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Manage drench resistance, don’t fight it

15 January 2003

Sheep farming periodicals only

Manage drench resistance, don’t fight it
(About 440 words)

The country’s largest drench manufacturer Merial New Zealand is advising farmers to get up to speed on parasite resistance.

Veterinary Technical Manager Dr Barry McPherson says resistance to all three drench families has now been reported in New Zealand.

“In most cases, only one parasite species develops resistance to one drench on one farm. But there are cases where a parasite is resistant to two drench families, or two or more economically important parasites are resistant to a single drench,” he says.

“This is serious. Resistance is irreversible. There are three drench families on the market and there are no new ones on the horizon.”

Dr McPherson says the development of resistance is an inevitable biological process but, through good management, its onset can be delayed and its impact can be minimised.

“It’s hard to over-state the importance of drenches to New Zealand’s high intensity livestock production systems,” he says.

“An animal health company might be expected to say this – but independent experts are on record as saying that up to a third of our sheep production is reliant on effective drenches.

“There has never been a pastoral farming system which is as productive as New Zealand’s. With our present knowledge, it is difficult to achieve high productivity without using drenches.”

Dr McPherson says the science behind resistance management has advanced in recent years and new information regularly comes available, but one key principle remains unchanged:

“Resistance is a family affair – if parasites develop resistance to one member of a drench action family, then they become resistant to all members of that family.

“If a farmer deals with resistance in a flock by using another drench from the same action family, then the problem becomes worse.

“It doesn’t matter whether a new drench is more potent, more persistent, or more anything else, if it’s from the same family it won’t automatically resolve the problem!”

To achieve high levels of parasite control, to delay the onset of drench resistance, or minimise its effects once it has occurred, an integrated pest (worm) management programme should be adopted.

“Drenches should be used strategically in association with other management techniques – they should not be the sole parasite control method on the farm,“ advises Dr McPherson.

“We also strongly advise the use of a triple combination drench and good drenching technique.”

He says it’s important for farmers to come up to speed with resistance and to develop a parasite management plan in association with their animal health advisors.

This may involve drench selection, monitoring drench effectiveness, reviewing grazing systems, selecting replacements from lambs with low faecal egg counts and the use of specialist lamb finishing crops.


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