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