Introduction In Australia, sheep production uses different genotypes, ranging from pure genotypes to second-cross Merino–British breeds managed under various production systems (Allden 1970; Cotterill and Roberts 1979). Profit from lamb meat production can be a major component of these systems even in the case of Merino genotypes bred primarily for wool production. The wide variations in climatic conditions, ecology and soil systems and management strategies (Allden 1979), together with different genetics, limit the consistency of carcass quality traits obtained from Australian lambs. In the last 2 decades there have been studies undertaken in lambs to maintain year-round production with improved growth performance and carcass quality traits of lambs. These include studies on genetic variability and sire selection (Fogarty et al. 2000a, 2000b; Hall et al. 2002), studies on nutritional manipulation using supplementary feeding (Rowe et al. 1989; Dixon and Egan 2000; Ponnampalam et al. 2003; Ponnampalam et al. 2005a) and studies on compensatory growth in altered management systems (Hodge and Star 1984; Thatcher et al. 1991). Under traditional commercial practices in Australia, lambs are slaughtered between 3 and 12 months of age, at a liveweight that will achieve carcass weights of greater than 18 kg. Although Australian Journal of Experimental Agriculture, 2007, 47, 1147–1154 0816-1089/07/101147 10.1071/EA07001 © CSIRO 2007 E. N. Ponnampalam A,B,F , D. L. Hopkins D , K. L. Butler C , F. R. Dunshea B,E and R. D. Warner B A Australian Sheep Industry Cooperative Research Centre, Armidale, NSW 2350, Australia. B Animal Production Sciences Platform, Department of Primary Industries, 600 Sneydes Road, Werribee, Vic. 3030, Australia. C Science Innovation and Quality, Department of Primary Industries, 600 Sneydes Road, Werribee, Vic. 3030, Australia. D NSW Department of Primary Industries, Centre for Sheep Meat Development, PO Box 129, Cowra, NSW 2794, Australia. E Present address: The University of Melbourne, Parkville, Vic. 3010, Australia. F Corresponding author. Email: eric.ponnampalam@dpi.vic.gov.au Abstract. The present study examines carcass quality traits in different genotypes of Australian sheep slaughtered at 4, 8, 14 and 22 months of age. Poll Dorset growth × Border Leicester Merino (PD g × BLM), Poll Dorset growth × Merino (PD g × M), Poll Dorset muscling × Merino (PD m × M), Merino × Merino (M × M) and Border Leicester × Merino (BL × M) genotypes were compared. Sires were selected on the basis of growth (PD g , M and BL) and muscle (PD m ) using estimated breeding values (EBVs). Approximately 150 mixed sex animals were slaughtered at each age (595 in total for the four slaughters) and carcass quality traits, including hot carcass weight, loin and semitendinosus (ST) muscle weights, GR fat, fat C, rump fat depth and eye muscle area (EMA), were determined at 45 min to 3 days postslaughter. With age, all carcass quality traits increased in magnitude. Merinos produced the lightest and leanest carcasses (P < 0.01) with lowest values in nearly all the carcass quality traits. At 4 and 8 months of age carcass and loin weights, EMA and GR fat measures were greater (P < 0.01) in second-cross PD g × BLM sheep than all other genotypes. However, traits related to muscle content were similar between first- and second-cross Poll Dorset genotypes at 14 and 22 months of age. The BL × M genotype had lower values than the PD × M genotypes for these traits, at all ages. Traits related to carcass fatness showed that sheep with Border Leicester genetics tend to deposit more fat (P < 0.001) in the carcass than those with Merino or Poll Dorset genetics from 14 months of age onwards. Most carcass quality traits did not differ (P > 0.05) between first-cross Poll Dorset genotypes selected for growth or muscling (PD g × M v. PD m × M). Sires with high muscling EBVs are associated more with shorter carcass length than low muscling EBVs. From ~8 months of age, wethers had greater values (P < 0.05) than ewes of the same age for most carcass quality traits related to size and muscle development, but no effect was present for carcass quality traits that were related to fat deposition. It was clear that the carcass weight of M × M at 8 and 14 months were similar to that observed in PD g × BLM at 4 and 8 months, respectively. Loin weights of M × M and BL × M at 13 and 8 months were similar to that weights observed in PD g × BLM at 8 and 4 months of age, respectively. Animals slaughtered to a muscle weight specification will reach the target at a younger age when they have more Poll Dorset than Border Leicester background genetics. Alternatively animals slaughtered at a specified fat score (using fat C or rump fat depth) will reach the specification at a younger age when they have more Border Leicester than Poll Dorset genetics. Genotype and age effects on sheep meat production 2. Carcass quality traits www.publish.csiro.au/journals/ajea CSIRO PUBLISHING