Actually, rotational deworming is considered outdated by almost all veterinarians as well as the American Association of Equine Practitioners. People are trying to do the right thing by making sure their horses have no intestinal parasites, but we have overused them to the point that we have created resistant strains that our current dewormers can't kill.
Speak with your vet, run fecal egg counts on your horses, and put together a deworming schedule appropriate for your herd. If proper husbandry is maintained, a lot of adult horses really only need to be dewormed a couple times a year (if even that). You do need to consult with a vet first though.