Vet's View with Ravensdown Vet Gavin Goble BVSc MRCVS

There is no doubt internal parasites, commonly referred to as ‘worms', are a major cause of lost productivity in NZ agriculture.

Costs, estimated at hundreds of millions of dollars per year, include lowered production, illnesses, deaths and costs of drenching (product and labour). It is estimated 30 per cent of sheep production are reliant on worm drenches.

Now, with various worms becoming resistant to the chemicals we use, costs will only increase further. There are many figures quoted; for example, using an ineffective drench decreases lamb carcass value by 14 per cent, compared to using an effective drench.

To maximise productivity in the short-term, it is important worms are adequately controlled, but what is good for worm control often selects for drench resistance. Alternatively, what is good at slowing the development of resistance may compromise worm control. So are you managing worms or managing resistance? For long-term sustainability, the ideal position is somewhere in between.

The industry initiative called Wormwise sets out ‘agreed principles' (facts about worms that most experts agree on), identifies high risk activities that speed up drench resistance and outlines strategies for sustainable worm control.

In short this means: avoid or minimise known high-risk activities; for example, routine drenching of adult animals, using long-acting products, drenching at less than 28-day intervals. Maintain refugia; for example, avoid drenching onto clean pasture, leave some animals in a mob un-drenched. Know your drench resistance status and only use fully-effective drenches.

Many experts state a fully-effective drench should be a combination drench. However this is an over-simplification, as single-active drenches can have their place if they're still working 100 per cent. The drench resistance status on every property is different, and drench resistance is a farm-specific and not a flock/herd-specific issue because the majority of the worm population is on the ground.

To determine what drenches are fully effective on your property, two tools are available; a simple ‘drench check' to check the effectiveness of a drench (usually 7-12 days after drenching); and a full Faecal Egg Count Reduction Test (FECRT), which looks at several drenches at once. Talk to your animal health adviser to discuss further.