Worm testing for ram breeders

Commercial sheep breeders, especially in the higher rainfall areas of Australia, increasingly want sheep that better resist parasite burdens. As a result, the number of ram breeders offering rams with genetic information about worm resistance is also increasing.

What information is provided to the ram buyer?

Worm Egg Count Australian Sheep Breeding Values (WEC ASBVs) are the measure that breeders should use when choosing rams for worm resistance.

Sheep that are more resistant to worms are able to

• prevent more worm larvae from establishing and maturing
• suppress the egg laying of worms that do mature

As a result, more worm-resistant sheep will have a lower worm egg count than their less resistant flock mates. Therefore, worm egg count is a direct measure indicating worm resistance. The more direct a measure is for the trait to be improved, the faster improvement can be made through selection.

A WEC ASBV is a far more accurate measure than just using a raw WEC figure because ASBVs take account of:

• the worm resistance of the animal’s relatives
• the age, sex and other management factors of the animal
• the heritability of worm egg counts and correlations with other traits

They also let you objectively compare animals from different studs or within a stud where they are from different ages or have had different management.

Raw WEC figures cannot do any of this and are not considered suitable for most ram selection purposes.

WEC ASBVs represent the % difference in worm egg count (WEC) an animal is expected to have based on its genes. For example, compare Ram A with a WEC ASBV of -50% to Ram B with a WEC ASBV of +30%.

The difference between them is 80%, but as the ram will only contribute half of the genes to its progeny, the difference is halved to 40% in the progeny. Ram A has the lower more negative value, so his progeny, on average, will be expected to have 40% lower worm egg counts than the progeny of Ram B.

What data needs to be collected by the ram breeder and when?

An individual dung sample needs to be collected from each of the animals to be tested. For the most accuracy:

• Test all appropriate animals within the drop—both males and females.
• All animals within a management group (e.g. based on sex) should have received the same prior management (including being drenched at the same time and run together).
• Test between weaning and 1 year old (around the time that worms are a problem on your property or region), however, in areas where worm-risk is very high after weaning, wait until after the next drench before conducting the test to avoid susceptible weaners being put at risk.
• Test when the mob has no more than 10% animals with a zero worm egg count.
• Samples from each management group must be collected on the same day.
• Samples must be tested through a suitable laboratory if data are to be analysed by Sheep Genetics.

To provide data to Sheep Genetics, you must follow their quality assurance procedures. Further details and refinements can be obtained from Sheep Genetics.

How are the data analysed and presented?

SheepGenetics members can have their data analysed with their other performance data at no extra cost. Worm egg count data for each animal are submitted (without Nematodirus results, if these have been provided separately by your laboratory) in a similar manner to other data.

These data will be analyzed in a normal LAMBPLAN or MERINOSELECT analysis run and your results will include WEC ASBVs. If your data are accurate enough for publishing (depends also on the amount of similar data from relatives) WEC ASBVs will be shown for weaning (W), post-weaning (PW) and yearling (Y) ages.

When providing information to your clients, generally choose the WEC ASBV age when your clients have the most worm problems.

Note that cross-resistance exists across worm species, therefore if a ram is tested, for example, in Victoria with few barber’s pole worms and is shown to be highly resistant, but then the ram has progeny in northern NSW, where barber’s pole worm predominates, the progeny will also be resistant to the barber’s pole worm in their environment.

What if you are not a Sheep Genetics member?

Firstly, if you have chosen not to be a Sheep Genetics member for your other performance data, be aware that you can still become a member and only have WEC data analysed (with other suitable pedigree information).

Ram breeders sometimes ask whether it is at least worthwhile using the raw WEC figures for their own within-stud selection purposes, even if they don’t publish them with sale rams. The answer essentially comes down to value.

The accuracy of raw WEC figures is much lower than, for instance, using raw fibre diameter figures, however the cost of conducting the WEC tests is much dearer (around $3 or more per head, depending on your laboratory).

If you are serious about breeding for worm resistance then it makes sense to gain the most from your test costs and have the data analysed by Sheep Genetics.

What about gene tests for worm-resistance?

The Sheep CRC, through its Information Nucleus and genomics pilot project have shown that gene tests can vary widely in accuracy across breeds and across populations within breeds—even across the different sheep bloodlines within Australia.

Currently there are no gene tests for worm resistance that have been tested in large populations of Australian sheep.

The WormSTAR test (Pfizer) was tested in New Zealand. Independent verification for Australian sheep has not been published.