Problem hidden from view

01-08-2010 | Jackie Harrigan

Two-thirds of New Zealand sheep farmers have a drench-resistance problem and it is impossible to find a beef farm without resistance to one of the drench families.

AgResearch applied parasitology researcher Dave Leathwick told farmers at the Massey University farmers' science day last month that the cost of drench resistance in one trial was a 20% drop in lamb growth over summer. A recent repeat trial showed a 14% loss of carcase value, totalling 2.8kg between January and May.

"A few years ago when lambs were worth $50 each we estimated losses at $1000-$2000 per 2000 ewes from drench resistance. Drench resistance can cost you thousands of dollars without you being able to see it."

He says he is getting tired of farmers telling him they haven't got a resistance problem because "their stock look fine".

Leathwick urged farmers to safeguard the efficacy of the cheaper drenches because without them they will have to go to the new expensive drench family.

"Farmers don't realise that they simply can't see the problem."

Every farmer needs to regularly test drenches on their farm to make sure they are not wasting money, he says. A drench test will cost about $1000 and needs to done every three years, thus $300 a year. Leathwick says the cost of undetected resistance is easily $5000 a year.

Minimising drench resistance can be managed on an industry-wide basis, he says, citing Landcorp which has been testing since early in the 1990s. In a recent survey 83% of Landcorp farms passed all test groups compared with 36% of farmers nationally when tested for drench resistance.

"The benefit for them today is substantial; they are able to use cheaper products and know they are going to work."

Leathwick told farmers to identify whether they have a problem, then use the three basic principles to keep resistance at bay and safeguard the effective drenches they have.

The first principle is to identify and mitigate high-risk practices. Using long-acting products (capsules, injections and pour-ons), treatment of ewes around lambing, drenching on to low-contamination pastures and running a monoculture of young stock and inadequate quarantine procedures are all high-risk, he says.

Preserving susceptibility is the second principle - and Leathwick suggests using refugia techniques to do this. Refugia means a selective breeding programme to dilute the gene pool, getting resistant genotypes to mate with susceptible genotypes.

There are many possible ways of getting refugia, he says, including extending the interval between treatments, drenching before or after the move, leaving some animals undrenched (drench the tail end and leave the rest), or leaving ewes undrenched and rotationally grazing them over the same pasture as the lambs.

Deciding how much refugia is needed is a difficult question to answer, Leathwick says, because it is affected by the efficacy of the drench. To get a 10-fold dilution of resistant eggs if the efficacy of the drench is 99.9% requires leaving only 1% untreated as refugia, but if the efficacy of the drench drops to 95% then 34% of the flock needs to be left untreated as refugia.

The use of combination drenches is the third principle. For farmers with a resistance problem the combination will maintain worm control while slowing the growth of resistance.

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