Drench resistance in sheep worms is now widespread across Australia, including Queensland. Producers who rely on single-chemical drenching programs are increasingly finding they have no effective treatment options left. Building a resistance management program now is essential.
Why Resistance Develops
Resistance develops when a small number of worms with genetic tolerance to a drench survive treatment and pass that tolerance to offspring. Every drench treatment applies selection pressure. The faster you rotate and the more accurately you dose, the slower resistance develops.
The Five Key Principles
- Rotate chemical classes — never use the same class back-to-back. Use benzimidazoles (BZ), levamisole (LEV), and macrocyclic lactones (ML) in rotation.
- Combination drenching — products combining two or three classes (e.g. Startect, Zolvix Plus) dramatically slow resistance development.
- Accurate dosing — weigh the heaviest animal in the mob and dose to that weight. Under-dosing is the fastest path to resistance.
- Targeted Selective Treatment (TST) — use FAMACHA scoring and body condition to identify and treat only animals that need drenching, leaving refugia untreated.
- Refugia — always leave some animals undrenched at each treatment to maintain a susceptible worm population in the system.
Worm Egg Counts
Drench efficacy testing via worm egg count reduction tests (WECRT) should be done every 2–3 years. Contact your vet or state agriculture department for guidance.
Products at TGRM
TGRM stocks Startect, Zolvix Plus, Tridectin, Matrix, Cydectin Oral Sheep Drench and more in our Sheep Products and Drench ranges. Our team can help you build the right rotation program for your flock. Gympie QLD — 07 5482 1692.
Related reading: How to Treat and Prevent Blowfly Strike in Sheep | Common Q&A for Queensland Farmers