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Again, time to do some reading. Again I recommend
www.thehorse.com as a concise place to find lots of good up to date information, although there are also plenty of other sources.
The reason to rotate wormers (in some situations; see below) is that different wormers kill different worms, and most importantly because repeated use of just one type wormer encourages the development of strains of worms resistant to that chemical. You really REALLY don't want to develop resistant worms, because then that class of chemical is useless against them. In goats there are some strains of worms in some regions that are resistant to EVERY KNOWN wormer chemical
which is a real problem. Don't want to go there with horses!
There is little or no point in rotating *within* a class, as resistance generally occurs to the WHOLE class if it develops at all. So just changing *brand* of wormer often achieves nothing whatsoever. You have to read the fine print and see what actual chemical a particular brand IS.
There are basically three general classes of wormers used in horses: compounds ending in -bendazole which are the old-style "cheap" paste wormers like Anthelcide, which affect the smallest number of types of internal parasites; pyrantel pamoate, such as Strongid, which is broader-spectrum and if used in high concentration for several days will kill tapeworms; and ivermectin and moxidectin, which a lot of horse owners treat as if they're totally different but actually are in the same *broad* chemical class and it is really debatable whether or not it's useful to rotate between them.
The ideal thing to do is to have several fecals done, such as by the vet, so you have a pretty good idea of what and how much worm burden your horse is carrying, and treat accordingly. So's not to be worming unnecessarily or with inappropriate compounds. If you don't do that though, then a good approach is to worm every 8 weeks (arguably skipping it in the absolute dead of winter.... but make sure you've taken care of bots and tapeworms first), using pyrantel for six months or a year and then ivermectin/moxidectin for six months or a year and going back and forth between 'em. (It used to be recommended to rotate *every worming*... current recommendations tend to be for a slower rotational cycle as being less apt to encourage resistance. Either way is probably better than totally not rotating.)
Make sure that what you're rotating to/between is going to do what you need it to do. If benzamidazole-resistant worms are common in your area (or where the horse came from), or if you expect bots to be an issue, then it would be stupid and pointless to use Anthelcide or another -benzadole compound in your rotation.
The reason for every 8 wks btw is based mainly on the life cycle of the parasites... the idea being you want to kill 'em off in the horse (the stages that are affected by the wormer, at least -- it is more complicated than I'm making it sound here, you should read more on the topic) BEFORE undue amounts of eggs are passed to reinfect the horse.
Moxidectin has had some issues in the past -- although I am unconvinced that part of it hasn't been a deliberate bad PR campaign
-- because of dosage sensitivity in very young foals and in minis, and because it kills off even encysted strongyles so if a horse has a LARGE worm burden it's a bad choice for the first time or two you worm the horse. But on the whole it is still pretty safe and does have some advantages over ivermectin in some circumstances.
These things -- nutrition, worming, behavior -- are not something it's smart to be relying on "general learning" from neighbors (however experienced) or the guys at the feedstore or a little article you read in a magazine five years ago. They are pretty important things to be up to date and knowledgeable about, if you are going to be in charge of a horse's life and health. I would really, REALLY recommend stepping back and reviewing basics like this from a reputable and up to date source.
Pat