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.

Identifying indicator traits for breech strike in Merino sheep in a Mediterranean environment

“…This estimate is slightly higher than the value estimated by Scholtz et al (2010) for absence of breech strike, but significantly higher than the heritability of 0.32 reported by Smith et al (2009) for breech strike. These differences in heritability estimates could possibly have been caused by different fly strike frequencies and different models, and also because Smith et al (2009) treated breech strike as a continuous trait, whereas Scholtz et al (2010) treated breech strike as a threshold trait, as was done in the present study. As this flock was composed of sheep from different unrelated genetic sources, strain or origin of flock effects could also have contributed to the higher heritability of breech strike in spite of the fact that the property of origin was fitted in the model to accommodate genetic groups.…”

“…The highest wrinkle scores were recorded on the neck of the animals. Scholtz et al (2010) showed that neck wrinkle is genetically strongly correlated with breech wrinkle in mulesed (r g = 0.80) and unmulesed (r g = 0.89) sheep. They also found a stronger genetic relationship between the absence of breech strike and neck wrinkle (r g = -0.57) in unmulesed sheep than that between the absence of breech strike and breech wrinkle (r g = -0.45), which strongly agrees with the results of the present study.…”

“…Several genetic studies have been carried out during the past 70 years to identify the causal factors of fly strike (Seddon et al 1931;Watts and Merritt 1981;Gherardi et al 1983;Lewer et al 1987;Raadsma and Rogan 1987;Raadsma 2000;James 2006;Scholtz et al 2010). This has led to the identification of body wrinkle, wool colour, fleece rot, dermatophilosis and fibre variability traits as potential indicator traits to breed indirectly for body strike resistance (Raadsma and Rogan 1987).…”

“…This was probably due to the effectiveness of the mulesing operation that was introduced in the early thirties, regular crutching and jetting sheep with chemicals. Several studies have indicated that faecal soiling (dags) in the breech, wrinkles and bare area in the breech (Scobie et al 2008(Scobie et al , 2002(Scobie et al , 2007Edwards et al 2009;Greeff and Karlsson 2009;Smith et al 2009;Scholtz et al 2010) are the main candidates as potential indicator traits for breech strike resistance. The size of the bare area in the breech received strong focus of attention as a solution for surgical mulesing, as removal of wool around the anus through breeding reduces the potential to develop dags, as was shown by Scobie et al (2008) in Romney sheep.…”

The adaption of the South Africa sheep industry to new trends in animal breeding and genetics: A review

“…Research by Scholtz et al (2010b; suggested sufficient genetic variation on the underlying liability scale where breech flystrike of Merinos is concerned. The presence of significant genetic variation in breech strike is supported by marked line differences in its frequency in lines of Merino sheep divergently selected for number of lambs weaned per ewe joined (Scholtz et al, 2010a).…”