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02.06.2023 Source: AWEX
AWEX EMI 1209 -5
Micron 17 2078 +15
Micron 18 1758 +33
Micron 19 1528 +1
Micron 20 1406 +8
Micron 21 1353 -11
Micron 22 1295n -24
Micron 26 522n -20
Micron 28 317 -6
Micron 30 288 -4
Micron 32 237 +2
MCar 738 -23
Breeding and Selection for Flystrike Resistance
Breeding for more flystrike resistant sheep plays a critical role in controlling flystrike in non-mulesed flocks as well as reducing the risk of flystrike in mulesed sheep.

Over 80% of flystrike occurs in the breech area of the sheep (breech flystrike). The AWI Breeding for Breech Flystrike Resistance Program identified that the most important traits in determining the risk of flystrike are:

  1. Dags
  2. Breech Wrinkle

Followed by:

  1. Urine Stain
  2. Breech Cover
  3. Wool Colour

While high dags and high wrinkle are the two main risk factors, significant interactions with and between the other risk factors can occur, which will increase the risk of flystrike of the flock.  As well, the importance of each individual trait in determining the risk of flystrike depends on the region. Dags are not normally found in Australia’s low rain wheat-sheep, pastoral areas and summer rainfall areas and in these regions wrinkle is the lead cause of flystrike, but care does need to be taken because once in every 10 years, conditions can lead to a significant presence of dags, which will require special management strategies. In some areas of Australia, where the risk of dags is high (high winter rain and high rainfall wheat-sheep areas), the risk of flystrike is impacted by both high risk factors, dags and wrinkle. In areas where diarrhoea/scouring and dags are prevalent and wrinkle is low, dags can be the most important factor in determining the risk of breech flystrike.

For woolgrowers who are working to phase out mulesing, the goal is to maintain or improve productivity outcomes, whilst breeding a flock that has enhanced resistance to breech flystrike and can be managed effectively, without the need for mulesing or for a high reliance on chemical control.

Sheep genetic research results indicate breeding for enhanced flystrike resistance can be successful, but in many cases will take a considerable amount of time to achieve, particularly for super fine and fine wool sheep and sheep in high dag environments. A range of strategies can be used to breed for flystrike resistance to suit sheep breed, wool type and environment and individual woolgrower breeding strategies must also integrate objectives for other health and welfare traits such as worm resistance, conformation and reproductive performance. Therefore, there is no one-size-fits-all sheep breeding program and breeding strategies need to be customised to the individual farm, guided by key principles identified from AWI’s investment to date in breeding for breech flystrike resistance.


The length of time required to breed sheep that no longer require mulesing not only depends on how intensive the breeding program is, but also on how susceptible the sheep flock is to breech flystrike at the start of the breeding program and the severity of environmental conditions favourable to flystrike likely on the farm. For example, sheep types with high wrinkle scores and / or high dag scores will take considerably longer to reduce susceptibility to breech flystrike to a point where mulesing could be stopped.

A project to model the rate of genetic gain in breeding for breech flystrike resistance, using the data from the AWI Breeding for Breech Flystrike Resistance Program, concluded that:

  • For ram breeding flocks that do not import breeding stock, meaningful reductions in flystrike incidence to a sufficient level to reduce reliance on mulesing, or to cease it, are possible over in a fully pedigreed and recorded flock in a 10 to 15-year period, whilst retaining competitive levels of genetic gains for other important health and welfare and productivity traits, by using appropriately constructed selection indexes.
  • For commercial flocks, selection and culling strategies such as buying high accuracy elite rams, culling heavily on breech straits and possibly changing the ram source need to be seriously considered to reach the required scores to reduce flystrike incidence for breech traits inside 12-15 years.
  • Both ram breeders and commercial producers with superfine sheep are currently more limited in making significant reductions in breech traits, particularly for breech wrinkle, as little genetic gains in breech traits are being achieved within that sheep type relative to fine/medium and dual-purpose sheep types.

Breeding and selection tools

A number of tools are available to support woolgrowers looking to move away from mulesing by breeding for breech flystrike resistance, including visual scores for breech flystrike indicator traits and Australian Sheep Breeding Values (ASBVs).

The FlyBoss Flystrike Risk Simulator is a comprehensive program that allows woolgrowers to compare two different management systems. The program uses the property’s local weather conditions to estimate the risk of flystrike, and woolgrowers can then make adjustments for:

  • Management options such as shearing and crutching
  • Breech modification procedures
  • Chemical preventive treatments including optimising the timing of treatments
  • Effect of breeding for reduced breech flystrike.

Visual Scores

Visual scores for important phenotype traits of sheep, including the breech flystrike indicator traits, have been developed to: 

  • Provide the Australian sheep industry with a standardised set of visual assessment scores for the consistent description of important phenotypic traits of all breeds of sheep;
  • Provide a quick and simple scoring system to help sheep classers and breeders select sheep on visually-assessed traits to accelerate genetic gain;
  • Enable sheep breeders and classers to record and submit visual score data and genetic information to Sheep Genetics to progress development of across-flock Australian Sheep Breeding Values* (ASBVs) for visually-assessed traits; and
  • Enable researchers to estimate the heritability of visually-assessed sheep traits, and to measure their relationships, if any, on important production traits such as fleece weight, fibre diameter, growth rate and body weight.

The visual scores for the key flystrike indictor traits are presented in Figure 1. Information on these, and others, are available in the AWI/MLA Visual Sheep Score Guide (AWI/MLA Visual Sheep Score Guide 2019).


 Figure 1. Visual scores for Dag, Breech Wrinkle, Urine Stain and Breech Cover  

The lower the score the lower the risk, and in most locations across Australia the following are the key visual score targets woolgrowers are encouraged to aim for, to reduce the risk of breech strike to levels where sheep may be considered naturally resistant without increasing reliance on chemicals:

  • Dags - score 2 and less,
  • Breech Wrinkle - score 2 and less,
  • Urine Stain – score 2 and less, and
  • Breech Cover - score 3 and less.

A dag score 4 animal can be up to seven times more susceptible to breech strike than a dag score 1 animal in the same flock.

Australian Sheep Breeding Values (ASBVs)

ASBVs are an estimate of an animal’s breeding value based on its own recorded data and that of the animals in its pedigree, rather than what the animal looks like. They allow woolgrowers to make a projection of how an animal’s progeny will perform over a range of traits relative to other animals across different flocks of the same breed. Woolgrowers submit the visual score data of their animals to MERINOSELECT, which is a genetic evaluation service run by Meat & Livestock Australia, which analyses this performance recorded information on animals, along with their pedigree information to generate their ASBVs for the various traits.

Woolgrowers then use this ASBV information to inform their ram selection decisions for breeding.  AWI’s genetic research into natural flystrike resistance and Sire Evaluation led to the development of three new ASBVs for the flystrike indicator traits - breech wrinkle, breech cover and dag. Released in 2009, they are now available for use by ram breeders and wool producers on 20-30% of recorded animals. Ram breeders are utilising these findings and are increasingly breeding lower dag and breech wrinkle animals with higher fleece weight to meet market demands as illustrated in Merino genetic trends.

More information on Breeding and Selection

See Flystrike Education, Extension and Promotion for more information on the education and extension programs that are readily available to woolgrowers wanting to more efficiently select and breed naturally flystrike resistant, profitable sheep, to reduce the reliance on mulesing, and for managing the risk of breech flystrike.

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