Lamb production is an important part of UK agriculture, contributing more than 10% of total livestock output. It is crucial for maintaining employment and infrastructure in rural communities, and in managing and enhancing the countryside and biodiversity. For the UK sheep industry to continue as a major producer and exporter of lamb, the improvement of its economic sustainability is essential and requires high product quality.

As one of the major terminal sire breeds in the UK, Texel sheep (TEX) have been selected over generations for rapid growth, muscularity and lean meat content. In contrast, Scottish Blackface sheep (SBF) have been selected for their hardiness and maternal ability, with less attention being paid to carcass traits. It is clear therefore that divergent selection goals are driving the breeding of TEX and SBF. Based on work in pigs in particular, where selection for lean tissue growth rate has been used extensively, it might be predicted that these two breeds would have diver-gent muscle fibre characteristics. The proportion of muscle fibre types and their size affect muscularity as well as functional properties of the musculature and meat eating quality (MEQ). Breed differences indicate a genetic basis for these traits. This study evaluates the effects of breed and sex, together with those of birthweight and litter size on fibre type characteristics.