Inheritance of breech strike, neck wrinkles, dags and breech cover over the lifetime of crutched Merino ewes in a Mediterranean environment
Johan Greeff DPIRD, Perth WA; Anthony Schlink (retired) and John Karlsson (retired)
Author correspondence: firstname.lastname@example.org
Breech strike continues to be a major problem for wool sheep. The main blowfly species that initiates 90% of all strikes in sheep is the Australian sheep blowfly (Lucilia cuprina), which is copper green in colour with reddish eyes.
The main methods to alleviate breech strike include mulesing lambs at an early age, ongoing crutching, and jetting sheep with an effective insecticide. However, mulesing is a sensitive issue with consumers, and continued use of chemical treatments has led to an increase in blowfly resistance to insecticides. Research in hogget sheep has shown that there are large differences between sire progeny groups in susceptibility to breech strike. Breech strike resistance is a heritable trait and makes breeding for resistance to breech strike the best long-term, permanent solution. As breech strike is also a repeatable trait, any sheep that were struck at a young age, are more likely to get struck as older sheep.
This study was undertaken to estimate the genetic parameters of breech strike and its indicator traits over the lifetime of crutched Merino ewes in a Mediterranean environment.
Breech strike records were collected on 3993 ewes over their lifetime where they were annually crutched. The ewes were scored at different times of the year for neck wrinkle, dag score and breech cover over their lifetime. A total of 13 577 breech strike records from birth up to five years of age were generated between 2006 and 2017. Face cover was only scored mid-year. Mixed model methodologies were used to estimate the genetic parameters of the different traits at different ages.
Results and discussion
Breech strike was heritable and repeatable on the observed scale, and as a binary trait on a logistic scale.
The incidence of breech strike for mature ewes in the Susceptible line was 8.1%, compared to 3.6% in the Rylington flock and 2.3% in the Resistant line (P>0.001). Breech strike was generically positively correlated across ages. The incidence of breech strike in 2-year-old ewes (9.2%) was approximately double compared to older ewes (4.8-7.7%). This study also showed that mulesing is a very effective management tool to prevent breech strike, with only 1.86% of the mulesed ewes being struck compared to 8.33% of the unmulesed ewes under the same management.
Positive genetic correlations were found between breech strike and the indicator traits of neck wrinkle, face cover, breech cover and dags.
Neck wrinkle and face cover, as well as breech cover and dag scored at different times of the year, were heritable and repeatable. Previous studies completed showed that neck wrinkle is a better predictor of breech strike compared to breech wrinkle, although high genetic correlations were found between different ages for neck wrinkle and post-shearing breech cover, these traits are not genetically the same traits at different ages. The ewes in this study were reasonably plain with a neck wrinkle score below two, only the weight of the lamb weaned had a significant phenotypic effect on neck wrinkle. Therefore, it was concluded that culling ewes on neck wrinkle score will result in a decrease in neck wrinkle, and according to Scholtz et al. (2010), it is genetically strongly correlated with breech wrinkles, which implies that breech wrinkles would also be reduced by selecting for a decrease in neck wrinkles.
The average breech cover score of the ewes in this study was approximately 2.5. Breech cover scored mid-year, before shearing and after shearing were genetically the same trait.
The genetic relationship between dag at different ages is unclear as a moderate positive genetic correlation (rg = 0.25 ± 0.12) was found between hoggets and 2-year-old ewes, whereas a moderate negative correlation (rg = –0.30 ± 0.14) was found between 3- and 4-year-old ewes. The genetic correlations amongst the dag traits in older age groups were not significantly different from zero. The average amount of dags in each age group was relatively low due to the ewes being crutched before the start of the winter rainfall. Mid-year dags were genetically strongly correlated with breech strike, whereas dag pre-shearing was not significantly different from zero, so mid-year dag qualifies as an indicator trait for breech strike.
Breech strike and its indicator traits; neck wrinkle, breech cover, face cover and dags, were all heritable. Breech strike appears to be genetically strongly correlated across ages, however, the correlations had large standard errors. It was positively correlated with the indicator traits. Moderate to strong genetic correlations were found between ages for neck wrinkle and for breech cover. Dags, however, were poorly correlated across ages, indicating that dags are not genetically the same trait in mature ewes at different ages.
This study showed that breech strike, neck wrinkle, dags and the wool cover traits are heritable at different ages and are low to moderately repeatable over a ewe’s lifetime. The results confirm that breech strike is genetically positively correlated with neck wrinkle, breech cover, face cover and dags. Therefore, breeding plainer sheep with less wool cover and less dags will reduce breech strike. However, although breech strike appears to be genetically the same trait at different ages, neck wrinkle, dags and breech cover did not always seem to be the same trait. Due to the low genetic correlation among dags across ages, breeding for low dags in hoggets will not result in a lowering of dags in adult ewes but as it is moderately repeatable, culling on dags will reduce dags in the current generation. As dags are the most important predisposing factor for breech strike in hogget sheep in a low wrinkle flock in the southern Mediterranean regions of Australia, the underlying causes of winter scours needs further research to determine why it is genetically a different trait in ewes of different ages. Ram breeders in the different environmental regions of Australia should therefore continue to score their adult sheep for dags at different times of the year to investigate whether selection for low dags, using hogget data, will carry over to older age groups.
Genetics is a powerful tool as any genetic gain is permanent and accumulates over time. These results imply that wool producers should not purchase or use wrinkly rams, or rams that are prone to develop diarrhoea. Ram buyers should visit the MerinoSelect website (sheepgenetics.org.au) which provides very useful breeding information on wrinkles, dags and production for a large number of potential rams. Breeders should use this information to breed more flystrike resistant sheep.
- Breech strike, neck wrinkles, dags and the wool cover traits are heritable at different ages. Breech strike is positively correlated with neck wrinkle, breech cover, face cover and dags.
- Neck wrinkle, dags and breech cover did not always seem to be the same trait, even though breech strike appears to be genetically the same trait at different ages.
- Breeding plainer sheep with less wool cover and less dags will reduce breech strike.
- Breeders should continue to score their adult sheep for dags at various times of the year to investigate whether selecting for low dags using hogget data will carry over to older age groups.
Full paper available here: publish.csiro.au/AN/AN20415