A Hamilton farmer who introduced chemical-naive worms to his maternal shedding composites Nudie breeding operation said that drenches have been significantly effective since he began the practice.
Over the span of three years Matt Kelly, Low Footprint Lamb, Croxton East, had been able to change chemical resistance from an original worm population to chemical naive worms, with "ewes going from one drench per year at lamb marking to no drench at all".
Mr Kelly, who said he had not heard of any other farmers deliberately drenching their sheep with worms, said the process had "a massive impact".
The process began when Mr Kelly met Josh Fittler from Resurrect Refugia, who specialises in preventing and reversing anthelmintic resistance in internal parasites, about five years ago.
"It dawned on me that I have a lot of trouble getting my young sheep wormy enough to put enough challenge on them to do an individual worm egg count before I make sire selection, and I need to be able to select those young rams because they're the most advanced genetics I've got," he said.
"I then said to [Josh], 'Could I buy worms to infect my young sheep in a controlled way?' and he said he could set that up easily,"
"Four weeks after we drench with worms, we checked our worm egg counts, and they were certainly climbing, and then six weeks later, they were loaded right up to about 2000-3000 eggs per gram," Mr Kelly said.
"We then individual egg counted them, drench them, and they were in a position to be healthy again, and grow fast."
Mr Kelly said it helped get early post-weaning weight measurements and other accurate growth data, allowing him to confidently drench with those chemical naive worms.
"I also need, in that timeframe [of six weeks], to also get my sheep extremely wormy so that I can test their worm egg count, and of course, they don't grow once they have a gutful of worms," he said.
"The quicker I can get them from no worms to loaded up with worms and back to no worms, the better it is for us.
"If you let nature do it, it can take three to four months."
While purchasing more worms in recent years had not been required because of a local wet season, Mr Kelly said drenching with chemical naive worms worked to where he had nearly 100 per cent effectiveness with Levamisole and Moxidectin drench types.
Drench resistance tests with Bendimadazole, Abamectin, and Ivomec also recorded an effectiveness of 96pc or higher.
Veterinarian Brodhi Carracher, who analysed the resistance tests, said he had never been involved with a test where the farm exposed sheep to worms.
"It was interesting to see the results where Matt now has worm populations on his farm that are a lot more susceptible to the commercially available drenches," he said.
"This is partly due to the fact that he's bought in some refugia, which is the naive worm population to those drenches and has diluted out the other worms that were previously resistant to some of the chemicals."
Dr Carracher said farmers should consult their vet before taking on a regime similar to Mr Kelly's, and it may be challenging for some farms, depending on many factors.
"It's an option for those farms that are seeing a lot of resistance for the commercially available drenches with chemicals," he said.
"Matt has done an ambitious thing that's worked for him in bringing in the naive worms, so that he can select those sheep that can combat and give more resistance to worms without drenches, but I wouldn't be doing it straight up."
Mr Kelly acknowledged there was some animal welfare pressure on his stud but he had a process to monitor and act accordingly if sheep became stressed.
"We also don't do footbathing, and if the animal is not coping, I take it out [of the stud]," he said.
"We'll make it a commercial sheep, and then if it needs to be footbathed, well, it is footbathed, and if it needs to be drenched, then it is drenched, so they get the animal health they need.
"Certainly, there'd be a production cost to all that as well but we do this to highlight the sheep that are robust enough to cope and find the softer sheep that get pushed out of the stud."
