Again, Dr. Don Bliss, one of the world's foremost parasiteologists says, "if you want to get lice, you have to pour them twice."
Usually we all get by with one application, but nothing truly eliminates them with one application. "Twice" means 14 days apart. I remember when Cydectin (I think that was what it was-my recall isn't what it used to be) had the lice guarantee, '6 months lice free'. The government made them pull that guarantee because it was bogus. We had a friend who sold it and she lost her job because their marketing tool was taken away. I know because she told us that was the reason she lost her job.
We used Cylence for years and poured the cows 2-3 times during the summer for flies. That broke the life cycle of the lice as much as anything. At that time it was cheap and easy to use. But we had to change because the cows built an immunity to it and we didn't think anything worked as well after that. We didn't want to have to treat them twice 14 days apart, so didn't and crossed our fingers one treatment would get us by. Remember, there are biting lice and eggs. Kill the biting (chewing) lice and you still have eggs that hatch 14 days later.
We used Epernix once and broke with lice really bad. Had the vet come out and scrape the cows. He said what we were seeing wasn't caused by lice. Anyway, sent it in and sure enough, it was lice. They replaced the product.........with Ivomec. Go figure.
Here is a good article on it, this guys says 3 weeks between 1st and second treatment. It's a good read.
Lice are spread by direct contact; calves pick up lice from their mothers or herdmates. Lice can infest cattle all year round, but their numbers are typically low in summer because most lice shed off in spring with winter hair, says Jack Campbell, a University of Nebraska professor emeritus and veterinary entomologist.
Cattle’s winter hair coat provides lice protection and an ideal environment for reproduction, he says. The life cycle is 20-30 days and the entire cycle takes place on the host, which makes lice an easier parasite to kill.
Adult females attach their eggs to hairs, which hatch in 5-14 days. When the nymphs emerge, they look similar to adults, but smaller, and go through three molts within a week. They advance to egg-laying adults in about 14 days.
Lee Townsend, University of Kentucky Extension entomologist, explains that pyrethroid pour-on insecticides are effective against all types of lice, whereas the avermectins, moxidectin and eprinomectin (systemic products) mainly kill sucking lice.
Townsend says diligent lice control can effectively eliminate lice in most herds. But, if a producer doesn’t ensure that all the cattle on the farm get treated, or if he fails to follow up with the second treatment, there will be residual lice populations to reinfect the herd.
“Giving two treatments three weeks apart is an important key. Most products don’t work against the egg stage,” he says. Thus, the eggs on the hairs survive, and hatch later.