Where do you live? Right now most mares are in anestrus which means they arent coming in heat at all or very little. Perhaps furthure south you go the longer average daylight is and might cause them to cycle a little but most mares arent going through fullblown heat cycles and probably wouldnt be fertile if they did. If she is under lights at night that could also make her still come in. In about a month mares around here (Missouri) will start to transition into regular estrus cycles about every 21 days. Some mares act like they are in heat all the time just because of their nature. I have one thats like that and has to be teased to tell if she really is in or just being a hussy. It is thought that some mares may experience cramping and be a little grouchy during parts of the cycle. Despite good training sometimes mares will just be mares.
Also, the anestrus starts to kick in again and regular cycles taper off around October or November.
They can start having cycles as early as a year old, again most generally arent fertile enough to actually conceive but it's not impossible. It would really depend on where their birthday falls in the year. A filly with a birthday in March might actually have some well defined heat cycles in June but a filly that was born in June probably wont have much in the way of heat cycles as she wouldnt mature soon enough to get in on the length of daylight required to successfully ovulate. Not that I would ever breed a yearling or even a 2 or 3 year old for that matter so its not usually something to be concerned about but our horse vet is really good equine repro. vet and have discussed heat cycles to some degree and thats why I know a bit about it.
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