The decision to castrate (or geld) a stallion, called an orchidectomy, is one that most horse owners, trainers and managers have had to make. While performance and manageability may be improved with castration, many feel the pressure of limiting future progeny to be a crippling choice if the horse has proven to be suitable as a breeding prospect.

Former tenured faculty member at the University of Guelph, Dr. Antonio Cruz, a board-certified equine surgeon and sport medicine specialist, states emphatically that not all stallions should be considered candidates for the breeding shed. “All horses that are not suitable as stallions, because of their genetic lineage and aptitude should be castrated,” he said. “This facilitates their management in a herd situation; it is safer and prevents unwanted pregnancies.”

When to Castrate

While Dr. Cruz recommends that horses be castrated at around one year of age, he noted that they may be castrated younger if necessary, and that waiting to make a decision may have negative effects in the long run. “A horse is more likely to retain stallion tendencies if he is castrated later in life, particularly if he has been used as a breeding stallion,” explained Dr. Cruz. “There is no reason to keep a horse as a stallion beyond a year of age.”

He added, however, that “if the owner wished to see the performance of the horse and as long as the horse has good genetics, then he could be castrated later on. If there are no plans to breed, then he should be castrated around a year of age.”

Pre-Op Exam

Prior to a castration, the veterinarian will perform a thorough physical examination, in which palpation will ensure there is no evidence of a scrotal hernia, which forms when a section of intestine protrudes from the abdomen into the scrotum. “If a hernia is present during the initial exam, the horse should be referred to a board-certified surgeon to castrate and fix the hernia under operating room conditions,” said Dr. Cruz. He added that “an alternative for preserving the testicle while fixing the hernia is to do a laparoscopic hernia repair.”

The veterinarian will also check to make sure both testicles are descended. “Both testicles should be in the scrotal pouch at birth,” said Dr. Cruz. “When this does not occur, the horse is deemed a cryptorchid, sometimes referred to as a ‘rig,’ and [this scenario] necessitates a special operation to remove the retained testicle. A retained testicle tends to form a tumour later on in life, and keeps producing testosterone responsible for aggressive male behaviour. Although [it is] a popular [belief], retained testicles do not descend after birth.”

Castration Methods

Whether the horse is anaesthetized and recumbent or left standing during castration often depends on the personal preference of the veterinarian and whether there are any increased risks of herniation following the surgery. When it comes to the actual procedure, there are several methods. “Anatomically, we talk about ‘open’ or ‘closed’ castration depending whether the soft tissue that covers the testicle is opened during the surgery or not,” explained Dr. Cruz.

“An open castration is faster and easier to do, but has bigger risks of herniation or infection into the abdomen. Many [vets] perform what is referred to as a ‘semi-open’ castration, which is a technique that opens some layers of tissue, but closes them surgically before finishing the procedure.

“Then, depending whether the scrotum is sutured or not, we talk about a ‘primary closure’ castration or an ‘open healing’ castration. The latter is the technique that is done most often, as in field situations it is not advisable to suture the scrotum. However, under the sterile conditions of an operating room, a primary closure castration offers the advantage of cleanliness, minimal swelling, easier after care, faster recovery and return to work. The trade-off is cost, as a primary closure castration performed in the operating room is more costly.

“The horse must be carefully handled, diminishing stress, pain and possibility of infection,” said Dr. Cruz. “It goes without saying that adequate sedation, anaesthesia and analgesia are paramount. The days when people used to wrangle horses and ‘cut them proud’ should be over. There is no need for this kind of brutality and abuse.”

Note that the Canadian Veterinary Medical Association regards castration as a veterinary medical procedure, which should only be performed by a veterinarian using appropriate surgical, anesthetic and analgesic techniques. In addition, the National Code of Practice for the Care and Handling of Equines urges owners to review provincial regulations before contracting anyone other than a licensed veterinarian to perform the procedure.

Post-Op Care

After surgery, Dr. Cruz said the horse should be put on 24 hours of stall rest, then turned out into a small area. He said it is imperative the horse has movement following stall rest to ensure the incisions are draining properly; turn out in a small paddock with supervision will allow for movement and gentle self-exercise, which will reduce swelling and encourage drainage.

The incision should be kept clean by hosing with cold water once or twice daily, and a combination of antibiotics and anti-inflammatories or analgesics administered as prescribed by the veterinarian.

Horses usually return to work within four to six weeks following surgery if there are no serious complications.

Potential Complications

Though castration is a common procedure, it should never be considered routine. Many complications can arise following surgery, but, with proper management and attention, are usually contained to mild swelling and discharge. “Some horses will develop a deep infection that leads to a condition called scirrhous cord that needs surgical excision,” explained Dr. Cruz. “In more severe cases, some horses may herniate, with the bowel coming out of the incision following the surgery. The latter complication is life threatening and requires immediate emergency attention and optimally a referral to a board-certified surgeon.”

