It all depends on the mare. From what I've read, mares are supposed
to only come into heat during the Spring and Summer months, but that isn't always the case and some cycle year round. My mare is currently kept with a 29 year old mare and shows no signs of heat around her. Last January, I moved her to a friend's place who had geldings and she was in full-blown heat (squeeling, peeing, etc.) She just didn't show it with her geriatric female companion. I think you would be wise to try and figure out an alternative water situation for winter, just in case.