Friday, 21 May 2021 07:55

Change to deal with drench resistance

Written by  Staff Reporters
Drench resistance is controllable with the appropriate management. Drench resistance is controllable with the appropriate management.

An emerging drench resistance issue has been identified on Beef + Lamb New Zealand's Future Farm Lanercost in North Canterbury and a holistic plan is being put in place to protect future livestock productivity.

Lanercost's management team was alerted to a problem last spring when lamb weaning weights were below what would have expected in an otherwise outstanding season.

Consultant vet Ben Allott looked at a number of possible issues, but parasitism identified during a post-mortem was deemed to be the most likely cause of poor lamb performance.

"I don't think drench resistance is limiting production here, but poor drench selection decisions could have. But I suspect a wider systems issue is all leading to a parasite challenge problem."

Allott says Lanercost is on the precipice in that it does have multi-drug resistance Osertagia and if the system is set up so that thee is a continued reliance on drench or chemical input, then the situation will rapidly deteriorate.

"The onus is on us to implement farm management that will halt the progression of drench resistance and ultimately we would like to improve." Tests have shown some resistance to benzimidazoles (BZs) and levamisoles (LVs), but Dr Dave Leathwick from AgResearch believes this resistance is manageable with the appropriate management.

This is an early warning, and you need to pay attention to it."

Leathwick says the management team is fortunate to have the option of drenching as they have effective drenches available to them, which is not the case on a growing number of farms.

Lanercost is far from unusal in having a degree of drench resistance. Leathwick says there are very few farms in New Zealand that don't have resistance of some sort.

"There might be 10% of sheep farms that don't have any resistance, but that is possibly being generous and in cattle farms there are none. I am sure every cattle farm in NZ has Cooperia resistance."

The focus at Lanercost is to develop long-term sustainable policies that will enable them to maximise productivity and profitability in their sheep flock without relying on drenches.

The management team on Lanercost will soon be documenting their parasite management decisions on the Future Farm page of B+LMNZ's website. This will give farmers the opportunity to see what worked, what didn't, and lessons learnt.

Preventing Further Drench Resistance

The following management strategies are being implemented on Lanercost to halt more drench resistance:

  • Sell non-replacement lambs as store.
  • Use Faecal Egg Counts to closely monitor worm burdens and extend the interval between replacement lamb drenches for as long as possible. Threshold FEC levels will be determined to trigger a drench treatment.
  • Place more rigor around Body Condition Scoring. This will be carried out around weaning, pre-tup, late autumn and pre-lamb.
  • Actively cros-graze using cattle to clean up sheep pastures and vice-versa.
  • No routine pre-lamb drenching. Decisions to drench pre-lamb will be made based on FECs.
  • No long-acting capsules to be used.
  • Quarantine drenching all stock coming onto the farm with an appropriate drench, e.g. Zolvix Plus or Startect.
  • Maintain a source of refugia (undrenched ewes are ideal).
  • Recognise the risk of 'clean' pastures or forage crops as a potential breeding ground for resistant worms and take appropriate management action e.g. follow the grazing with undrenched ewes or tail-end two-tooths.
  • Include drench resistance in ram selection criteria.

For more information on parasite management go to

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