Making the right call on combinations

A common sentiment that vets come across on farms is the idea from farmers that they "will stick with a double combination for now so we have the triple up our sleeve for later". This might seem like good logic, but there are some reasons why this is not sound...

The New Zealand science on resistance management indicates that resistance is delayed for the longest period by using a combination of as many effective actives as possible while you still have low levels of resistance and in conjunction with the other known resistance-delaying strategies.

The chance of resistant worms having the combination of genes to survive three completely different actives is exponentially lower than the chance of having the genes to survive one or even two actives. If this tiny number of resistant survivors is either not allowed to breed, or to be greatly diluted by unselected worms, the rate of resistance development is massively slowed and modelling indicates that this approach can be protective for decades.

We haven't had enough decades go by yet to prove this in real life, but our own experience has been that farms where combination drench use was adopted early (back in the mid-90's when they first came out) that have avoided extensive use of long acting products pre-lamb and have only used moxidectin in a targeted way, have maintained good drench testing results.

Double combinations are not "as many effective actives as possible" because they only contain two drugs. Furthermore on many (in fact most) farms, at least one of the actives (typically, but not always, white drench) is failing to such an extent that it does not remotely fit the definition of "effective"!

So, at best with a double combination, you might be using one "effective" active and one that is partially effective. This puts unnecessary pressure on the effective drug, when you can protect it better by using a combination of more actives. You can improve your resistance-delaying power by moving to a triple for your routine drench. Even if there is some resistance to each of the individual actives, far fewer worms (often none) will survive than if you stick to a double.

The next question is - What happens when worms start surviving the triple? The key is to put in place strategies that minimise the chance of those resistant survivors of drench breeding with each other and their population building up! These strategies include:

  • Good feeding and body condition management of adult stock to minimise the need for drench treatment (especially whole flock drenching and especially long acting pre-lamb treatments).
  • Use of the refugia concept, that is, deliberately leaving a small percentage of worms free to breed without being screened by a drench first. Where lambs are grazed on permanent pasture over the summer/autumn, sharing that grazing area with un-drenched ewes (as opposed to lambs only) will reduce selection for resistant worms. This strategy, combined with leaving five percent of lambs un-drenched has been shown in AgResearch trials to greatly reduce the rate at which resistance continues to develop, without reducing lamb growth performance.

Use of knockout drenching. Administer a knockout drench to lambs that remain on the farm at the end of a summer/autumn drenching programme to remove any resistant worms that have established in the face of your routine drench. As far as possible the knockout drench needs to be a combination of drench actives unrelated to the ones in the routine drench.

Speak to your vet today about what product will be most suitable for your situation.