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Thoroughbred growth rates: Ready to Race?

Thoroughbred growth rates

In the racing industry, horses are backed and trained very young to begin their racing career. There are several influencing reasons for this. However, the main one is money driven. The sooner a horse is racing, the less time it’s sitting in a field and the sooner at can ‘win’ money and/or go to stud. Owners also want horses born as early in the year as possible and preferably a colt. This is reflected by how the racing breeding season starts considerably sooner than the sports horse season and stud fee terms. Stud Fee Terms are offers a stallion owner may offer a mare to make the stallion more attractive. These often include; No Pregnancy by 1st of October you don’t need to pay/free try next year, no live foal at birth don’t pay/free try next year or filly foal get half money back.

I thought it would be interesting to see the thoroughbred growth rate is different to other horses and to determine if there are any reasons for these preferences the racing industry.

Fillies v Colts Thoroughbred Growth Rates

As with many species, the males tend to be bigger and stronger. It is therefore a good hypothesis that stallions are better athletes than mares. However, it is also important to consider, that a good stallion is worth a lot more as a breeding animal than a good mare.

The two papers I looked at found that from as early as 14 days of age, colts were taller, heavier and had more bone than fillies. These papers also found that as the foals got older, the difference between colts and fillies increased. This backs up the idea that colts/stallions are more desirable as racehorses. However, Pagan’s paper also found that fillies tended to have a higher body condition score than the colts.

Early v Late Foals Thoroughbred Growth Rates

All thoroughbreds officially age on the 1st of January. Therefore, all horses born in the same calendar year will race in the same age groups. Horses born closer to the 1st of January are more desirable as in the 2 and 3 year old races, being 5-6 months older than the competition can make a huge difference in their muscle development and training.

However, when you look at the growth rates recorded in these papers, foals born earlier in the year tend to be shorter and lighter in the early stages than those born later in the season. The difference is big enough that a January foal may take 6 months to catch up with the weight of a March foal. A March foal could take 9 months to catch up to a June foal. I would suggest that this happens due to the fact that the later foals are closer to the natural breeding season. But with earlier foals sometimes taking months to catch up to later born foals, I find it interesting that being born earlier in the year is still an advantage. I therefore think that younger foals are desirable as they mature sooner and can have more time in training before their first race.

Maturity Rates

Assuming that the average TB stallion weighs 545kg and average TB mare weighs 500kg, thoroughbred foals reach 44% of their body weight at 6 months, 63% at 12 months and 79% at 18 months. Taking into consideration that many flat race horses begin training at 18 months, I think it’s interesting that they are only at 79% of their full weight. Despite some peoples beliefs that TBs mature quicker than other breeds, this is false. Lighter breeds such as Arabians or ponies tend to reach mature weight earlier than TBs. Whereas, heavier breeds such as shires, reach their mature weight much later.

The average height for a TB is between 160-162cm. This equals roughly 15.3-16h. The average TB reaches 83% of their mature weight at 6 months, 90% at 12 months and 95% at 18 months. This is similar across most breeds of horse. I think this is interesting in terms of growth plates, as at 18 months a horse is almost at full height, however, only growth plates from the knees down should have finished going through growth plate fusion. (Learn more about growth plates here!) Therefore, it could be suggested that the bone growth and growth plate closure in the rest of the body has very little to do with height.


Pagan, J. (1996) A summary of growth rates in thoroughbreds in Kentucky. Kentucky Equine Research.

Hintz, R., Hintz, L. and Van-Vleck, L. (1979) GROWTH RATE OF THOROUGHBREDS.

Last Updated on 05/04/2019

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