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Tasmania worms, flies and lice update - December 2020

Tasmania WormBoss Worm Control Programs

Tasmania WormBoss Drench Decision Guides





Perth: Paul Nilon, Nilon Farm Health (

November was dry and December is heading the same way, although it can all change quickly. Days of endless wind have quickly changed the fields of green to fields of beige with irrigators going flat out. Still, a spring for the ages.

Weaning is in full swing. The rapid turnaround in pastures should convince those inclined to delay weaning to think again. There are three correct times to wean: first, at 10 o’clock after a cup of tea. Second, wean early most years. Third, wean even earlier in hard years. There’s no room for late weaning, even in the most abundant seasons. Apart from the risk (or likelihood) of deteriorating pastures, it can bugger things parasitically if unweaned lambs graze saved paddocks with their mothers.

Drench choice: The weaning/first summer drench needs to work to maximise the strategic effect. A ‘working drench’ is normally defined as greater than 95% efficacy. This is therapeutically fine, but falls short when it comes to decontaminating paddocks (which, after all, is the reason for the first summer drench). We need an efficacy of around 99% to maximise pasture decontamination.

Without current resistance data choosing a drench that is 99% effective is problematic. The options for Tasmania are:

  • Derquantel (Startect™) or monepantel (Zolvix™ or Zolvix Plus™) should work. There has been so little use of them here in Tas that it beggars expectation that there would be resistance. Watch the latter’s export slaughter interval (ESI). Do a 14 day post drenching count.
  • An abamectin/moxidectin triple should come close, but it is not guaranteed. The end of 2020 is not a good time to trot out the excuse that you cannot afford more expensive options.
  • Single actives (except Zolvix™) are unlikely to work; avoid them, and even use Zolvix™ with the Plus component.

So, bite the bullet and go all out.

While on the subject I heard a rumour that derquantel was not working. Remember that there is no residual activity with derquantel, or any of the short-actings (OK, there is some residual activity with moxidectin oral). If you have any doubt about drench efficacy do a 14-day post-drenching count. Much better than spreading rumours!

It’s complicated: Previous notes have mentioned the need to remember the wool harvesting interval apropos fly and lice treatments. Simple! Well no, because end-users may place additional requirements that confound the thinking.

For example, one supermarket chain has both a maximum pelt length, and time since shearing. As there are big penalties for lambs that breach the contract specs, it may necessitate more than one shearing and/or fly suppressive treatment time based on weaning weight. This raises the prospect of a shorter acting product for the early shorn lambs, and a medium-long acting for the late shorn lambs.

As crossbred lambs will go through any fence that is not waterproof and some that are, you need good separation between groups treated with different products. EID tags and auto-drafts may help. Or an additional tag. Either way it may not be easy.

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