

It is not just about the age that we back the horse, but evidently what we request of them in their early years. With the addition of a saddle and rider, a horse will naturally hollow its back. This automatically puts the horse’s vertebrae at risk of impingement. This is the case with any horse at any age, however with the added aspect of the unfused growth plates and immature vertebral structures, the risk of damage to the young horses back is greatly heightened. For this reason, it is vital we teach the horse good posture and equip it with solid muscular development.

Muscular support is required in order to round and stabilise the horse’s back. Encouraging the hind legs to step through and under and lifting the abdominals all pay a valuable part in strengthening, stabilising and supporting the back. In theory this is great but with the constant changing skeleton, it can take longer for young horses to develop and they often need time to establish and tone their musculoskeletal system.

Balance and co-ordination comes with time, while in the wild a horse is capable to running away from predators almost from birth, the degree of stopping and turning we ask of them is far greater than nature requires and therefore effectively steering a young horse can be tricky, especially if corners have been cut during groundwork. Yanking or pulling on an immature horse’s head and neck will cause strain or even long term damage. Remember the vertebrae growth plates on the base of the neck are the last to fuse!

Asking a horse to not only carry a rider, but perform in various disciplines before the horse is ready, can cause issues in later life such as lameness, kissing spines, pelvic problems, joint pain, and explosive reactions such as bucking, bolting and rearing. Research shows that kissing spines (impingement of the dorsal spinous processes) is clinically more likely to be found in horses under the age of 5 years old.

Having said all this, it is often unrealistic to wait until a horse is six years of age to begin backing and training, as financial implications and what the industry requires often dictate when a horse is started. Most people are aware that the majority of horses, particularly competitive horses are backed at a young age allowing them to start their career early, but also to allow the handler to defuse any negative responses before the horse gets too strong to manage.

What can we do to help protect these young horses?

When working a young horse throughout any discipline it is important to be aware of the implications on its body and try to minimise all risks.

Building strong muscles correctly is crucial to support and protect vulnerable joints that are still developing. Correct training and regular muscular maintenance from a qualified physiotherapist will help to relieve any muscle tension, identify soreness and reduce the risk of injury. Therapists that get to know your horse can also identify reoccurring issues, discuss with your vet and hopefully resolve quickly and effectively.

It is also crucial that tack is fitted correctly at all times. Young horses are constantly developing and changing  shape during the early stages of training, therefore the originally fitted saddle may not be suitable a few months or even weeks later. Having tack checks carried out regularly by a qualified saddle fitter will limit the risk of damage to the horse. Tack can be very influencial, on the horses development and whilst it can be tempting to ‘make do’ as the horse will change anyway, this can bring about issues preventing positive progression.

Other important factors to consider are the surfaces you ask the horse to work on. If a large amount of work is done on the incorrect surface (e.g. too hard or too deep) major strain will be put on the joints and the body’s suspensory apparatus that are already susceptible to weakness. Soft tissue injuries such as muscle/ ligament/ tendons strains can be avoided by keeping in mind these implications.

The duration of training sessions is something else to consider and should be built up slowly. Increasing the workload slowly will ensure correct muscle development without tiring and therefore protecting the joints and immature skeleton. Time off is just as important as time in work as this will give the horse time adjust to the new challenges.

In conclusion

There are mixed opinions within the equine industry regarding the correct age to ‘start’ a horse and the extent of what they are asked to perform. Maybe this is down to the industry itself, with more and more young horses being asked to complete more advanced performances at a young age.

Every horse is an individual and quite often when new objectives and experience presented are accepted, the rider believes the horse is ready and able to continue with its in-depth education, however, although the horse may be mentally ready, we need to repeat the question, is it really physically ready?

If the horse is required to perform at a young age, it is of most importance we help them along the way with correct muscling, appropriate equipment and facilities and professional assistance as well as being mindful of the pressures on their body.

Animal Therapy Magazine