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Deer farmers are being advised to use three drench families in combination to keep parasites under control.
It followed four years of research showing that internal parasite resistance was becoming widespread across the industry.
The use of a single drench family - mectins - applied as pour-ons, along with poor application technique, were the cause of resistance, Deer Industry New Zealand (DINZ) producer manager Tony Pearse said.
''Replacing a mectin pour-on with an injectable can dramatically improve growth rates, but the best bet, based on recent on-farm trials, is to use a triple mix: a mectin injection, plus a white/clear combination oral drench,'' he said.
He knew of farmers who had increased weaner growth rates from 250g a day to 400g a day by switching to a mectin injectable. Better weaner health and a reduction in losses from yersinia and Johne's disease had been reported, too.
Before using the preferred triple mix, Mr Pearse said farmers should consult their veterinarian. Southland research veterinarian Dave Lawrence, who has conducted the parasite control trials, said deer on all farms tested so far had internal parasites resistant to mectin drenches.
Dr Lawrence advised farmers to adopt integrated parasite management that reduced the need for drenching and the risk of resistance developing. That includes quarantine drenching of incoming animals, minimising adult drenching and grazing with other species.
The trials have been funded by DEEResearch - the agency that channels farmer levies into research - with additional funding from New Zealand Deer Farmer Association branches and the Elk and Wapiti Society.