When breeding for worm resistance, animals are selected for lower WEC ASBV or EBV. Dag ASBV does not affect worm resistance, rather, reducing dag through selection of lower DAG ASBV decreases the susceptibility of sheep to flystrike, as dag is a major predisposing cause of flystrike.
Stock resistant to worms can
ASBVs and Goat EBVs:
What do the WEC ASBV and Goat EBV figures mean?
They describe how different the worm burden of the progeny of one ram or buck will be compared to another ram or buck. WEC ASBV and Goat EBV are shown as percentages.
The illustration below compares the progeny of two rams, A and B, which had been mated to a group of similar ewes, with the ewes all having WEC ASBV of zero.
The difference between the rams themselves is 100%, 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 100% 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, 50%.
If Ram A’s progeny averaged about 1000 epg, then Ram B’s progeny will be about 50% higher: about 1500 epg.
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.
The WEC of sheep or goats in a mob can vary considerably over time depending on external factors, in particular, the time since the mob were drenched and the level of worm challenge they recently faced. This is unlike traits like live weight or fibre diameter that vary over time to a much smaller extent. This makes the actual egg count, measured in eggs per gram, far less useful than the relative difference between animals at any time. The percentage difference between animals is more consistent than the actual difference in eggs per grams.
In addition to the environmental effects discussed above, raw WECs are fairly crude measurements for individuals, and the WEC trait has a low to moderate heritability. This results in the raw data having a far less accurate predictive value than its corresponding ASBV or Goat EBV.
A sheep or goat's resistance to worms directly affects worm egg count. The points on the image describe what will happen in more worm-resistant animals (those with lower WEC ASBV or WEC Goat EBV):
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%.
All of the above factors result in fewer eggs passing in the dung of more worm-resistant animals, with less pasture contamination than by less worm-resistant animals.
Dag resulting from scour worms is not an indicator that the sheep are more or less resistant to worms. It indicates the presence of a worm infection, but the amount of scouring in individuals has no correlation with the worm burden carried.
Most young animals will scour as their immunity develops. Scouring will also occur at any age if the animals have a very large burden of scour worms and the degree of scouring can reflect the amount of damage caused to the gut. But some animals, once their immunity is developed, become hypersensitive to larvae and will scour for a short period when re-exposed to scour worm larvae each new season, even at low levels; these are best culled from a breeding program. There is no relationship between sheep that 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.
Dags can also be the result of other factors unrelated to worm infection, such as nutritional and/or toxin scours from pasture or bacterial and protozoal infections.