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Castrating a horse is intended to focus its mind on racing. As young male animals colts can be distracted by the opposite sex and lose focus. Horses that are underperforming may be castrated so that they can concentrate on racing without the distractions of fillies.
The operation is surgical and the horse feels no pain. A horse is said to be coltish when aroused by a female, so castration renders this no longer an issue.
A castrated male horse is called a gelding, while an entire horse with testicles is called a colt. More jumps horses are gelded than flat horses. In fact, most horses that compete over jumps have been gelded, as male horses can be tentative when approaching fences and other obstacles out of fear of physical pain if they were to collide with the obstacle mid-jump.
Geldings cannot run in Classics and other major flat races. The Classics are important for the breeding industry so the winners must be physically able to reproduce, so some races are framed for colts and geldings while others are just for colts. The Triple Crown for colts is made up of the 2,000 Guineas, Derby and St Leger. Fillies can run in these Classics but they usually compete in their own Classics, which are the 1,000 Guineas and Oaks.
Castration can improve the level of performance of a male horse because the distraction from fillies is eliminated. A male horse with everything intact is called a colt until the age of four and then the terminology is stallion. Gelded horses cannot be stallions and run in certain races. Fillies have their own races with mares and both sexes are eligible for other races.