Breech strike genetics
Johan Greeff, DPIRD, South Perth, WA
Corresponding author: email@example.com
The Breech strike project was initiated in 2006 with the establishment of the research flock on the Mt Barker research station in Western Australia and supported by Australian Wool Innovation. Six hundred Merino ewes were sourced form 10 industry and three research station flocks from the Department of Agriculture of Western Australia.
The third phase (2010-2014) of the experiment focussed on identifying those factors that contributed to susceptibility and resistance to breech strike in the absence of dags, i.e. when animals are crutched or shorn just prior to the fly season. This is representative of production systems in winter rainfall regions where all sheep are normally crutched before the onset of winter/spring.
Effect of crutching and mulesing on breech strike
Figure 1 shows the incidence of breech strike in different management groups that were mulesed or not mulesed, and that were crutched or not crutched, for both males and females. A crutched plus mulesed group was not included as this would have distorted the genetic information necessary for studying the inheritance of breech strike and identifying the potential indicator traits.
Differences within treatment groups across years (i.e. within crutched or mulesed groups for females and males) can be explained by differences in environmental conditions between years.
However, large differences were also found between management groups. Both the un-mulesed and un-crutched female and male groups experienced high incidences of breech strike – an average of 27% (light orange) vs 15% (grey), respectively. Mulesing sheep decreased the incidence of breech strike to 6% for both males (light blue group) and females (orange group), while crutching alone decreased the incidence of breech strike to 11% for females (green group) and 5% in the males (dark blue). No difference between rams that have only been mulesed (light blue) or have only been crutched (dark blue) were found.
In summary, not crutching ram or ewe hoggets increased the risk of breech strike by two to three fold, while not mulesing ewe hoggets increased the risk of being struck by four to five fold.
Large differences exist between sire progeny groups
Figure 2 shows the differences in the incidence of breech strike from birth to hogget shearing among 148 sire progeny groups from 2006 to 2014 for animals that were not mulesed. The sires were sourced from research flocks (homebred), commercial industry flocks and from different ram breeding flocks (studs).
The progeny were classified as being resistant or susceptible based on their breeding value for breech strike from birth to hogget age. Additional sires were progeny tested (Homebred) to assess their resistance to breech strike. The sheep that were born in 2006, 2007, 2008, 2009 and in 2014 were not crutched and those born in 2010, 2011, 2012 and 2013 were crutched at yearling age. Thirty sires were used in multiple years to generate genetic links across years.
Figure 2 clearly shows the large differences that were found between sires within each year. A very high breech strike rate was experienced in 2008 (39%) whereas a very low rate of 4% 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. 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.
Do not breed from any struck animal
Figure 2 shows the large differences between sire progeny groups and Table 1 shows the incidence of breech strike of the un-crutched and un-mulesed progeny of the two most resistant and two most susceptible sires that were born in 2008, over their lifetime in the flock.
The progeny of the two most resistant sires experienced a strike rate of 5.7%. 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. In contrast, the progeny of the most susceptible two sires’ had an average strike rate of 98.6% at hogget age, which means that virtually all progeny of the two most susceptible sires in this study, were struck prior to hogget age.
It is clear that if sheep are struck early in life, then there is a high likelihood that they will be struck again at later ages. Therefore, one can say with reasonable accuracy that any sheep that had been struck in the breech is very susceptible to future breech strike and that all such animals should be culled and not used for breeding in future.
In subsequent years, Table 1 shows that when these animals were 3, 4 and 5 years old, none of the two most resistant sires’ daughters were subsequently struck in the breech even though they were not mulesed or jetted, but only crutched. However, a significant proportion of the daughters of the two most susceptible sires were again struck at 3, 4 and at 5 years of age. The differences in breech strike within the two groups from year to year were probably due to different environmental conditions from year to year.
Important indicator traits of breech strike in winter rainfall regions
Figure 3 shows the important indicator traits for breech strike up to weaner shearing and the amount of variation these indicators explain in un-crutched and un-mulesed weaner sheep. All the factors explain about 25% of the variation in breech strike up to weaner shearing, of which dags at weaning (W) was the most important indicator trait in ram lambs. In ewes lambs dags was less important and instead, differences in urine stain and tail wrinkle contributed significantly to breech strike. However, a large proportion of the variation (>70%) remains unexplained.
Figure 4 shows the indicator traits for breech strike from weaner to hogget (H) shearing and the amount of variation they explained in crutched ewe hoggets. It is clear that in crutched hogget ewes, breech wrinkle (PBRWR) was the most important factor in breech strike from weaner to hogget shearing.
This implies that whilst the removal of the wool by crutching hogget ewes reduces breech strike, probably through preventing the wool from getting too wet from urine and enabling it to dry much quicker, the presence of wrinkles negates this drying out effect of the urine from crutching. This then increases the risk of being struck.
However, in crutched hogget rams (Figure 5), dags, breech (PBCOV) and crutch cover (HCCOV) explained only about 10% of the total variation in breech strike. This leaves about 90% of the variation in breech strike unexplained.
Take home messages
- 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 un-mulesed and un-crutched sheep, is a heritable trait similar to fibre diameter (~50%). It is lower in crutched sheep (~20%) 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.
- Dags in a winter rainfall region 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.