Manage Worms & Drench Resistance

The farmer’s guide to manage worms & drench resistance

A flock of sheep grazing on a New Zealand farm, related to managing worm and drench resistance.

Realising the drench isn’t working anymore, is a tough moment for a farmer.

But there is a way forward from here.

Compiled by farmers across the WormFEC Gold network – and explained in more detail through the Beef + Lamb NZ Wormwise programme – here are the five key steps for managing worms and drench resistance long-term.

  1. Act like you have drench resistance already
  2. Know the signs of worm and parasite burden
  3. Test
  4. Breed for parasite resistance
  5. Use good management practices

Act like you have drench resistance already.

With more than a third of New Zealand sheep farms now showing triple drench resistance – there’s a good chance your farm has it too.   

Sadly, many farmers are choosing to ignore the issue. Don’t be one of them.

A strategy to manage worm burden and protect (or reverse) drench resistance is possible. But it won’t come out of a drench gun.

In the long-term, the profitability of your flock & farm will come down to the basics – knowing the signs, regular testing, breeding for worm-resistance and good management practices.

The sooner you get started, the better.

Drench resistance can cause lamb carcase weights to drop by 2.8kg – 4.7kg1,2,3. At the five-year average farm-gate price of $7.50 per kilo, that’s a loss of $20 – $35 per lamb.* Pop this out as a side box or similar element.

Scouring and poor health – know the signs of worm burden and drench resistance.

Signs of continued worm burden following a drench, can signal drench resistance. Always be on the lookout for:

  1. Weight loss: Sheep with a heavy worm burden often have low weight gain. or can lose weight, due to competing with gut worms for nutrients.
  2. Scouring (diarrhea): Common in young lambs, scouring is often caused by worms like Trichostrongylus and Nematodirus. This leads to dehydration and further health deterioration​.
  3. Anemia: Particularly associated with barber’s pole worm (Haemonchus contortus), sheep may show pale gums, eyelids or lethargy due to blood loss.
  4. Bottle jaw: This is fluid build-up under the jaw and can be a sign of severe worm burdens, particularly barber’s pole.
  5. Poor coat: Sheep with worm infections may develop a dull, patchy coat due to poor health.
  6. Coughing or respiratory Issues: Lungworm in particular can cause coughing and difficulty breathing.

Test

You can’t manage what you don’t know.

Regularly testing your flocks for worm burden and drench resistance is essential. Following a drench, a faecal egg count (FEC) test will show you the level of worm burden remaining – a good measure of that drench’s effectiveness. A more detailed Faecal Egg Count Reduction Test (FECRT) can confirm whether worms are resistant to specific drenches.

Need advice on testing? WormFEC Gold breeders take on average 300 FEC samples each per year and work closely with Techion – providers of leading testing kits. Contact a member today to help make testing the first-step in your long-term strategy.

Breed for parasite resistance

“Worm resistant genetics is like compound interest – every year it just gets better.” Robert Peacock, Orari Gorge Station

Genetic worm resistance is one of the strongest long-term strategies farmers have to manage worms and drench resistance. Selecting for genetic resistance means flocks are better able to withstand worm burden, without relying on chemical drenches. This inbuilt resilience, combined with strong production traits, earn WormFEC flocks +$6 in NZMW (NZ Maternal Worth) value over the national average for dual purpose flocks.

Find your local WormFEC Gold breeder

Use good management practices

Testing and parasite-resistant genetics are just two parts of the toolkit needed to manage worm burden and guard against drench resistance. For a full list of management practices. visit Beef + Lamb NZ Wormwise programme. The common practices WormFec Gold breeders recommend to farmers are:

  • Refugia. Leaving a portion of your flock (and their worms) untreated so that non-resistant worms can compete with resistant strains.
  • Strategic Grazing. Worm larvae survive on pasture, so rotating sheep between paddocks and allowing fields to rest reduces exposure to infective larvae. Mixing cattle with sheep can also help disrupt the parasite life cycle.
  • Feed on crops. Some crops provide a less favourable habitat for worms and can support better nutrition in animals fighting parasite burden.
  • Rotate drench families or using combination drenches. Even parasite-resistant sheep will need a drench under some conditions. To avoid worm populations developing drench resistance, don’t rely on a single type of drench. Instead, rotate between different drench families, or use combination drenches (containing multiple active ingredients) to help slow down the development of resistance.
  • Quarantine drenching. Drenching new stock on arrival can prevent resistant worm strains being introduced into established flocks.

*Sutherland, I.A., Shaw, J., Shaw, R.J., The production costs of anthelmintic resistance in sheep managed within a monthly preventive drench program. Veterinary Parasitology 2010.

2Sainsbury’s FECPAK project 2014-2017.

3Miller, C.M., Waghorn, T.S., Leathwick, D.M., Candy, P.M., Oliver, A-M.B., Watson, T.G. Lamb productivity responses and benefits to monthly STARTECT treatment of grazing lambs exposed to mixed anthelmintic-resistant nematode parasite pasture challenge. Veterinary Parasitology 2012.

*Five year weighted average lamb farm-gate price. Beef & Lamb New Zealand New Season Outlook 2023-24. October 2023.