12 NOV 2022

Let’s talk slowing drench resistance

Extremely wet conditions across parts of southern and eastern Australia are giving rise to unseasonably high instances of internal and external parasites in livestock. Wet conditions increase the survival rates of worm eggs and larvae on pastures, causing production losses and animal health issues.

Drenching is a common strategy used to control worm burden in sheep and cattle, however it’s important to consider the chance of animals developing resistance to certain chemicals.

Livestock Production Advisor, Emma Shattock, has seen her fair share of drench resistance and suggests the following steps to ensure ongoing efficacy, and the best health outcomes for livestock.

Drench only when necessary

Faecal egg counts are a simple way to understand worm burdens over the year and avoid an unnecessary drench. These tests provide an estimate of the average burden across a mob and can be used to determine the potential impact on performance. Mature, dry ewes tend to have some natural immunity and therefore may not require drenching as often as a younger animal. Larval cultures, where the eggs are hatched and their species determined, can provide further insights into the relative burden of multiple worm species within a mob of sheep.

Quarantine drench on arrival

Don’t buy into someone else’s resistance issues. Give new stock a quarantine drench with at least four unrelated active constituents, including one of the newer drench groups. Hold stock on feed and water in yards for one to three days before turning them out onto a ‘dirty’ pasture (one that is contaminated with worm larvae from your own sheep). Please note, if your stock arrive stressed or dehydrated, it is safer to let them settle in on hay and water for a day or more before drenching.

Use drenches according to label instructions

Ensure sheep are getting a full dose of drench by calibrating drench guns, weighing sheep and drenching to the heaviest animal. Use combination drenches (of two or more different drench groups) that are effective on your property. An effective drench is one that reduces the worm burden by at least 98%.

Use short acting products and limit the use of long-acting drenches which can provide greater exposure to sub-lethal levels of chemical. If you do need to use a long-acting drench, ensure you are monitoring efficacy with a faecal egg count every 30 days and using an effective primer and exit drench.

Include non-chemical strategies

Employ an integrated pest management strategy rather than relying on drenches alone. Pasture worm burdens can be reduced by spelling paddocks through cropping or pasture renovation. Other strategies for managing worms include cross grazing with other species, avoiding overgrazing and breeding for worm resistant sheep.

Please note: this article contains information of a general nature, and does not take into account your personal objectives, situation or needs. Before acting on any information, you should consider the appropriateness of the information provided, and seek advice on whether it is fit for your circumstances.

Meet the Author

Emma Shattock

Livestock Production Specialist
  • Elders Burra
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