What determines a horse’s height? *H&H VIP*

Breeding a foal or buying a youngster is always an exciting prospect, but it can prove challenging to get all the aspects of feeding, training and management exactly right. Friends and observers along the way will no doubt give their opinions on how nice the horse is, what height they think he will make and whether or not he will be any good at his job.

While it may be fun to speculate, the reality is that final mature height is influenced by many factors — from the moment of conception through to full wither height at somewhere between five and seven years.

Each horse has a certain genetic potential for growth that is inherited from his sire and dam. Heritability of wither height is thought to contribute only 26% of the “phenotypic variation” of the end product — that is, accounting for just over a quarter of adult final wither height. The remainder is controlled by environmental factors including nutrition and exercise management.

Predicting height

There is no “one size fits all” model to accurately describe the growth of horses. Factors such as breed, management and owner objectives all influence growth.

Understanding growth patterns is important when balancing economic and perceived athletic advantages of large size and rapid growth against the potential associated risk of skeletal abnormalities. This raises questions about how patterns of growth can be controlled, and if certain patterns may be more beneficial or detrimental than others regarding the future health and performance of the horse.

The first growth spurt and the most rapid rate of gain occurs just prior to birth, emphasising that attention must be paid to the nutrition of the broodmare throughout gestation. Foal height at birth positively correlates with final height and is thought to account for approximately 60% of height at maturity. In the commercial thoroughbred industry, birth height is an accurate indicator — yearling wither and hip height can even predict lifetime earnings.

Foals born to maiden mares (first-timers) are usually smaller than those of multiparous mares (those who have bred before), so this must be borne in mind if considering breeding for maximum height.

In all breeds there is a great deal of importance attached to the early growth period. By 12 months, most riding horses will have already attained 85-90% of adult height. By two years, approximately 95% of the mature height may have been reached, and at three years 98.6%.

While it is possible to estimate your young horse’s fully grown height with a relatively simple calculation, it may be that the last centimetre causes him to measure out of his class.

It is recommended that the growing horse should be fed to achieve moderate and consistent growth rates without experiencing any checks — where growth is stopped or reduced temporarily — which are often followed by growth spurts and thought to be detrimental. If a foal or yearling has a period of ill-health that affects his appetite for a significant duration, care should be taken to smooth the recovery phase and avoid a catch-up growth spurt.

Wither height is achieved by growth of the long bones of the forelimbs. Many youngsters have croup-high conformation; it is hoped that this levels out by the time the horse starts work, as such conformation is thought to predispose to forelimb lameness.

Nature or nuture?

In his first two months, a foal receives most of his nutrition from the mare’s milk. During this time, the foal may nibble on the mare’s concentrate and forage.

At about two months of age, the mare’s milk will no longer meet the foal’s nutritional requirements. Left to nature, there is a growth check at this stage until the foal is able to forage well, before another check at around six months — usually in autumn — at weaning. Winter grazing gives a further reduced nutritional period, followed by a growth spurt when the spring grass comes.

It is thought to be better for consistent and quality growth to nurture rather than rely on nature. Introducing foal creep (supplementary) feed between one and two months helps the transition for the foal from a liquid to a solid diet at weaning, avoiding any changes in growth rate and maintaining good bone development.

Creep feed must be introduced gradually on a free-choice basis, to mimic the suckling behaviour of frequent small meals day and night. Properly balanced creep feed formulations are now available, aimed at steady growth.

Suckling foals fed poorly balanced, high-energy creep feeds will be deficient in essential amino acids and minerals, which will lead to over-conditioning with inadequate musculoskeletal development. The impact of inadequate nutrition on growth and development of a young horse can manifest itself in many ways, ranging from retarded overall growth to changes in the metabolism and conformation, which may not produce apparent disease until later in life.

Failing to meet the nutritional requirements of the post-weaning foal by under- or over-feeding is known to have a detrimental impact on bone maturation and structure, and a profound effect on the horse’s future use and longevity. This should act as a motivation for producers to get this growth period right — and not to tailor feed to produce a certain height at measuring time.

Ref Horse & Hound; 4 May 2017