An investigation of tissue mobilisation in Scottish Blackface ewes using X-ray computer tomography

This thesis reports on an investigation of seasonal changes in tissue levels in Scottish Blackface hill ewes throughout their reproductive lifetime. X-ray computer tomography (CT) scanning was used to produce accurate estimates of total body weights of muscle, carcass fat and internal fat. A calibration trial was undertaken which identified five anatomical sites on the ewe that provided the most informative cross-sectional CT scans to allow the derivation of prediction equations to estimate total weights of bone, muscle and fat in different body depots. Seasonal tissue changes, estimated from CT measurements, were then investigated during one annual cycle, in ewes producing different numbers of lambs. Carcass fat (subcutaneous and inter-muscular) and internal fat depots were depleted during pregnancy and early lactation and repleted from mid-lactation to mating the next year. Muscle was mobilised only when fat reserves were severely depleted. Genetic parameters were estimated for fat (carcass and internal) and muscle weights during the first production cycle of ewes (at pre-mating, pre-lambing, mid-lactation, weaning and pre-mating the following year), and for total tissue weight loss (pre-mating to midlactation) and weight gain (mid-lactation to pre-mating). Heritabilities were moderate to large for most traits except total tissue weight gain of fat and muscle, which showed little genetic variation. Genetic and phenotypic correlations with lamb production traits were then investigated. Muscle weight in the ewe throughout the production cycle, and internal fat weight at events before lambing, tended to be positively correlated with number of lambs born and total weaning weight of lambs. No clear relationships were apparent between total carcass fat weight and lamb production traits. Moderate to large genetic correlations were estimated between weight loss from each tissue and total weights of lambs reared to midlactation or weaning. Changes in tissue levels were modelled over the reproductive lifetime of ewes using random regression, and genetic and phenotypic parameters estimated at different timepoints. Heritabilities were moderate to high for each tissue and were highest during mating for fat weights, and during the dry period and at lambing for muscle weights. Tissue levels in the ewe were affected by the number of lambs produced in the current and previous years and this effect increased as ewes aged. Correlations were estimated for ewe tissue weights throughout their reproductive lifetime (modelled using random regression) with total weights and growth rates of lambs produced. Genetic correlations with lamb growth traits were moderate to large and positive for muscle weights throughout the productive lifetime of the ewe, and small to moderate and positive for internal fat weights. Correlations between carcass fat weight in the ewe and lamb growth traits were not significantly different from zero. The findings from these analyses have implications for hill sheep breeding programmes. To achieve the aims of increasing litter weights at weaning whilst improving carcass quality, it may be preferable to increase muscle weight in the ewe flock, decrease carcass fat weight and maintain, rather than reduce internal fat.