Breeding thousand pound animals can be challenging. One of the risks is that if the mare isn't ready, she might kick the stallion - causing injury to both of them and potentially delaying other breedings.
Traditionally, thus, stud farms would use a teaser. A teaser was a cheap, less valuable stallion who would be used to see if the mare was receptive - then the real stallion would be switched in. (Poor guy - I once heard it compared to having a bunch of women in lingerie paraded past you and not getting laid). In some cases, the teaser is vasectomized so if he manages to get to the mare she won't become pregnant.
Some studs use the real stallion for teasing, but keep the two animals at a distance or separated by a barrier. One popular method involves simply leading the stallion past the paddock full of mares - only mares that are in heat will approach the fence.