From: Sheila
Hi Jessica, Thank you for Horse-sense. I have gained so much knowledge about so many horse matters from your practical and sound (no pun intended!) advice. I have looked in your archives for an answer to my query and have'nt had much luck. I asked my vet and she directed me to a few web sites, but no luck there either. I have a 9 year old mare and I was just curious about a mare's heat cycles. She comes into season every 4 weeks or less and lasts about 7 days or so. Does this time correspond to the ovulation cycle? And why (if it the time of ovulation) does it last so many days. Do mares have a period of time when they shed the lining of the uterus when not fertilized like humans do?
I hope you can help me understand her better by knowing what she is going through.
Thank you Sheila
What your mare is going through is simply her estrus cycle, which takes place more-or-less year-round. I say "more or less" because the strength and predictability of the cycle is dependent on day length, thus her cycles will be stronger and much more predictable during "breeding season" - from late spring through the summer months and into the early autumn. Mares don't menstruate, so - contrary to the opinion of many riders - they don't get PMS. ;-)
In the winter, which is not a normal breeding season, mares tend not to show signs of "heat" or estrus. Even those mares that do show signs are usually not actually ovulating, and those few mares that show signs and accept stallions during the winter months rarely become pregnant. Nature intends for mares to conceive during the spring and summer, so that foals will be born the following spring when there is abundant new grass. It makes very good sense.
Your mare's cycle may seem erratic and odd during the early spring - that's normal. During the winter, it's typical for no follicles to develop on the ovaries. As the days lengthen into spring, follicles begin to develop, but these follicles typically come and go without ever maturing or releasing an egg. Because of this, the cycle will be unpredictable - your mare may not come into heat all winter, then become completely erratic! You might see one mare come into heat for two or three days, go out of heat for a week, come back into heat for another two days, etc. Another mare might come into heat in early March and remain in heat for ten days or two weeks - or longer.
By the time spring is firmly established, usually in early or mid-April, most mares will actually be ovulating - and their cycles will begin to take on a regular pattern. Most mares will continue to ovulate and show predictable heat cycles through the summer and sometimes through the early fall as well. As October approaches, mares will typically show fewer and weaker signs of heat, and by the time winter comes around, they may show no signs at all until spring comes and the cycles begin again.
An estrous cycle is the time from one ovulation to the next one. For most mares, this cycle lasts about 21 days. If your mare's cycle lasts only 19 days, or lasts as long as 22 days, don't worry - she's still in the normal range.
During the cycle, the mare will be "in heat" (estrus) for 5-7 days, then will be "out of heat" (diestrus) for the next 14-15 days. Breeders will often speak in shorthand - "Is she in? Is she out?" This is important information! While a mare is "in" she will accept the stallion; when she is "out" she will not.
During the five or six or seven days of estrus, there is steady enlargement of a follicle (containing a developing egg) on one of the mare's ovaries. When the follicle reaches its maximum "ripe" size, it bursts, and the egg is released - this is called ovulation. Once the mare has ovulated, she will be in estrus for only one or two days longer.
Hormones control the cyle. When estrogen is the predominant hormone, the mare is in estrus. After ovulation, another hormone takes over: Progestrone. When the mare has ovulated, a progesterone-producing structure called the corpus luteum (CL) will form on the ovary. As long as the progesterone is produced, the mare will be in diestrus - "out" of heat. This period normally lasts 14-15 days - unless the mare is pregnant. If the mare was bred whilst she was "in", and if she became pregnant, then the CL will remain on the ovary and continue to produce progestrone - and the mare will not come back into heat. If the mare was not bred, or was bred but did not conceive, whilst she was in estrus, the CL will last the usual 14-15 days and then regress, allowing estrogen to dominate once again, and estrus to begin. Neither the buildup to ovulation nor the regression of the CL is instant - it takes time for the follicle to ripen and bursst, and time for the CL to grow and the hormone balance to shift - and that's why the estrus period lasts 5-7 days.
Mares can be touchy and easily distracted by other horses - particularly stallions - when they are in heat. I say "can be", because many mares do not show any particular behavioural changes during their cycles. Some mares can be erratic performers in the show ring whilst they are "in", but many other mares simply go on with "business as usual". If you own a mare that shows strong changes in her performance whilst she is "in", it would be a good idea to consult your veterinarian. Estrous cycles and "mareish" behaviour are often blamed for changes in a mare's performance, but there are many other possible causes - hormonal problems, cysts, tumours, etc. Some mares do seem to experience pain at the time of ovulation; others become very tight and tense in their backs at that time. Be aware of your mare's normal cycle and her typical behaviour, and consult your veterinarian if she begins to show unusual behaviours or changes in her typical cycle - if, for example, a mare with a typical cycle goes into heat early in July and doesn't come out of heat for several weeks. There are treatable medical conditions that will cause mares to show some or many of the behaviours associated with being in heat, so don't just write this off as "being mareish" or "having PMS" (which, by the way, mares don't get) - always make the effort to find out what is actually going on with your mare.
I hope this helps - it's probably more information than you actually wanted. ;-)
Jessica
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