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Understanding WEC and DAG breeding values (ASBV, Goat EBV)

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.

What is an ASBV or EBV?

ASBV stands for Australian Sheep Breeding Value and Goat EBV stands for Goat Estimated Breeding Value. ASBVs and Goat EBVs are the national language for benchmarking sheep and goats based on their genetic merit and are produced by Sheep Genetics. ASBVs and Goat EBVs describe an animal’s breeding value for a trait, e.g. worm egg count (WEC), fleece weight or body weight, and express the relative breeding value of animals across different breeding flocks of that breed (or across breeds in the case of Terminal breeds). They reflect the true breeding value of the sheep or goat rather than what the particular animal looks like. They are equivalent to estimated breeding values (EBVs) used in other livestock industries e.g. BREEDPLAN in the beef cattle industry.

Why is WEC ASBV and WEC Goat EBV expressed as % rather than eggs per gram?

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 which 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.

Why are ASBVs and Goat EBVs more effective than raw measurements when selecting for worm resistance?

ASBVs and Goat EBVs incorporate all of the available pedigree and performance data of the animal and its relatives, making the genetic estimate far more accurate than raw measurements alone. The calculation of ASBVs or Goat EBVs removes environmental effects that can obscure an animal’s breeding value; these include birth type, rear type, dam age and differences between management groups. Because management differences between drops of progeny and flocks are removed, ASBVs and Goat EBVs can also be directly compared across age groups and flocks or herds, which is not possible if using raw measurements.

In addition to the environmental effects discussed above, raw WECs are fairly crude measurements for individuals, and the WEC trait has only 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.

How does selection for lower WEC ASBV or WEC Goat EBV improve resistance to worms?

A sheep or goat's resistance to worms directly affects worm egg count. In more worm-resistant animals  (those with lower WEC ASBV or WEC Goat EBV)

  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 animals, and the pasture will also be less contaminated than by less resistant animals.

Is dag an indicator of worm-resistance in 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).

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.

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.

What do the WEC ASBV and Goat EBV figures mean?

WEC ASBV and Goat EBV are shown as percentages. They describe how different the worm burden of the progeny of one ram or buck will be compared to another ram or buck.

This example 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.

Ram A has a WEC ASBV of –30% and Ram B has a WEC ASBV of +50%.

All ewes and their progeny were managed equally and were run together—ewes were removed at weaning.

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.

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.