Auburn University researchers recently reported on an unusual method for controlling “mare-ish” behavior: placing a large glass marble in a mare’s uterus to keep her from entering estrus.

The unusual research project was initiated by Gary Nie, DVM, PhD, after he read a message posted on a veterinary email list.

“The message was from a Dutch veterinarian describing a method of using glass balls to prevent mares from cycling,” recalls Nie. “Of course, most list participants had never heard of it before. The Dutch veterinarian had learned the technique years ago from another veterinarian, but I could find only anecdotal evidence that it worked.”

Intrigued, Nie and some colleagues decided to research the topic themselves. “Personally, I think a lot of misbehavior is erroneously blamed on a mare’s cycles,” he says. “But I also know that horse owners frequently seek means of controlling estrous behavior in their mares, sometimes utilizing hormone products that are not approved or even tested on horses. I was hoping to find a safer alternative for those owners.”

For the study, Nie placed sterilized glass marbles in 12 healthy mares with normal reproductive histories. A “shooter”-size (35 millimeter) marble was inserted into each mare’s uterus 24 hours or less after she had ovulated. “At that point the cervix is closing, which helps keep the ball in,” says Nie.

Blood was taken daily to monitor the mares’ plasma progesterone levels, which normally drop during estrus. Five of the 12 mares had elevated progesterone levels for an average of 88 days. The remaining mares had elevated levels an average of 20 days, the approximate length of a normal equine estrous cycle.

“What this means is that 40 percent of these mares did not cycle for three months. Once a mare cycled again, we took the ball out. I don’t know what would have happened if we had left them in place. Perhaps the mares would have gone another three months before ovulating again,” says Nie, explaining that, after the marbles were removed, examinations were done to determine whether any damage to the uterus had occurred. “We took endometrial biopsies before and after the balls were put in place,” he says. “When we compared the two, we found no damage and, if anything, healthier tissues after the balls had been in.”

Eleven of the study mares were bred after the balls were removed and 70 percent conceived. “This practice appears to have no ill-effects on a mare’s uterus or fertility,” says Nie.

He is not sure how the marbles keep a mare from cycling, however. “One theory is that the physical presence of the ball signals to the mare that she is pregnant,” says Nie. “But this is contrary to what we know about chemical messengers produced by the conceptus in other species to keep a female pregnant.” A second theory is that mild inflammation caused by the marble keeps the mare from cycling, but Nie says that inflammation can actually cause a mare to short-cycle. “The bottom line,” he says, “is no one knows how this works, but it does seem to in some mares.”

Nevertheless, Nie says, the procedure must be performed by a veterinarian: “Properly placing the glass ball is essential to avoid problems. Taking a ball out is even more difficult and can only be done by someone experienced at palpating mares.”

This article originally appeared in the March 2002 issue of EQUUS magazine.