Internal parasite management: Cost-benefit analysis | Beef + Lamb New Zealand
Animal Health

Internal parasite management: Cost-benefit analysis

Given that you don’t “see” drench resistance in animals until the problem is extreme, what is the cost that we cannot see? AgResearch’s Dr Dave Leathwick has crunched some numbers – and the figures are so compelling, they don’t seem real.
Friday, 21 October 2016

Despite most New Zealand farms now having some degree of drench resistance, very few farmers ever test for drench efficacy. This means many farmers are using drenches that are not working as expected. 

Spending up to $1500 (ideally repeated every three years) on a drench test to manage a problem that you cannot see is a hard pill to swallow.

So, what’s the cost of not testing?

We calculated the numbers, based on a farm finishing 3000 lambs annually. 

Trial data – comparing lambs treated with a drench compromised by resistance, with lambs treated with a fully effective drench revealed:

  1. All the lambs looked fine. You could not pick the two groups apart.
  2. The production loss in the lambs treated with a compromised drench was 10-15% carcass value at the works.
Per lamb annual cost of testing Per lamb cost of resistance
17 cents

($1500 every three years – so $500 annually. $500 across 3000 lambs = 17c/lamb)


(Based on a $90 lamb, with a 10-15% production loss = $9-$13.50/lamb)

It makes no sense to save 17c per lamb on a proper drench test every three years, when you could be losing at least $9 per lamb in carcass value.

The story is the same in cattle. That is, the cattle look great, yet the carcass yield shows their growth has been compromised.

What about combination drenches?

Yes, in the short term, these are a good option. They contain multiple active ingredients, so they are more likely to deal with any resistant parasites. 

However, by now, resistance to more than one active ingredient is widespread throughout New Zealand.

Which drench is most effective on your farm?

You don’t know, unless you carry out a drench resistance test (FECRT).

Talk to your vet or farm consultant. Your $1500 outlay will yield you the best return on investment you’ll never see. 

If you have questions in relation to internal parasite management, please email them to and we will address these at the conclusion of the series.

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