Sheep worms – breeding worm resistant sheep

Page last updated: Friday, 30 June 2017 - 11:41am

Please note: This content may be out of date and is currently under review.


Ram breeders

  • Ensure a controlled and adequate worm challenge.
  • Select for reduced faecal WEC at weaning and/or hogget age.
  • Selection for reduced scouring in the winter rainfall environment is best done from yearling to hogget age during their second winter challenge period.
  • Increase the selection response by using trait ASBVs incorporated into an overall selection index.
  • Allocate at least 30% of the total selection pressure to faecal WEC selection and a further 20% to scouring if necessary where worms and dags are a problem.
  • Ask for information on worm resistance where a sire was progeny tested in a sire evaluation scheme.

Ram buyers

  • Ask for ASVBs and indexes to suit your breeding objective. If ASBVs are not available, then ask for information on the average faecal WEC of the test group (to check that there was adequate selection pressure from worms when the worm resistance selection was done).
  • Ask for information on a ram’s deviation from the group mean rather than actual faecal WEC (this allows ‘within group’ comparison such as considering if the ram is better than average).
  • Ask for information on worm resistance where a sire was progeny tested in a sire evaluation scheme.


  • Any genetic improvement program, such as selecting on fibre diameter to reduce the average micron of a flock and selecting sheep for worm resistance will take time before major improvements are apparent.
  • By including worm resistance in breeding programs now, significant progress can be made before existing drenches become totally ineffective and it is no longer profitable to run sheep because worms have become uncontrollable.


  • Australian Wool Innovation
  • Rylington Merino contributors
  • Sheep Genetics
  • Curtin University
  • Department of Agriculture and Food, Western Australia (DAFWA) staff

Contact information