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Home Breeding and Selection of Sheep to Lower Flystrike Risk Breeding to Reduce Flystrike Susceptibility

Breeding to Reduce Flystrike Susceptibility

It is possible to select and breed your flock for increased flystrike resistance by focussing on the traits that increase the risk of breech strike (wrinkle, dag and breech cover) and body strike (fleece rot).

Urine stain is also heritable, but currently no ASBV for this trait is available.

Despite the slightly unfavourable relationship between fleece weight and wrinkle it is possible to maintain or improve productivity and breed plainer sheep at the same time. The industry’s experience in breeding for reduced fibre diameter, whilst increasing fleece weight is evidence that this is possible.

Small changes in average flock wrinkle score will have a large impact on the distribution of wrinkle scores in the flock and reduce the number of high risk sheep within the flock.

Any gains made in breeding are permanent and cumulative and will reduce the treatment and management costs associated with flystrike in the future.

How do I make a start?

For a commercial breeder there are four components to rapidly breeding a flock with low flystrike susceptibility.

1. Select rams with low wrinkle, low dag and low breech cover ASBVs

Preferably your ram breeder can provide ASBVs for wrinkle, dag (in areas where dag is common) and breech cover—make use of them in selecting your rams. Be sure to select rams that also have favourable fleece rot resistance and performance in production traits (eg fleece weight and fibre diameter) that is consistent with your breeding objective. If ASBVs for wrinkle are not available one of the following options (in descending order of value) can be used.

a) If no ASBVs are available for wrinkle—ask the ram breeder to provide breech or body wrinkle scores for the rams you are inspecting, as this assessment can be made off shears when the score can be more accurately assessed.

b) If no wrinkle scores are available, and breech wrinkle has been removed by mulesing—make a preliminary assessment for wrinkle using neck wrinkle and body wrinkle. When a ram is identified that matches your needs for all traits, including wrinkle, make a final assessment using breech wrinkle. Making an initial assessment on neck and body wrinkle will save time and not compromise accuracy.

c) Consider another ram source if your current ram breeder cannot supply rams that meet your needs. Ensure the new source meets the required standard for all traits in your objective.

When assessing the wrinkle breeding value of your ram source, first compare the wrinkle of your existing ram team with the wrinkle standard of the ewes they bred in your flock. In general, rams have a higher wrinkle score than the ewes they breed, particularly in commercial flocks and this needs to be considered when selecting rams or considering a change in ram source.

Note: For ram breeders. Collecting data on breech, body and neck wrinkle (which are highly correlated) will increase the accuracy of the ASBVs generated.

2. Assess ewes for fleece rot, wrinkle, dag and breech cover

a) Review the traits that are risk factors for flystrike and decide what selection emphasis you will put on each trait (fleece rot, wrinkle and dag {where dag is common} are the most important):

Then use the Visual Scores Guide (with the Flock Scoring Chart at the bottom of the Scores page) to make an assessment of your ewes and determine the average and distribution of scores for those risk factors.

b) Having assessed your ewes:

– tag the ewes according to wrinkle score, so that they can be permanently identified. This will allow you to cull the more wrinkled ewes when there is an opportunity, or identify those that may need preventative treatments.


– Immediately cull, or tag for later culling, the ewes with the most fleece rot, wrinkle, dag and fleece cover. Identifying them, if they are to be kept on, will also allow you to give them preventative treatments as required, and these could be the first sheep to be culled if flock numbers need to be reduced.


Wrinkle score, fleece rot score, dag score and urine score are indirect traits for flystrike susceptibility. The incidence of flystrike itself is the most direct indicator of susceptibility. Consider culling struck sheep from the flock, where the proportion is small enough to be practical.

3. Develop a joining strategy for breeding replacement ewes

  1. Determine which sheep you want to breed replacement Merinos from (e.g. wrinkle scores 1 and 2 and fleece rot scores 1 and 2.)
  2. Join your best ewes to your best rams to breed replacements.
  3. Develop a strategy for the remaining ewes with higher susceptibility to flystrike. For example, you may decide to cull all ewes with wrinkle scores 4 and 5 and join score 3 ewes to terminal sires.
  4. It may be necessary to retain your plainer ewes to older ages in the early years, to maintain the required number for breeding ewe replacements.

