What owners need to know about castration in horses, including the risks and side effects
Surgical resident Elise Rodden MRCVS explains the equine castration procedure along with the risks involved, and equine reproduction consultant James Crabtree FRCVS explains the physical and behavioural impact of gelding
Castration of horses and donkeys is the most common surgical procedure in equine veterinary practice performed worldwide. However, there is widespread variation in how the surgery is performed, and in which medications are given pre- and post-operatively.
Elise Rodden MRCVS, a surgical resident at Bell Equine Veterinary Clinic and B&W Equine Hospital, explains: “There is a vast variation used by vets in the methods to geld or castrate a horse, presumably because no one protocol is perfect.
“Certainly, different options are available depending on the clinician’s choice and the exact circumstances. For instance, castration can be performed standing or under general anaesthesia (GA), in a hospital setting or on yards, placing sutures or not, using local anaesthetic or not, or leaving the skin incisions open or closing them.”
Equine castration essentials
In Britain, the law dictates that an appropriate anaesthetic must be used to geld a horse of any age and that castration is a surgical procedure that must be performed by a vet. Gelding without veterinary involvement and without use of painkillers or anaesthetics is barbaric and illegal.
● Painkillers must be given before, during and after the procedure
● Check tetanus vaccinations prior because any wound is a potential entry site for the infection responsible
● A sterile surgical technique and a clean environment should be prioritised over the use of antibiotics as a preventative measure
● Castration can be carried out at any age although between six months and two years old is usually the norm
● The best times to operate are spring and autumn as the presence of flies in warmer weather increases the risk of infection
● Keep a close eye on the patient for 12 hours, then turnout is generally recommended to encourage drainage and reduce the likelihood of swelling
What can go wrong during castration?
Although a routine procedure, castration is not risk-free and although most complications are mild, some are serious and can even be fatal. A review undertaken by Elise in 2025 found 17 different reported complications following castration procedures.
“Among the less concerning complications are those that cause no distress to the horse but are a concern for the owner, such as swelling or continued undesirable behaviour, and mild complications that resolve with minimal intervention (oedema or localised infection),” says Elise.
“More seriously, severe and life-threatening complications such as bleeding, prolapsed gut or abdominal infection/inflammation can occur.”
Despite castration being a routine surgical procedure, reported mortality rates are up to 1%.
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“Perhaps unsurprisingly, scrotal swelling is the most common problem reported, followed by infection of the surgical site,” adds Elise. “The size at which the scrotal swelling changed from being a normal post-operative finding, which is expected after surgery, to being considered to be a complication varies tremendously.
“Although less common, some of the other observed complications can be more serious if they are severe. These included bleeding and evisceration (when intestine or some abdominal tissue comes out through the open skin incisions), which can be a non-survivable complication.”
The risk of evisceration is thought to be higher for certain breeds, such as draught breeds and standardbreds. A prior history of a scrotal or inguinal hernias would also increase this risk.
“The risk can be minimised by suturing the tunic (which surrounds the testicle and is continuous with the lining of the abdominal cavity) closed once the testicle is removed,” explains Elise. “This prevents any intestines from the abdominal cavity from escaping into the abdomen via the incision at the castration site.
“To place such sutures safely and in a sterile fashion, a GA is recommended, which obviously increases the cost of a routine castration. A more complex procedure in a hospital under GA is more appropriate in these cases.”
Talking in more general terms, Elise says risks can be reduced by “gelding young, and ensuring that the colt is well-handled and castrated in a clean environment.
“Ultimately, the surgical choice should be based on a risk analysis for the individual horse, alongside your vet’s preferred approach and the facilities available, as well as economic factors.”
Castration is a common procedure, but not risk-free
Why do we castrate colts?
When colts are gelded they can no longer produce large quantities of testosterone, making them easier to handle.
“Testosterone is a driver for sexual behaviour and reproduction,” explains Dr James Crabtree, junior vice president of the British Equine Veterinary Association and an independent vet consultant in equine reproduction and stud medicine.
“Horses are very big animals; colts that are a bit lairy don’t have their minds on the job and can be unpredictable. If you remove that testosterone, you remove that hormonal driving force behind the unwanted behaviour and the potential barrier to onward training.”
After castration, testosterone levels drop very quickly.
“It can be within a few days,” says James. “The reason we talk about not putting a newly castrated colt in with mares isn’t because they are still producing sperm, it’s because they could still have sperm left in the reproductive tract and the behaviour may not change immediately.
“Behaviour isn’t just the hormones; it’s also learned. It will vary according to individuals and environment. If the environment is very stimulating, with a lot going on, the horse may take longer to calm down.”
A further complication is that testosterone isn’t produced only by the testes. Mares also have testosterone, produced by their ovaries and adrenal gland, and a gelding will continue to have very low levels of it.
Is a horse’s growth affected by castration?
As well as being androgenic (responsible for promoting male sexual characteristics), testosterone is also anabolic – that is, responsible for muscle growth. So castration inevitably has a physical impact as well as a behavioural one.
