Veterinary Corner from the Vet Corner Archives Part of Horse Previews Magazine website. A stallion should be an outstanding example of his breed. This is very important, since potentially he can breed hundreds of mares in his lifetime, affecting the gene pool significantly more than a mare, who can have a maximum of one foal per year. Additionally, stallions can be aggressive toward handlers and other horses, and unless managed carefully can escape confinement and seek random mares to breed, thus producing unwanted pregnancies. For these reasons, castration is the most common surgery performed on the horse.
Much controversy surrounds the decision about the appropriate age for castration.
Most colts have reached puberty by the age of 18 months. Most veterinarians agree that there is no advantage, and, in fact, are disadvantages to waiting beyond this time. Once colts have reached puberty they are often difficult to handle, and must be separated from the group in order to prevent fighting and unwanted breedings. Some veterinarians have noticed an increased incidence of hernias as a complication of castration when it is performed before 6 months of age, however. Given these facts, it appears that the optimum time for castration is after 6 months of age but before 18 months.