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South Australia worms, flies and lice update - March 2020

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





Adelaide: Colin Trengove, Sheep Health Lecturer (UA Roseworthy campus) (trengovet@icloud.com)

The 20–70mm rainfall in late January is now a distant memory with surprisingly few worm issues directly stemming from that significant precipitation period. Amid bushfires and now pandemics there still has been a good number of samples submitted for WECs in the last month and about a third of these – mostly from higher rainfall areas – have indicated a need for a drench.

Autumn is asserting its change of weather now that the equinox has passed, with cooler evenings and morning dews starting to become more apparent. As a result, any worm eggs are more likely to develop into infective larval stages on soil/pasture for unsuspecting grazing ruminants. There is little pasture or ground cover across the State and so infective larvae have little protection in those situations. However, any moisture in the next month will encourage regrowth in perennial species or potential germination of annual species assisting consumption of infective larvae by hungry stock grazing close to the ground. There is also a significant risk for the increasing number of livestock being confinement fed these days and so they too need to be monitored or drenched prior to entering their confinement areas. Unmonitored or undrenched stock could develop significant worm burdens over the next month or two under these conditions.

That begs the question...is a pre-lambing drench warranted? The green pick that followed the summer rain will have given ewes a little boost and follow-up condition scoring will indicate their nutritional status prior to lambing. The decision whether to drench should be based on egg count monitoring; ewe condition (ideally average of 50 at random = 3); feed on offer (preferably > 1,000 kg DM/Ha or dense pasture > 3cm average pasture height); and the intended management of the paddock currently being grazed i.e. are lambing ewes to remain in this paddock or will it be spelled for several months. 

It is difficult to predict weather patterns at this time of year, but if you believe long term forecasts the rain outlook is bleak, therefore, with expected poor nutrition, worm counts greater than 100 epg indicate drenching is warranted. These decisions need to be made on an individual mob basis using local knowledge. However, it is important to remember, if you are drenching at this time of year, use a double or triple active drench (including a different drench group to that used  at the start of summer) and check if it is effective, i.e. do a Drench Check by performing a WEC 14 days after drenching to see if any worm eggs remain. At least then you will have a retrospective indication of drench efficacy.

Another consideration is the risk of worm pickup associated with high intensity stocking/grazing i.e. increased risk of faecal and hence worm larval contamination with trail feeding or irrigation. Once again WEC monitoring will soon indicate whether these are significant risks for ewes in the lead up to lambing and/or the season break.