The period of time drench-selected worms lay eggs without competition from non-selected worms.

Worms can only reproduce inside an animal. Only worms that survive a drench can therefore reproduce (i.e. lay eggs). These resistant worms have a reproductive advantage over susceptible worms.

The longer resistant worms enjoy a reproductive advantage, the faster drench resistance can develop.

It requires about 21 days for roundworms to develop from ingested larvae to egg-laying adults. This “prepatent period” is the minimum period of reproductive advantage after a short-acting drench is given.

Some oral, injectable, pour on and all capsules keep killing susceptible larvae for an extended period. Persistent drenches allow mostly resistant worms to establish. This gives a reproductive advantage for at least the activity period stated on the label but often for longer.

The more often an animal is drenched and the longer the period of persistant activity for a drench, the longer resistant worms have a reproductive advantage.

Increasing host immunity may also increase reproductive advantage as the animal’s immunity prevents most new larvae from developing. This is one reason why drenching of adult stock (which have a better immune response) is considered a greater risk for drench resistance development.

Reducing the period resistant worms can reproduce without competition from susceptible worms will slow the development of resistance.