Breeding for flystrike resistance in sheep
Genetic selection for flystrike resistant sheep is not a new idea and a large amount of work was completed in the 1930s, 1940s and 1950s on genetic selection for resistance to breech strike. However, with the advent of mulesing, most efforts then focused on non-genetic methods such as the use of chemicals and crutching.
A breeding program by DAFWA at Mt Barker Research Station and the Commonwealth Scientific and Industrial Research Organisation's (CSIRO) Livestock Industries at Armidale, New South Wales, with support from AWI, is currently being carried out to determine whether sheep can be bred for breech strike resistance, and if so, which indicator traits will be the most effective to select indirectly for resistance. This experiment will also indicate how long it takes and what the trade-offs may exist with other commercial traits such as fleece weight.
For current results and recommendations please read the Breech Strike Resistance newsletter available as an attachment to this webpage.
Read further general information on managing flystrike.
Selecting indirectly for resistance to fly strike
Research has shown that selection of breeding animals that have low dag score, breech and body wrinkle will have immediate and long term effects on the incidence of flystrike in the breeding flock. These animals themselves will have a lower risk of flystrike and so will their progeny.
Research is continuing to find additional indicator traits to select indirectly for resistance to flystrike. However, the results to date have shown clearly that selecting on these traits will increase your flock's genetic ability to resist breech strike.
Breech and body wrinkle
Recording breech wrinkle score is best done after crutching or shearing or at marking of animals. Wrinkle scoring lambs at marking is a great way to reduce your flock wrinkle score as it is a repeatable trait. Consider culling all wrinkle score 4 and 5 animals. Breech wrinkle is related to body wrinkle and neck wrinkle score. This means that reducing breech wrinkle score will also reduce body wrinkle and hence susceptibility to body strike is reduced.
Reducing the overall wrinkle score of your flock will also have added benefit. Past research has confirmed that the number of lambs weaned per ewe joined decreases as the breech wrinkle score of the ewe increases.
Selecting against wrinkle score does not necessarily mean you will decrease your fleece weight. Analysis of sires across Australia has shown there are individuals within a mob with low breech wrinkle scores and higher than average fleece weights. Sheep in the top left quadrant of the below graph have both high greasy fleece weights and low wrinkle score, so it is possible to have both.
The best time to assess dag score in a winter rainfall region is in the spring time prior to hogget shearing when at least 20% of the flock have an average score of 2 to 3 (see picture below). Consider culling all score 5 sheep prior to mating. Mark score 4 and score 3 ewes and note the number in the flock so that an assessment of how many of these could be culled from the breeding flock can be made. Read specific information on breeding sheep for worm resistance.
The importance of a bare breech in breech strike was investigated and it was found that it was less important than dags, urine stain and skin wrinkles. However it does play an important part as it can exacerbate the effect of wrinkles and dags.
Bareness can be scored by using the breech cover scoring system to score sheep prior to hogget shearing.
Culling score 5 animals will also have a beneficial effect on reproduction rate as shown in the following graph. Ewes with low breech cover scores wean more lambs than ewes with high breech cover scores.
The number of animals that are culled is determined by the selection pressure that you are prepared to put on the flock. Higher lambing rates allow greater selection pressure due to more sheep available to choose replacements from. All sheep with a score 5 in any of the indicator traits should be culled.
The relative importance of each of these traits is still being assessed and research is underway to assess the heritability and if there are links between traits. Some flocks will have higher incidence of some traits compared to others. Wool colour is also important for body strike and some producers have identified animals with large bare areas around the breech and are investigating the heritability of these traits.
Urine stain is also a key attractor for flies. Consider culling all ewe lambs with score 5 for urine stain. It is possible to identify high urine stain lambs on the marking cradle. These should be culled upon weaning. Only a relative few ewes will have urine stain problems. Identify them prior to crutching and consider culling high scoring animals.
In general, the greater the selection pressure and heritability of the trait, the faster the genetic progress.
Current research is continuing to find other indicator traits that could be used to select directly for flystrike resistance. However, the results to date have shown clearly that selecting on the known indicator traits will increase your flock's genetic ability to resist breech strike.