Managing non-mulesed sheep

Page last updated: Wednesday, 7 December 2022 - 12:05pm

Please note: This content may be out of date and is currently under review.

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 DPIRD at its Katanning Research Facility 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.

Body wrinkle score: A sheep with Score 1 has a plain body with no wrinkles. A Score 5 sheep has extensive wrinkles and heavy folds of skin over its entire body.
Body wrinkle score: A sheep with Score 1 has a plain body with no wrinkles. A Score 5 sheep has extensive wrinkles and heavy folds of skin over its entire body.
Breech wrinkle score: A sheep with Score 1 has no wrinkle. A Score 5 sheep has extensive wrinkle at the tail set, sides of the tail (bat wings), adjacent to the anus/vulva and down the hind legs.
Breech wrinkle score: A sheep with Score 1 has no wrinkle. A Score 5 sheep has extensive wrinkle at the tail set, sides of the tail (bat wings), adjacent to the anus/vulva and down the hind legs.

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.

Number of lambs weaned per ewe joined in a winter rainfall environment at Mt Barker, WA, for ewes with different breech wrinkle scores at hogget age
Number of lambs weaned per ewe joined in a winter rainfall environment at Mt Barker, WA, for ewes with different breech wrinkle scores at hogget age

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 variation in progeny greasy fleece weight compared to the breech wrinkle score from 560 sires involved in sire evaluation trials across Australia. Source: www.sheepgenetics.org.au & www.merinosuperiorsires.com.au
The variation in progeny greasy fleece weight compared to the breech wrinkle score from 560 sires involved in sire evaluation trials across Australia. Source: www.sheepgenetics.org.au & www.merinosuperiorsires.com.au

Dag score

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.

Dag score: A sheep with Score 1 has no dags. A Score 5 sheep has extensive dags in the breech area, and extending right down the hind legs to the pasterns.
Dag score: A sheep with Score 1 has no dags. A Score 5 sheep has extensive dags in the breech area, and extending right down the hind legs to the pasterns.

Wool cover

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.

Breech cover: A Score 1 sheep has natural bare area that extends outwards around the anus and vulva and right down to the bottom of the breech area (the channel). A sheep with Score 5 has complete wool cover i.e. no natural bare area at all
Breech cover: A Score 1 sheep has natural bare area that extends outwards around the anus and vulva and right down to the bottom of the breech area (the channel). A sheep with Score 5 has complete wool cover i.e. no natural bare area at all

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.

Number of lambs weaned per ewe joined (NLW/EJ) in a winter rainfall environment at Mt Barker, WA, for ewes with different breech cover scores at hogget age
Number of lambs weaned per ewe joined (NLW/EJ) in a winter rainfall environment at Mt Barker, WA, for ewes with different breech cover scores at hogget age

Culling pressure

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.

Contact information

Julia Smith
+61 (0)8 9368 3449
Katherine Davies
+61 (0)8 9690 2169