Breeding for worm resistance

Worms cost the Australian sheep industry more than any other parasite or disease in lost production and treatment costs.

For effective worm control a multi-faceted approach is taken. One of the tools that can be used is to breed or purchase rams that are more resistant to worms.

A sheep that is more resistant to worms will carry fewer worms than the average of its flock mates. A worm egg count is used when calculating the worm resistance of an animal.

Australian Sheep Breeding Values (ASBVs) allow producers to effectively breed for worm resistance; they take into account not just the animal’s own performance, but that of its relatives as well.

A Worm Egg Count Australian Sheep Breeding Value (WEC ASBV) shows the expected difference in the worm egg count for progeny of different animals (with different levels of worm resistance).

For instance, let us compare two rams. If ram A has a WEC ASBV of –40% and ram B has a WEC ASBV of +20%, there is a difference of 60% between the two animals. If these 2 sires are mated to an even line of ewes and their lambs are reared together, you can expect that ram A’s lambs, on average, will have worm egg counts 30% lower than lambs sired by ram B.

Note that the result in the progeny is 30% difference, that is half of the original 60% difference between the rams, as the rams only account for half of the genes in the lambs, the other half coming from the ewes (in this case each group of ewes was similar for worm resistance).

While sheep in different areas of the country can be affected by different worm species, resistance to worms is relatively consistent across these species, that is, resistance to one type of worm species is highly correlated with resistance to the other worm species.

This allows a ram from northern NSW, selected for a high level of resistance to barber’s pole worm, to be used in another part of the country where scour worms predominate, with the assurance that the progeny will also be resistant to the scour worms they encounter.

To breed for worm resistance:

  1. Choose a ram source that provides rams with Australian Sheep Breeding Values (ASBVs) for Worm Egg Count (WEC).
  2. Choose rams that have a more negative value (e.g. a WEC ASBV of –50% indicates more worm resistance than a value of –20%, which in turn shows more resistance than a zero value. Animals with positive values for WEC ASBV are less resistant to worms, with the higher numbers being those most susceptible to worms).
  3. Also take into account ASBVs for other traits important to you to ensure a balanced selection.

More information can be found on the ParaBoss website

Practical Wisdom Notes / Fact Sheets

PW1.4 Parasite control in sheep

In this series:

  1. WormBoss: Australia's sheep worm control resource
  2. FlyBoss: resources for integrated flystrike control in Australian sheep
  3. LiceBoss: Australia’s sheep lice control resource

PW4.1 New Opportunities in Genetics and Genomics

In this series:

  1. Genomics and DNA testing: new tools for ram breeders to accelerate genetic gain
  2. Breeding towards a poll flock with the Sheep CRC Poll test

