Riverina Equine Vet



Suppressing Oestrus in Performance Mares

Throughout Spring and Summer we frequently take phone calls from owners who find that their mare’s behaviour has changed and they have suddenly become difficult to handle. Ridden work (especially at competitions) may be impossible. These behavioural changes may be associated with the mare’s oestrus cycle, or ‘season’.

The challenge for the owner and the vet in these cases is to determine if the unwanted behaviour is truly related to the mare being in season or if there is an underlying, undiagnosed illness causing behavioural problems.

Signs may be quite vague and non-specific and include:

  • Kicking, angry vocalisation, swishing the tail and urinating frequently.
  • Oversensitivity.
  • Refusal to move forward (‘planting’ and rearing), bolting and bucking.
  • Aggression.
  • Showing an excessive interest in other horses.

A full and thorough veterinary examination is necessary to determine whether the behaviour is related to the reproductive tract.


Conditions to rule out..

There are several clinical conditions unrelated to the mare’s seasons that can cause behavioural changes. These should be considered and if appropriate, investigated to rule out a more serious illness. Examples of such conditions include:

  • Low-grade lameness - May be difficult for the rider to assess but clues may include a lack of forward movement, unwillingness to jump, or asymmetric musculature.
  • Dental problems - Overgrown enamel points, gaps between the teeth (diastemas) and discomfort caused by the bit can all lead to behavioural changes due to pain.
  • Gastric ulcers - Signs may be very vague but include resentment of tightening the girth, recurrent colic, difficulty maintaining weight, sporadic inappetence or general malaise. Gastric ulceration has been shown to be very common in our horse population.
  • Back problems - E.g. poorly fitting saddle, kissing spine.

The vet’s approach to assessing the moody mare is therefore likely to include a general clinical exam, an evaluation of lameness and back pain and an oral examination with a gag. Gastroscopy may be performed if there is a suspicion of gastric ulcers. An ultrasound scan of the reproductive tract may also be undertaken.


Treatments to Suppress Oestrus Behaviour

Once it has been confirmed that the abnormal behaviour is likely to be related to the oestrus cycle, there a number of treatment options available to us.

The preferred treatment will depend on several factors, including:

  • The use of the mare and restriction under competition rules.
  • Cost of treatment.
  • Feasibility of daily treatment.
  • Intention of future breeding.


HORMONAL - Progesterone supplementation

Altrenogest is a synthetic progesterone which effectively prevents the mare from coming into season.

  • Highly effective for suppressing oestrous for the duration of treatment.
  • Mares come back into heat when drug is discontinued.
  • Safe for long term use if the mare is reproductively healthy.
  • Contraindicated for use in mares with uterine inflammation or infection.
  • There may be competition rules restricting its use e.g. it is banned in racing fillies.


Daily oral altrenogest liquid - "Ovumate Oral"

  • Must be administered daily by mouth.
  • Can be messy.
  • Human health risk – gloves should be worn to prevent accidental absorption.
  • COST: Approx $25/week for a 500kg mare.


Weekly IM altrenogest injection – “Readyserve Injection”

  • Administered by intramuscular injection every 5-7 days, varying from mare to mare.
  • Can get mild inflammation at injection site.
  • COST: Approx $50/week for a 500kg mare.



IMMUNOLOGICAL  - GnRH Immunization

“Equity vaccination”

  • Immunization against GnRH will suppress oestrus hormones therefore resulting in “immune mediated” oestrous suppression.
  • Primary vaccination involves two doses of vaccine administered by your vet 4 weeks apart.
  • Highly effective.
  • Oestrous activity will start to decline, or cease altogether, within about two weeks following the second vaccine dose in most mares.
  • Repeat vaccination may be required every 3-6 months (or longer) thereafter, varying from mare to mare.
  • NOT TO BE USE IN MARES INTENDED FOR BREEDING - the effect of this product on future fertility is unknown.
  • COST: $355/dose (minimum of 2 doses required).


SURGICAL – Ovariectomy

Ovariectomy/“spay surgery”

  • Invasive surgery involving surgical removal of the ovaries.
  • Usually reserved for removal of an abnormal ovary (e.g. tumours) but occasionally performed in fillies or mares which have particularly bad behaviour during oestrus.
  • Several approaches that can be performed under standing sedation or general anaesthesia.
  • Higher risk – haemorrhage, infection etc.
  • Best performed by a specialist veterinarian in a hospital environment - $$$.


MARBLE - Intrauterine glass marble

“Glass marble”

  • Very low efficacy - studies have shown oestrus is prevented in only 8-33% of mares.
  • Sterile glass marble inserted into the uterus 0-48 hours after ovulation.
  • Mimics pregnancy and prevents the mare from coming back into oestrus.
  • Mare usually needs to be monitored closely during oestrus and may be given a drug to induce ovulation which ensures accurate timing of marble insertion.
  • The marble may be expelled by the mare (losing her marbles!)
  • Ideally the marble should be removed at the end of the breeding season – this is not always easy!
  • By placing a foreign body into the uterus, we risk introducing a uterine infection - future fertility may therefore be compromised.
  • Due to the poor response to therapy in the majority of mares, we do not recommend using a glass marble to suppress oestrus behaviour in mares.