“You can look at lots of different species and see this. For example, bulls are stocky with lots of muscle and bullocks are leaner,” says James. “Of course, that doesn’t mean to say that, with training, you can’t put the muscle mass on a gelding.”
Testosterone also plays a role in bone development as it indirectly helps close the growth plates (the cartilage at the end of the long bone, which fuses upon maturity). If a colt is gelded relatively early, before the plates have closed, the plates can stay open for longer and the gelding may grow taller than if he had been left entire.
These and other factors are taken into account when deciding at what age to geld.
“In racing, some people don’t want to geld too early, as that testosterone might give the horse a competitive advantage,” explains James. “In contrast, some colts’ testicles aren’t fully descended until they’re three years old, which can present a comfort issue for them in training, and which can be resolved by castrating.”
Some racehorses may be gelded late, to retain the benefits of testosterone
Why can some “coltish” behaviours remain?
Despite the lack of testosterone, some geldings do continue to demonstrate sexual behaviour long-term after castration and James says the reason behind this isn’t fully understood: “It might be remembered, as some geldings were stallions for a time, or learnt or genetically programmed.”
In other cases, it may be because the horse has undescended testicles; that is, they are not in the scrotum.
“If a gelding is showing dominant behaviour – often colloquially described as being a bit ‘riggy’ – it can be because there is testicular tissue still there, which you can’t see. In those individuals, we have to do some diagnostics.”
Using a combination of physical examination, ultrasound and hormone tests, a vet will assess whether any testicular tissue is present. If so, it’s removed surgically.
As testosterone is just the last link in a hormonal chain set in motion by longer days in spring, geldings remain affected by seasonal fluctuations in hormone levels even after castration
What are the side effects of castration?
Testosterone is part of a “negative feedback loop”. When the testes produce a lot of testosterone, it signals to the brain and pituitary gland to slow down production of the LH and FSH hormones. Without testes, that feedback loop is interrupted and less inhibited.
Research in dogs reveals some interesting differences between entire males and castrated males.
“There’s good evidence that intact males [dogs] have a lower risk of several immune-mediated and autoimmune diseases, making canines one of the veterinary species in which testosterone appears to exert a genuine protective immunological effect, although testosterone also increases the risk of several androgen-dependent conditions, particularly those affecting the prostate and perianal region.
“However, there’s currently no evidence to suggest that testosterone has a protective role in the long-term health of male horses.”
“Clinically, I don’t see a lot of geldings who lack confidence”
James Crabtree FRCVS
There’s also some evidence with dogs that the drop in testosterone following castration can cause increased anxiety and fearfulness, but again James says this isn’t typically seen in horses.
“Clinically, I don’t see a lot of geldings who lack confidence – if anything, you could argue they are not burdened by testosterone or oestrogen.
“It could be because horses have reasonably good socialisation from a young age, with their dams and other horses, so they don’t need testosterone to feel confident. Whereas dogs are often raised in single dog households and lack confidence when taken to a park full of other dogs.”
What about social licence?
James points out that we need to consider the gelding procedure from the point of view of public perception, given that it may be regarded as unkind or unnecessary by the general public. But he explains the safety considerations behind it and the social benefit for the horses themselves.
“As vets, we think about surgical procedures on horses a lot, with social licence in mind,” he says. “Primarily it is performed for human and animal safety reasons; however, it is also an intervention to ensure we can keep male horses in mixed sex environments and in sport. Keeping stallions entire is easier with experience and the right environment.”
An alternative to surgical castration
A vaccination that could be an alternative to surgical castration has been used with some success. However, it’s currently not available for use in horses, meaning that any use is “off licence”.
It acts on the GnRH, which signals to the pituitary gland to release the hormones that stimulate the ovaries and testes, essentially limiting testosterone production (and oestrogen production in mares) from further up the hormonal chain.
“The GnRH vaccine is really interesting,” says James. “Anecdotal evidence suggests that it may suppress more than surgical castration, as it acts on a greater number of hormones. Also, its use is not without some significant risks.
“In some cases, the vaccine could be used for a time and you could still breed from the horse in the future, but there are no guarantees and its effect should practically be considered irreversible.
“Most people, if they want the stallion to have a breeding career later, will collect semen and freeze it first and then castrate, either surgically or chemically.”
Elise is a surgical resident at Bell Equine Veterinary Clinic and B&W Equine Hospital. She graduated from the University of Liverpool in 2020.
James is junior vice president of the British Equine Veterinary Association and an independent vet consultant in equine reproduction and stud medicine. He graduated from the Royal (Dick) School of Veterinary Studies, Edinburgh in 2001.
Our equine veterinary expert has more than 30 years experience in practice. She is an FEI vet and has attended the past three Olympic and many other major championships in her role. She has a particular interest in anaesthesia, pre-purchase examinations, ophthalmology and dermatology amongst other clinical concerns. She has been Horse & Hound’s official veterinary advisor since 1991. Karen is part of the Bell Equine team.
