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South Australian Winter Rainfall: Breeding worm-resistant sheep
Worm resistance explained including how to choose rams to breed worm-resistant sheep.
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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.
Worm egg counts of rams need to be measured; the mob should be run and managed together so that their results can be compared.
Worm Egg Count Australian Sheep Breeding Value or WEC ASBV.
A sheep’s resistance to worms directly affects worm egg count. In more worm-resistant sheep (those with lower WEC ASBV)
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.
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.
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.