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.
Castration is a routine procedure if both testicles have descended in to the scrotum.
Before puberty the testicles may be too small to be seen, but can be felt. Testosterone, the hormone responsible for male breeding behavior and some forms of aggression, diminishes to minimal levels within 48 hours of surgery. It is possible for geldings to exhibit stallion-like behavior, including breeding mares. If the horse is extremely aggressive, hormone tests may be indicated to determine whether or not a retained testicle is present. Most stallion-like behavior in the gelding is attributed to a small amount of testosterone secretion by the adrenal glands near the kidney.