Drench resistance is an inevitable consequence of using drenches in an unsustainable way. Parasite resistance to drenches develops slowly on some farms and rapidly on others, depending on drench usage and cattle management factors.
Introducing cattle from other properties can also increase the proportion and type of resistant worms. Different worm types also have biological features that can make them more, or less, likely to develop resistance to different chemical groups.
There are several strategies to manage resistance, but by far the most effective measure is to minimise the number of times that drenches are applied.
Strategies for delaying the emergence of drench resistance include:
- Reduce the need for treatments by using non-chemical worm control methods to reduce larval contamination on pastures, and by providing the appropriate level of nutrition for growth and maintenance of the stock class.
- Follow a planned annual program for high-risk classes of cattle and management types (younger age classes) in regions where worms are known to be a common cause of disease or production loss. Strategically timed treatments will have a preventative effect on the development of large worm populations.
- Time treatments for lower risk classes or management types so that you drench only when necessary, such as when faecal worm egg counts are high, poor weight performance is observed or worms are found post-mortem.
- Avoid drenching older, worm-immune cattle unless parasitic problems are confirmed.
- Use the appropriate drench for the species of worm present. Different worm species have developed resistance to different chemical classes.
- Consider a combination drench to maximise efficacy, and reduce the build-up of resistant worms within the population.
- Manage long active products to kill any resistant worms.
- Maintain refugia (a population of worms that have not been exposed to drenching) by ensuring populations of worms, not recently exposed to drenches, are present when other mobs are drenched.
- Consider other parasites when treating, to avoid unintentional exposure of non-target parasites to chemicals. It is best to use different actives for internal and external parasites.
- Follow product label instructions to avoid under-dosing.
- Quarantine drench new arrivals to prevent the introduction of resistant worms.
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