ABSTRACT: The objective of this study was to quantify the effect of inbreeding on carcass quality, growth rate, live conformation measures, and calv- ing performance in purebred populations of Charolais, Limousin, Simmental, Hereford, and Angus beef cattle using data from Irish commercial and pedigree herds. Variables analyzed are reflective of commercial farming practices. Inbreeding was included in a linear mixed model as either a class variable or a linear continuous variable. Nonlinear effects were nonsignificant across all traits. Inbred animals had decreased carcass weight and less carcass fat. The effects of inbreeding were more pronounced in the British beef breeds. Effects for carcass weight ranged from -0.87 kg (Charolais) to -1.90 kg (Hereford) per 1% increase in inbreeding. Inbred Charolais and Hereford animals were younger at slaughter by 3 and 5 d, respectively, per percent- age of increase in inbreeding, whereas the effect of in- breeding on age at slaughter differed significantly with animal sex in the Limousin and Angus breeds. Inbred Limousin and Angus heifers were younger at slaughter by 5 and 7 d, respectively, per percentage of increase in inbreeding. Continental animals were more affected by inbreeding for live muscling and skeletal conforma- tional measurements than the British breeds; inbred animals were smaller and narrower with poorer devel- oped muscle. Calf inbreeding significantly affected peri- natal mortality in Charolais, Simmental, and Hereford animals. The effects were dependent upon dam parity and calf sex; however, where significant, the association was always unfavorable. Dam inbreeding significantly affected perinatal mortality in Limousin and Hereford animals. Effects differed by parity in Limousins. Inbred first-parity Angus dams had a greater incidence of dys- tocia. Although the effects of inbreeding were some- times significant, they were small and are unlikely to make a large financial effect on commercial beef pro- duction in Ireland. Key words: beef cattle, calving performance, carcass, growth, inbreeding depression ©2008 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2008. 86:3338–3347 doi:10.2527/jas.2007-0751 INTRODUCTION Inbreeding occurs when related individuals are mated and is defined as the probability that 2 alleles at any locus are identical by descent (Falconer and Mackay, 1996). Inbreeding depression refers to the reduction in mean phenotypic performance as a result of inbreed- ing. The effects of inbreeding on production traits of beef and dairy cattle have been well documented (Mac- Neil et al., 1989; Smith et al., 1998; Mc Parland et al., 2007a). However, the majority of studies on the effects of inbreeding in beef populations are dated and general- ly involve experimental herds and thus are not entirely representative of real populations (Dinkel et al., 1968; Krehbiel et al., 1969; MacNeil et al., 1989). The effects of inbreeding on preweaning production (Keller and Brinks, 1978), production up to 1 yr of age (Nelms and Stratton, 1967), and varying definitions of cow fertility (MacNeil et al., 1989) have previously been reported in beef cattle. However, the effect of inbreeding on carcass quality, animal conformation, and calving performance, particularly in relation to beef animals, is less well documented. A review of the literature on the effects of inbreeding in beef cattle detailed only linear effects (Burrow, 1993). However, inbreeding has been shown to have nonlinear effects on production traits in dairy cattle (Croquet et al., 2006), as well as weaning traits in beef cattle (Dinkel et al., 1968). Furthermore, Burrow (1993) reported a paucity of information con- cerning the effect of inbreeding on some economically important traits in beef cattle. Few studies (Burrow, Inbreeding effects on postweaning production traits, conformation, and calving performance in Irish beef cattle S. Mc Parland,*† 1 J. F. Kearney,‡ D. E. MacHugh,† and D. P. Berry* *Teagasc, Moorepark Dairy Production Research Centre, Fermoy, Co. Cork, Ireland; †Animal Genomics Laboratory, School of Agriculture, Food Science and Veterinary Medicine and Conway Institute for Biomolecular and Biomedical Research, College of Life Sciences, University College Dublin, Belfield, Dublin 4, Ireland; and ‡Irish Cattle Breeding Federation, Bandon, Co. Cork, Ireland 1 Corresponding author: sinead.mcparland@teagasc.ie Received November 23, 2007. Accepted July 31, 2008. 3338 Published December 5, 2014