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South Australia worm update - December 2012

Colin Trengove, Sheep Health Lecturer, UA Roseworthy campus (

A dry start to summer in most parts has suited harvest with yield and quality varying from lower rainfall areas to more favoured districts. Worm egg counts (WEC) have also been variable from 0 - 1200 in recent weeks across SA and even within flocks. This highlights the importance of checking several mobs to assess the need for a summer drench. 

Any mobs less than 100 eggs per gram going into summer will most likely not need a drench, but this can only be assessed by monitoring. If a drench is required it is a good practice to minimise the risk of developing drench resistance by leaving 5-10% of the mob undrenched ie those apparent healthy sheep in good condition can be passed by. This ensures that a susceptible population of worms remains in the mob to contaminate the next paddock and dilute the drench resistant worms that may survive the drench. An alternative is to leave the mob in their current paddock for a week after drenching before moving to the next paddock. However, this practice is less popular at this time of year especially where limited time and moving onto stubble. Be sure to use a drench that you are confident is working. This may be a "mectin", double or triple combination, or the new monepantel drench group. Discuss with your advisor if uncertain.

The unpredictability of summer rainfall means that further monitoring is best done 3-4 weeks after a significant amount of rain, eg 20mm, to see if worm larvae pickup has followed. In the event of no significant rainfall it is still good practice to do worm egg counts again in early February to check for unexpected worm burdens. It is preferable to avoid a second summer drench, but a WEC greater than 100 may indicate it is necessary. In this event, make sure to use a different drench group to the first summer drench to minimise the risk of developing drench resistance. Note: worms which survive in the gut or on pasture over summer are the basis for worm burdens next winter and so careful parasite and paddock management over summer can minimise time and expense with worm control for the rest of the year.

Editor: you have to admire the dedication of some of our contributors – Colin sent this from Nepal Airport while waiting for his plane.  He has been there on a FMD (Foot and Mouth) study for the Sheep Meats Council along with some colleagues.  It’s good to know that, in the event of any exotic incursions, we will have some people who know what to do.

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