I used Regu-Mate(altrenogest) is a liquid oral medication
There is birth control for mares. Consult your Vet.
I personally would first test for ulcers.... the vet can do a quick test, and once my mare was diagnosed and treated she was a different horse!!
I also use raspberry leaf from a natural health product store and make a tea in my crock pot stirring with a plastic laddle. NOTE THE CROCK POT IS VERY IMPORTANT AS METAL FROM OTHER COOKING UTENSILS WILL ALTER THE TEA! Store in a milk jug, tea leaves and all, and use in the grain a few days before, during, and a few days after the mares cycle period.
Also, there's a feisty mare supplement
check out smartpak's mare magic i believe it's called. their website has a great guide to their products