Our dairy cows are going through a lice epidemic as well. I used "eprinex" at the beginning of February because the vet said it doesn't affect the meat or milk and is way better than ivomec. Guess what $70 jug of crap later it didn't work and the lice is worse! So I'm back to dusting with DE, apple cider vinegar, mms in their water. Can't wait for the green grass to come. Lots of farmers in the area are having lice trouble, apparently the critters are becoming immune to all the drugs that are dumped into cows routinely. Working on a better solution...