Effective Drenching - The Key To Resistance Prevention

Key Points:

  • Use Faecal Egg Counts to determine worm infestations
  • Conduct drench resistance test to select most effective drench
  • New drenches will not cure resistance problems
  • Good drenching practices ensure drenching effective and economical


Drench resistance is a widespread problem. Sheep Producers are experiencing large production losses, when drenches have 'suddenly' let them down during an occurrence of acute infestations. Drench resistance can happen after a product has been used often, especially in drier seasons, or when the product has been used inappropriately. It can also happen when drench resistant worms are introduced onto a property. This can be prevented by ensuring that purchased or re-introduced sheep are quarantine drenched. Using drenches effectively is the key to managing worms and preventing drench resistance.

A FEC (Faecal Egg Count) is one of the most useful worm management tools a sheep producer can use. It is a count of the number of worm eggs in a sample of sheep dung. The result is usually expressed as 'eggs per gram' (epg) of dung. Conducting a larval culture, will identify all the species present in the sample and can further refine a FEC result.

Choosing a drench

Cost is no indication of effectiveness. There is no guarantee that even an expensive 'multi-active' drench will contain a single ingredient that will be effective against the worms on your property. Each worm population is different and drench resistance varies from property to property - even to be more specific in some cases - Paddock to Paddock. Base the decision to drench on the results of a drench resistance test. This test will tell you which active ingredients will work on your property and let you select the most effective and economical drench. Drench resistance tests should be done once a year, around early November before conditions become favourable for worm infestations. However in some years it may need to be conducted in February, before worm numbers build up.

In recent times new drench combinations have been more readily available. However the new actives as we know have become few and far between. We need to pro long the effectiveness of drenches by improving farm management practises therefore reducing the chance of resistance worm populations.

Drenching practices

Good practices will ensure your drenching is effective and economical. Management should include:

  • Monitor sheep before drenching with a worm egg count.
  • Select a highly effective drench based on a drench resistance test.
  • Reduce feed before drenching to increase drench effectiveness (BZ and mectin groups only).
  • Test the drench gun's accuracy.
  • Weigh the sheep so that the dose can be calculated accurately.
  • Read the label carefully and follow any precautions or contraindications.
  • Use the recommended dose rate and dose to the heaviest sheep in the mob.
  • When drenching, place the drench gun over the tongue to increase effectiveness.

For further information on drench resistance go to ParaBoss