Many older mares become pregnant but do not stay pregnant. In the first 14 days of pregnancy, EED occurs in about 9 percent of young, fertile mares. There are many reasons that older mares have fewer foals than younger mares. Lesions is the oviduct increase with age and may retain the embryo longer then normal. Usually, the embryo takes six days to travel through the oviduct.
Progesterone is essential for the maintenance of pregnancy.
Lower progesterone levels have been observed in older mares. The importance of including sufficient fat in the diet to support ovarian function has been shown in beef cattle. Benefits of adding fat to the rations of horses has recently been recognized. Fat is utilized in the body's production of progesterone. There was not any difference in the amount of progesterone produced by either group of mares.