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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

Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.

  1. Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred. The more effective a drench is the fewer drench-resistant worms will remain in the animals after treatment. If drench effectiveness is unknown, conduct a DrenchCheck-Day10 after drenching.
  1. Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. The higher the efficacy of each drench group, the greater the benefit for slowing drench resistance. The chance of a worm being resistant to all active ingredients in a combination is much lower than for each individual active on its own. For goats, be aware of what drench groups are registered or permissible with a veterinarian’s prescription.​
  1. ​​​Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year. Persistent products provide a long time during which ingested resistant larvae can survive and reproduce. There is little need to use mid-length or long-acting treatments if animals are being moved to low worm-risk paddocks.

A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.

While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the sheep or goats may go to slaughter.

Search for drenches based on the worms or other parasites targeted, drench group or active and product name.



Using drenches

Use 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.