Had a discussion with Dr. Tom Yazwinski at UofArk today, regarding a research project I have to evaluate - two different dewormer treatments on stocker calves.
His recommended treatment for stocker calves from any source, at any time (we didn't discuss bred heifers, cows, pre-weanlings) is to give both a benzimidazole(fenbendazole, albendazole, oxfendazole) drench and a macrocyclic lactone(ivermectin, doramectin, moxidectin, eprinomectin) at the same time. Resistance to the MLs is really strong in Cooperia and Haemonchus - but the benzimidazole drench will get them; the benzimidazoles won't get arrested/encysted Ostertagia - but the MLs will.
If you're gonna do a pour-on, do it RIGHT! Apply the correct dose in the proper manner, on clean dry cattle, with no rain in the recommended time frame. Majority of the drug gets to the target to kill worms by ingestion, as the cows lick/groom themselves and one another... more gets there that way than by being absorbed through the skin.
So... if you're only treating a few here and there, and not treating them all... none of them get the appropriate dose.