Dun,
We treat all the cows in the fall once and never had to retreat such a large group of cattle in any other year except for that time we used Cydectin. Lice are common up here, as the moose and deer seem to pass it along to the cattle ie. rubbing on the bush, bales, etc. So it's not unheard of to have to retreat two or three by the end of winter. But it wasn't just the breds who were affected that time, also the bulls too, and they were penned in a different area seperate from the cows. Also the affected animals were patchy very early in the winter, which is also an oddity.
Another difference in pour ons is that some (now I can't remember which brands) have an oil base rather than a water base. I believe the Cydectin has a water base, and if the cows get snowed or rained on after you treat it doesn't tend to stay on them as well as the oil based ones. But the fall we used it, the cows' backs were dry the day we processed and pregged them, and we didn't get any precipitation for a couple of weeks after, so who knows what really went on with it...
I don't know if Dectomax is water based or not, would have to check one of the jugs, but I'm pretty sure that Ivomec is oil based. Anyone know for sure?
Take care.