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1. Why does drenching in dry weather impose a greater selection pressure for drench-resistant worms than drenching in wet weather?
2. Why do ‘multi-active’ drenches help delay the development of drench resistance?
3. Is it better to treat for flystrike on the same date every year, or to wait and see?
4. When restocking, should you consider that all introduced sheep present a possible risk of introducing lice?
A drench is essentially a tool that selects for resistant worms, and like any other selection tool, drench resistance is a numbers game.
The worm population is divided into 2 subgroups: one within sheep (ingested lavae and adults) and the other on the pasture (eggs and larvae). Ideally, to slow the development of drench resistance, drenching is done when the worm population in sheep is much lower than the worm population on pasture—that there are many worms in refugia on the pasture.
For example, when pasture conditions are favourable for the development of eggs and survival of larvae i.e. the grass is green and temperatures are mild, the proportion of the worm population within sheep will be smaller than the proportion on pasture.
Conversely, when pastures are dry and temperatures are hostile i.e. too hot or too cold, worm eggs on pasture are inhibited from developing. Consequently, the proportion of the worm population within the sheep can be larger than the proportion on pasture.
On many farms, combinations, or drench groups with more than one active, are likely to be more effective than just the individual ones—unless the individual drench group is 100% effective. Combinations, therefore, will generally give a better kill of the worms in your animals than any of the single components of that drench and will leave behind fewer resistant worms.
Ideally, if drenches killed 100% of worms in 100% of sheep, 100% of the time, there would be no drench-resistant worms.
Research has found that the higher the efficacy of each drench group in the combination and the more drench groups included in the combination, the greater will be the benefit for slowing drench resistance.
Any drench, no matter how effective, should always be used in conjunction with a range of strategies (IPM-integrated pest management) including pasture management, animal management, and quarantine treatments of purchased stock.
There is no best answer, but there are a few guidelines.
As with all introductions, but particularly when sheep lice prevalence is high or the demand for restocking is high, it is important to take steps to avoid bringing lice onto your property. Introductions that pose a risk for lice are purchased sheep, and sheep brought in from other properties including sheep returning from agistment.
Good biosecurity will not only prevent the introduction of lice and the consequent associated costs, but avoid problems from introducing lice that are resistant to specific chemical treatments.