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Breeding for Flystrike Resistance

The risk of breech strike has increased over the last 50 years with improved pastures and higher stocking rates. There is also a greater focus on the welfare of our animals. Long term breeding is one of the solutions to help address breech strike and welfare challenges that producers are faced with.

Two research flocks have been bred over time for the purpose of breeding for flystrike resistance one in Mt Barker, Western Australia(WA) (in a winter dominant rainfall pattern area) and one in Armidale, NSW (in a summer dominant rainfall pattern area).

The WA flock was established in 2006 and its purpose at that time was to identify and quantify indicator traits for breech strike in unmulesed sheep. The traits that were focused on included dags, urine stain, wrinkle, cover and wool colour. These sheep were not crutched in the first four years, were not mulesed from 2008 onwards and no jetting was conducted in any of the years.

The WA flock data has shown that females are more susceptible to breech strike but overall males are more susceptible to flystrike due to the high incidence of poll strike. Certain rams will breed sheep that are more susceptible or more resistant to breech strike. In the NSW flock some individual sheep have been repeatedly struck up to 15 times over the duration of the trial. Breech strike is very heritable and for the most part the rams cannot be told apart visually nor can their progeny. Within this flock there are rams that have the production traits we are looking for in addition to the breech strike resistance.

Dags and urine stain are the two most important indicator traits when it comes to flystrike, followed by wrinkle, and then face cover. It is possible to select for worm resistance and lower dags.

It is well known and proven through the trial work conducted in the WA flock that low wrinkle and breech cover animals have a higher number of weaned lambs per joined ewe.

The indicator traits are an effective means of selecting for breech strike resistance and breech strike is heritable. However this does not explain all breech strike, as the WA flock has shown that 45% of breech strike is still unexplained. Moisture, wax and suint have a limited impact. This has led to further work where preliminary trials using trained dogs have shown that resistant lines of sheep can be differentiated from susceptible lines of sheep through smell. This promising preliminary work may lead to further work into odour studies.

Micro-organisms found on the skin and wool have also been looked at with promising results. There are micro-organisms found on resistant lines of sheep that are not found on the susceptible lines of sheep.

Interestingly fly populations have also been looked at, and with no surprise to many, fly populations are higher in paddocks where sheep are present. Although fly populations are higher in the open or around trees compared to near where water is located.

Breech strike is highly repeatable and heritable and struck sheep should be culled where possible. There is a strong relationship between breech trait scores and breech strike and reducing the score reduces the risk. If wrinkle scores are reduced to 2 or below, the dag score is reduced to 2 or below, and the breech cover 3 or below, then those sheep are at the same risk levels as mulesed sheep.

Odour and bacteria may be important but further work needs to occur in these areas before the industry can use them. So for now focusing on low dag, low wrinkle and high fleece weight rams (with the use of breeding values to assist rate of gain) and having a balanced approach to other traits is the best way forward.

Genomics research is working on a test to increase the accuracy of selecting for flystrike resistance allowing for faster genetic gain. It will be particularly useful for traits which do not express a wide variation in all environments. Genomics may also allow for detection of underlying genes involved in flystrike resistance or susceptibility, which will also link in with the odour and bacteria work and genes associated with traits like urine stain. Mapping co-evolution of the fly and the sheep genome is also being looked at. The accuracies of genomics will need improvement and validation before being offered to the industry.

This article was developed by Tiffany Bennett from information presented at the AWI Breech Strike Prevention Update held in August (2014). All of the Update presentations are available from the AWI website.

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