Colin Trengove, Sheep Health Lecturer, UA Roseworthy (firstname.lastname@example.org)
Corresponding with a dry November and the completion of hay making we see a tapering of the worm egg counts. This is not to say that worm populations have diminished, but worm egg production tends to fall as Summer approaches. It is still the best guide that we have to monitor worm burdens and certainly it is reflective of worm burdens in lambs, weaners and hoggets. Monitoring now will determine the need to drench these most susceptible age groups as we enter Summer with the aim of minimising worm numbers through to the Autumn break.
In contrast we do not drench adult sheep at this time unless they have relatively high egg counts as we also know that drenching in Summer is the quickest way to promote worm resistance. If monitoring indicates that adults have significant worm burdens we can either drench the entire mob and then return to their current paddock for a week before moving to a fresh paddock, or we can leave 10% of the mob (the biggest, fittest and healthiest sheep) undrenched before moving to a fresh paddock. The principle is called "refugia" and is to encourage the survival over Summer of a worm population not exposed to drench chemical. The purpose is to minimise drench resistance development by maintaining a population of susceptible worms to dilute the presence of any resistant worms. This is recognised as the most effective strategy to delay the onset of resistance if there is a need to drench.
In addition we endeavour not to give another drench during Summer as this will compound the risk of developing resistance. Monitoring for the presence of worms in January/February is still encouraged as it may avert a disaster and especially after significant summer rainfall that can encourage Barbers pole worm to emerge.
A long dry Summer may not be desirable in all situations, but it is certainly the ideal to prevent worms impacting sheep health and production. Irrigation is an entirely different situation and you should consult your local sheep health advisor on how best to deal with worms under these situations.