Double Deworming after wet season

I have for some time believed in rotating not just brands but classes of dewormers.Recently I've read about people using the two types at the same time but wasn't going to because of the theory"What do you do when you develop an immunity to both types?"
This spring,when I had the vet out to do preg checks,I asked her opinion.She recommended that for this year I might want to do just that.After the year we had with poor condition cows and a lot of rain,the worm load would probably be excessive.I took her advice and used Dectomax injectable and Synanthic drench on all my cows and yearlings prior to turn out.Cattle a re doing good and shedded of well except for some of the yearlings which will get another round soon.
For those unfamiliar with dewormer classes,there are two basic classes used now.The avermectins,Ivomec,Dectomax and Cynanthic plus the "white wormers" in drench or paste like Panacur/Safeguard,Synanthic and Valbazen.I used to think I was doing right by using an avermectin one time then a drench the next but now think maybe should stick to one class for several cycles then switch to the other.Hopefully LuckyP or someone like him will be along to give a better answer.
One thing I think has led to some ivermectin resistance is the use of pour ons.If they are done carefully and the weather window is observed,they may do a lot of good especially for those who lack the time or help to catch them individually.If, however, they are "shot at" or it rains then the dose might be too low to kill well.
I'm blessed to have a several pens where I can get up close or load them and haul home to the main pens plus my grandsons are getting big enough and want to help work with the cattle.Just the other day,we were treating some Pinkeye cases I hauled home.The youngest on was filling syringes and I showed the oldest where to give the injection then told him to go ahead.Proud of both of them.

'Rotating' dewormers has been misinterpreted and misused, and many of my veterinary colleagues are to blame for giving erroneous information to their clients regarding this practice.
Using a different dewormer or class of dewormers every time you deworm is a recipe for speeding down the road to selecting for a population of worms that are resistant to everything; ask the goat people. Better to stick with one class and use it until it is no longer effective, then switch.
And... with the avermectins and the benzimidazoles(white/drench)... once you've selected for a resistant population... it's forever. It appears, however, that with the imidazothiazoles(levamisol, etc.), if you stop using them for 10 years or so, the level of resistance genes in the population abates.

That said, the combination of a macrocyclic lactone(ivermectin, dectomax, cydectin) and a benzimidazole(panacur/safeguard, synanthic, valbazen) has shown, in trials with stockers and weanling/yearling heifers, to be as effective - and less expensive - than LongRange. I've done the combo with calves and weanling heifers, but not cows.

I'm still of the opinion that adult cows rarely, if ever, NEED deworming. Yes, trials have suggested increased weaning weights in calves of dewormed cows, but I only deworm calves, yearling heifers and first-calf heifers nursing a calf. Mature cows... can't recall the last time I dewormed one.
I haven't seen 'worms'(with the exception of liver flukes) kill a mature cow...or even have a significant health impact on one... in over 25 years. Used to see Type II ostertagiasis causing weight loss and diarrhea...to the point that they looked like a Johne's Disease cow... but the ML/avermectin-type dewormers have mostly eliminated that issue... same as with Strongylus vulgaris in horses... it's largely 'extinct' on most premises, due to frequent use of the ML/avermectin type dewormers.

Thank you for your reply.
I had hoped you would see this and give us a professional opinion.
I don't want to ever spread any misinformation.

LP - I have heard that there is very little resistance being seen in NE area (mine). I use the "forbidden" ivermectin pour on (genetic). I realize my mature cows "probably" don't need deworming, but it is soooo helpful with lice and flies. I do the cows in July and then in fall. Is this a bad method?
Also, I use Valbazen & Pour On for all calves and yearlings during summer & both for calves in fall.

The double wormer has been recommended in sheep and goats for a while. Somehow the combo does not allow as much resistance building to either of the two individual wormers. I'll try to look up an article on it.

Ebenezer said:

The double wormer has been recommended in sheep and goats for a while. Somehow the combo does not allow as much resistance building to either of the two individual wormers. I'll try to look up an article on it.

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The idea is that it's extremely unlikely for a parasite to have resistance to both, so worms that are resistant to one or the other don't survive to reproduce. Of course, if you keep doubling up on the dewormers long enough, eventually you'll develop a worm population that can withstand it.

Every time we use dewormers or antibiotics we promote resistance. Improper use can accelerate it, but there's no way to prevent resistance unless we use them extremely sparingly.