The problem with lice is that they have survived since the beginning of time and cattle do need something to help keep them controlled. As for the pour-ons only working on lice and grubs 50% of the time, I have never heard that. NEVER; and I attend lots of meetings on this type of thing. Now it is known that pour-ons only kill 50% of the worms, but lice and grubs is a different story. They are easy to kill.
In fact, years ago we were told by Dr.Don Bliss AND he folks selling pour-on that just waving the stuff over the back of the cattle is enough to kill grubs; and a 1/5 of a dose will kill lice. Now that the price has gone wayyy down, you can afford to use the full dosage. But you still can use 1/5 of a dose to get lice and grubs if you want.
Don Bliss, one of the world's formost authorities on parasites says that
the only way to kill lice totally is to pour them TWICE 13-14 days apart
to kill the life cycle of the lice. He says that is the way it is, regardless of what you use. The way I understand it, that is why the pour-on's had to take back their lice guarantee. I think you get the biting lice when you pour the first time, then in 2 weeks the eggs hatch and by pouring them again, you get those as well. But most of us don't do that, because who wants to run cattle through the chute again? So we mostly get by with one application.
Cold weather seems to cause more lice and of course, as any parasite, they attack poorer, thinner cows more than fat, healthy ones.
I know, I don't want lice on my cows. Just pulls them down and doesn't do anything but harm their immune system.