Spin or Not? Is that the question?

Rotating Pesticides – Weed-killers, Drenches, Insecticides etc.

It’s not about rotating ‘names’ – it’s all about rotating modes of action and maybe not actually rotating at all.

And is it effective for all pesticides?

Over the next few Smith & Georg AgChemNews editions we’ll consider some current wisdom gleaned from industry and industry support experts.

In this edition of the Smith & Georg AgChemNews let’s look at sheep drenches and in-a-spinan article that rocked my understanding of drench rotation: Published by Lewis Kahn, ParaBoss Executive Officer in October 2016 (full article available at WormBoss.com.au.

I’ve a small flock of breeding Dorpers and normally, once the grass dries off in late November/early summer, I drench them to clean out worms from winter and set the ewes up for mating. Then each year I’ve been rotating drenches white/clear/mectin – with the occasional ‘multi-drench’ used ‘for good measure’.

Lewis Kahn’s article leads with the headline:

‘Drench rotation does little to combat drench resistance’

What’s that all about? I thought I was doing the right thing simple changing drenches regularly.

The article continues:

‘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 belowa 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.’
Further information on the development of resistance is followed by an explanation for combining drench groups and again I quote the article:
‘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.


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. ‘


Much of this was new to me and may be to you too – the WormBoss.com.au website has valuable tools to assist sheep and goat producers and the Smith & Georg Chemical Accreditation Course includes a free, optional, module for persons who perform Animal Health Treatments.