Behavioural Changes

Residual testosterone will remain in the horse’s body for a short period of time following castration. Learned behaviour or unwanted stallion-like tendencies may still be present after castration; the age of the horse at the time of castration will also play a part in how quickly noticeable changes in behaviour occur. Most horses experience a progressive behavioural change, but a period of six weeks without contact with mares is usually advised.


Performing an ovariectomy on a mare, also referred to as spaying, is a relatively uncommon procedure when compared to castration.

The primary reason owners, trainers and managers opt for this procedure is to suppress behavioural signs of estrus. Common complaints include a change in attitude, tail swishing, difficulty in training, squealing, excessive urination, kicking and a decrease in performance.

Dr. Tracy Plough, who offers mobile ambulatory and advanced reproductive services, has experience as a resident in equine reproduction at the University of Vienna and completed a residency in private practice at JCS Veterinary Reproductive Services, under the mentorship of Dr. Juan Samper. Her training and work in equine sports medicine, while specializing in equine reproduction, allows her the perspective of seeing mares as performance athletes, while keeping in mind the value of preserving good genetics and reproductive soundness.

With the goal of improved performance being the main factor for those considering the procedure, Dr. Plough cautions that going as far as an ovariectomy to suppress estrous behaviour may not have the desired effect.

“Mares are a little different than most domestic animals,” she explained. “They, of course, have the ovarian-derived, estrogen-induced signs of behaviour, but many [ovariectomized] mares will also exhibit paradoxical estrous behaviour, which is associated with hormone secretion from the adrenal cortex.

“This unseasonal estrous behaviour can look exactly the same as that of intact cycling mares. This is important to keep in mind because clinically suppressing ovarian follicular activity or removing the ovaries may not ensure the elimination of estrous behaviour.

“If a mare does not respond to pharmacological estrous suppression methods, it is unlikely that she will respond to ovariectomy. The surgery is the last thing I recommend, and, generally, I do so only if the mare has a tumour or some other kind of pathology within the ovaries that makes it necessary.”

Dr. Plough recommends a less invasive treatment for mares whose cycle-related behaviour is interfering with performance, and suggests that mares be assessed to rule out other causes of behavioural issues. “Before opting for ovariectomy, mares should be examined for any musculoskeletal pain, and it is important to determine if the issues are indeed related to the estrous cycle,” she said. “A reproductive examination should be performed by a qualified reproduction specialist when the mare is showing the behaviours described.”

Though there are several options for estrous suppression without resorting to surgery, the most common pharmacological aid is the administration of altrenogest (Regu-Mate®), which is administered orally. “Regu-Mate® is a synthetic progestin that will suppress estrus or heat signs,” explained Dr. Plough.

She added, “Some of the downsides to Regu-Mate® include that it requires a consistent program, as it has to be administered orally every day; it is oil-based, so can be messy; and it absorbs through the skin, so you have to consider human safety.”

Dr. Plough added that “Regu-Mate® will not suppress follicular growth. Therefore, pain is still possible if related to enlargement of the capsule.” She also cautioned that it shouldn›t be used in mares with endometritis or uterine inflammation.

“Another method of estrus suppression is to use deslorelin implants, such as Ovuplant™,” said Dr. Plough. “Originally intended to induce ovulation in mares, a side effect was discovered in some mares to be prolonged estrus suppression. By placing two or three inside of the vulva, subcutaneously, so they can easily be removed, the deslorelin implants will cause a decrease in the circulating concentrations of follicle stimulating hormone [FSH], and this will suppress follicular development in mares. The estrus suppression effects last anywhere from 30 to 90 days before treatment [needs to be] repeated.”

If response to clinical treatment is favourable and ovariectomy is elected as a permanent solution to estrus suppression, the breeding potential of that mare is lost, but “both mature and immature oocytes can be collected from excised ovaries in domestic species,” Dr. Plough explained.

“Ovaries can be stored for up to 24 hours post collection and still have viable oocytes collected,” she said. “However, the recovery rate and rate of embryonic development is compromised for every hour post-collection the oocytes are not recovered.

“In cattle and sheep, simple aspiration from the follicles with an 18g needle is usually sufficient to collect oocytes. In the horse ovary, aspiration alone is not usually successful in the collection of oocytes due to the firm attachment of the immature equine oocyte to the follicular wall. Efficient collection of equine oocytes requires opening of the follicle and scraping free the granulose cell layer. These oocytes would then need to undergo in vitro maturation and advanced procedures such as ICSI [intra cytoplasmic sperm injection], as in-vitro fertilization in the horse is currently not successful. Vitrification of vitro-produced embryos is possible for transfer at a later date.

Dr. Plough cautioned that, “with every step in the process, the pregnancy rates are reduced and the cost to owner is increased. Also, this maturation process followed by ICSI is only available in a few specialized labs and, thus, the ovaries would need to be transported and processed in these labs within the 24-hour window.”

She concluded, “It is likely that an ovariectomy would not be recommended to a valuable mare with good genetics if the goal was to modify estrus behaviour.”