Study Sheds Doubt on Marbles' Effect on Mare Heat Cycles

Many people have offered solutions to overcome the problems of inconsistent performance while mares are in heat. One method popular with owners today is the implantation of marbles as glass intrauterine devices (IUDs).

"The original paper that described this technique suggested that the marble might behave like an early pregnancy and delay the mare’s return to heat," explained Caroline M. Argo, MRCVS, Dipl. ECAR, of the University of Liverpool, U.K. "Some mares seemed quite extreme in their heat behavior, yet most of the owners reported a distinct improvement in their attitude and 'trainability' after the marble was implanted."

But new research by Argo and colleagues disputes this claim. The researchers monitored nine Welsh Mountain pony mares over several natural reproductive cycles then either performed cervical dilation without inserting an IUD (sham-treatment) or performed cervical dilation and implanted IUDs. Then they monitored them through several cycles for physical, behavioral, reproductive, and endocrine parameters.

"All of our treated mares continued to show regular heat and we could find no difference in any aspect of their biology or behavior after marble implantation," Argo said, adding that marbles had little or no real effect on the mares' handling.

A type of placebo effect might have influenced owners--belief that a treatment worked enabled them to handle mares with greater confidence, which was the real reason why behavior improved.

If an owner has a problem mare, Argo suggested that he or she keep track of when the behavior problems occur and whether the mare shows heat. Then, have the animal checked by a veterinarian to make sure there are no problems with her reproductive system.

"Marbles have been grasped as a 'cure,' because compared with other treatments they are considered relatively benign, reversible, and without a lasting effect on fertility," she said.

She noted that long-term treatment with a synthetic progestagen might be more reliable.

The study, "The effect of intra-uterine devices on the reproductive physiology and behaviour of pony mares," was published online in August ahead of print in The Veterinary Journal. The abstract is available on PubMed.  

About the Author

Marie Rosenthal, MS

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