This is due to the presence of the ampulla, a sperm reservoir at the end of the vas deferens. Because the ampulla is not removed during gelding, a gelding can potentially settle a mare for up to one month after castration. After one month, the sperm that were stored in the ampulla at the time of castration are no longer viable. Improvement in quality and performance in the equine gene pool must begin with selection of only the very best individuals for breeding purposes. Because up to 90 percent of male horses are not of breeding stallion potential, gelding is very common.
Gelding makes a male horse suitable for a greater range of uses.
Male hormones are responsible for much more than the desire and capability to breed mares. Athletic performance can be helped or hindered by testosterone. It can make a stallion perform with more energy and brilliance than a gelding yet it can also serve to distract a stallion from the work at hand. Similarly, secondary sex characteristics, such as muscle bulk, that are influenced by testosterone production can manifest as desirable muscle definition and strength or as an undesirable cresty, thick, and inflexible neck. Gelding is often used as a means to modify a horse's behavior.