4. Assess your lambs and make a decision about mulesing and hogget classing

  1. Breech wrinkle score your lambs at lamb marking using the Visual Scores Guide (with the Flock Scoring Chart at the bottom of the Scores page) .
  2. Use coloured tags so you can identify the wrinkle score groups at an older age.
  3. Make a decision about which lambs need to be mulesed—e.g. do score 1 Merino lambs, first cross lambs, wethers for slaughter require mulesing?
  4. Think about the most suitable management for lambs based on their flystrike risk – and aim to reduce the reliance on mulesing in your flock.
  5. Determine how much selection pressure you want to put on wrinkle and fleece rot relative to other production traits. Develop criteria for classing your young ewes. An example of selection criteria is given in the table below. An example of the type of classing criteria that can be developed for young ewes:
Trait *Criteria
WrinkleCull the worst 7%
Fleece RotCull the worst 7%
ProductionCull the worst 15%
Conformation Cull the worst 4%
This figure is less than the sum of the percentage
for each trait as many cull sheep have multiple faults.
* Traits primarily responsible for culling sheep in a commercial flock with a breeding objective placing equal emphasis on improving breech strike resistance, body strike resistance and production (e.g. fleece weight and fibre diameter). Plus maintain the flock’s high standard for structural conformation.

How long will it take?

Commercial producers can make rapid progress in reducing wrinkle and susceptibility to flystrike by:

  • using rams with low ASBVs for breech wrinkle or low visual wrinkle scores
  • scoring and segmenting the ewe flock based on wrinkle scores and flystrike risk
  • applying culling and breeding approaches that rapidly reduce the proportion of high wrinkle score ewes and maintain or improve productive traits, such as fleece weight and fibre diameter.

The following work is from modelling conducted by the Sheep CRC and presents a small number of options.

However, changing your ram source to one that is already not susceptible to flystrike and selecting ewes on the same basis that those rams have been selected can bring very rapid changes. Some flocks have been able to almost eliminate flystrike and remove the need for mulesing in about 5 years.

The table below illustrates the rate of change for a 19 micron flock starting with an average wrinkle score of 3, using in-flock selection and the same ram source.

The following work is modelling conducted by the Sheep CRC and presents a small number of options.

At the start the flock would have 5% flystrike resistant sheep (Score 1), 24% low risk sheep (Score 2) and 71% sheep at risk of flystrike (Score 3+).

After five years of selection and breeding, the proportion of at-risk sheep (Score 3+) will have declined to 47%, whilst resistant and low risk sheep will represent more than half the flock.

After 10 years of selection and breeding, the proportion of at risk (Score 3+) sheep will have declined to 30%, whilst resistant or low risk sheep will represent 70% of the flock.

The graphs below provide some more information on the rate of change that can be expected in a commercial flock in response the various strategies of ram selection and the combination of ram and ewe selection in a self-replacing flock starting with an average wrinkle score of 3.

The three options are summarised below:

  • Purchase rams with low breech wrinkle
    >Low breech score and, preferably low breech score breeding values
  • Select plainer ewes
    >Select replacement ewes with low wrinkle score
  • Mating Strategy – mate selected rams with selected ewes
    >Nucleus flock (wrinkle-free, scores 1 or 2) to breed replacements
    >2nd tier animals (score 3 and above) are used to breed sale animals

The figure below demonstrates three selection strategies that can be adopted to reduce breech wrinkle and their speed with which this can be done. If relying on average stud rams to reduce wrinkle score within your flock, the process may take a long time. By doing some research and aggressively selecting rams for reduced wrinkle this process can be sped up a little. The biggest improvement is seen when this aggressive ram selection is used in combination with ewe selection for reduced wrinkle as well.

The results represent the application of ‘best practice’ genetics and within flock selection and not all producers will achieve this rate of change. The introduction of genomic selection may speed up rate of genetic gain but is unlikely to significantly change these predictions.

Whilst the flock is becoming plainer and less susceptible to flystrike it is important to constantly review:

  • the annual management program that will be required to provide added protection to those sheep in the flock that do not have adequate flystrike resistance
  • your policy on mulesing and which sheep within the flock still require mulesing for adequate flystrike protection.

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