Castration in any species involves the removal of one or both testicles and associated structures, such as the epididymis, writes Dr. Christina S. Cable in the April edition of The Horse. Part of the spermatic cord also is removed. The spermatic cord is a tube-like structure that contains the blood supply (veins and arteries), ductus deferens, and nerve supply to the testicle. Castration is usually performed in male horses of inferior breeding stock, in cases of cryptorchidism, or in horses that are difficult to manage. The testicles are the organs responsible for producing testosterone, the main hormone that creates stallion-like physical features and behavior. Removal of both testicles usually rids the horse of unwanted stallion-like behavior, including screaming at and fighting with other horses, attempting to mount other horses, erection, masturbation, and potentially aggressive behavior toward humans. However, when castration is performed at a late age and/or after the horse has been used to breed mares, it is not always successful in abolishing learned stallion-like behavior. Even though the horse will not be able to reproduce, the behavior of mounting mares in the pasture in an attempt to breed and aggressive behavior with other horses might not cease. Horses can be castrated at any age. But most veterinarians will agree that castrating horses at a young age (less than one year old) is ideal. Male horses at that age have smaller testicles that are easier to remove and have a smaller chance of severe post-operative bleeding. Many people castrate horses when they become a management problem -- around two to three years of age. If a horse is gelded early in life, it will grow taller because castration delays the closing of the growth plates in the legs. The ideal time to castrate a horse is variable and will depend on several factors, including the management of your farm, the climate, and training schedules. The most basic requirement for a normal castration is for the horse to have two testicles descended into the scrotum. Your veterinarian must confirm this before surgery is begun. If one or both testicles are "missing," then the horse is considered a cryptorchid. A cryptorchid castration requires more extensive surgery than a routine castration, so such a procedure is not usually performed in the field. If the horse has two fully descended testicles, then the second decision to make is whether or not he will be anesthetized for castration. Castrations can be performed with the horse standing or recumbent (lying on his side or back) and under anesthesia. The main advantage of standing castrations is the horse does not have to be anesthetized. The horse will have to be sedated and restrained with a twitch, but he will not have to lie down, therefore avoiding the possibility of harming himself when he attempts to stand after the procedure. The main disadvantage is that if a major problem occurs, such as severe bleeding, then it is nearly impossible to correct with the horse standing. The horse will have to be anesthetized quickly for treatment. Care after surgery is extremely important, especially if the skin incisions are left open to heal on their own. The horse should rest quietly in his stall for 24 hours following castration. After that time, however, it is imperative that he has forced exercise for at least one hour every day to keep the incisions draining appropriately. Skin incisions in horses that are not exercised can close prematurely, and the area can become swollen, painful, and infected. Daily cold water hosing also helps keep the swelling down and will help clean the incisions.