What are the options for oestrus suppression in mares?

A review of the indications for oestrus suppression in mares, including the pros and cons of various treatment options

19 March 2018, at 3:41pm

Indications for oestrus suppression

Some mares seem to experience back or colic-like pain in the peri-ovulatory period. A mare being “in season” (ie displaying signs of behavioural oestrus) is often perceived as a problem by owners, who feel that it makes the mare difficult to manage and/or detracts from her performance. Equally, mare owners often blame behavioural issues or poor performance on the mare being “in season”, when in fact there is another underlying cause (such as lameness or poor rider ability). It is therefore important, before embarking upon treatment to suppress oestrus, to verify that episodes of pain, behavioural problems or poor performance do indeed correlate with being in oestrus. This can be achieved through a combination of ultrasonography and serum progesterone analysis. Once verified, there are numerous treatment options available.

Progestin treatment

High plasma progesterone levels in mares suppress oestrus behaviour. Progesterone also has an inhibitory effect on the secretion of luteinising hormone, via a negative feedback effect on the hypothalamic-pituitary axis.

The oral synthetic progestin altrenogest (Regumate Equine, MSD Animal Health) is the only treatment licensed for the suppression of oestrus in mares in the UK. The license is for use during the vernal transition period, for 10 days, although oral altrenogest is often used (off-licence) for prolonged periods in mares who would otherwise be cycling regularly during the physiological breeding season.

Despite common use, oral altrenogest has significant disadvantages. It can cause positive drug test results for in-contact horses via feed contamination. It also poses risks to pregnant women, women of childbearing age, and those with certain types of tumour and thromboembolic disease. It is crucial that horse managers are aware of these risks, and that the drug is handled according to the data sheet.

The use of altrenogest may exacerbate endometrial infection if a low level of (sometimes undiagnosed) infection was present when treatment was initiated. Its use is contra- indicated in mares which have been diagnosed with uterine infection. Furthermore, the use of oral altrenogest is not permitted by the governing bodies of some competitive equine sports, and others require special permissions for use to be issued. This must be discussed with the horse owner/manager.

In Australia, an injectable form of altrenogest is available (Readyserve, CEVA Animal Health). This has reduced risk of cross-contamination compared to the oral form. The same concerns otherwise apply. Injectable altrenogest can cause discolouration of the coat, alopecia and hair thinning at the site of injection – mare owners should be warned of this.

GnRH vaccination

The injection of the GnRH hormone (or a modified form) plus an adjuvant induces GnRH antibody formation. The antibodies bind to endogenous GnRH, and prevent endodogenous GnRH from acting on pituitary receptors. This removes the normal stimulus for gonadotrophin (LH and FSH) production, and thus ultimately abolishes behavioural oestrus by reducing steroid hormone secretion.

There is no GnRH vaccine currently licensed in the UK. A GnRH vaccine (Equity, Zoetis) is licensed in Australia for the suppression of oestrus in mares. Injection site reactions have been reported. There is great individual variation in response to vaccination. Some mares continue to display behavioural oestrus, despite ovarian inactivity. Some (particularly older mares) require repeated vaccination for the treatment to work. Others enter very prolonged suppression of reproductive cyclicity, or fail to regain ovarian activity at all. Owners should be warned of this – the vaccination is not recommended for use in mares intended for future breeding.

Repeated oxytocin injections

Repeated injections of oxytocin during dioestrus using various regimes have been shown to suppress oestrus in some mares (Gee et al., 2012; Vanderwall et al., 2016). Normally, oxytocin mediates the endometrial prostaglandin release, which induces luteolysis and brings a mare back into oestrus. Repeated injections of oxytocin disrupt this mechanism, probably either by down-regulating oxytocin-receptor synthesis, or by decreasing receptor sensitivity. The main disadvantage of this technique is the need for repeated injections, which may produce injection site reactions and muscular soreness.

Injection with human chorionic gonadotrophin

Injection with human chorionic gonadotrophin has also been used as a method of suppressing oestrus in mares. It has been reported in one study using a small number of mares, with variable results (Hedberg et al., 2006).

Insertion of an intrauterine device (eg marble)

The introduction of a 30-35mm sterile marble or plastic ball into the uterus immediately after ovulation has been reported to suppress oestrus in mares (Nie et al., 2003). The mechanism of oestrus suppression was believed to be that the marble blocked the release of PGF2α from the uterus, either by mimicking an embryo moving through the uterus, or by mildly damaging the endometrium.

In recent years, it has become apparent that the use of an intrauterine marble or similar device is not an efficacious nor reliable method of suppressing oestrus in mares (Argo and Turnbull, 2010). There have been a number of reports of adverse side effects including marbles fracturing, and adverse effects on future fertility by damaging the endometrium. There are ethical issues surrounding failure to declare the insertion of an intrauterine marble at the time of sale, or during competition. Removal of marbles when they are no longer required can be difficult; conversely, some mares seem to quite literally “lose their marbles” because the treatment fails, they return to oestrus, and the marble escapes through a relaxed cervix.

Intrauterine infusion of plant oils

One paper reported that intrauterine infusion with either fractionated coconut oil or peanut (arachis) oil at day 10 post-ovulation suppressed oestrus in the majority of mares for up to 30 days post-ovulation (Wilsher and Allen, 2011). It was postulated that this worked by monounsaturated fatty acids and polyunsaturated fatty acids found in the plant oils interfering with prostaglandin synthesis, and thus blocking luteolysis.

Subsequently, two separate publications (Diel de Amorim et al., 2016; Campbell et al., 2017) reported that neither fractionated coconut oil nor peanut oil reliably suppressed oestrus in mares when infused into the uterus at 10 days post-ovulation. Both papers also suggested that intra- uterine infusion of plant oils causes at least a temporary inflammation in the uterus. Veterinarians considering the use of intrauterine plant oils as a method of oestrus suppression in mares should make owners aware of these facts, and alert them that sports governing bodies could consider intrauterine plant oils to be medication.


The permanent surgical removal of the ovaries from a mare may suppress oestrus behaviour, but does not always do so. It is therefore advisable to evaluate the mare’s response to non-surgical methods of inducing ovarian quiescence (such as GnRH vaccination) before proceeding to surgery – if behavioural oestrus is not significantly diminished by medical methods, it is unlikely to be by surgery. While ovariectomy avoids problems of medicine administration, it carries associated surgical risks, and obviously results in irreversible loss of fertility for the mare. Some sports governing bodies may require ovariectomy to be declared.


Before embarking upon suppression of oestrus as a treatment for behavioural or performance issues in mares, veterinarians should verify the correlation between the mare being in oestrus and the occurrence of the episodes of pain/undesirable behaviour/poor performance. There is currently only one licensed method of oestrus suppression for mares in the UK. All the methods described above have significant disadvantages associated with them.