Mares in performance careers are sometimes a frustration to their trainers and riders because during estrus, they can have difficulty concentrating on their work or have “behavioral problems.” Many horse owners resort to hormone therapy to keep mares from coming into heat while training or showing. The most commonly used drug is a synthetic progestin (altrenogest, marketed as Regumate) given daily by mouth or in feed. Some of its drawbacks include cost and risk to humans, especially women. Contact with this drug (which is easily absorbed through the skin) can disrupt the menstrual cycle or cause miscarriage. However, a new way to keep mares out of heat has been introduced to the horse world–marbles.

Placing a glass marble in the mare’s uterus to suppress estrus was discussed at the 2001 American Association of Equine Practitioners convention by Gary Nie, DVM, MS, PhD, Dipl. ACT, Dipl. ABVP. He learned about this technique from a veterinarian in Holland and tried it on 24 mares in a two-year study with half using a 25 mm glass ball and half using a 35 mm glass ball.

Nie says that this method for suppressing estrus (in horses and other species) has been used for a number of years in Europe and the Middle East, but he had not heard of it being tried in this country, so he wanted to do a study.

How Does It Work?

A marble is placed in the uterus (via the cervix) within 24 hours after ovulation, while the mare is still in heat. The glass ball causes the corpus luteum (CL) on the ovary to remain, producing progesterone–the hormone that keeps a mare out of heat during pregnancy and between heat periods.

“In the study, five of the 12 mares (41.7%) that had a 35 mm ball experienced prolonged luteal function, which lasted on average three months,” says Nie. The Dutch veterinarian who inspired Nie to try the technique had greater success, with the procedure suppressing estrus in 75% of mares.

Nie used a 35 mm ball instead of the 30 mm size recommended. “The web site I went to for finding these only had 25 or 35 mm-diameter marbles, and no 30 mm,” recalls Nie. “I tried the 25 and the 35 mm, but the latter size was more effective, and was also retained better by the mares. We lost about 50% of the 25 mm balls; they got pushed out of the cervix within 24 hours of placement. Two of the remaining six (33%) that had the 25 mm had prolonged luteal function. We didn’t lose any of the 35 mm balls. They were heavier and would sink down to the front of the uterus.”

Andrea E. Floyd, DVM, of Serenity Equine in Evington, Va., tried the procedure on a couple of mares last year, but also had trouble finding the right size marbles. “The ones I used were 5 mm too small and the mares slipped them,” she says. “Both mares remained out of heat while the marbles were in the uterus. In one mare the marble was placed at the end of the heat cycle when the uterus had tone, but the cervix was closed down, close to ovulation. I think this is the best time to place them. My speculation is that the uterus is most likely to recognize the marble as a pregnancy if placed soon after ovulation. That mare stayed out of heat for six weeks. I did an ultrasound at that point and found the marble missing. The mare soon came back into heat.”

David Haynes, DVM, of Meridian, Idaho, has also used the procedure. “It was easy to place, and fun to see on ultrasound,” he says. “The owner felt it worked for the rest of that year, but the next year the mare went back to her normal cycling.” He didn’t know if she lost the glass ball, since he had no chance to examine her after that.

Regarding the mares in Nie’s study, he says, “We suspect that if marbles were left in, each mare would have experienced another prolonged luteal period. Another researcher told us about two mares in which a glass ball had been placed and they experienced prolonged luteal function. Following administration of prostaglandin, both mares returned to estrus, retained the marbles, ovulated, and again experienced prolonged luteal function.”

The marble can be easily removed when the mare returns to heat. It can be moved (via rectal palpation) to the cervical opening and removed at that time. “Occasionally a mare may need to be lightly sedated during the removal procedure,” says Nie.

All marbles were removed from the mares in Nie’s study by mid-fall of 2000. Twenty-three of the original 24 mares were bred in 2001 and 17 conceived (74%). All cycled normally, with no reproductive impairment. Ultrasound and tissue samples showed that none of the mares had any damage to the uterine lining. “No detrimental effects were observed,” reports Nie.

The advantage of this technique is keeping mares out of heat without drugs. The disadvantage is that it does not work in every mare. “When it is effective, however, the need for daily administration of progestin products or intermittent use of off-label products is avoided,” says Nie.

Anyone considering the use of marbles for this purpose should seek his or her veterinarian’s assistance, he advises. “Time of cycle is very important for placing the balls, and especially for getting them out again, whenever that time comes. It should be done by someone who has experience palpating mares per rectum,” he says.

The glass balls are just marbles, like those used in marble shooting contests. Nie got his from the web site and says he learned about a whole world of marbles he didn’t know existed. They come in all colors, or clear–some with colorful swirls inside them. Nie used plain white ones, and paid $1 apiece for them.

He says these are just shooter-sized marbles, but they work just as well for postponing estrus in mares as they do for marble shooting contests.