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Online learning: Tasmania—Drench resistance management

This strategy helps to preserve the effectiveness of your drenches for more years.

Structured reading

For those who like to see all the information and simply read through it in order. Each heading is a link to a page of information—the dot point provides a summary of the page.

Tip: Keep this page open and open the links in new tabs.

Tasmania: Managing drench resistance
A comprehensive guide to the problem of drench resistance and how it develops; and how you can slow the development.

Testing drench effectiveness with a DrenchTest
How-to guide to check the efficacy of a drench just used.

Checking for drench resistance with a DrenchCheck-Day10
How-to guide to test the efficacy of drenches using a Worm Egg Count Reduction Test (WECRT).

Drench groups and actives
A table listing each of the groups or classes of drench treatments as well as the specific actives in the class, the worms they target and some example products.

Drench mixtures and combinations
An explanation of how mixtures and products differ and why this is important when choosing drenches.

Resistance status of drench groups
A general overview of the drench-resistance of worms to the drench groups listed at November 2012. 

Drench Database
This tool allows you to search for commercial drench products according to various criteria with the results showing a wealth of information including drench resistance notes, withholding periods and dose rates.

Question and answer

For those who prefer a problem based approach to learning, answer the following questions.
Each of the questions below links further down the page to the answers.

Questions:

  1. How often should a DrenchTest be carried out on a property?
  2. To check if a drench has worked after you have used it (DrenchCheck), what is the minimum and maximum number of days after the drench that a follow up WEC test sample be taken?
  3. Name 3 factors associated with choosing and using drenches that can contribute to the development of drench resistance.
  4. How many drench actives should be used in a quarantine drench?
  5. What is the difference between a drench combination and a drench mixture?
  6. List the different groups of drenches.

Answers:

You can also click on each question below to go to WormBoss pages with related information.

1. How often should a DrenchTest be carried out on a property?

A DrenchTest is used to assess the effectiveness of a number of drenches that you might use on your property in the next 2–3 years. The DrenchTest:

  • Examines each drench’s effectiveness for each worm type present.
  • Is the most accurate way to test for drench resistance.
  • Uses the procedure called a Worm Egg Count Reduction Test or WECRT
  • Should be conducted on each property every 2–3 years.

2. To check if a drench has worked after you have used it (DrenchCheck), what is the minimum and maximum number of days after the drench that a follow up WEC test sample be taken?

The first WormTest within the DrenchCheck-Day10 is done up to 10 days before a mob is drenched with a short-acting drench and the second is done exactly between 10 and 14 days after the mob is drenched (testing earlier or later than 10–14 days can be inaccurate).

3. Name 3 factors associated with choosing and using drenches that can contribute to the development of drench resistance.

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 DrenchTest. 
  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).

4. How many drench actives should be used in a quarantine drench?

‘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 (Zolvix®) or derquantel (with abamectin—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.

5. What is the difference between a drench combination and a drench mixture?

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.

6. List the different groups of drenches.

  • 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

 


Links to the learning topics for Tasmania

  1. Introduction
  2. Grazing management
  3. Breeding for worm resistance
  4. Worm testing
  5. Drenching
  6. Drench resistance management (you are currently on this page)
  7. Sheep worms