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Oh - there was one method of controlling or preventing heat cycles in mares that the vet said he'd look into if we wanted, if Cyn's heat cycles continue to be disruptive to the peace... Putting a marble in her uterus! It's a lot easier and better than giving hormones.

I looked it up on Google to see what's said about it.

C. Intrauterine placement of Glass Marbles is a method of estrous cycle manipulation that has been used for a number of years in Europe and the Middle East and introduced to the many US veterinarians at the 2001 AAEP convention by Dr Gary Nie. It involves the placement of a sterile, 30 to 35 mm diameter glass marble through the cervix and into the uterus within 24 hrs after ovulation. It is effective in increasing the interovulatory interval from 21 days to approximately 90 days for about 40 to 70% of mares. It seems to work by causing the uterus to react to the marble as if it were an embryo. Accordingly, the CL is then maintained for a substantially extended period. During estrus, the marble can be removed by a veterinarian who is experienced in transrectal uterine palpation. The veterinarian will manipulate the marble, per rectum, though the uterus and out through an open cervix. No adverse effects of this treatment have been reported.

Source: Estrous Cycle and Manipulation