Lewis Kahn, Associate Professor, Animal Science, UNE, Armidale
The key principles for choosing a drench are:
Use drenches most effective on your property; ideally use those shown to reduce worm egg count by at least 98% as shown by a DrenchTest. If drench effectiveness is unknown, conduct a DrenchCheck-Day10 after drenching. The more effective a drench is, the fewer drench-resistant worms will remain in the sheep after treatment.
Use a combination of two or more actives from different drench groups where possible, as the chance of a worm being resistant to all active ingredients in the combination is much lower than for each individual active on its own.
Remember that a drench group refers to drenches with different activity. For example, benzimidazole, levamisole and macrocyclic lactones are all different drench groups, however, within each group there are different but related active ingredients. For example, within the macrocyclic lactone group the most common three active ingredients are: ivermectin, abamectin, moxidectin.
There may be confusion about the relative importance of drench efficacy and the value of combination products for slowing drench resistance. The most important factor is drench efficacy and where this can be achieved with combination products then all the better.
For example, consider the case of a particular farm where the combination product of benzimidazole and levamisole (white + clear) has an efficacy of 96% versus the single active product monepantel (Zolvix) with an efficacy of 99.9%. With this example, the efficacy of monepantel is higher than for the combination and on that basis, development of resistance to monepantel will be slower than development for the combination. This doesn’t mean you should totally rely on the most effective treatment.
Rotate among all effective drench groups each time a mob is drenched (and for each paddock where possible). An effective drench from a different group may kill worms that were resistant to the last treatment. These may be worms that survived treatment in the sheep or were picked up from the paddock. When rotating drenches the current drench ideally would exclude any groups that were used the previous time. However, in practice, ensure the current drench has at least one effective active from a drench group that was not used the previous time.
Use short-acting treatments where possible, and restrict the use of persistent products for specific purposes and high worm-risk times of year where other management practices to reduce worm-risk have not been adopted.