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South Australia worms, flies and lice update - May 2019

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





Adelaide: Colin Trengove, Sheep Health Lecturer (UA Roseworthy campus) (

Rain at last! Many parts of South Australia received 30+mm during May. This follows the driest period experienced in 50 years for much of the State. Seeding is now well underway for all who intend to crop, but long-range forecasts are less than encouraging for a bountiful winter.

Not deterred by the dry start, worm egg counts have risen in the last month as per normal. This rise has corresponded with the increased worm risk as ewe immunity wanes around lambing time. It may have also been exacerbated by the shortage of hay and grain after the long dry period, and combined with the worm risk associated with confinement feeding, it is anticipated that most stock owners will have drenched pre-lambing.

The exceptional lack of ground cover combined with the emerging green pick will be contributing to larval uptake in ensuing weeks, and so worm egg count (WEC) monitoring will be critical to maintaining optimum worm control. The choice of worm drench should it be required during winter is not critical as most of the worm population is in a larval form on pastures rather than in the gut. However, high WECs usually indicate a drench is required to give the animal at least temporary relief from a worm burden.

As a general rule strategic use of highly effective “triple active” drenches (i.e. three active chemical ingredients) are recommended to prevent worm burdens. In contrast, salvage drenching is required in the face of high worm burdens, and a single active drench may be sufficient to knock it down, but be aware of the potential to leave behind a large number of resistant worms.   

Even in these tough years, a drench to lambs at lamb marking is not indicated. If you’re concerned, check the egg count in 15–20 lambs in the lead up to marking. Drenching ewes at lamb marking is similarly unnecessary unless the mob is being moved to a low worm risk pasture. Once again use monitoring to assist your decision making.


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