If a horse is ‘supplemented’ for a race, it is entered into the race at a late stage, known as the ‘supplementary stage’, after the original entry stage. Supplementary entries typically involve a substantial fee, which must be paid by connections before their horse can run in the race in question.
The initial entry stage for the Derby, for example, takes place eighteen months before the race, when the horses that will participate are still yearlings and have yet to set foot on a racecourse. Yearling entry is, understandably, the most economic way to enter the Derby but, even by the second entry stage, in early April in the year of the race itself – by which time the horses are three years old – a trainer may not know he has a Classic prospect on his hands. However, he or she has one last chance to enter a horse in the Derby; five days before the race, when the remaining horses have their entries confirmed, a horse can be supplemented for £85,000, or just under £5,000 more than the prize money for fourth place.