Modelling of breech strike risk and protective efficacy of mulesing in adult Merino sheep B. J. Horton A,B,G , R. Corkrey A,C , J. Smith D , J. Greeff E and L. J. E. Karlsson F A CRC for Sheep Industry Innovation, Homestead Building, UNE, Armidale, NSW 2350, Australia. B Tasmanian Institute of Agriculture, Private Bag 1375, Launceston, Tas. 7250, Australia. C Tasmanian Institute of Agriculture, University of Tasmania, Hobart, Tas. 7005, Australia. D CSIRO Agriculture, FD McMaster Laboratory, Chiswick, Locked Bag 1, Armidale, NSW 2350, Australia. E Livestock Industries, Department of Primary Industries and Regional Development, Western Australia, 3 Baron-Hay Court, South Perth, WA 6151, Australia. F RMB 314, Bridgetown, WA 6255, Australia. G Corresponding author. Email: brian.horton@utas.edu.au Abstract Context. Mulesing provides a high degree of protection against beech strike, but wool producers need to be able to predict the level of risk of flystrike if they cease mulesing. Aims. To use data from trials of mulesed and unmulesed sheep to estimate the distribution of adult breech scores (wrinkle, breech cover and dag) from mean flock values, and use those scores to estimate the risk of breech strike in Merino sheep flocks. Methods. Breech scores and breech strike from flocks with comparable mulesed and unmulesed sheep were examined using logistic functions to relate breech scores in mulesed sheep to those in unmulesed sheep, and to estimate the risk of breech strike in sheep for any given combination of breech scores. Key results. Functions are given to estimate whole flock adult breech wrinkle and breech cover using combinations of the breech traits that are available. Unmulesed Merino sheep have a 6-fold increased risk of breech strike compared with mulesed sheep under the same conditions. There were increased relative risks of breech strike of 2.0 for each unit increase in dag score, 2.2 for each unit increase in breech wrinkle score and 1.7 for each unit increase in breech cover score. Mulesing reduced each of these risk factors, but had an additional 3.2-fold benefit independent of these factors. Adult breech wrinkle was most reliably estimated from neck wrinkle or from marking breech wrinkle (before mulesing). Breech cover was estimated from face cover, or from marking breech cover. The risk of breech strike if adult sheep were to be left unmulesed in a commercial flock was predicted using a centred log ratio transformation to estimate the distribution of dag scores in the unmulesed sheep, given the actual distribution in the mulesed flock. If the distribution was not known, then the mean dag score of the mulesed sheep could be used to estimate the distribution of dag scores in unmulesed sheep using a 2/3 power function to transform the mean dag score. Conclusions. A flock of unmulesed sheep would need to lower breech wrinkle, breech cover and dag scores by 0.5 for each score to achieve a comparable level of risk of breech strike in mulesed sheep. Implications. The use of the model will allow wool producers to selectively breed for sheep with lower risk of breech strike, while continuing to mules the sheep as long as it is necessary to do so. They will be able to estimate what the risk of strike would be if they cease mulesing, due to the proportion of sheep with potentially high wrinkle score, high breech cover or high dag scores at any stage of their breeding program. Additional keywords: breech wrinkle, breech cover, breeding, dags. Received 7 August 2018, accepted 14 October 2019, published online 13 February 2020 Introduction Mulesing involves the removal of loose skin on the breech of the sheep, in order to stretch and permanently enlarge the bare area on the breech and tail (Phillips 2009). Mulesing provides a high level of protection against breech strike, a form of flystrike (Reid and Jones 1976; Smith et al. 2009), primarily due to the reduction of breech wrinkle (Morley and Johnstone 1983), increased bare area (or decreased breech cover) (Scobie et al. 2008) and reduced accumulation of dags (faecal soiling) around the breech (Morley and Johnstone 1983). The proportion of Merino sheep properties that do not mules sheep is low, but is increasing, and there are some price premiums CSIRO PUBLISHING Animal Production Science, 2020, 60, 1051–1060 https://doi.org/10.1071/AN18488 Journal compilation Ó CSIRO 2020 www.publish.csiro.au/journals/an