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As well as providing information about form, draw and choice of jockey, racecards for two-year-old thoroughbred races also list the month in which each runner was born.

There is a good reason why racegoers are interested in this. If a race is being run in April but the horse’s birthday isn’t until May, then he is not yet even two years old. How can he be mature enough to be in intensive race training, at an age when most horses are barely broken to a headcollar?

The answer is a combination of nature and nurture. Traditionally, thoroughbreds are considered a quick-maturing breed. In addition, good nutrition, healthcare, worming and exercise all help speed up growth and muscle development.

Another answer might be that these horses are not yet mature and are likely to pay a price in later life for racing so young. Joint wear and tear, sore shins and both chip and stress fractures are all common problems for two-year-olds in training, while horses that race a lot early in life are much more likely to suffer early-onset arthritis and to have a shortened ridden career.

Two-year-old races are possible because they are relatively straightforward — going flat-out in a straight line does not require high levels of schooling. Dressage and jumping, by contrast, demand that the horse is both physically and mentally more mature. How can we tell when a horse is sufficiently grown-up for more advanced work?

The most commonly quoted guide to maturity is based on the closure of growth plates. These are the cartilage discs near the end of the bones from which bone growth occurs.

Once a growth plate has closed, or “fused”, the bone can no longer increase in size (cross-sectional area) or length. The general rule is that bones near the ground fuse first. The growth plates in the pastern bones, for example, are fused by one year of age, while those in the femur and humerus stay open for three years. The slowest to close are those in the vertebrae, which can still be open after four or five years.

The age at which growth plates close is relatively uniform across different breeds and types, which would seem to contradict the belief that some breeds are faster-maturing than others. The truth is that thoroughbreds and Arabs often appear to mature earlier than warmbloods or draught horses because they are smaller and lighter.

In any event, maturity cannot be judged simply on growth plate closure. Horses continue to fill out, adding muscle and gaining strength, for two to three years after bone growth has stopped. Some large sport horses still appear to be growing until seven or eight, although this is usually more “bulking up” rather than any significant increase in height.

Tailored exercise

So how much should a young horse be worked before his growth plates have fused? We know that exercise is important during growth — it helps joints and tendons to strengthen and is protective against bone and cartilage problems such as osteochondritis dissecans (OCD). Children are encouraged to play sport and some train intensively and compete at a high level, so why not horses?

The difference is that horses, even when young, are large and heavy. The forces transmitted through their joints and growth plates when they are jumping or galloping are far beyond what any human frame would experience. For this reason, the level and frequency of work should be tailored to the horse’s degree of development. The larger the horse, the longer he will take to fill out — and the more prone to injury he will be if overused when young.

The first couple of winters are particularly important. Optimal care and feeding will ensure that growth is not delayed, whereas a youngster “roughing it” outside during winter will inevitably grow more slowly. Poor-quality pasture and high worm burdens can set youngsters back further, while those kept warm and well-fed will reach their mature height more quickly.

The good news for those who do have a tough start to life is that studies have shown that they will still attain full adult size, albeit taking longer to do so. But overfeeding a young horse can have an adverse effect on his health, of course. In exercise and diet, as with most things in life, moderation is key.

There is also the issue of mental maturity. Just as we would not expect a 10-year-old child to sit his GCSEs, so it would be unfair to expect a young horse to have the trainability, concentration and intelligence to perform advanced dressage moves or to cope mentally with a technical cross-country course.

Many of the skills that we ask horses to exhibit in competition demand a combination of both mental and physical maturity. Smaller, lighter types are not just faster to mature, but they often seem mentally quicker than their bigger, heavier counterparts and enjoy a natural advantage that comes with their smaller size.

Long limbs and a heavy frame are much harder to control, as anyone who has ever tried to steer a large warmblood youngster around a 20mx40m arena will testify.

Racing ahead

The reason that thoroughbreds race as two-year-olds is chiefly economic. Racing is more openly commercial than most other equestrian disciplines, and the high cost of keeping and training a horse means that owners are unwilling or unable to wait longer for a return on their investment.

But the commercial pressures on producers are growing in the other disciplines. The resulting proliferation of highly competitive classes for four- and five-year-olds can lead to overtraining of horses that are not yet ready.

Assessing a horse’s maturity and deciding on an appropriate level of work is not an exact science — and there is no formula or veterinary test to help.

The best Flat trainers have an eye on a racehorse’s potential, keeping back those that are too immature to race as two-year-olds. Likewise, it’s down to us as owners and riders to bring on a “slow developer” with appropriate patience and care.

Case study: ‘He wouldn’t stop growing’

“He was small but quite leggy as an 18-month-old colt,” says Holly Norris of her Hanoverian dressage horse, Rubuloo. “At four years old, he was a nice little package, but still just 15hh. We are only now at elementary because he wouldn’t stop growing — he grew two inches just last year, at the age of seven — and he is so immature.”

Rubuloo, known as “Ruin” because of his destructive tendencies, is now 16.3hh. According to Holly, his growth spurts followed a distinctive pattern.

“He would go ribby, almost overnight, and it was clear that all his energy was going into growth,” she says. “He has always been quite juvenile in the head, but he would lose the ability to pick up canter, as if he couldn’t organise his legs. After giving him a high-protein diet and a lighter workload for a few months, I would find that he’d gained another inch in height.

“I never thought he would reach this size and it has taken a long time to get here,” adds Holly. “He has filled out nicely now and has finally stepped up mentally, although he remains slightly babyish and is often away with the fairies.

“It has been worth the wait, though. I think he could have been ruined if someone had pushed him too hard.”

Ref Horse & Hound; 5 July 2018