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South Australia worms, flies and lice update - January 2019

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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


A blast of scorching temperatures in mid-January has erased memories of rainfall received in mid-December, but the impact persists with a third of recent worm egg counts being high enough to warrant a strategic drench. Some counts have been surprisingly high (i.e. into the 1000s) indicating the likelihood of Haemonchus being present.

The recent worm egg counts (WECs) indicate many mobs drenched in spring may need a follow-up summer drench. However, it is recommended to monitor for the presence of worms before considering a drench.

It will be essential to check egg counts again in early February as a strategy to monitor for worm contamination going into autumn—even more so with grazing close to the ground in feedlots and confinement feeding. To minimise the risk of drench resistance developing it is important that any drench used in summer is fully effective. If rotating between drenches with different chemical actives, only drenches that you know will work should be considered. If no recent drench resistance has been done, checking worm egg counts after drenching will show this.

The dilemma arises that summer drenching  can promote drench resistance, i.e. any worms that survive a summer drench will be directly responsible for the majority of the (drench resistant) worm population in the following winter. However, delayed drenching will encourage a larger proportion of worms surviving through to winter, either in the gut or on pasture and so causing a higher worm risk in 2019.

And so the long-held principles of worm management remain paramount:

  • Monitor WECs frequently to decide if drenching is necessary;
  • Where possible, use an effective combination drench, rather than a single active;
  • If rotating between drench groups, use only drenches you know will be effective;
  • Summer drench only if monitoring indicates the need; and
  • Adopt refugia management, i.e. either delay moving a mob to a clean paddock for a week post-drenching or only drench 80% of the mob. This is to ensure “drench susceptible” worms are transferred to the clean paddock to dilute the impact of any “drench resistant” worms that survive drenching. It’s worth talking to an adviser to plan refugia programs.


For January 2019 state outlooks, please follow the links below:
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