FWIW compared to dairy cattle, cystic ovaries in horses are not common. Cystic Ovaries can cause a mare to act like she is in heat all the time or like she never comes into heat (the latter being less typical). This is not to say mares never have cystic ovaries, they certainly do!
As to KigerQueen'w comment I believe she is located in AZ so heat cycles could be year round as the day light periods are longer and the climate is warmer.
I used Regumate to synchronize mares coming in heat. They go off regumate and the first heat cycle is very predictable.
In cattle if you want to synchronize heats you inject a prostaglandin product.. shot #1 is followed 14 days later with shot # 2. The reason for two shots is prostaglandins cause the CL to recede if a CL is present. At the first shot some of the cattle will have bare ovaries (just after or just b4 a heat cycle). The second shot they should all have a CL present and all should come into heat.
Prostaglandin shots CAN be used for mares, but the shot will cause a reaction in the mare (she will sweat profusely).
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer