To be fair to my Vet this mare always showed a regular heat cycle so we (all of us) assumed incorrectly that she had already kicked it out....that's what is supposed to happen. Had she not come back into season regularly after the marble was inserted of course we would have thought it was still there doing its thing.
Despite many, many, many ultrasounds (including a full very thorough Vet repro workup the year before we were going to breed to ensure everything was good to go) that darn thing was never seen. Not once! I guess it was moving around and it was only by chance we glimpsed it on the ultrasound one day after the 5th non sucessful breeding.
It was the weirdest most expensive thing I have ever witnessed with a broodmare.
The marble is not intended to be expelled by the mare; that does happen sometimes, but it certainly isn't meant to come out without being removed. I'd be asking for my fees back, if my vet made that mistake (unless my embarrassment at not figuring it out myself prevented me from bringing that up!).
I have a mare that went into a heat in spring of her 4yo year and stayed in heat for nearly 5 months. She was hard to handle 24/7 at the barn, and at times unsafe to be around--not just a question of preference or inconvenience, but really a "never ever a mare again" kind of girl. And this is while I was starting her undersaddle....
We started with raspberry leaves and quickly progressed to Magnesium supplement (which helped focus but did not stop heat).
Then we marbled. This brought her out of the constant heat and helped bring down the level of crazy from red zone to yellow zone and would be safe if in daily work. Although she did go into heat again 2 months later. I called the vet out thinking she's lost her marble (as opposed to lost her marbles, which is pretty common), but US there it still was. She didn't stay in heat this time.
Because if she was not in daily work she still tended toward dangerousness around the barn (and she was needing rest/handwalking for the various mare-kicking-stall injuries she caused herself) I felt like I needed further steps on top of the marble. I chose to do an implant of a formulation of the hormones in the cattle product Ral-Gro (inserted under skin in neck). This should work for 6 months to a year and the vet and I decided that would give her time to grow through this intense puberty phase and be safer around the barn. She is not allowed to compete in the Tour de France now, as the consequence is she is getting male hormones. The marble is still in. We chose this over Regumate because of the safety of handling it at the barn where we have a lot of teen girls (including my own) hands-on with care, and the other option was regularly sedivet-ing her for work/handling, which I have a distaste for. It also cost significantly less over a year.
It has worked. No one mistakes my hot red mare for a gelding, mind you, and never will, but she isn't the constant in-heat screaming rearing beayotch anymore either. She is progressing nicely, for a hot sensitive red mare.