Estrus:
Time when a mare is in heat and receptive to stallions. Usually lasts about a week, or 5 to 7 days. Estrogen is the primary hormone.

Diestrus:
Time when a mare is out of heat. Usually lasts two weeks, or 14 to 16 days. Progesterone is the primary hormone.

Anestrus:
Time when mare ceases all ovarian activity and is unable to be bred. During the winter months, 80 percent of mares experience anestrus.

Progestin:
Synthetic progesterone hormone. Two types exist – oral (altrenogest) and injectable.