Online learning: Rangelands—Breeding for worm resistance

This strategy helps your flock to withstand the worms that are present.

Structured reading

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Rangelands: Breeding worm-resistant goats
Worm resistance explained including how to choose bucks to breed worm-resistant goats.

Question and answer

For those who prefer a problem based approach to learning, answer the following questions.
Each of the questions below links further down the page to the answers.

Questions:

  1. What is the difference between resistance and resilience?
  2. What would a stud breeder need to measure to see the worm resistance of their sale bucks?
  3. What do you call the figures they would supply at their sale for you to see the worm resistance of each buck?
  4. Will worm resistant goats reduce the level of worm contamination on pastures?
  5. Are daggy goats less resistant to worms?
  6. What % difference would you expect to see between the average WEC tests for the progeny of sire group A and sire group B in the example shown below?
  • Two bucks (A and B) were mated separately to 50 does each.
  • Buck A has a WEC EBV of –30 and Buck B has a WEC EBV of +50.
  • All of the does had WEC EBVs of 0 (zero).
  • All does 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.

Answers:

You can also click on each question below to go to WormBoss pages with related information.

1. What is the difference between resistance and resilience?

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

Resilience to worms: Goats that are resilient to worms can grow and produce with less ill effects from worms despite carrying a worm burden. 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 Estimated Breeding Values (EBVs) for growth, a more resilient animal will perform better than a less resilient animal when both have high worm burdens. Resilience is independent of worm resistance so must be selected separately by choosing better production performance.

If you regularly score individual goat eye colour as part of your barber’s pole worm management, you can select for resilience to barber’s pole worm by choosing animals with darker eye colour scores as they are likely to carry fewer worms and require fewer drenches for barber's pole worm.

2. What would a stud breeder need to measure to see the worm resistance of their sale bucks?

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

3. What do you call the figures they would supply at their sale for you to see the worm resistance of each buck?

Worm Egg Count Estimated Breeding Value (WEC EBV). Choose the WEC EBV age that corresponds to the time of most worm-challenge on your property, e.g. weaning (WWEC), post-weaning (PWEC), yearling (YWEC).

4. Will worm resistant goats reduce the level of worm contamination on pastures?

Worm resistant goats will reduce the level of worm contamination on pastures.

A goat’s resistance to worms directly affects the number of worms it carries. In more resistant goats (those with lower WEC EBV), the worm egg count will be lower, fewer eggs will be passed into the dung and onto the pasture. In more worm-resistant goats:

  1. Fewer worm larvae eaten with the pasture are able to establish in the gut and become adults. In goats with poor immunity, 50–60% of the worm larvae are able to become adults in the gut. In goats 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 goats over a period of days to weeks.

5. Are daggy goats less resistant to worms?

Dag resulting from scour worms is not an indicator that the goats are less resistant to worms.

The propensity to scour has a substantial genetic component that is independent of both resistance and resilience to worms. Only fibre goats will show dags, but meat and dairy goats can display soiling of their hindquarters and under their tails, but these effects do not last long and are not available as EBVs for goats.

6. What % difference would you expect to see between the average WEC tests for the progeny of sire group A and sire group B in the example shown below?

  • Two bucks (A and B) were mated separately to 50 does each.
  • Buck A has a WEC EBV of –30 and Buck B has a WEC EBV of +50.
  • All of the does had WEC EBVs of 0 (zero).
  • All does 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 Buck A’s progeny were about 40% lower than the average WEC of Buck B’s progeny.

The difference between the bucks 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 bucks is expected in this case (as the does were all equal).

In simple terms, Buck A is likely to have 80% less worm eggs himself at any time than Buck 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 Buck A’s progeny averaged about 1000 epg, then Buck B’s progeny will be about 40% higher: about 1400 epg.

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


Links to the learning topics for Rangelands

  1. Introduction
  2. Grazing management
  3. Breeding for worm resistance (you are currently on this page)
  4. Worm testing
  5. Drenching
  6. Drench resistance management
  7. Goat worms