With some mares, it is impossible to tell when they are in season. Others make it perfectly obvious. Most are manageable when being handled or ridden during their oestrus period (often called being in heat, or in season), yet the hormonal ones can leave you wishing you had bought a gelding instead.
The fact is that while geldings are not always predictable, they do not have oestrus cycles.
Unwanted behaviour and poor performance from a mare may well be related to her cycle, yet determining the link between oestrus and an undesirable attitude can be difficult. It is important to remember that erratic or aggressive behaviour could be due to another cause, such as colic or back pain, even if the signs on display are typical of an “in-oestrus” mare.
Angel to devil
Keeping a diary of your mare’s behaviour is vital to aid diagnosis.
Include detailed information about when the behaviour occurs and under what circumstances, along with her stage of oestrus.
Record the time of year and her exercise routine, her stable management including nutrition, her yard companions and all supplements and medication administered, including any changes noted.
Consistent bad behaviour is unlikely to be related to oestrus cycles but, if your angelic mare turns into a little devil for five to seven days every three weeks, her hormones could be playing a role. A vet check, including an ultrasound exam, may be useful to confirm that she is in season when you think she is.
Low-grade pain can result in similar clinical signs, poor performance and aggression. A complete physical examination should be conducted, including an orthopaedic and neurologic assessment and possibly a blood test to evaluate organ function. An analgesic trial can be performed by giving anti-inflammatory painkillers such as phenylbutazone (bute) daily for a period of 10 to 14 days, during which time she should be monitored closely to determine whether there is any behavioural improvement.
Discomfort of the urinary tract can result in tail lifting, frequent urination or straining to urinate — all of which can be confused with signs of oestrus. Aggression and stallion-like behaviour, such as sniffing and marking other horses’ excrement by urinating, can be due to high levels of reproductive hormones.
Although rare, an abnormally high level of these hormones can be caused by a granulosa cell tumour of the ovary. Ultrasound examination of the ovaries, combined with a blood test to measure the hormone levels, is necessary to diagnose these tumours.
Fearfulness and submission can be displayed via “squirting” of urine, but the mare’s tail is usually clamped down and she will move away from the cause of this emotion rather than approaching it as she would if in season. Removing the mare from this environment and patiently reintroducing her to the source of fear, such as loading into a trailer, is the best way to manage this situation.
Treatment options
Once other causes have been ruled out, addressing your mare’s difficult behaviour will depend on her requirements and your own.
If she underperforms when she is in season but is otherwise manageable and content, it may be advisable to simply give her those days off. This becomes more difficult for competition horses who have training requirements, or if the displayed behaviour is unsafe or threatening during the oestrus period.
For oestrus to occur, progesterone must be low. Oestrus can be suppressed, therefore, by giving the mare progesterone or a synthetic equivalent such as altrenogest (prescribed under a brand name, usually Regumate).
Treatment should be started when the mare is in dioestrus (between periods of oestrus) to prevent the return to heat. It can be used in the lead-up to a competition or an important event, or administered from spring to autumn to prevent the mare from cycling at all. Oestrus cycles resume approximately 10 days after the synthetic progesterone is stopped.
Progesterone-like hormone products can be administered orally or by injection into a muscle. The effects of injectable progesterone are longer lasting, but injections can result in muscle soreness — making this method less suitable for horses in ridden work. Nor are injections licensed for use in mares.
Gloves must be worn when handling progesterone products as it is absorbable through human skin, and it is not safe to handle at all if you are pregnant.
If it can be confirmed that the ovaries are causing a hormone imbalance and subsequent behavioural changes, an ovariectomy can be performed, where both ovaries are surgically removed. This is an absolute last resort and means that the mare can never be used for breeding. It is vital to first ensure that the ovaries are to blame, as they are not the only source of reproductive hormones and it is possible for the problem to persist after their removal. A vaccine which prevents the ovaries from functioning is currently not licensed for general use in the UK.
A large marble can be inserted into the uterus to act as a false pregnancy, although this is not always effective and generally only works for up to 90 days before oestrus cycles resume.
Some owners of moody mares opt for nutraceutical products, also known as supplements, containing substances such as magnesium, valerian root and camomile leaves. These nutrients do not influence your mare’s hormone balance but can have a calming effect which can lead to an improvement in behaviour.
Magnesium has been shown to work against receptors in the brain, enabling adjustment to stress, although it does not have a visible effect on all horses. Not all supplements will be allowed in competition horses as they may contain prohibited substances.
If you are unsure or if your mare is dangerous, contact your vet.
Ref Horse & Hound; 22 March 2018
