Trials by the Co-operative Research Centre for Sheep Industry Innovation and the WA Department of Agriculture and Food in the Great Southern, show treating only those sheep with worm symptoms could delay the development of drench resistance.

Department senior veterinary officer, Brown Besier, said the findings were now being used to develop practical strategies for farmers to target their worm treatments, rather than treating their whole flock.

"By treating only those sheep that show signs of worms, farmers are able to create a section of their flock with a 'refuge' for non-resistant worms," Dr Besier said.

"This helps to delay the development of drench resistance in the flock, while at the same time using less labour and reducing treatment costs."

While the research showed some production losses from targeting worm treatments, they were not considerable.

"In the Mount Barker trial involving two groups of 150 head of 12-month-old ewe lambs, the target treatment flock's weight gain was 5pc below the flock that received a normal treatment," Dr Besier said.

"Never the less, the target treatment group still achieved good weight gains and have since caught up with the normal group.

"Importantly, there was a benefit in reducing the rate of resistance development due to the targeted treatment."

Further research is now focused on identifying what portion of the flock should receive targeted treatments at different times, based on the flock worm egg count and the body condition score.