Most farmers rely on drenching as part of their worm management. This means the emergence of drench resistance can seriously compromise production.

As drench resistance is an increase of the proportion of resistant worms in a population, the number of resistant worms compared to the number of susceptible worms is more important than the total number of resistant worms.
If we could measure the genetic makeup of worms, this proportion would be calculated by dividing the number of resistant worms by the total number of worms in the population (resistant and susceptible).

(See How Resistance Develops for an illustration of how the proportion of resistant worms increases.)

To reduce this proportion, we need to either reduce the number of resistant worms or increase the number of susceptible worms that survive and contribute to the next generation.

However, to maintain production, the total population of worms on a farm must be maintained at acceptably low levels.

If this is done with drench alone, the ratio of resistant to susceptible worms will increase.

The key is to use a range of tools to reduce both the level of larval contamination in your pastures, and the effects of worms on the stock.

Every management decision can play a role in determining the farm worm population.

Grazing Management

Immunity to Worms

Drench Policy

Drenching

Equipment and Technique