Avoiding anthelminthic (worm drench) resistance is balancing optimal stock performance and reducing exposure of parasites to anthelmintic products.
The brief recommendations below are aimed for optimal/near optimal young stock performance while avoiding unnecessary/ reduced exposure of parasites to anthelmintic products. Discuss on farm implementation with your vet.
General notes on worm drenching;
We strongly recommend doing a Faecal Egg Count (FEC). This is to determine worm challenge and allows to choose timing for an anthelminthic treatment.
Calves on pasture (targets Cooperia)
From 6 months to 18 months (targets Ostertagia)
Over 18 months – when indicated
The objective of refugia is to leave some parasites unexposed to drench product so they can breed with parasites exposed to drench. Parasites still alive after anthelmintic exposure will have resistance to that anthelmintic and we don’t want to populate the grass with just resistant worm larvae.
There are a few methods of achieving refugia on your property/with your animals.
Pasture length – Shorter pasture length means more worm larvae are eaten because they are in the bottom 2 cm of pasture. Fully fed animals are more resistant to fighting off a parasitic burden.
Quarantine drench – Pick a product that has more active ingredients compared with your usual product of choice. For example, if you normally use a dual active then use a triple active for the quarantine. Apply the anthelmintic on arrival and hold on a designated quarantine area for 24+ hours.
Pasture management to reduce overall challenge – cross grazing with cows or other stock, 3+month spelling, cropping, pasture renewal.
Note: a FEC estimates the burden of adult worms in the animal (not immature worms).
Check your anthelmintic is effective by taking ten to 15 faecal samples seven to 12 days after oral drench (12-14 days after if using pour-on).
Use the same method if sampling ahead of drenching to determine timing of treatment.