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Drench rotation versus combinations to combat drench resistance

Rotation between different drench groups generally does little to slow development of drench resistance and should not be used at the expense of these three highly effective practices:

  • use products most effective on your property
  • use multi-active (combination) products
  • use short-acting products

However, there are times when using a drench group that is different from the previous treatment is important and this is discussed later in this article.

Drench rotation is the practice where consecutive drenches are used from different drench groups.  A simple example of a drench rotation is for the first drench to be from the BZ group and the next time a drench is given, a product from the levamisole group is used.

It’s tempting to think that rotation, once considered to be drench best practice, would slow the development of drench resistance on the basis that using a different drench group would kill any worms that were resistant and survived the previous treatment.  While this is true, and this effect can still provide value under certain circumstances (discussed later), advances in the understanding of drench resistance indicate that the practice of rotation itself will not delay the development of drench resistance.

To provide context to the scenarios below, a treatment with resistance at 10–20% (in other words, the drench is 80–90% effective) will result in a large loss of production from worm infection.

Development of drench resistance

At the ParaBoss Technical Forum held in March 2016, Dr Yan Laurenson of the University of New England presented the output of a model that predicts development of drench resistance.

This model broadly confirms the results of other models developed in Australia, New Zealand and elsewhere over the last 20 years.

Two scenarios from the presentation are relevant to this article and are provided in the figures below.

The first scenario is sequential drench use (no drench rotation), where drench A was the only drench group used for 15 years before being replaced by drench B as the only group for the next 15 years. 

In the second scenario (annual drench rotation), drench A was used in an annual rotation with drench B for 30 years.  For example drench group A in year one, group B in year two, group A in year three and so on.

Although producers have more drench groups than this at their disposal, the concepts shown here will still apply.

Complete resistance is predicted to have developed to both drench groups A and B after 30 years regardless of whether a rotation was practiced or not; but let’s look more closely at these scenarios.

When the drenches are used alone (sequential), resistance to drench A increased after 10 years to about 80% (shown as the blue circle on the Figure 1), but when rotated with drench group B, resistance had only increased to about 30% (grey circle on Figure 2).

Figure 1: 'Sequential'—Development of drench resistance when drenches are used sequentially.

Figure 2: 'Annual Rotation'—Development of drench resistance when drenches are used in rotation.

The main reason for this apparent benefit at this stage is that only half the number of doses of drench group A were given when used in a rotation and so there was less opportunity for sheep worms to develop resistance.

At the 10 year-mark of the simulation a drench rotation looks highly effective at slowing the development of resistance, but remember, in the sequential use situation, while resistance to drench A is 80%, drench B is still 100% effective because it has not been used.

Now let’s look at the predicted situation after 20 years. By then, drench group B had only been used alone (sequential) for 5 years and resistance had developed to about 30%, see the red circle on Figure 1, (andresistance to Drench A is 100%). When used in rotation, resistance to both drench groups had increased to about 80% (black circle on Figure 2).  A complete reversal of the situation that was predicted at the 10-year mark.

On average, across the entire 30-year period, there was no predicted advantage from a drench rotation.

Use drench groups in combination

In contrast, using drench groups in combination has a much greater effect in slowing the development of drench resistance.

Using two drench groups in combination at each treatment reduced resistance after 10 years to 20% and when using three drench groups in combination, resistance had only developed to the level of 5% over the same period (see orange oval in Figure 3). Once again, a closer look at this figure is informative. 

In a situation where you had three drench groups at your disposal and you decided to use only one until resistance to that drench group developed to 20% (purple circle on figure 3) (when major production loss would be occurring due to the drench being only 80% effective), then each of the three drench groups could be used for about 4 years each, giving 12 years before major production loss was likely to occur from poorly controlled worm infection.

In contrast, if you used all three drench groups in combination (green line on Figure 3), it is predicted that major production loss from drench resistant worm infections would not occur until after 30 years (green circle, Figure 3):  this is an 18 year advantage over the singular use of drench groups.

