Best practice parasite management
November 2022

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.

  • Drench to weight – choose heaviest weight in the group. (Take care with levamisole and abamectin safety margins.)
  • Clean and calibrate guns before each use.

Calves on pasture (targets Cooperia)

  • Worm drenching is not needed during the first 3 weeks on pasture.
  • We recommend doing a FEC to determine first indicated drench.
  • Drench every 4 weeks for 4 drenches.
  • Products of choice:
    - Triple oral (Ivermatrix) from 80kg BW whilst stocks last.
    - Dual oral – Turbo initial or advance.

From 6 months to 18 months (targets Ostertagia)

  • We recommend doing aFEC to determine timing of drench.
  • Products of choice:
    - Triple oral (Matrix C) or
    - Dual injection (Eclipse E) or
    - Dual pour-on (Eclipse Pour-On)
    - Dual pour onTurbo initial pour on

Over 18 months – when indicated

  • Animals with low BCS.
  • Challenged animals – poor nutrition/diarrhoea/sick.

Other considerations – Refugia

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.

  • When drenching, leave the top 5-10% of the mob undrenched if looking fit and strong, and feed supply is good. Mark animals that are left undrenched to avoid them missing out twice. Alternatively growth rates can be used for leaving animals undrenched.
  • Return animals to same paddock for 24+ hours after drenching.
  • Cross-graze youngstock with (older) untreated cows.

Other considerations – Management

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.

Faecal egg count (FEC) and DrenchCheck

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).

  • Collect fresh warm faeces, 3 tsp per sample (ear tag numbers are not needed for a drench check).
  • Store individually in freezer bags/containers.
  • Keep cool and/or in the fridge, especially if overnight (not freezer).
  • If eggs are detected, there is an indication to investigate further.

Use the same method if sampling ahead of drenching to determine timing of treatment.