Brown Besier, DAFWA, Albany (firstname.lastname@example.org)
Worms generally take a break during the hottest and driest part of the year, much like many of the owners of the sheep. With no risk of pick-up of more worm larvae to add to a worm burden, any worm effect is entirely due to what has been picked up in previous months.
The main likelihood of problems is where summer drenches were not fully effective. We recommend giving drenches in summer only to weaners and hoggets, but as these have the highest worm numbers of any class of sheep and will be most affected by them. Any worms surviving a summer drench are by definition drench-resistant, and will have a big influence of the worm population in coming months. Allowing even a small burden (say, an average of more than 50 eggs per gram) can lead to significant worm problems in sheep of this age in winter, and a significant increase in drench resistance.
It is therefore a good idea to check that the drench has worked effectively: this applies to any drench except the new group (monepantel or ‘Zolvix’), and generally the ‘triple combinations’ (3 different drench groups, all with abamectin and a white drench, plus either a clear drench or naphthalophos). Drench resistance now involves all other drench types including the more potent of the ML drenches (moxidectin and abamectin).
A DrenchCheck is simple a worm egg count, with the samples collected from the paddock. Although the usual period for checking is 10–14 days after the drench is given, this is less important in sheep grazing totally dry paddocks, as they won’t be picking up more worm larvae. If counts are more than 50 eggs per gram, a re-drench is needed before the end of summer.