Body composition changes in Scottish Blackface ewes during one annual production cycle

Tissue depletion and repletion were investigated in 142 Scottish Blackface ewes using computed tomography (CT). Ewes of two ages (2 or 3 years) and differing reproductive status (barren, single- or twin-bearing) were studied through one annual production cycle to investigate mobilization of carcass fat (subcutaneous and inter-muscular), internal fat and muscle. Ewes were CT scanned five times during the 1-year study period: pre-mating; pre-lambing; mid-lactation; weaning; pre-mating the following year. For each animal at each of the five scanning events cross-sectional CT scans were taken at five anatomical sites (ischium, hip, 5th lumbar vertebra, 2nd lumbar vertebra and 8th thoracic vertebra). CT images were analysed to yield areas of carcass fat, muscle and internal fat and total weights of these tissues were estimated at each scanning event using prediction equations derived from a separate calibration data set. The results show that both carcass and internal fat depots were depleted during pregnancy and early lactation and repleted from mid-lactation to mating the following year. In proportionate terms, internal fat was most labile, but carcass fat contributed more to total weight change because it was a bigger fat depot. Subcutaneous fat was the largest and most labile of the carcass fat depots. Muscle reserves were depleted only when fat reserves had fallen to very low levels. Older ewes carried more carcass fat in total than younger ewes when reserves were low. Mobilization of tissue reserves in twin-bearing ewes was less than in single-bearing ewes, probably due to preferential feeding.