Preventive Veterinary Medicine 103 (2012) 257–264 Contents lists available at SciVerse ScienceDirect Preventive Veterinary Medicine j ourna l ho me pag e: ww w.elsevi er.com/locate/prev etmed Factors affecting dystocia and offspring vigour in different sheep genotypes Cathy M. Dwyer ∗ , Lutz Bünger Sustainable Livestock Systems Group, Scottish Agricultural College, King’s Building, West Mains Road, Edinburgh EH9 3JG, UK a r t i c l e i n f o Keywords: Genotype Dystocia Lamb vigour Sheep a b s t r a c t Birth difficulty and poor lamb vigour are significant causes of perinatal lamb mortality. In this study we investigated whether sheep breeds differing in appearance, muscularity and selection history also had differences in dystocia and lamb vigour, and considered some of the factors that may contribute to the variation in these traits. Data were collected at birth from a total of 3252 lambs of two terminal sire breeds selected for lean growth (Suffolk [S], n = 500 and Texel [T], n = 1207), from a Hill breed (Scottish Blackface [B], n = 610), which has been mainly selected for hardiness, and a crossbred (Mule × T [M], n = 935) representing a maternal line. For each lamb the degree of assistance at delivery, lamb presentation, amount of assistance to achieve successful sucking, sex, litter size and birth weight were recorded. T lambs required the most, and B and M lambs the least assistance at birth, S lambs were intermediate (% lambs assisted: T = 55.7, S = 30.7, B = 22.7, M = 24.9, P < 0.001). T and S lambs were equally likely to be malpresented at birth (29% of births) and more likely to be malpresented than B or M lambs (20%; P < 0.001). In T and S breeds lambs requiring veterinary assistance at delivery were mainly heavy and singleton lambs, whereas in B and M breeds these were exclusively low birth weight lambs in multiple litters. Although heavier lambs needed greater birth assistance, T lambs were lighter than S and M lambs, but heavier than B lambs (birth weight (kg): S = 4.66, M = 4.56, T = 4.32, B = 3.67, P < 0.001). S lambs were more likely to require assistance with sucking than other breeds, and T lambs also required more assistance than B or M lambs (% lambs assisted to suck: S = 56.0, T = 31.6, M = 19.8, B = 18.4, P < 0.001). Heavier lambs were more likely to suck unaided than lighter lambs (P < 0.001). The data suggest that the two terminal sire breeds, selected narrowly for greater productivity (muscle growth and conformation), are more likely to experience birth difficulty and poorer lamb vigour than the breed selected for hardiness, or the cross breed. Whether these effects arise as a consequence of genetic selection (e.g. for specific lamb conformation), or as a result of management practices to achieve selection goals (e.g. increased intervention at lambing) is unknown. Specific actions to improve birth difficulty and lamb vigour, such as including these traits in the selection index, would be beneficial in improving the welfare of ewes and lambs of the terminal sire breeds. © 2011 Elsevier B.V. All rights reserved. ∗ Corresponding author at: SAC, Roslin Institute Building, Easter Bush, Midlothian EH25 3RG, UK. Tel.: +44 131 5353228; fax: +44 131 5353121. E-mail address: cathy.dwyer@sac.ac.uk (C.M. Dwyer). 1. Introduction In livestock species farmed for meat production, increased productivity is achieved by selection for increased number of offspring, increased growth rates and increased muscling, such that larger numbers of off- spring reach market weights quicker with a greater meat yield. However, there is evidence that selection for high 0167-5877/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2011.09.002