Every year we get asked this question!!
In our area, if January is damp and warm, Barbers Pole populations will begin their initial increase in late January. Given this fact we normally advise farmers to change to a Barbers Pole persistent drench in late January or early February. Note that all drenches for sheep kill Barbers Pole worm on the day of treatment (provided drench resistance isn’t present) however Moxidectin (Vetdectin and Exodus) and Closantel (Genesis Ultra) are the only 2 chemicals that give a prolonged kill of Barbers Pole after treatment and are the best products to use where Barbers Pole is a known threat.