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.