In horse racing, a ‘gelding’ is male horse that has been castrated or, in other words, has had its testicles surgically removed, so that it is unable to reproduce and has no breeding value. Castration not only makes a male horse sterile, but typically inhibits stallion-like behaviour, making it easier to control and easier to train, especially around female horses. In Britain, the majority of male horses that participate in National Hunt racing have been gelded for this reason.
The castration procedure is typically performed on horses that are mature, skeletally, so that they reap the benefits of the hormone testosterone, which is produced by the testicles, on their physical development. However, the procedure is best performed soon rather than later – between the ages of six months and two years is typical – because castration at a late age may not eradicate aggressive, masculine behaviour that has been learned in the meantime.