Scientist showing the wrinkle-free breech area of sheep bred for breech strike resistance.

CSIRO's Dr Jen Smith has developed methods to breed sheep with wrinkle-free breech area resistant to flystrike.

Breeding for Breech Strike Resistance

We collaborate with the sheep and wool industry to develop alternatives to mulesing, including breeding sheep that are genetically resistant to breech flystrike.

  • 21 January 2011 | Updated 26 October 2012


Blowfly strike is caused by sheep blowflies laying their eggs in moist fleece, which commonly occurs around the sheep's breech area. When the eggs hatch, the maggots feed on the sheep's skin causing severe pain and death if left untreated.

To reduce the risk of breech strike in Australian Merino sheep, lambs have conventionally been mulesed – having skin surgically removed from their hindquarters. Mulesing reduces skin wrinkles and increases the amount of bare skin on the breech area.

Although mulesing provides animal welfare benefits by reducing breech strike, the practice is now generally viewed as unethical because of the pain associated with the surgical procedure.

Since the 1980s, CSIRO has been researching alternatives to mulesing as well as effective drug treatments for alleviating the pain caused by mulesing.

Selective breeding for sheep with a naturally bare and wrinkle-free breech area, resistant to flystrike, is widely considered the best long-term alternative to mulesing.

Breeding genetically resistant sheep

The original Breeding for Breech Strike Resistance project (2005–10) investigated the effectiveness of selective breeding programs to produce breech strike resistant sheep.

The research was aimed at evaluating potential indicator traits and developing 'best practice' guidelines for inclusion of breech strike resistance into Merino breeding programs.

Selective breeding for sheep with a naturally bare and wrinkle-free breech area, resistant to flystrike, is considered the best long-term alternative to mulesing.

This project was based around an experimental selection flock of 600 ewes in three selection lines and usage of industry genetics.

The sheep were evaluated for differences in the breech strike indicator traits, breech strike itself, wool production and cost of production.

Project outcomes and further work

The results of this project are promising, confirming breeding for breech strike resistance using indicator traits as a useful alternative to mulesing for reduction of breech strike.

Selective breeding will be particularly effective if combined with other breech strike management practices such as well-timed shearing and crutching, worm control to reduce dags and strategic use of preventative chemical treatments.

Several of the traits investigated have sufficiently high variability, heritability and correlation with breech strike itself to enable successful selection for breech strike resistance. These traits include breech wrinkles, dags and breech cover.

There are antagonistic relationships, or trade-offs, between some breech traits and production traits, but these are only low to moderate. With an appropriately balanced breeding objective, concurrent genetic gain in both wool productivity and flystrike resistance is therefore achievable.

Further work in this area includes research into genomic-assisted selection for breech flystrike resistance. Genome-wide analysis using the Ovine 50K Bead-chip will enable identification of single nucleotide polymorphisms (SNP – variations in the DNA sequence at a single site on the genome) that have significant associations with flystrike and three key breech flystrike indicators (breech wrinkle, breech cover and dag).

Prediction equations will be derived for calculation of genomic breeding values for flystrike and the three indicator traits. Genotype-assisted selection can potentially provide considerable improvement to breeding and selection for flystrike resistance compared with what we have already achieved through the quantitative genetic methods used to date. 

Our partners

Our key collaborators in this project were:

Project updates

CSIRO and the Department of Agriculture and Food Western Australia have been producing an annual newsletter on breech strike genetics, funded by the AWI.

The Breech Strike Genetics newsletters, along with presentations delivered to industry representatives – available for download below – provide information to Merino breeders and wool growers about selective breeding for breech strike resistance.