This is an important area to be comfortable with evaluating, especially in reproductive mares. As mares age, the anus tends to recede deeper and the vulva tilts. This is especially true of mares that have had multiple foals. This tilting and sinking of the reproductive tract is partly responsible for older mare’s decreased ability to clear fluid from the uterus after breeding, a critical part of maintaining fertility.
Before foaling, there is a noticeable lengthening and some swelling of the vulva. After foaling, this area must be evaluated again. Bruising and small tears are common after foaling, and heal well. Larger wounds and tears through into the rectum require veterinary evaluation and treatment.
Perineal conformation is also important due to its ability to maintain a vulvar seal. A good seal is important as the first barrier between the outside world and the sterile uterine environment of pregnancy. The second is a functional cervix. You cannot evaluate the cervix but you can certainly look at the shape and appearance of the vulva and perineum and assess its implications for fertility. Lastly, tumors tend to appear in this area in older mares.
Halter your mare, and with an assistant standing on the same side as you make contact with your horse and move to her back end. Stand by the mare's hip and lift the tail with one hand. Inspect the vulva and perineum for symmetry and the angle of slope between anus and vulva. Do the lips of the vulva look as though they seal well? Look for any wounds, masses or reddening associated with the area.