'Rotating dewormers' is a concept that's been shown to be detrimental, but unfortunately, many of my veterinary colleagues continue to recommend it - largely out of ignorance of the studies that have been done in recent years.
It's an even bigger problem for small ruminants than it is for cattle - I almost never see a cow or calf DYING of worms, and I'm not convinced that it's even economically advantageous to deworm adult cows on any sort of regular basis. 95+% of death losses in goats, particularly across the Southeast, are due to infection with the barberpole worm, Haemonchus contortus. We frequently see populations of Haemonchus in many of our goat flocks that are resistant to all three classes of dewormers, due to producers 'rotating' dewormers in quick succession - and that at the recommendation of the veterinarian (I know; I used to make that recommendation - before I knew better.).
If you're concerned about the development of 'resistance' in the populations of 'worms' present on a farm/ranch, rotating between classes of dewormers will just move you down the road toward resistance to multiple (or all) classes all that much faster.
If you deworm with, say, Panacur/Safeguard one time, you kill off all the benzimidazole- susceptible worms, and all that are left to breed and produce the next generation of infectious larvae are those that are resistant to the benzimidazoles(Panacur, Valbazen, Synanthic, etc.) - resistance to one member of the class translates to resistance to all members of that class of dewormers. Then, if you deworm with an ivermectin the next time, you kill off the worms that are susceptible to ivermectin, and you're left with a population that is resistant to both the ivermectin-type dewormers and the benzimidazole dewormers.
It's far better to pick ONE type of dewormer and stick with it until it is no longer effective - before switching to another class. Depending upon frequency of treatment, you might get 2, 3, 5 years or more out of a class of dewormer before you need to switch.
cfpinz - Eprinex, Dectomax, Cydectin, and ivermectin are all in the same class, though Cydectin (at least at this point in time) seems to still have some effectiveness against populations of ivermectin-resistant nematode parasites.
I don't even bother with ivermectin in my herd anymore; as my hero Dr. David Pugh says, "If it doesn't kill the worms, why waste your time or $$$ putting it in 'em?!"
