Online Learning: Pastoral—Drench Resistance Management

Each property has its own drench-resistance profile based on its own drenching history and that of properties from which sheep were sourced. The profile of neighbouring properties can be quite different.

The extent of resistance is only known by testing. Obvious worm control failures may only occur when resistance is quite advanced. In this region, a DrenchCheck-Day 14 is the preferred method to check individual drenches at any time. DrenchCheck-Day14s should be considered when any drench is given and it is the most practical and cost-effective method of testing drenches in this region.

While a DrenchTest or Worm Egg Count Reduction Test (WECRT) is the most accurate test for drench resistance, this test is rarely feasible in this region as infections are often not high enough and when they are, they may be unexpectedly high and need swift treatment or are in lambs at weaning, which should not be put at risk in a DrenchTest.

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The first WormTest within the DrenchCheck-Day 14 is done up to 10 days before a mob is drenched with a short-acting drench; and the second is done exactly 14 days after the mob is drenched (testing earlier or later than 14 days can be inaccurate).

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Choosing drenches

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 
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. 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.
4. 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.
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

1. Avoid unnecessary drenching
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).

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‘Quarantine’ drench all sheep (including rams) new to the property.

Use a combination of no less than 4 unrelated drench actives with at least one of these being the newest drench actives: monepantel (e.g. Zolvix®) or derquantel (with abamectin—e.g. Startect®). This can be done using multi-active (combination) and/or single-active products concurrently—up the race with one product, then up the race again with the next.

Do not mix different drenches unless the label states you can, as different products may be incompatible.

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A combination contains two or more active ingredients that each target the same worms. 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.

A mixture contains two or more active ingredients, but the actives target different worms. These give the convenience of a single drench when quite different worms are targeted; however, they should be considered ‘single-active’ against each worm.

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• BZ or benzimidazole group (‘white’)
• LEV or levamisole group (‘clear’)
• ML or macrocyclic lactone group (sometimes called ‘mectins’)
• AAD or amino-acetonitrile derivative group
• SPIRO or spiroindole group
• OP or organophosphate group
• BZ or benzimidazole group (flukicide)
• SAL-P or salicylanilides/phenols group
• ISO or isoquinolone group

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