Genetic Analysis of Absence of Breech Strike and Breech Strike Indicator Traits in South African Merino Sheep

The sheep blowfly is an economically important ectoparasite of sheep, and impacts on animal health and welfare under pastoral conditions in South Africa. The absence of flystrike in the breech (ABS) was recorded in 2198 Merino hoggets on the Tygerhoek Research Farm. This trait was analysed together with dag score (DS; n=1623); neck wrinkle score (NWS; n=2162); midrib wrinkle score (MWS; n=2162) and breech wrinkle score (BWS; n=2162) in a five-trait threshold model, using Bayesian inference and Gibbs sampling. ABS occurred at a frequency of 0.89. Estimates of direct heritability (s.e.) amounted to 0.20 (0.06) for ABS, 0.24 (0.07) for DS, 0.31 (0.08) for NWS, 0.34 (0.06) for MWS and 0.36 (0.08) for BWS. The genetic correlation of ABS with DS was favourable at -0.47 (0.20) and significant (P < 0.05). The genetic correlations of ABS with NWS, MBS and BWS were all favourable at -0.78 (0.21), -0.74 (0.16) and -0.78 (0.18) respectively (all P < 0.05). Genetic correlations among wrinkle scores (WS) were all high (> 0.88). Selection for a reduction in DS and in wrinkles on any part of the body will benefit ABS.