Over the next few months, I am going to post information about reproduction, pregnancy, and foaling, to prepare myself for Hailee Jr. Okay, so, let's begin.Mares' reproductive cycles are closely tied to nature, and geared so that they become pregnant at a time of year rich in nutrients and then deliver in the same time of year. So, their fertility closely cycles with the sun and their latitude on earth. Northern horses have shorter fertile periods each year, and Southern horses longer. Deep winter, with December 22nd (the winter solstice) is the period of deep infertility for mares. Most mares are in anestrus.
When days lengthen, they go through periods of estrus when they are receptive to stallions, and periods of diestrus, the moment when they are not ovulating. Ovulation occurs in a very tight window right toward the end of her heat. Twenty-four hours after ovulation, blood fills the egg follicle in the ovary and in 5 days is replaced by the yellow Corpus Luteum which sends out progesterone to prepare the uterus to receive the fertilized egg, if there is one. If she isn't pregnant, the CL rapidly regresses in 12 to 14 days and she begins a new estrous cycle in another 3 days. If she is pregnant, the CL keeps operating and producing progesterone until the uterus is capable on its own. The transition between CL and uterus in progesterone production occurs somewhere between 70 and 90 days after gestation!
Okay, so how can you tell when a mare is in heat? A healthy mare does not hide. She "winks." She squats, raises her tail, presents her hindquarters to the stallion. She urinates and sprays. In Hailee's case, her estrous was so fast and strong that she would literally lean against walls, buckle her knees, and swoon. When she's in Diestrus, after the ovulation occurs, she rejects the stallion, pins her ears, kicks, squeals, wheels, bites, and paws. I saw Hailee doing this activity this summer. She was drinking from a barrel and a stud pony leaned over to give her a kiss and she just threw up her front legs, pinned her ears, and roared her displeasure. She's making a baby!