Are breech strike, dags and breech wrinkle genetically the same trait in crutched, uncrutched and mulesed Merino sheep?

Abstract

Breech strike is caused by the infestation of maggots from Lucilia cuprina on the skin of susceptible sheep. Crutching and mulesing modify the potential expression of breech strike, because crutched and mulesed sheep have a lower risk of being struck than uncrutched sheep. The inheritance of and genetic correlations among breech strike from birth to hogget shearing, and dags (breech soiling) and skin wrinkles scored at yearling age were estimated in unmulesed and crutched, unmulesed and uncrutched, and in mulesed and uncrutched Merino sheep to determine whether these traits were genetically the same under these three husbandry systems. The heritability estimates of breech strike in uncrutched, crutched and mulesed sheep were very similar (0.11 ± 0.02, 0.09 ± 0.02 and 0.08 ± 0.05 respectively). Breech strike in uncrutched sheep is genetically strongly correlated with breech strike in crutched sheep (0.80 ± 0.16). The genetic correlation between breech strike in uncrutched and in mulesed sheep was high (0.98 ± 0.40), but the high standard error makes this estimate unreliable. Dags and wrinkles are genetically moderately to strongly correlated with breech strike, which confirms that they are indicator traits for breech strike. Dags in uncrutched sheep was genetically strongly correlated with dags in mulesed sheep (0.84 ± 0.11). Breech wrinkle in uncrutched and crutched sheep, and tail wrinkle in mulesed sheep were not genetically strongly correlated with each other, as the correlations varied between zero and 0.48 (±0.18). The key outcome from this study indicates that all breech strike, wrinkle and dag data in mulesed, crutched or uncrutched flocks may be used in future to estimate a breeding value directly for breech strike.