Moody mares: Understanding seasons

By Gayle Hallowell

23 February 2010 11:34

Seasons are the regular cycles of a mare becoming fertile and preparing for pregnancy.

Around 20 to 25 per cent of mares have seasons all year round, and this is most likely to occur in Arab and Thoroughbred mares. The other 80 per cent of horses have different stages of their cycle throughout the year.

During the spring, summer and early autumn, they come into season every three weeks.

In late spring and early autumn, mares have the most intense and shortest periods of being in season – this time correlates to the best natural fertility.

During late winter, early spring and late autumn, mares enter what is called
a transition phase – eggs develop but may not be released from the ovary, and oestrogen concentrations at this stage are variable.

Some mares in this phase show unpredictable behaviour – they may show no signs of being in season, or be in season continually or erratically.

During the winter, most mares enter anoestrus, where they don’t produce eggs or come into season, and have low concentrations of oestrogens and progesterone. At this stage, they should not have any fluctuations in their behaviour due to these sex hormones.

Young animals from puberty often initially start with erratic and unpredictable cycles while their reproductive systems are developing. The cycles then become more predictable and regular for the remainder of their lives.

Many mares will have seasons into their 20s, although it becomes more
difficult for them to conceive as they get older.

Menopause in horses is not traditionally reported, although certain diseases of the older horse, such as Cushing’s Disease, may result in erratic or no reproductive cycles.

During pregnancy, it’s often reported that the behaviour of difficult mares markedly improves. This is not surprising as they do not come into season and are under the effects of the hormone progesterone.

In the phase post-foaling, mares have a lot of the lactation hormone prolactin so, although they are having reproductive cycles, this hormone is thought to give a feeling of calmness and happiness.

Following weaning, some owners report an improvement in the mares’ behaviour whereas others do not.

Whether improvements are due to increasing age and settling into their cycles or as a direct consequence of having a foal is unknown.