Lamb production is an important part of UK agriculture, contributing >10% of total livestock output. It is also important for maintaining employment and infrastructure in rural communities and helping manage and enhance landscape and biodiversity. However, for the UK sheep industry to continue as a major producer and exporter of lamb it is essential to improve carcass quality, since currently only ca. 55% of UK lambs meet core target specifications. Improving carcass quality by increasing lean meat yield (LMY) without a corresponding increase in fatness has been the focus of breeding programmes since the mid 1980's. This has been achieved utilising new technologies such as ultrasound and more recently CT scanning. While such breeding results in cumulative and permanent gains, the penetration to date has been modest, not least because of the belief that current payment systems do not reward producers for increased LMY. This project will help to change this situation. Firstly, there is one highly promising technology (video image scanning and analysis; VISA) currently being evaluated as the basis for a future value-based marketing system in the UK. Secondly, funding bodies, in the UK and elsewhere, in recent years have invested heavily in research to identify genes or chromosomal segments (QTL) responsible for a significant part of the genetic variation for important production traits. The challenge now is to successfully integrate these QTLs into breeding programmes. An earlier LINK project has identified a number of QTLs affecting muscle growth, the one on chromosome 18 in Texels (TM-QTL) being the most promising. Development of the best strategies for introducing and managing this QTL requires reliable information on the magnitude of its effects on LMY as well as possible indirect effects on other important traits. This comprehensive evaluation of new QTLs is important as genes have direct AND indirect effects, e.g. the callipyge mutation having negative effects on meat quality. There are suggestions that some of these mutations may also have negative impacts on fertility and animal welfare. It is therefore essential to understand both the direct and indirect effects of TM-QTL before exploitation. Recent business developments will soon make it possible for UK farmers to exploit other QTLs (e.g. LoinMaxTM and MyoMaxTM have recently been identified and validated in NZ, and are known to enhance muscle growth and decrease fatness in some NZ breeds. In recognition of the stratified crossbreeding structure inherent in the UK sheep industry in which terminal sire rams are mated to Mule ewes within the lowland sector to supply around 70% of slaughter lambs, we will here test the effects of these QTLs in crossbred lambs under UK conditions. This project will link together the comprehensive evaluation of the TM-QTL and other QTLs with the development and evaluation of a VISA system. It is essential that any improvements in LMY arising from exploitation of the QTLs can be differentiated by the VISA system that is likely to become the industry standard. The project will produce a range of carcasses that will be subjected to detailed evaluation through both CT scanning and dissection; providing ideal data sets for investigating relationships between CT and VISA traits and to provide the first estimates of genetic parameters for VISA traits so that these can be built into selection programmes. The project will also investigate the mode of inheritance of the TM-QTL, as well as it's interactions with other genes in the different genetic background of different breeds. For example, are the magnitude of the QTL effects measured in crossbred lambs out of Mule ewes the same as those in purebred Texels? This project will provide the essential information, which is required to avoid potentially inappropriate developments and recommendations, as well as to provide the industry with appropriate new technologies that have been fully evaluated.