Choosing and using drenches

The WormBoss Drench Decision Guides will provide you with recommendations on when to drench as well as what length protection drench may be required.

The specific drench group and/or active is not recommended as this will be quite specific to your property, depending on:

  1. The effectiveness of each active on your property (which should be tested each 2–3 years with a DrenchTest [Worm Egg Count Reduction Test] and DrenchCheck-Day10s in between).
  2. The drench actives you used last time.

To assist you to find out more about drenches (including withholding periods and export slaughter intervals), go to the Drenches section on the WormBoss web site where you can search on the type of worms and other parasites you want to target, drench groups and actives, and on drench product names.


Choosing drenches

Integrate all 5 principles where possible:

  1. Use drenches most effective on your property; ideally use those shown to reduce worm egg count by at least 98% as shown by a DrenchTest. If drench effectiveness is unknown, conduct a DrenchCheck-Day10 after drenching. The more effective a drench is, the fewer drench-resistant worms will remain in the sheep after treatment.
  2. Use a combination of two or more drench groups, as the chance of a worm being resistant to all active ingredients in the combination is much lower than for each individual active on its own.
  3. Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. There is little need to use mid-length or long-acting treatments if sheep are being moved to low worm-risk paddocks.
  4. Rotate* among all effective drench groups each time a mob is drenched (and for each paddock). An effective drench from a different group may kill worms that were resistant to the last treatment. These may be worms that survived treatment in the sheep or were picked up from the paddock.
  5. Choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the sheep may go to slaughter.

*When rotating drenches the current drench would ideally exclude any groups that were used the previous time. However, in practice, ensure the current drench has at least one effective active from a drench group that was not used the previous time.


Using drenches

Follow all 5 principles where possible:

1. Avoid unnecessary drenching, especially

  • adults
  • during droughts or prolonged dry periods
  • immediately before or after moving sheep onto very clean, low worm-risk paddocks (such as ungrazed cereal stubbles or paddocks that have been sheep-free for extended periods). See points i) and ii) below for further discussion on this.

2. Calibrate drench guns to ensure the correct dose is delivered.

3. Calculate the dose based on the heaviest animals in the mob. Split mobs for drenching if there is a large weight range, so sheep are not under-dosed.

4. Follow the label instructions to ensure correct dose and use of treatments.

5. After sheep have been drenched, graze them initially on paddocks already contaminated with worms likely to be less resistant to drenches (except in the cases of paddocks that specifically being prepared as low worm-risk). Eggs from surviving drench-resistant worms will be diluted by eggs and larvae already on the paddock and therefore not exposed to the drench (i.e. ‘in refugia’).

If sheep must be drenched onto low worm-risk paddocks, such as lambing, weaning or winter weaner paddocks, do both of the following:

i. When the sheep eventually leave these low worm-risk paddocks, treat them with an effective drench that is from a different group to the drench used when the sheep first went onto the paddock. The aim is to remove any drench-resistant worms surviving in the sheep after the first drench.

ii. Ensure that the next time the paddock is grazed it is with a different mob of sheep. This second mob should have a moderate to high worm burden and their last treatment must be different from the treatment used on the first mob that grazed the low worm-risk paddock. This will dilute drench-resistant worms already on the paddock with more susceptible worms that the second mob is carrying. Note that grazing with cattle will not dilute the proportion of drench-resistant worms, but they will decrease the total number of worm larvae on this paddock.