Factors explaining the incidence of breech strike in a Mediterranean environment in unmulesed and uncrutched Merino sheep

Abstract

Factors responsible for an increase in breech strike were investigated in an unmulesed Merino flock in a Mediterranean environment that were not crutched in 4 years but were crutched in the subsequent 2 years. No chemical preventative treatments such as dipping or jetting were applied in order to assess the susceptibility of the different progeny groups. The number of breech strikes for each animal was recorded from birth to hogget shearing at 17 months of age. Production traits were measured and a large number of visual wool and body conformational traits were subjectively scored to determine whether they contribute to variation in breech strike. Large differences (P < 0.01) in the incidence of breech strike were found between years. A significant interaction (P < 0.05) was found between year of birth and sex for breech strike to weaning, as rams had a higher strike rate than ewes in 1 year only. Across years ewes generally had a higher strike rate than rams. No consistent pattern in potential indicator traits could be found across years, as many traits each explain a small proportion of the variation in breech strike. Dags and skin wrinkle in the tail and breech area contributed most to the variation in breech strike. In uncrutched sheep, dags scored at yearling age for rams, and before hogget shearing for ewes, explained 16% and 10% of the variation in breech strike, respectively, whereas tail wrinkle scored post-weaner shearing explained 21% of the variation in breech strike in both ewes and rams. In sheep that were crutched at yearling age, about 4 months before the normal fly season, tail wrinkle was the most important indicator trait of breech strike in rams, whereas dags at yearling and dags before hogget shearing were the most important indicator traits in ewes. This research confirms that dags and wrinkles are major predisposing factors to breech strike in a Mediterranean environment and that sheep with higher dag and wrinkle scores were consistently more likely to be struck. A large proportion of variation in the incidence of breech strike remains unexplained in uncrutched and unmulesed sheep.