Pat Coleby's book "Natural Horse Care" is a pretty nice book to have if your interested in natural equine care. My copy is in storage right now but I believe she recommends copper and sulfur for deworming. I think DE would definitely be worth a try too.
Getting a microscope and a fecal egg count kit would be a good way to see your horses current parasite load, then try a deworming method that you want to try out, then do a recheck fecal egg count to see how effective your deworming method was. You could try several methods out, different dosages, etc. until you found something that worked best for your horses and situation. It's a lot more effective than just guessing if your horses have parasites and if your deowrming method works or not.
