Testing for drench effectiveness prompts change | Beef + Lamb New Zealand
Animal Health

Testing for drench effectiveness prompts change

Ian and Paul Roy are converts to sustainable parasite management, after a drench resistance test revealed they were throwing good money away on drenches that weren’t doing the job.
Monday, 3 July 2017

Like most farmers, Macraes’ Paul Roy doesn’t like throwing money away. But, that’s exactly what he was doing a decade ago.

In 2006, Paul got a drench resistance test carried out and discovered that his flock had significant resistance to the ivermectin drench family. This meant that internal parasites (ostertagia species, mainly) were not being well controlled. 

This was the start of raising awareness around sustainable worm control measures for Paul and his brother Ian. 

“We’d been using single-active capsules across the whole flock, but now only use them strategically. Throughout the year, we use the most effective combination drench.”

Roys farm 1600 hectares in the Moonlight Valley, between Middlemarch and Macraes. The property is 500m above sea level with a relatively short growing season, thanks to dry spring and summer seasons. It’s intensively stocked, wintering 7800 Coopworth ewes and 2000 replacement hoggets, and finishing sale lambs, as the season allows. They make and feed a lot of silage, to even out the seasons.


Paul attended a Beef + Lamb New Zealand (B+LNZ) Wormwise workshop several years ago and works closely with vet Dave Robertson to keep on top of internal parasites. 

His key advice to other farmers is: “Talk to your vet. We use our vet a lot and he has overview of our internal parasite programme. Second, do a resistance test. You’ve got to know if your current drench is working. Otherwise, you may be wasting money on cheap drenches.”

Resistance tests

Roys carried out resistance tests again in 2010 and 2017 and resistance levels have held. Father Bruce also takes faecal egg counts regularly, so drenching can be targeted. More recently, Roys are getting their heads around the concept of refugia, which involves consciously leaving some stock undrenched each year. 


Refugia builds up the population of parasites that are still susceptible to drenches. While it seems counterintuitive, it means that over time the problematic resistance parasites become a smaller proportion of the overall parasite population. This keeps more drenches effective for longer on the property. Healthy, robust stock are used for refugia, so there is minimal impact on overall production. 

Heading into winter, pre-tup, ewes receive a combination drench. The pre-lamb worm control programme can vary, depending on season and ewes condition. Generally, all ewes receive a triple mix oral drench and the bottom 20 per cent are capsuled with a double-acting drench.

Pre-weaning, lambs receive triple combination plus tape, then Arrest double action at weaning three weeks later. 

Hoggets receive novel-acting drench Startect as they move onto winter crop as an exit drench strategy. These new actives can be used to extend the life of the current drench families. 

Request a workshop

There is a lot more to worm control than simply choosing which drench to use. If you want to understand how to tackle internal parasites in a sustainable and profitable way, request a B+LNZ Wormwise workshop in your area by emailing B+LNZ Extension Manager, Laura Gray, at Laura.Gray@beeflambnz.com