This is something you need to be able to work out if you want to have your mare served by a stallion at a stud.  It is also something that may be able to explain certain behaviours in your mare.  Or it may simply be something you are interested in understanding…

Mares are what is known as seasonally polyoestrous.  This means that they will cycle a number of times each year, but will have a period of time when they are cycling and a period of time when they are not.  Cycling is dependent upon day length.  So as the hours of daylight each day get longer, mares will start to cycle.  They will usually cycle from around mid spring until the end of summer, but some mares will start cycling earlier than this and some will cycle for longer.

The normal oestrous cycle in the mare is 19 – 22 days long with most cycles lasting 21 days.  The duration of oestrus (when the mare is in heat) can vary but will usually last 5 – 7 days.    Oestrus tends to last longer when the hours of daylight are less and become shorter as the hours of daylight get longer.  The most consistent part of the cycle is dioestrus (when the mare is not in heat).  Dioestrus tends to last 14-15 days.  So the best way to determine when your mare will next come into heat is to count 14-15 days from the day she comes off heat.

Signs of your mare coming into heat can include some or all or the following:

• A winking vulva
• Tail raising, frequent urinating and a subsequent wet tail and hind end
• Squatting down, dropping her hips and generally adopting a breeding position
• Increased interest in stallions and mature geldings
• Squealing, having her ears back, kicking and aggression
• Looser bowel motions
• Being difficult to handle or ride
• Nervousness or anxiety
• Unpredictable behaviour

But the only way to determine 100% that your mare is on heat is by ultrasound.

Ovulation is usually said to occur on around day 5 of the mare’s cycle, or approximately 24 – 48 hours before the end of oestrus.  No matter how long she is in heat for, ovulation will only occur shortly before the end of oestrus.  So if you are breeding your mare then it is necessary to breed her shortly before she comes off heat, as sperm will only live in her reproductive tract for about 24 hours.  In order to get your mare in foal it is recommended to breed her on the second day of her heat cycle and then every other day until she goes off heat.

Most mares are not intentionally bred until they are at least 3 years old, although mares will reach puberty earlier than that.  Most will begin cycling soon after they are 1 year old.   Age is a factor in mare fertility, and there tends to be a decline in the fertility of mares from around 12 years old.  So if you are looking to purchase a broodmare, probably the ideal would be one between 4 and 10 years old that has successfully foaled at least once already.