The Mare Cycle


Mares are seasonal breeders in contrast to cattle and other farm animals. Under natural conditions they have an anoestrus period (no or minimal follicular activity) from October until February. Around March time depending on environmental factors such as day length, temperature and nutrition mares will start cycling again. This phase where mares change from being non-cycling to cycling and vice versa is called the “Transition Period”. The change in day length or photoperiod has an important role in the mare’s cyclicity. Longer days in spring time are the key for the mare to cycle regularly every 21 days and that throughout the whole reproductive season until the days are getting shorter and the mare returns into anoestrus again.

The oestrus cycle

The oestrus cycle is usually 21 days and can be defined as the period between two ovulations. The follicular phase or oestrus last about 7 days and in this period the mare is receptive to the stallion and ovulation occurs. This period is mainly driven by one hormone called “oestadiol” (=OESTROGEN) produced by the dominant follicle. In the remaining 14 days or di-oestrus the mare will be non-receptive to the stallion. This period starts just after ovulation (= when the dominant follicle pops) with the formation of a corpus luteum that is the source of a hormone called PROGESTERONE. When the mare is not pregnant her progesterone levels decline and the cycle starts again.

“Moody” mare

Mares may occasionally be referred to as “Moody”, difficult to train, show unwanted behavior as tail swishing, kicking etc … when they are in heat. The level of behavioral changes and attitude will vary between mares. In a survey in America, 90% of the respondents (671/751) believed that the oestrus cycle played a role in the mare’s athletic performance (Jorgensen et al., 1996). The challenges here for the owner and for us as vets is to determine whether the unwanted behavior is linked to the normal oestrus cycle of the mare or is there a pathologic cause for this behavior such as an ovarian tumor or whether there is another issue underlying such as lameness, dental problems, gastric ulcers etc…

Once other causes have been ruled a treatment can be started. Ideally what we want is a product that can suppress the expression of those oestrous behavioural changes. As we described above Progesterone is capable of keeping mares out of oestrus, maintaining pregnancy and has in general a calming effect. Regumate® is the trade name for a synthetic analogue for progesterone called altrenogest. Regumate® is a liquid that has to be administered daily at the appropriate dose of 1 ml of Equine Regumate per 50 kg Bodyweight horse. This means that an average 500 kg horse will require 10 mls per day. Avoid skin contact and always wear gloves when administering Regumate®. Pregnant woman should not handle this product. Equine Regumate® is a licensed product for the use in horses and will effectively suppress oestrus in 95% of mares. Mares ( NOT STALLIONS) on Equine Regumate® can compete under FEI regulation when an FEI 2 document (authorization for administration of altrenogest (Regumate®) to mares competing in FEI competitions – is completed, signed and approved by the veterinary delegate at the event. The jockey Club does not allow mares to race on Regumate®. It is regarded as a “prohibited substance”.

Other treatments to suppress oestrus behavior are: 1) Sterile plant oils intrauterine 2) Intrauterine marbles 3) Oxytocin injections 4) Surgical removal of the ovaries

  1. The administration of sterile plant oils on day 10 post-ovulation (Wilsher and Allen, 2011). This should prevent the mare from coming into season for about 3 months. Disadvantage: mare has to be followed up and scanned multiple times to find out where she is in her cycle. The exact mechanism of action is not known. Advantage: No restrictions for competing.
  2. Placing intrauterine marble was a popular technique in the past. A glass marble was placed in the uterus ideally about between 1-4 days after ovulation. It is speculated that the marble mimics the equine pregnancy and triggers the corpus luteum to produce progesterone. Very low efficacy and an approximate oestrus suppression of about 90 days. More recent studies are more promising about the use of plastic water filled ball. Success rate of 75% was reported in 2008 (Rivera Del Alamo et al) but only on average 60 days of estrus suppression. Disadvantage: mare has to be followed up and scanned multiple times to find out where she is in her cycle. The device has to be removed from the uterus prior to breeding activities. The mare may expel the marble. Introducing a foreign body to the uterus can induce a uterine infection and compromise the mare’s future fertility. Advantage: No restrictions for competing.
  3. Repeated and high dose (60 units = 6 ml) oxytocin injections twice daily between day 7 and day 14 after ovulation. Approximate oestrus suppression of about 30 days. Disadvantage: Not animal friendly to inject your mare twice daily with high doses of oxytocin. Mare has to be followed up and scanned multiple times to find out where she is in her cycle. Very short oestrus suppression.
  4. Bilateral removal of the ovaries (=ovariectomy) will permanently stop cyclicity in the mare but not always stop the signs of oestrus especially in the presence of a stallion. 

    Granulosa cell tumor

    Granulosa cell tumor is the most common tumor in the ovary in the horse. It is a slow growing tumor, involves one ovary and can occur in mares of all ages. Clinical signs: stallion-like behavior, cresty neck, aggressiveness and prolonged oestrus. The presence of a granulose cell tumor can usually be diagnosed by its typical appearance on ultrasound scan (honeycomb appearance) whether or not in combination with a blood sample for hormone analysis (inhibine, testosterone, progesterone and anti-mullerian hormone). The only treatment option for a granulosa cell tumor is surgical removal by standing hand assisted laparoscopy. The majority of mares will return to a normal reproductive function after a couple of months and can successfully be bred.

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