First, lots of questions--is your horse kept with other horses who are not regularly dewormed or on the same worming schedule? Is manure picked up daily? Is he/she on pasture or on dry lot and fed hay? How many horses per acre? If he/she is kept by him/herself in a dry lot and fed hay and if you pick up manure daily, once every three months is sufficient. Rotate, but at least twice a year use ivermectin/moxidectrin. That's my program, anyway. Also, if your horse is younger versus older, tapeworms have to be addressed.