Mares commonly have behavior problems and most of these occur only when they are in heat, and they act normally when they are not in heat. However, some mares have behavior problems continually and that may also be related to the reproductive tract. These mares can have a tumor on an ovary called a granulosa theca cell tumor. These tumors can cause all sorts of different problems. An ovarian tumor can cause a mare to exhibit stallion-like behavior, she can act like she is in heat all the time, or she can never come into heat. And although most tumors in horses occur as they get older, this is not the case with ovarian tumors as these commonly occur in horses 5 to 9 years of age.

Your vet can perform an ultrasound exam of the ovaries and get an idea if your horse has an ovarian tumor. Sometimes blood tests are also helpful. If a tumor is on the ovary, the only treatment is to remove the ovary surgically. The good thing about these tumors is they are rarely malignant, and if they are small they can be removed through the mare's flank while she's standing. However, some of these ovarian tumors are so large that the mare must be placed under general anesthesia. Regardless, after removal of the ovary, these mares usually return to normal. So if you have a mare that is not cycling normally, seems to be in heat all the time, or seems to act like a stallion, she may have an ovarian tumor. Contact your veterinarian for an exam to see if an ovarian tumor could be causing a problem in your mare.