If drenching killed every worm, those carrying resistant genes would not have a reproductive advantage and resistance could not develop.

This is impossible on most farms today. The aim, therefore, is to kill as close to 100% as possible, as fewer will survive and mate with other resistant parasites and contaminate pasture.

To achieve the best kill every time you drench, you must know what worms are resistant on your farm, use short-acting drenches in combination and avoid underdosing by using good equipment and drenching technique.

Monitor the efficacy of current drenches with a minimum of two drench checks a year.
As drench resistance can develop rapidly, is farm specific and different parasite species are prevalent at different times of the year, do at least one drench check in spring and another in autumn. Data from the national survey strongly suggests that resistance in sheep detected early in the season would frequently be missed late in the season due to the changing proportions of parasite species.

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Ascertain your farm’s status by doing a faecal egg count reduction test (FECRT).

You must know your farm’s resistance status! Using any drench, especially long-acting products, without this knowledge can result in production loss and pasture contamination with resistant worms.

A FECRT gives a detailed profile for your property, including the resistance status of individual worm species.

Veterinary advice should be sought to perform and interpret an FECRT accurately, and to help plan a drench strategy. This extensive investigation should be repeated every two to three years.