Figure 3: 'Combinations'—Development of drench resistance when one, two or three drench groups in a treatment.

What is important from these model predictions is the superiority of multi-active combinations for delaying drench resistance. Regardless of whether the development of drench resistance occurs more slowly or rapidly on your property to that presented here, the relative benefit from using combination treatments will remain the same.

WormBoss recommendations

This is why WormBoss highlights the importance of using drenches in combination or using more than one product concurrently (up the race with one and then the other/s) to combine different drench groups.

But remember, these models started with fully effective drenches. Use combinations before each of the single actives in them become ineffective on your property.

The higher the efficacy (effectiveness) of each drench group used in a combination and the more drench groups included in the combination, the greater the benefit for slowing drench resistance.

This is why it is important to test the efficacy of each drench group (that is, you should test the single active products, even if you know they are no longer fully effective) to know how protective each drench group is in a combination product. For example, if you use a triple combination, where two of the drench groups in it are already very ineffective (say 30-40% effective or less) it will be more like using a single active product and drench resistance won’t be slowed a great deal, even though the combination product might still be effective at killing worms.

Just because a product is a combination drench, doesn’t mean it is effective on your property. You need to do a DrenchTest to find out.

When does a drench rotation provide value?

There are specific situations when using a different drench group/s from the previous treatment can have a major benefit.

The first of these situations is the choice of an exit or tail-cutter drench used at the end of the protection period of a mid- or long-acting drench. Under this situation WormBoss recommends that you use an exit drench and that it is from a different drench group/s to the persistent treatment.

Persistent treatments provide a long period when only resistant worms (if present) can establish and reproduce in the sheep, with only their eggs contaminating the pasture and developing into resistant infective larvae.  Removal of these resistant worms from the sheep will require the use of a product containing different drench groups to those in the persistent treatment, and importantly, that product itself must be highly effective.

The second situation is when using a different drench group/s from the previous treatment can have a major benefit  for managing animals that have been drenched into a low worm-risk paddock (such as a prepared lambing or weaning paddock).  Under this situation, there are few worms in refugia meaning there will be few worms on the pasture to dilute those that develop from the eggs laid by any resistant survivors of the treatment of the sheep going into the paddock.  While, this is a good outcome for worm control, it does create the situation where drench resistance may develop more rapidly.

WormBoss recommends two actions to deal with the resistant worms now in the sheep and on the paddock. Firstly, drench the sheep as they leave that paddock with a product containing different drench group/s to that used on these sheep when they were drenched into this low worm-risk paddock. For example, drench groups AB into the paddock and groups CD out of the paddock.

Secondly, the paddock potentially has a high proportion of resistant worms and should now be spelled or grazed with cattle for as long as practical prior to being grazed with sheep that have a moderate worm egg count.  The spelling or cattle grazing will allow for some of the infective larvae to die, making it easier for the worms that develop from the next grazing of sheep to dilute the resistant population and reduce the level of resistant worm larvae on the paddock.

The third situation is when rotational grazing management is practiced. A small benefit for slowing the development of drench resistance can be gained by rotating drench groups providing you also rotationally graze stock across the property. This creates situations where the drench resistance status of worms in the sheep differs from those on the pasture.

Despite being a useful practice, it is important to keep in perspective that drench rotation practiced with rotational grazing management might slow development of resistance by 15–20%.  In practical terms, this will add 2–3 years to the expected period of useful efficacy for a drench group.  In contrast, using a combination product with three efficacious drench groups will add an extra 18 years over the singular use of drench groups.


Rotation between different drench groups does not play a significant role in slowing the development of drench resistance. The three principles for choosing drenches to slow the development of drench resistance are:

  • Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred.
  • Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups. The higher the efficacy of each drench group and the more drench groups included in the combination, the greater the benefit for slowing drench resistance.
  • Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year.