Genetic parameters of breech strike, neck wrinkles, dags and breech cover over the lifetime of crutched Merino ewes in a Mediterranean environment
Authors: Greeff J. C. , Schlink A. C. , Karlsson L. J. E. 
Published: July 2021

Aims 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. Methods 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 5 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. Key results Breech strike was heritable (h2) and repeatable (t) on the observed scale (h2 = 0.07 ± 0.01; t = 0.13 ± 0.01), and as a binary trait on a logistic scale (h2 = 0.51 ± 0.10; t = 0.68 ± 0.10). Breech strike was genetically positively correlated across ages. 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. 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. However, breech cover scored mid-year, before shearing and after shearing are 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. Conclusions Breech strike and its indicator traits (i.e. 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. Implications This study shows that selecting ewes for low breech strike, and for reduced neck wrinkle, lower breech cover and less dags will reduce breech strike in mature Merino ewes in future generations. However, as the phenotypic correlations were relatively low, culling ewes on these indicator traits will result in little gain in the current generation. It appears that dags at different ages is not genetically the same trait. Thus the benefit of selecting or culling ewes on dags will not be carried over to subsequent ages.