Genetic parameters for carcass composition and meat quality traits were estimated in Scottish Blackface sheep, previously divergently selected for carcass lean content (LEAN and FAT lines). Computerized X-ray tomography (CT) was used to obtain non-destructive in vivo estimates of the carcass composition of 700 lambs, at ca. 24 weeks of age, with tissue areas and image densities obtained for fat, muscle and bone components of the carcass. Comprehensive measures of meat quality and carcass fatness were made on 350 male lambs, at ca. 8 months of age, which had previously been CT scanned. Meat quality traits included intramuscular fat content, initial and final pH of the meat, colour attributes, shear force, dry matter, moisture and nitrogen proportions, and taste panel assessments of the cooked meat. FAT line animals were significantly ( P , 0·05) fatter than the LEAN line animals in all measures of fatness (from CT and slaughter data), although the differences were modest and generally proportionately less than 0·1. Correspondingly, the LEAN line animals were superior to the FAT line animals in muscling measurements. Compared with the LEAN line, the FAT line had lower muscle density (as indicated by the relative darkness of the scan image), greater estimated subcutaneous fat (predicted from fat classification score) at slaughter, more intramuscular fat content, a more 'yellow' as opposed to 'red' muscle colour, and juicer meat (all P , 0·05). All CT tissue areas were moderately to highly heritable, with h 2 values ranging from 0·23 to 0·76. Likewise, meat quality traits were also moderately heritable. Muscle density was the CT trait most consistently related to meat quality traits, and genetic correlations of muscle density with live weight, fat class, subcutaneous fat score, dry matter proportion, juiciness, flavour and overall liking were all moderately to strongly negative, and significantly different from zero. In addition, intramuscular fat content was positively genetically correlated with juiciness and flavour, and negatively genetically correlated with shear force value. The results of this study demonstrate that altering carcass fatness will simultaneously change muscle density (indicative of changes in intramuscular fatness), and aspects of intramuscular fat content, muscle colour and juiciness. The heritabilities for the meat quality traits indicate ample opportunities for altering most meat quality traits. Moreover, it appears that colour, intramuscular fat content, juiciness, overall liking and flavour may be adequately predicted, both genetically and phenotypically, from measures of muscle density. Thus, genetic improvement of carcass composition and meat quality is feasible using in vivo measurements.

Genetic parameters for LM fatty acid composition were estimated in Scottish Blackface sheep, previously divergently selected for carcass lean content (LEAN and FAT lines). Furthermore, QTL were identified for the same fatty acids. Fatty acid phenotypic measurements were made on 350 male lambs, at approximately 8 mo of age, and 300 of these lambs were genotyped across candidate regions on chromosomes 1, 2, 3, 5, 14, 18, 20, and 21. Fatty acid composition measurements included in total 17 fatty acids of 3 categories: saturated, monounsaturated, and polyunsaturated. Total i.m. fat content was estimated as the sum of the fatty acids. The FAT line had a greater i.m. fat content and more oleic acid, but less linoleic acid (18:2 n-6) and docosapentaenoic acid (22:5 n-3) than did the LEAN line. Saturated fatty acids were moderately heritable, ranging from 0.19 to 0.29, and total SFA were highly heritable (0.90). Monounsaturated fatty acids were moderately to highly heritable, with cis-vaccenic acid (18:1 n-7) being the most heritable (0.67), and total MUFA were highly heritable (0.73). Polyunsaturated fatty acids were also moderately to highly heritable; arachidonic acid (20:4 n-6) and CLA were the most heritable, with values of 0.60 and 0.48, respectively. The total PUFA were moderately heritable (0.40). The QTL analyses were performed using regression interval mapping techniques. In total, 21 chromosome-wide QTL were detected in 6 out of 8 chromosomal regions. The chromosome-wide, significant QTL affected 3 SFA, 5 MUFA, and 13 PUFA. The most significant result was a QTL affecting linolenic acid (18:3 n-3) on chromosome 2. This QTL segregated in 2 of the 9 families and explained 37.6% of the phenotypic variance. Also, 10 significant QTL were identified on chromosome 21, where 8 out of 10 QTL were segregating in the same families and detected at the same position. The results of this study demonstrate that altering carcass fatness will simultaneously change i.m. fat content and oleic, linoleic, and docosapentaenoic acid content. The heritabilities of the fatty acids indicate opportunities for genetically altering most fatty acids. Moreover, this is the first report of detection of QTL directly affecting fatty acid composition in sheep.

