Good management can delay drench resistance

Good management can delay drench resistance

The country’s largest drench manufacturer, Merial New Zealand, says farmers should not rely solely on drenches to control worms.

Veterinary Technical Manager, Dr Barry McPherson, says farmers should integrate as many ways of controlling parasites as possible into their farm management.

“Integrated management practices play an important part in parasite control, particularly in delaying the onset and impact of resistance.

“Drenches are still essential. Management alone will not control parasites, but it will reduce the need for their use and prolong the working life of drench compounds.”

In New Zealand, there are strains of all economically important roundworms that are resistant to one or more of the three drench action families.

“Because there are no new drench families on the horizon, everyone must use drenches responsibly. Resistance is forever – it cannot be reversed.

“To maximise productivity at the same time as delaying the onset of resistance, the best strategy is to adopt integrated management practices and to use a triple combination drench before resistance is present,” Dr McPherson says.

“Triple combination drenches are more effective against a wider range of worms than any single or dual combination drench.

“It’s simple mathematical odds. If one worm in 1000 carries a gene for resistance to one action family, then one in 1,000,000 will carry genes for resistance to two action families, and one in 1,000,000,000 will be resistant to three families.

“The chances of a resistant parasite surviving a triple combination drench are extremely low.”

On the management front, Dr McPherson says there are many practices which farmers can adopt. But probably the most important of these is good nutrition.

“Well-fed animals are much more resistant to parasite infections than those that are poorly-fed, so high quality feed should be available at all critical times.”

Alternating grazing sheep with cattle or deer will reduce the worm challenge that sheep, especially lambs, are exposed to, he says.

“Sheep parasite larvae are destroyed when they are eaten along with pasture by cattle or deer.

“Farmers should also graze drenched lambs on pasture ahead of mature sheep, and use specialist lamb finishing crops with high levels of tannins, such as Lotus corniculatus, which act like natural anthelmintics.

“Selecting replacements from lambs with low faecal egg counts will build up the flock’s resistance to roundworms.”

Good drenching technique will also help delay the development of drench resistance.

Dr McPherson says under-dosing as a result of poor technique is one of the major causes of drench failure and may lead to drench resistance.

“Drench guns should always give the correct dose and should be calibrated on the basis of the weight of the heaviest animal in the mob.”

Once resistance is present, he says it should be managed so the killing ability of the drench is maintained for as long as possible without compromising control efforts. Once again, this means use of a triple combination drench and the adoption of intergrated parasite management techniques.

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