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South Australia worm update - December 2013

Colin Trengove, Sheep Health Lecturer, UA Roseworthy campus (

Recent worm egg counts (WEC) indicate it has been a relatively good year for worms with moderate to high counts (up to 1400 epg) recorded from the SE up to the Mallee through the Fleurieu to the mid north and across to the lower Eyre Peninsula. Similarly many counts have been zero or negligible highlighting that worms are anything but predictable. One stock owner was delighted to find that Levamisole was still effective, further demonstrating that worm control throws up some surprises.

These vignettes only serve to reiterate that every flock is unique and monitoring is essential to achieve efficient worm control regardless of rainfall, enterprise or region. Having said that, I am constantly amazed at how many stock owners still manage worms on the basis of a management calendar rather than surveillance. For example, some are busy drenching now in an effort to get various tasks done before Christmas, while others drench before harvest or hay making.

The only mandatory drenching that can be justified is a weaning drench, but even this drench may be arbitrary depending on enterprise mix and grazing management strategies. "Drenching by the book" became obsolescent with the discovery of drench resistance and the sooner that all graziers become more astute with their parasite monitoring, grazing and chemical use strategies the better off we will all be.

Egg counts generally diminish with the onset of summer in a Mediterranean and temperate rainfall pattern and so now is not the ideal time for doing drench resistance testing. However, it is a simple procedure if a count exceeds 250 to drench a portion of the mob with one chemical and leave another portion undrenched. Ten days later check the WEC of the drenched sheep compared to the undrenched and establish if that chemical is effective ie has reduced the egg count by at least 98% compared to the control.   

In order to maintain this chemical efficacy (effectiveness) it is important to ensure that a susceptible population of worms are maintained in the flock ie it is as much about sheep worm farming as it is about sheep health management! With reference to the Levamisole example in the opening paragraph, even relatively old chemical groups can retain their efficacy if used infrequently and resistance is not introduced through sheep purchases.

The most pertinent strategy at this time of year is to not move "worm free" sheep onto clean stubbles ie do not drench sheep with an effective chemical and move them into a fresh stubble paddock. If sheep are drenched prior to stubble grazing ensure they remain in their paddock of origin for a week before moving in order to allow them to acquire an insignificant but tactically important burden of worms that were not exposed to the chemical before they change paddocks. This will assist in maintaining the efficacy of your drench(es) in 2014.

On that note may you have a worm-(worry)-free Christmas and New year!

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