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Mares are known as seasonally polyestrus animals.
They cycle and ovulate for only a portion of the year. Natures way of saying a foal born in winter is a bad idea. Their seasons are triggered by the lengthening summer days. However even then they will only be receptive to a stallion when they are actually in season during a portion of their cycle.
Terminology
Anestrus – The period of the year your mare is not cycling at all, in New Zealand this is the winter months from April to September
Estrus – The “on heat” period of about 5 days in a mares cycle when she is receptive to a stallion
Diestrus – The period during a mares 21 day cycle when she is not receptive to the stallion.
Ovulation – When the egg is released from the ovary
In New Zealand mares generally start to cycle in about September and finish by April.
Each cycle during the spring/summer phase lasts 21 days.
She will have 5 or 6 days where she is in oestrus and 15 or 16 when she is not. Generally mares will ovulate towards the 5th day of that in season cycle.
Day 1 to Day 5 or 6 Estrus – Receptive to a stallion
Day 5 or 6 to Day 20 or 21 – Diestrus, unreceptive to a stallion.
Signs your mare is in season are lifting of her tail, winking, splaying her hind legs and repeated urination.
Try to keep a record of your mares cycles as she starts to come into the spring. There will be a period in early spring as she starts to transition into regular cycles where she is receptive to a stallion but not actually producing any eggs. Older mares can take a few more weeks than younger mares to come into strong regular cycling. A good general rule of thumb in New Zealand for breeders is that after labour weekend your mare should be settled into regular cycles and be ready to breed.
The stud and/or vet will be very appreciative if you know approximately when she is due to come into season. In some mares this can be very difficult to ascertain because they have a silent heat and may need other methods of detection.
Methods of heat detection
You can use a teaser stallion. Make sure he is calm and knows how to chat to a lady nicely! Some aggressive forward stallions can put a shy maiden mare off totally. Be patient with your teaser it can take up to 20mins to get a shy mare to show you what she’s up to.
I prefer to use the more modern method of detection – The ultrasound. I will watch for as many signs as I can naturally but then getting her scanned will give you a much better idea of the size of follicle she likes to grow and when she ovulates. A follicle above the size of 40mm is very close to ovulating and a good sign for insemination.
The last method you can use to get your mare cycling is stepping in with hormones and actually controlling it all. This is usually done with a Prostaglandin injection (commonly known as PG) and will bring your mare into season with 2-4 days after the injection.
However it should be noted that this will only occur if she is at least 10 days into her cycle, so you may find you inject her then wait several days and ultrasound her to discover she doesn’t have a follicle.
This means you will have to wait another 10 days and then re-inject her. This is why its important to try and get a rough idea of where she is in her cycle.
If you do PG your mare make sure you watch her carefully afterwards it can cause her to sweat excessively and even have mild colic symptoms. Please also keep the injection away from any human women. It will be absorbed through the skin and you will end up in the same state as the horse, its not pretty I can assure you!
Sometimes people wish to breed their mares early in the season when they normally wouldn’t be cycling strongly or at all. A mares seasons are controlled by day length so stabling your mares under lights will cause them to start cycling earlier in the season. You need about 16 hours of daylight in total, so if she is out and about from 8am to 3pm then you will need lights on until midnight. The lights need to be bright enough that you can read a newspaper easily in every corner of the stable. At least 60-90 days under lights is necessary to produce strong regular cycles. Remember to keep her under lights after she has been bred, for at least another 40 days.
This method of early cycling is more common in the northern hemisphere than New Zealand.
No matter which method you use, always try to keep a track of your mare’s cycle, it will make breeding much easier and more cost effective I promise!