Quantitative trait loci (QTL) were identified for traits related to carcass and meat quality in Scottish Blackface sheep. The population studied was a double backcross between lines of sheep divergently selected for carcass lean content (LEAN and FAT lines), comprising nine half-sib families. Carcass composition (600 lambs) was assessed non-destructively using computerized tomography (CT) scanning and meat quality measurements (initial and final pH of m. semimembranosus, colour, shear force value, carcass weight, lamb flavour, juiciness, tenderness and overall liking) were taken on 300 male lambs. Lambs and their sires were genotyped across candidate regions on chromosomes 1, 2, 3, 5, 14, 18, 20 and 21. QTL analyses were performed using regression interval mapping techniques. In total, nine genome-wise significant and 11 chromosome-wise and suggestive QTL were detected in seven out of eight chromosomes. Genome-wise significant QTL were mapped for lamb flavour (OAR 1); for muscle densities (OAR 2 and OAR 3); for colour a*(redness) (OAR 3); for bone density (OAR 1); for slaughter live weight (OAR 1 and OAR 2) and for the weights of cold and hot carcass (OAR 5). The QTL with the strongest statistical evidence affected the lamb flavour of meat and was on OAR 1, in a region homologous with a porcine SSC 13 QTL identified for pork flavour. This QTL segregated in four of the nine families. This study provides new information on QTL affecting meat quality and carcass composition traits in sheep, which may lead to novel opportunities for genetically improving these traits.

This paper describes an investigation of the use of computer tomography (CT) to genetically improve carcass composition and conformation in Scottish Blackface sheep. After 5 years of selection on an index designed to improve both composition and conformation (the 'CT index'), a large response was observed in the CT index, with genetic progress equivalent to 0.11 phenotypic standard deviations per year. Heritabilities for the index and for the component traits of average CT-assessed muscle area, ultrasonic muscle depth and ultrasonic fat depth were 0.41 (s.e. 0.08), 0.38 (s.e. 0.07), 0.41 (s.e. 0.05) and 0.30 (s.e. 0.05), respectively. The index was positively genetically correlated with ultrasonic muscle depth and carcass weight and negatively genetically correlated with fat class. The genetic and phenotypic correlations among ultrasonic measurements were positive and moderate. However, many of the genetic correlations tended to have large standard errors. Selection on the CT index moderately improved conformation and was successful at decreasing fat class of the carcass. Equivalent selection on live weight at ultrasound scanning would improve carcass and slaughter weight, and total price received, but would have a slightly deleterious impact on conformation score. The results of this study demonstrate that genetic improvement of carcass quality can be achieved in hill sheep using CT assessed traits.

Genetic parameters for eating quality assessed by trained taste panellists were estimated on longissimus thoracis et lumborum (LTL) muscle in Scottish Blackface lambs, comprising lines previously divergently selected for carcass lean content (FAT and LEAN lines) as well as crosses between these lines. Also, relationships between eating quality assessments and fatty acid composition were investigated. Eating quality and fatty acid phenotypic measurements were made on 350 male lambs, at ca. 8 months of age. Eating quality measurements included 18 descriptive terms and fatty acid composition measurements included in total 17 fatty acids of three types: saturated, monounsaturated and polyunsaturated. The FAT line had juicier meat and more vegetable flavour than the LEAN line. Most of the eating quality traits were moderately to highly heritable, with heritabilities ranging from 0.21 (lamb flavour) to 0.92 (sweet flavour). Lamb flavour, juiciness and overall liking were strongly negatively correlated with individual polyunsaturated fatty acids, with the correlations being significantly different from zero. Overall liking was strongly positively correlated with the proportion of total monounsaturated fatty acids. This study provides new information on genetic parameters for eating quality traits in sheep, which may lead to novel opportunities for genetically improving these traits.