Online Learning: Western Australian Winter Rainfall—Breeding for Worm Resistance

Resistance to worms: Sheep that are resistant to worms can prevent some or all worms from establishing and as a result have lower worm egg counts.

Resilience to worms: Sheep that are resilient to worms can grow and produce with less ill effects from worms. An animal’s performance for a particular trait, such as growth, will also be dictated by its genetic merit for that trait. So, when comparing two animals with similar Australian Sheep Breeding Values (ASBVs) for growth, a more resilient animal will perform better than a less resilient animal when both have high worm burdens. It is independent of worm resistance so must be selected separately by choosing better production performance.

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Worm egg counts of rams need to be measured; the mob should be run and managed together so that their results can be compared.

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Worm Egg Count Australian Sheep Breeding Value or WEC ASBV.

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A sheep’s resistance to worms directly affects worm egg count. In more worm-resistant sheep (those with lower WEC ASBV)

1. Fewer worm larvae eaten with the pasture are able to establish in the gut and become adults. In sheep with poor immunity, 50–60% of the worm larvae are able to become adults in the gut. In sheep with good immunity, this establishment rate may be as low as 5–10%.
2. The immune response is able to reduce the number of eggs laid by established female worms.
3. The immune response develops to a stage where established adult worms are expelled from the gut of the sheep over a period of days to weeks.

This results in fewer eggs passing in the dung of more worm-resistant sheep, and the pasture will also be less contaminated than by less resistant sheep.

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Dag resulting from scour worms is not an indicator that the sheep are more or less resistant to worms. It results from a hypersensitivity response in sheep that have previously developed immunity, and who are subject to a larger larval challenge after a period of little challenge (as can occur after the autumn break following a dry summer).

There is no relationship between sheep who suffer from hypersensitivity scours and those with higher resistance to worms.

Nevertheless, it is useful to select against dag using DAG ASBV, as dag is an important factor predisposing sheep to flystrike.

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• Two rams (A and B) were mated separately to 50 ewes each.
• Ram A has a WEC ASBV of –30 and Ram B has a WEC ASBV of +50.
• All of the ewes had WEC ASBVs of 0 (zero).
• All ewes were managed equally and progeny were run together.
• At 8 months old the progeny were individually WEC tested and the average WEC was calculated for each sire group.

The average worm egg count of Ram A’s progeny were about 40% lower than the average WEC of Ram B’s progeny.

The difference between the rams themselves is 80%, but because the progeny gain only half of their genes from their sire and the other half comes from their dam, only half the difference between the rams is expected in this case (as the ewes were all equal).

In simple terms, Ram A is likely to have 80% less worm eggs himself at any time than Ram B if they were managed and run together. As only half of his genes are passed on, only half (on average) of the additional level of resistance is passed on; in this case, 40%.

If Ram A’s progeny averaged about 1000 epg, then Ram B’s progeny will be about 40% higher: about 1400 epg.

Note: This is a simplistic mathematical explanation of the impact of using rams with different WEC ASBV. Actual differences used in developing ASBV are calculated in a more complex manner.

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