Subcutaneous fat distribution as assessed by ultrasound in three genotypes of sheep : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

A trial was undertaken to provide data for an objective description of subcutaneous fat distribution in sheep and to examine the effect of genotype and age on subcutaneous fat distribution. Two hundred and twenty eight female sheep of three genotypes (Border Leicester -BL, Dorset Down Selected - DDS and Dorset Down Control - DDC) and two ages ( 7 1/2 mo, hogget and 20 mo, two-tooth) were ultrasonically scanned at six sites (tissue depth over the fifth rib caudal to the major tuberosity of the humerus and dorsal to the olecranon of the ulna (R), depth of muscle over the last rib (B), depth of fat over B (C), tissue depth over the last rib ll cm from the midline (GR), depth of fat above muscle over the penultimate lumbar vertebra (L), depth of fat on the rump lateral to the eight coccygeal vertebrae and medial to the ischiatic tuber of the tail (T)). The BL and DDS have been selected for high lean tissue growth rate using an index that combines information on liveweight, B and C. Fat distribution was defined as a shape variable that is independent of size. It was characterised by examining the relative size of differences in absolute tissue depths, tissue depths adjusted to a common liveweight (to remove the effect of variation in body size), or ratios of fat depth to muscle depth (shape). BL and DDS were at similar stages of developmental growth in terms of the lean body as evidenced by B divided by liveweight ¹/³ (6.98 and 6.92 for BL and DDS hoggets, 7.62 and 7.71 for BL and DDS two-tooths). While BL and DDS hoggets showed similar relative fatness (fat depth/muscle depth), the BL had relatively more fat (P<0.05) at GR (-7%) while the DDS had more at R (+9%) and T (+6%). At the two-tooth stage DDS were relatively fatter (fat depth/muscle depth), the greatest differences being significant (P<0.01) at R (+13%), L (+18%) and T (+37%). These data demonstrated two effects. Firstly that the BL preferentially deposits fat near the sites of C and GR relative to other sites and secondly that the DDS showed greater increases in fatness than the BL between the two ages. There was a general trend for fat distribution to change with age whereby relatively more fat was deposited toward the midline and tail as animals matured. This effect was greater in the DDS. No response to selection for leanness was found between the DDS and DDC for either fatness or fat distribution. This was attributed to the selection programme being at an early stage. Implications of these findings to scientific research and commercial meat production are discussed.