As the old horseman's saying goes, "Good stallions make great geldings," or castrated males. That means that all but the very best male horses should be gelded, leaving only the cream of the crop to reproduce. Stallions require extra precautions in handling, training and general management. While they're not a horse for the novice, stallions can make fine sport horses for the experienced rider, in addition to their breeding duties.
Colts hit puberty between the ages of 1 year and 18 months. While a young colt can frolic with fillies in the field -- or mares -- it's best to separate him from equine females before he reaches his first birthday. While younger stallions can breed, stallions don't fully mature in terms of scrotal size and sperm production until the age of 6. By that age, the stallion produces two or three times the amount of sperm daily as a 2 or 3 year old.
In the wild, stallions run free with a band of mares. That's not the situation for most domestic stallions. With new reproductive technologies readily available, fewer stallions actually breed mares. Instead, they are trained to mount "dummies," where the ejaculate is collected and either shipped fresh to mare owners or frozen for future use. One major exception to this brand new world of reproduction is the Jockey Club, the governing body of thoroughbred breeding. Thoroughbreds must still reproduce the old-fashioned way, through live cover, in order to enter the registry.
While fertility varies with the individual stallion, during the traditional spring through summer breeding season a horse is more sexually responsive and produces more sperm in each ejaculate than during fall and winter. In North America, stallion fertility peaks in May and June. As stallions age, they don't produce as much sperm but the overall quality of the semen shouldn't decline in healthy animals. However, the physical act of breeding -- whether naturally or on a dummy -- might become difficult for arthritic animals.
A colt's testicles usually descend shortly after birth, but it's perfectly normal for them not to drop until the horse is 2. If one or both testicles have not dropped by that time, the horse is considered a cryptorchid, either unilateral or bilateral depending on whether one or both testicles didn't descend. While a unilateral cryptorchid can breed and produce offspring, this condition is hereditary and breeding such animals is a bad idea. Even if these horses -- also known as ridglings -- have good pedigrees or athletic ability, they should be gelded. Cryptorchids retain the traits of stallions with none of the benefits. Because one or both testicles is still in the abdomen, such a procedure must be performed in a veterinary surgery, rather than on the farm in a typical gelding scenario.
Although temperament varies by the individual horse, there's a reason stallions have a reputation for aggression. That's not just toward other stallions, but toward humans, as well. The University of Pennsylvania School of Veterinary Medicine's website states that stallions require "Consistent, firm, judicious, and skillful training." While that's true of any horse, it's especially important for studs. You must always pay attention to a stallion's behavior and not drop your guard, even if he's usually good-natured. There's another old horseman's saying: "Never turn your back on a stallion."