In general, geldings tend to be unaffected by the presence of mares. However, some will show unwanted stallion-like behaviour, such as performing the flehmen response, snorting, arousal and even mounting mares. At best, this behaviour can be inconvenient and frustrating, at worst, it can be extremely dangerous for both horses and handlers. In this article we discuss why this may happen and what we can do about it. 

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Is the horse a true gelding?

Many horses have an incomplete history at the time of purchase so it is not always possible to say with certainty if they have been castrated or not, or to know details about the surgery. The veterinary surgeon who has performed the procedure will usually amend the passport to reflect this, but passports may be lost or this information may not be available. 

In the absence of prior medical knowledge of your horse, your vet will most likely start by palpating the horse’s inguinal region under light sedation to see if they can palpate any testicles or scars that may indicate a castration has taken place. The absence of testicles, visible or palpable with an associated scar is a good indicator that the horse is likely to be a gelding, but we cannot be absolutely sure. 

Horses can retain one or both testicles, known as unilateral or bilateral cryptorchidism

It is extremely rare, but not impossible for a horse to have only one testicle, known as a monorchid. So, the horse may have retained testicle(s) or it may have had a single descended testicle removed, leaving an internal one. The latter is extremely unlikely in the UK as veterinary surgeons do not usually remove a single descended testicle without first removing the retained testicle, to avoid any unwanted confusion in the future if the horse is sold on. Under the Veterinary Surgeons Act 1966 it is an offence for an individual who is not a qualified veterinary surgeon to castrate a horse. 

If a testicle cannot be palpated externally, rectal examination or ultrasonography may be used to attempt to find a retained testicle, however, this can be difficult as retained testicular tissue tends to be flaccid and difficult to identify. The presence of retained testicular tissue post castration is extremely rare, although this is more likely to occur if the horse had a retained testicle at the time of castration as these tend to be abnormal and it is possible that the testicle may be inadvertently incompletely removed. 

So if a testicle cannot be observed or palpated, we need to use another method to find out if testicular tissue is present or not. 

How do we test for the presence of testicular tissue? 

A single blood test for a hormone called Anti Mullerian Hormone (AMH) is usually sufficient to rule out the presence of testicular tissue. If this test suggests that testicular tissue is present, then your vet may consider further investigation followed by surgery to locate and remove any remaining tissue. This can be an extremely complex operation in some cases as the testicle may be difficult to localise if it is in the abdomen.  

What if blood tests rule out the presence of retained testicular tissue?

If laboratory testing indicates that this is not hormonal in nature, the problem may be psychological. It has been reported that up to 20-30% of horses can display some degree of stallion-like behaviour after castration. 

Testosterone production starts long before puberty, while the foal is still in the womb, so by the time the horse is castrated, testosterone has already played a significant role in shaping the animal’s brain. Many of the unwanted behaviours we see are learnt and therefore not always eliminated by castration. Geldings that exhibit such behaviour in the absence of testicular tissue are known as “false rigs”. Anecdotally, many cases of false rigs are horses in their teens. It may be the case that horses become more established in the herd as they mature and are more confident about expressing such behaviour by the time they reach their teenage years. 

Treatment

If your horse is demonstrating unwanted behaviour, speak to your vet so that they can rule out any physical or hormonal causes. If your horse is mounting mares, it is very important to ascertain whether or not he is fertile. Bilateral cryptorchids usually infertile. Unilateral cryptorchids can still produce at least some viable sperm, but may be sub fertile. As previously mentioned, the absence of palpable testicles is suggestive that he is unlikely to be fertile, but it is always prudent to investigate.

Separation from mares is advisable in the first instance

This may not always be possible or feasible. Ideally the gelding should be kept in a separate field that does not share a boundary with the mares. If this is not possible, it may be worth considering the removal of hind shoes from all horses, if they can cope unshod, to reduce the risk of trauma from kicks. Care should also be taken in shared yards where the gelding may inadvertently come into contact with a mare during grooming or tacking up. Be careful when introducing new mares to the yard. Completely isolating a horse from any others is not always advisable as many horses prefer contact with other animals, even if this contact is only visible. 

This has been shown to be ineffective and possibly dangerous as it may exacerbate aggressive behaviour. It is worth consulting an equine behaviourist or your vet to advise on training methods and to create a tailored plan for your horse. In general, positive reinforcement can be very helpful in eliminating undesirable traits. With this method of training, the horse is rewarded every time they show the desired behaviour, in this case, standing still and quietly when a mare approaches. If they become excited around the mare, stay calm, move the mare or your horse away and reward him when he becomes settled. Above all, stay safe as horses when excited can be unpredictable and dangerous. It is advisable to seek professional help to reduce the risk of injury. 

Medications, such as GnRH vaccines, can be used to control the expression of sexual behaviour in stallions. However, as these agents affect testicular tissue, they are of no use in true geldings. 

In summary, if your horse is displaying unwanted behaviour towards mares, it should be taken seriously. Speak to your vet, they will need to rule out retained testicular tissue as a cause before behavioural modification can be used to improve the situation. 

Further Reading