Normal Estrous Cycle
1. Mares are “long day, seasonally polyestrous breeders.”
As the days get longer (more light in a 24 hour period), they start cycling. As the days get shorter (in the autumn), they stop cycling. Their cycle lasts spring, summer and early fall. In late fall and winter, the mare is in anestrous – there is no cycling. In the spring, they go through a transition phase, where they go from anestrus back to estrus cycling, This also occurs in the fall when they go out of estrus cycling.
These two transition phases will have an abnormal cycling times and that leads to a stressful time for mares – it can last for 60-80 days, but can be in and out of heat within only a few days for example.
The usual estrus and diestrus days are not on schedule. They are irregular in timing, making behavioral patterns erratic.
2. In anestrus (late Fall & Winter), even though mares are not cycling, they often will have behavioral problems. They may be mildly receptive to a stallion still or even be fully receptive all winter.
3. Polyestrous means they will have many cycles during the long day times of the year – each estrus cycle is 21 days long.
For 4-6 days, a follicle forms on an ovary during estrus and mares are receptive to a stallion.
For 16-17 days, the mare is in diestrus where the corpus luteum forms on the ovary and the mare is not receptive to a stallion.
4. Control of estrus depends on control of hormones. In estrus, the main hormone is estrogen. In diestrus, the main hormone is progesterone.