Complications Using Glass Balls to Suppress a Mare's Estrus

June 27, 2005 (published) | May 11, 2016 (revised)

Mares begin cycling in spring as most do not cycle in the winter. And if you have a mare that acts crazy and has behavior problems when in heat, you may be looking for some help. Some of these mares are so severely affected that they cannot even be ridden when in heat and many of them do not perform up to their potential. There are several things that can be done to suppress heat.

One of these things that has been common is to insert a sterile glass ball or marble in the mare’s uterus. The glass ball or marble is believed to simulate a pregnancy and keep the mare from cycling. This technique was studied and published. I have used the procedure and it seemed to be effective in about 50% of the mares into whom I inserted a marble. However, a recent report indicates using these marbles may be dangerous and one report indicated the mare developed a severe infection, while in another report two glass balls were used and they broke apart and the glass became embedded in the uterus. Whether this was related to using two balls, which was not tested in the original research, or just something that happened, we don’t know. However, I think it is enough of a concern that I will be reluctant to use glass marbles in the future.

The most common method used to prevent heat in mares is to give a commercial hormone or progestin that must be given daily and is expensive. There is a compounded long-acting product that is effective in preventing heat. The newest technique is to use oxytocin to keep mares from cycling. If you have a mare with behavior problems when she is in heat, ask your vet for help.

VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email

Information and opinions expressed in letters to the editor are those of the author and are independent of the VIN News Service. Letters may be edited for style. We do not verify their content for accuracy.