Lamb consumption has declined in recent decades partly due to consumer perception of lamb as overfat. Lamb production is an important part of UK agriculture so, to help safeguard the industry, it is necessary to produce carcasses that better meet market demands. This issue was addressed in two main ways in this work. Firstly lamb growth and development and breed and feed effects on these were studied. Secondly, use of X -ray computed tomography (CT) scanning for in vivo measurement of carcass traits in meat sheep, and incorporation of CT scanning into genetic improvement programmes, were explored. This work aimed to (i) explore consequences of some breed and feed choices on lamb growth and carcass composition, (ii) examine changes in carcass quality traits during growth in meat sheep, (iii) identify ways in which CT scanning information can be used to measure carcass quality traits in meat sheep, and (iv) optimise two -stage selection strategies for incorporating CT scanning into breeding programmes for meat sheep.
Consequences of some breed and feed choices on lamb growth and carcass composition were explored using experimental data where growth rate and carcass composition had been measured at various stages of maturity in lambs of different genotypes (Suffolk and Scottish Blackface and their cross) in different nutritional environments (different dried, pelleted forages indoors and different swards outdoors). Lambs fed indoors on dried Ryegrass or dried Lucerne showed no genotype or diet effects on carcass composition when compared at the same stage of maturity. However, lambs on Ryegrass had lower intakes (0.878 as great) and slower growth (0.851 as fast) than those on Lucerne. Genotype effects on feed intake and growth rate were related to mature size differences. When lambs were grazing different swards outdoors, sward type did not affect carcass composition at any stage of maturity. At 0.30 mature weight, genotype differences in carcass composition were small but by 0.45 mature weight, Scottish Blackface lambs had less fat (0.749 as much), more lean (1.065 as much) and more bone (1.055 as much) than did Suffolk lambs. Genotype by sward interactions existed for growth rate, Suffolk lambs having higher growth rates than Scottish Blackface lambs on Clover but not on Ryegrass. Growth rate declined to a greater extent in Suffolk than Scottish Blackface lambs as nutritional environment became poorer; that is, Suffolk lambs expressed greater environmental sensitivity than the Scottish Blackface.
Carcass composition, tissue distribution and fat partitioning, and the way in which these attributes changed with growth in live weight, were studied in three breeds of terminal sire sheep. Data used were from 160 lambs from a serial slaughter and dissection trial. Texel lambs, at similar live weights, were leaner than Suffolk or Charollais lambs but any significant differences in tissue distribution and fat partitioning were small. Proportion of carcass weight and lean contained in the higher priced joints declined while intramuscular fat content increased with growth in live weight. Lambs became fatter overall, with partitioning of carcass fat tending more towards the subcutaneous depot, with growth in live weight. The way in which carcass composition, tissue distribution and fat partitioning changed with growth were similar in all breeds.
The best means to utilise CT scanning to predict carcass lean, fat and bone weights, tissue distribution and fat partitioning in vivo in terminal sire sheep were tested using data from 160 terminal sire breed lambs that had been CT scanned prior to slaughter and carcass dissection. Carcass lean, fat and bone weights can be predicted with a high degree of accuracy (R2 values of 0.924, 0.978 and 0.830 respectively) using a set of three CT scans which included a scan in each of the three main carcass regions: ischium in the hind leg, 5`h lumbar vertebra in the loin and 8`h thoracic vertebra in the shoulder. Using information from the same three scans, proportion of carcass weight contained in the higher priced joints and intramuscular fat content had moderate accuracy of prediction (R2 0.547 and 0.553 respectively). However, partitioning of fat between subcutaneous and intermuscular depots was not well predicted (R2 0.065).
Although CT is much more accurate in determining carcass composition than ultrasound scanning, it is also more expensive. A two -stage selection strategy for carcass composition in terminal sire sheep breeds was designed. This selection strategy includes a first round of selection using ultrasound and live weight measurements to identify a proportion of the best animals to go forward for CT scanning, and a second round of selection for the elite animals based on CT scan results. This selection strategy should enable much of the benefit of CT scanning to be obtained in a cost -effective way.