Phenotypic and genotypic analysis of a barebreech trait in Merino sheep as a potential replacement for surgical mulesing

Abstract

The potential for adopting a genetic solution to protect sheep from blowfly strike on the breech was investigated in a flock of sheep that contained several animals expressing a trait characterised by low wool coverage over the breech and through a wide channel from the anus to the udder or scrotum. A scoring system (1, bare to 5, woolly) was developed and used to determine the heritability of the trait and its phenotypic and genetic correlations with other traits of importance in a sheep enterprise. In comparison to animals with woolly breeches, the skin in the breech of animals with a low bareness score was characterised by a low density of follicles producing short, medullated fibres, with histological evidence of immune rejection and follicular atrophy. The bareness score of progeny was influenced by the score of their respective sires suggesting a strong genetic component. The heritability of bareness score was moderate to high (h2 = 0.45 ± 0.02, 0.53 ± 0.01 and 0.38 ± 0.02 at lamb, hogget and adult ages, respectively). The lactation status and age of ewes influenced their bareness score, resulting in a low repeatability (0.42) of the trait between ages in females. Genetic correlations between bareness score and most other economically important traits were low. The weight of belly wool and the weight of skirtings was genetically related to bareness score (rg = +0.52 and +0.48 respectively), indicating that animals with barer breeches tend genetically towards lighter belly wool weights and lower weight of skirtings at wool classing. Selection and breeding for bareness score should achieve relatively rapid progress towards fixing the trait in a flock and without adverse effects on other important traits. Caution should be exercised in extrapolating these results to other bloodlines and environments where genetic mechanisms or environmental influences may be different.