  3. Sheep CRC genomic test for Merinos —what are the benefits?

  4. Sheep CRC genomic test for maternal breeds —what are the benefits?

  5. Sheep CRC genomic test for terminal breeds —what are the benefits?

  6. Using Australian Sheep Breeding Values

  7. Fat and eye muscle depth in Merino breeding programs

  8. Benefits of Reproductive Technologies in  Closed Nucleus Sheep Breeding

  9. Merinotech: Strategies to achieve a 100% PP nucleus

  10.  Using selection decisions to improve ASBVs

  11. Genomic testing ram lambs helps increase rates of index genetic gain by 50% at Centre Plus

Products & Training Resources

ASBV Pocket Guide

ASBVs - A guide for ram buyers

Marketing your sheep with genetic data

Sheep DNA Testing Information

Reports, Articles and Presentations

Getting Started with Sheep Genetics

Sheep CRC Genomics Breakfast Workshop - LambEx 2012

News Releases

DNA Flock Profile cements the foundations for new flock
Developing a flock with the right mix of genetics can take generations to perfect, but DNA testing has put Geelong sheep and wool producer Jack Briscoe on the fast track to success. Mr Briscoe is only a year into breeding his own Merino flock after branching out from his sheep contracting business by cobbling together a mixture of 800 home-bred and yard-bought ewes as his breeding base. Click Here To Read Full Article »
RamSelect keeps track of flock performance at Parkes
The RamSelect app is not only assisting with better genetic decisions for the Dunn family of Parkes, NSW, but also assisting with their inventory management and industry benchmarking activities. The Dunn family, trading as Reedy Creek Partners, operates a mixed farming enterprise between Manildra and Parkes, and has been using the app since it was first launched in 2015 to assist with their ram selections. Click Here To Read Full Article »
Faster sheep DNA testing with new Australian GeneSeek lab
Sheep breeders using DNA testing to improve their genetic selections can look forward to faster turnaround times for test results thanks to Neogen Corporation’s decision to establish a genomic testing laboratory within Australia. GeneSeek Australasia, a wholly owned subsidiary of United States-based parent company Neogen, has acquired the assets of the Animal Genetics Laboratory (AGL), based at the Gatton campus of the... Click Here To Read Full Article »
Dramatic increase in early season searches on RamSelect
Inquiry for rams through RamSelect has never been stronger with a dramatic increase in the number of buyers searching the site so far this season compared to last year. More than 1129 ram searches were conducted on RamSelect during July, compared to just 154 at the same time last year. Click Here To Read Full Article »
DNA testing to drive new era of ewe competitions
The traditional ewe competition is set to enter the innovation era with the inclusion of DNA Flock Profile tests as part of next year’s Doug Bicket Memorial Ewe trial in Parkes, NSW. The Cooperative Research Centre for Sheep Industry Innovation (Sheep CRC) will provide DNA profiling for all flocks entering the 2018 competition, currently valued at $800 plus GST per flock, as a means of supporting improved genetic selection... Click Here To Read Full Article »
The predictive power of ASKBILL immortalised in song
An off-the-cuff comment and a spur of the moment decision to “have a go” have resulted in the sheep industry’s new predictive app, ASKBILL, being brought to life in song by Uralla musician Rhonda Brooks. Performed in a bush ballad style by Coffs Harbour musician Mal Winckle, the track is proving a popular addition to the ASKBILL website, helping to capture producers’ imagination as to the possibilities on offer... Click Here To Read Full Article »
RamSelect moves towards commercial business model
The popular RamSelect app will move to a commercial funding model ahead of this season’s ram sales, as a first step towards ensuring the genetic selection tool’s financial viability and availability to producers in the long-term. RamSelect Plus is an easy to use web-based application,, which allows ram buyers to find and rank rams based on Australian Sheep Breeding Values (ASBVs) that match their own... Click Here To Read Full Article »
Faster, more accurate DNA Parentage and Poll test to be released in July
Sheep breeders will have access to a faster, more accurate DNA test for parentage and poll/horn status from July, which also includes new features to detect a range of genetic defects. First launched in 2012 by the Cooperative Research Centre for Sheep Industry Innovation (Sheep CRC), the DNA Parentage and Poll test has proven very popular with sheep breeders seeking to improve their genetic selections through more precise... Click Here To Read Full Article »
‘Eye opening’ DNA Flock Profile delivers new breeding insights at Beechworth
DNA testing to obtain a genetic flock profile has changed the way Victorian Merino breeder Stuart Warner not only looks at his flock, but how he will approach his genetic selection decisions in the future. As a registered deliverer of Bred Well Fed Well and RamSelect workshops, Mr Warner has a close understanding of the power of Australian Sheep Breeding Values (ASBVs) as an objective genetic selection tool. Click Here To Read Full Article »
The power of prediction now in sheep producers’ hands – just ASKBILL
A new app launched today will give sheep producers the power to see their stock’s future, all from the palm of their hand. The app, available at, was made live to the public today, marking the start of a limited offering of the ASKBILL platform as part of a pre-release user feedback program – a vital step in putting the finishing touches to the product prior to ASKBILL’s full commercial release later... Click Here To Read Full Article »
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