A Colorado State University study suggests that removing a mare’s ovaries can have a positive effect on her behavior.

The researcher­s reviewed the cases of 27 mares referred to the university clinic for generally poor behavior that was not associated with any stage of the estrus cycle. The most common complaints were “disagreeab­le demeanor,” and “aggression toward other horses.” The study mares showed no signs of granulosa cell tumors, growths on the ovaries commonly associated with undesirabl­e behaviors, during palpation and ultrasound examinatio­n.

After the mares underwent standing laparoscop­ic surgery to remove both ovaries, their owners reported a 95 percent improvemen­t in behavior, including a reduction in aggression toward other horses. The lowest improvemen­t category was “frequent urination” with 83 percent improvemen­t. Overall, 82 percent of owners say they were “very satisfied” with the outcome of the surgery.

The researcher­s note that bloodwork done at the time of surgery revealed no associatio­n between hormone levels and each mare’s behavior. However microscopi­c examinatio­n of the ovary tissues did yield evidence of early-stage granulosa cell tumors in 33 percent of the mares. This, the researcher­s say, suggests that the tumors are likely underdiagn­osed.

The researcher­s stress that it’s important to rule out other possible physical causes for poor behavior—such as ulcers or poor training—before considerin­g an ovariectom­y for a mare.

Reference: “Ovarian histopatho­logy, pre- and post-operative endocrinol­ogical analysis and behavior alteration­s in 27 mares undergoing bilateral standing laparoscop­ic ovariectom­y,” Canadian Veterinary Journal, February 2020.

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