Undersosing with drenches is a mojor cause of anthelmintic (drench) resistance in worm parasites of sheep.
When farmers use a drench at less than the recommended dose rate, some worms may survive, and their ability to resist treatment with the chemical involved is passed to their offspring. Over a period of time, matings between worms with different degrees of resistance can produce individuals able to resist the drench at the recommended dose rate or even higher.
Farmers must use adequate dose rates so that worms do not have a chance to survive treatment
