Dr John Karlsson with a flystrike resistant Merino ram.

SHEEP producers should cull any sheep which have been struck by flies and their progeny, the latest Australian breech flystrike research has recommended.

The research indicated that producers could not just cease mulesing to eliminate flystrike, but needed to “plan, plan and plan, and use the tools available to reduce lifetime breech strike.”

The breech strike project was initiated in 2006 with the establishment of a Merino research flock on the Mt Barker Research Station in Western Australia.

Six hundred Merino ewes were sourced from 10 industry and three research station flocks from the Department of Agriculture of Western Australia, now known as the Department of Primary Industries and Regional Development. The project went through three different phases to identify the role of potential indicator traits in breech strike.

The project was a collaborative research effort of Department of Agriculture and Food Western Australia, University of Western Australia, CSIRO Animal, Food and Health Sciences, Armidale, New South Wales, and supported by Australian Wool Innovation Limited. AWI funding for this project has finished, but researchers want to continue research into fly attractant factors such as odour to potentially develop more effective methods to trap blowflies and to identify sheep that are less attractive to blowflies.

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Cull sheep that get struck by flies

Senior DPIRD geneticist Johan Greeff said producers should not breed from any sheep which are struck, or from their progeny.

“And you have to cull any animals that have excessive skin or dags, that’s very important.

“That should become part of the normal selection techniques and culling decisions during the year.”

Dr Greeff said some very plain mulesed sheep would still get struck, due to other reasons.

“Wrinkles only explain a portion of the variation, but if you’ve got very wrinkly sheep, then you have to still select to reduce that wrinkle as much as possible.

“And you don’t necessarily have to reduce your fleece weight by reducing your wrinkle, that’s not true anymore.”

The research compared the incidence of breech strike of un-crutched and unmulesed progeny of the two most resistant and two most susceptible 2008-drop sires over their lifetime in the flock.

“The progeny of the two most resistant sires experienced a strike rate of 5.7 percent.

“This is very low considering these sheep were not mulesed, crutched, or jetted while strongly challenged and assessed in a high flystrike season prior to hogget shearing,” Dr Greeff said in WA’s July Breech Strike Resistance Project Newsletter.

“In contrast, the progeny of the most susceptible two sires’ had an average strike rate of 98.6pc at hogget age, which means that virtually all progeny of the two most susceptible sires in this study, were struck prior to hogget age.

The research showed that large differences that were found between sires within each year of the trial. A very high breech strike rate was experienced in 2008 (39pc) whereas a very low rate of 4pc was experienced in 2010.

“However, in every year there were sires whose progeny were highly resistant and highly susceptible.

“All flocks are likely to have a similar distribution in sires for breech strike, and identifying the susceptible animals are very important,” Dr Greeff said.

“Culling sheep that have either been struck in the breech or, where records exist, have had significant numbers of progeny struck, is a simple way of reducing the incidence of breech strike in the flock.”

Key messages for breeders

Large differences in breech strike exist between breech strike susceptible and resistant sire progeny groups, irrespective of whether or not they have been crutched.

Breech strike, in unmulesed and uncrutched and sheep, is a heritable trait similar to fibre diameter (~50pc). It is lower in crutched sheep (~20pc) but it still provides useful information to make selection and culling decisions.Therefore, struck sheep are likely to produce progeny that are also susceptible to breech strike and so should be culled.

• It is difficult to visually identify genetically resistant or susceptible rams unless the animals are struck.
• Progeny testing is currently the only method for accurately identifying genetically resistant sires.
• In a winter rainfall region, the presence of dags is the most important indicator trait for breech strike.
• In crutched yearling ewes, skin wrinkle is the most important indicator trait of breech strike.

Wrinkle is a highly heritable trait and breeders can breed plain ewes by selecting high productivity rams that are free from wrinkles.

• It is not always possible to score breech wrinkle accurately in sheep with long wool. It should be done after crutching. Alternatively, neck wrinkle can be used as indicator trait. Wrinkle at birth or marking is also a good indicator of subsequent wrinkle score.
• Cull all sheep that are struck on the breech or tail. Breech strike is not well correlated to horn, body or pizzle strike.
• Time of crutching or shearing should take place just prior to the periods of high breech strike risk.
• Use the Sheep Genetics breeding values to select for low wrinkles, dags, breech cover and high production traits, in order to breed productive and more breech strike resistant sheep.
• Don’t just cease mulesing…plan, plan and plan, and use the tools available to reduce lifetime breech strike.

An AWI spokesman said “the latest outcomes from this research are being reviewed, with a forum to discuss this valuable work and next steps to be held later this year.”

• Click here to read the full July Breech Strike Resistance Project Newsletter.

Click here to see the differences in breech strike between mulesed and crutched groups.

Click here to see the differences in breech strike rates between sire groups.