DRENCHING POLICY: Think about refugia and reproductive advantage before you drench.

When determining the timing of drenching and deciding which stock to drench, the following points should be taken into account. These decisions should be made with your animal health advisor.

I: What proportion of the total worm population am I exposing to a drench?

Drenching during droughts, with extensive use of cropping, or drenching all classes of stock exposes a higher than normal proportion of worms to drench, accelerating drench resistance.

Allowing non-selected worms to reproduce by not drenching certain stock classes or mobs delays resistance by maintaining the ratio of susceptible to resistant parasites on a farm.

Deciding which class or mob not to drench requires care. The most obvious choice is adult stock.

Don't drench adult stock unless it is justified. Consider:

• What risk is there of challenge given the mob(s) that have grazed the paddock in the last 3-6 months?
• What is the drenching history of these mobs?
• What factors are influencing their immune status?
• What egg count information is available?
• How successfully will eggs hatch? (<2% of eggs develop in the winter and early spring, compared with 10-30% in a moist summer)

II: How clean is the paddock stock are grazing on after drenching?

Drenching stock and then shifting them onto pasture with little or no larvae has the potential of “seeding” a paddock with resistant worms only.

Consider seeding paddocks with non-selected parasites by:

• drenching stock two weeks prior to moving onto clean pasture
• delaying drenching until two weeks after shifting
• not drenching all stock prior to shifting
• grazing non-drenched stock, e.g. adult stock on the clean pasture prior to arrival of drenched young stock.

Not drenching the Entire Mob? A recent Australian suggestion is to leave a proportion (5 – 15%) of the best-conditioned animals undrenched within a mob. This policy has been shown to slow the development of resistance in recent New Zealand trial work^[17]
The effect on production in these trials was not great. However, it is likely that production effects will vary. Leaving a percentage of young stock undrenched may only be appropriate if worm management, including “non-drench” methods, is effective in reducing the larval challenge. Conversely, if non-drench methods of worm control create very low larval contamination levels in pasture, policies such as leaving some stock undrenched to increase refugia become more important.

III: If resistant eggs are produced, what is the risk of them developing into the next generation of worms?

Know your drench resistance status and consider the climatic conditions, pasture type, and the susceptibility or immunity of the next grazing mob following drenching.
For example, ewes on pasture with a low parasite burden given an ineffective persistent-activity drench prelamb. Their lambs would be exposed to a disproportionately high number of resistant worms. As the lambs lack immunity they will multiply these worms.