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Drenching Sheep

Effective drenching will control a worm burden and also minimise the incidence and impact of drench resistance. There is an abundance of information available in relation to when to drench, what to drench with, pasture management and drench rotation for sustainable worm control in sheep. However this information does not always cover the crucial aspects such as management of sheep prior to and after drenching to maximise the worm control outcome and minimise any post-drenching losses.

Overdosing sheep or lambs with drench or incorrect pre or post drenching management can be fatal so it is important to consider how you drench as well as ‘when’ and ‘what with’.

Key drenching principles include:

  • Always read the product label for both dosage and safety information
  • Calibrate drench guns to ensure the correct dose is administered 
  • Calculate the dose based on the heaviest sheep in the mob, use a scale rather than visual weight estimate.

Some drenches, such as levamisole and OP’s have lower safety margins and it is very important to follow dose recommendations and precautions with these actives.

  • When there is significant variation in weights within the mob (10kg for lambs, 20kg for adult sheep) draft into heavier and lighter lines and set the gun to the appropriate dose for each group.
    DO NOT give ‘½ squeeze’ for smaller or a ‘bit extra’ for heavier sheep – this is not accurate and does result in sheep losses (from overdosing) or poor worm control (due to under dosing).
      • Ensure correct pre-drenching and post-drenching sheep management:
             DO NOT restrict feed or water in heavily pregnant, stressed or poor sheep.
  • Levamisole and OP (organophosphates): DO NOT hold off feed overnight as this can and does result in post-drenching losses. Always allow access to feed and water before and after drenching. DO NOT treat stressed or debilitated sheep.
    • Ensure the drench gun is correctly inserted with the tip of the nozzle over the base of the tongue. Do not rush; allow time for the dose to be swallowed before moving onto the next sheep.
    • Quarantine drenches are vital to prevent bringing resistant worms onto a property. Always remember newly arrived sheep have been off feed and water for varying periods of time. Sheep should be allowed access to feed and water overnight before quarantine drenching the following day.
    The ideal quarantine drench should contain at least 4 unrelated actives. Particular care should be taken if you are considering including an OP drench as part of the quarantine drench, as the risk of toxicity could be increased by the stress, transport and limited food and water in preceding days.
    After quarantine drenching hold sheep in yards / secure paddock for 3 days and ensure sheep have adequate feed and water post-drenching.
    • Do not drench lambs in the marking cradle or sheep ‘on-the-board’ at shearing/crutching. This increases the risk of inhalation, leading to pneumonia and possibly death. This can also stimulate the oesophageal reflex causing the drench to by-pass the rumen making it less effective but also causing it to be more rapidly absorbed and increasing the risk of toxicity, particularly with levamisole or OP drenches. (Lambs generally do not require drenching at marking time – conduct a WEC (worm egg count) to determine the need to drench).

Prior to every drenching, fully review dose details, safety information and precautions on the product label.

Wormboss is an excellent resource for all things relating to sheep worms, sustainable worm control, drenching programs and drenching practices.

Author Jane Morrison BVSc, Technical Veterinarian, Coopers Animal Health.