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.

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