Different areas and different environments require different deworming schedules.
Have your horse get a fecal count done by a vet, and ask the vet what deworming schedule and type your horse should be on.
For my horses, my vet told me to use:
March - ivermectin 1.87%
July - Panacur fenbendazole
Nov (after first frost) - Quest Plus moxidectin / praziquantel
and my horses have had fecals done this year, so we know that that is an appropriate plan for them.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.