Regardless of the cause, basic management is critical in handling these mares. You must be able to determine when the mare is receptive to a stallion and you must be able to document ovulation. The first requirement is best accomplished by teasing on a regular schedule, and the second by having your veterinarian examine the mare. Then you must keep accurate records. If you haven't used lighting systems to advance the season, don't get impatient and breed the mare before she has established a good cycle with short heat periods and predictable ovulation.
Much damage is done by breeding mares too often during a given estrus.
These determinations require rectal palpation of the reproductive tract, visual vaginal examination, a swab to collect a cytology specimen, and ideally, ultrasound evaluation of the uterus. If there are signs of inflammation, a culture is indicated to identify the cause. Mares that have positive signs of inflammation may require treatment before the are bred. The decision to treat, and with what, must be based on history, the cause of the inflammation, and the overall assessment of its significance. You and your veterinarian need to discuss the situation and agree on a course of action.