Males horses typically undergo puberty or, in other words, attain sexual maturity at or beyond the age of 15 months. Adolescent horses learn quickly, so this is an ideal time for training. However, as testosterone levels in the blood, and sperm production, increase, adolescent colts often display mating behaviours, such as biting, kicking and rearing, and generally become distracted, unruly and difficult to train. Thus, once it is clear that they will not be used for breeding, many colts are gelded, or castrated, to improve their demeanour and work ethic.
Gelding is a common surgical procedure, which involves removing the testicles and epididymis, and can be performed under standing sedation, rather than general anaesthetic. Testosterone is produced in cells in the testicles and, in their absence, its level in the blood drops rapidly, resulting in abrupt behavioural changes.Testosterone is also a growth hormone, which stimulates muscle growth, so geldings may also benefit from physiological changes, such as more proportioned musculature, which can enhance their performance on the racecourse. In Britain, most male National Hunt horses are gelded but, on the Flat, geldings are prohibited from running in several prestigious Group One races, including the Derby.