Drench resistance to all drench families is increasing.
Drench resistance development can be evaluated and steps can be taken to minimise it.
Methods of delaying drench resistance
The concept of refugia refers to a worm population not exposed to drenching.
Using undrenched animals to create a refugia population will ensure there are non-resistant worms around, and can be a useful tool in delaying resistance.
The aim is to ensure this reservoir of drench-susceptible larvae significantly out number drench-resistant larvae on the pasture.
Balancing the need to reduce the risk of drench resistance and yet manage worms so that production and animal welfare do not suffer involves compromise.
Leave a portion of the mob undrenched – try 5% up to maximum of 10% of the mob. Do this only when conditions are good in terms of animal condition, feed supply and during lower risk periods.
Move undrenched ewes onto lamb paddocks to dilute any resistant worms left behind.
Return lambs to ‘infected’ paddocks after drenching for one week to dilute resistant worms with those already on pasture.
There are many other ways to create and maintain refugia depending on the farm system.
B. Quarantine Drenching
One of the most likely ways to get drench resistant worms on your farm is through introduced stock. A quarantine drenching policy must be part of your worm management plan. Apply the quarantine drenching policy to all new stock brought onto the farm, including rams. It doesn’t matter how long the animals will stay on the farm – it takes less than a day to deposit resistant worms on pasture.
C. Other Tips for Delaying Drench Resistance
Avoid drenching lambs onto “clean” pasture unless you have a plan to introduce/maintain refugia.
Avoid drenching more frequently than every 28 days unless there is a special need.
Avoid treating the whole flock pre-lambing with a long-acting drench.
Use effective combination drenches, even if drench resistance has not been identified on your farm.
Weigh animals so they are not under-dosed.
Apply the correct dose using correct technique as per the label instructions.
Use FEC to check the effectiveness of drenches, at least annually for each drench used.