With any of them, do a fecal before and after to find out what you need and what worked. If you are doing a big group you do not have to have a sample on each one.
We always rotate wormers and used Dectomax pour on last fall. It was the least effective of anything we have used in the past few years. Dosage was correct as cattle were on the balanced scale when they were poured.
Rotating wormers is a great way to build immunity to all possible wormers out there, ask sheep producers in Australia and New Zealand and some goat producers in Texas. Most wormers work fine if used correctly and you do the fecals as previously mentioned, the generics tend to have resistance issues sooner. Drenches are better than shots are better than pour ons. Surprising number of studies show that cattle lick more of the pour-on off than what a guy thinks. Depending on where you live also cows shouldn't have to be wormed every year. Calves yes of course worm in spring and fall.
Do you have any links with scientific info about the resistance because that is why we rotate and is what our vets have said to do for years. I will ask one of the vets next time he his here as well because that is the first I have heard of it.
hamburgman is right on most accounts.
Pour-ons are the least effective - sure, it's easy to squirt it on 'em, but hellfire, if the majority of the drug doesn't get to 'em to kill 'em, why use it? Never used 'em, never will.
'Rotating' dewormers has been misrepresented and misunderstood - and too many veterinarians do it wrong and make inappropriate recommendations to their clients. Proper 'rotation' does not mean "Use something different every time you deworm." - doing that really does speed you down the road to selection of a population of parasites that's resistant to everything.
Rather, you should pick ONE product, and use it until it no longer works on the parasites on your place (hopefully 2 years or longer), then switch to another dewormer in another class and use it 'til it no longer is effective, then switch to another class of dewormer