Genetic (co)variances between wrinkle score and absence of breech strike in mulesed and unmulesed Merino sheep, using a threshold model

Abstract

Heritability estimates for, and genetic correlations among neck wrinkle score, body wrinkle score, breech wrinkle score and the absence of breech strike were determined on 2918 16-month hoggets from a selection experiment with South African Merinos. Data were obtained from the lamb drops of 9 years between 1998 and 2007, born as the progeny of 247 sires and 1250 dams. All progeny born until 2002 were subjected to the Modified Mules operation as lambs (n = 1799). However, due to international pressure to phase out the Mules operation, animals from 2003 onwards were not mulesed (n = 1119). During both periods, the experimental animals were maintained in single flocks (separated on sex) after weaning. Progeny were subjected to visual appraisal of wrinkle scores (range) on the neck (1–6), body (1–5) and breech (1–6) at an age of ~16 months. The occurrence of blowfly strike in the breech area (breech strike) was recorded in all animals during the wool growth period of a year from shearing as weaners. Two four-trait animal models were fitted to obtain (co)variance components and ratios from the separate datasets with neck wrinkle score; body wrinkle score, breech wrinkle score and the absence of breech strike, all defined as threshold traits. Estimates of heritability for breech wrinkle score amounted to 0.27 and 0.45 for the mulesed and unmulesed groups, respectively. The absence of breech strike on the underlying scale was also heritable for the mulesed (0.33) and unmulesed (0.46) groups; translating to heritability estimates of 0.074 and 0.157, respectively, on the observed scale. The genetic correlations of wrinkle scores with absence of breech strike were generally favourable, but significant only for neck wrinkle score (–0.57) and breech wrinkle score (–0.45) in the unmulesed group. It was concluded that South African Merino sheep would respond to selection against breech strike under conditions of adequate challenge. Based on results from unmulesed sheep, selection for plainer sheep is also expected to result in a favourable correlated response in breech strike.