Producers are reminded to weigh up their worm control options for sheep over summer, with recent tests revealing increased resistance to frequently used drenches.
Department of Agriculture and Food Veterinary Parasitologist Brown Besier, of Albany, said drenches given to young sheep in summer set the scene for worm control for the coming winter and spring.
“It’s important that they are fully effective,” Dr Besier said. “In general, a summer drench should be routinely given to both the current year’s lambs (blue tags) and, unless a worm egg count indicates otherwise, also to the previous year’s drop (red tags).
“Young sheep are highly susceptible to the effects of worms, and ineffective worm control can reduce their resilience during summer and autumn, when the quality of pasture nutrition is at its lowest.”
Drench resistance has been detected in WA to all drench groups and combinations other than the new types introduced in the last few years (monepantel and the derqantel-abamectin combination).
“Many producers rely on the ‘macrocyclic lactone’ (‘ML’) drenches, abamectin and moxidectin, without knowing whether they are still effective,” Dr Besier said.
“Recent laboratory results indicate resistance to abamectin in nearly 50 per cent of tests, and to moxidectin in nearly 30 per cent of tests.
“Unfortunately, drench resistance is largely an invisible problem – even a partially-effective drench will usually reduce the worm burden sufficiently to prevent obvious signs of worm disease.
“However, worms surviving summer drenches often lead to worm problems both earlier in winter and more severe than usual.”
Where there is no drench resistance testing history, sheep producers are advised to use a new drench type, or a drench group where resistance is not yet common, followed by a worm egg count to check whether a follow-up treatment is indicated.
“Recent rains across most of the major sheep areas adds another consideration,” Dr Besier said.
“It’s important that summer drenches are given while sheep are on dry pasture, to prevent any development of worm eggs. If recent rains lead to pasture germination, wait until it has dried off before giving the additional drench.”
The present recommendation for adult sheep is to delay drenching until autumn (late March-early April), as summer drenching all sheep on the farm can rapidly increase the level of drench resistance.
If adult sheep are drenched in summer, a proportion of those with the highest condition scores (above 3.0) should be left untreated to minimise the likely increase in drench resistance.
More details of summer drenching strategies and drench group options can be found on the department website, or on the WormBoss website.
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