DIRECT AND CORRELATED RESPONSES TO SELECTION FOR HIGH OR LOW ULTRASONIC BACKFAT IN DEPTH IN SOUTHDOWN SHEEP

Direct selection for high or low liveweight-adjusted backfat depth (LA BF), assessed by ultrasound at Position C over the longissimus dorsi m uscle at the last rib, in Southdown sheep resulted in highly significa nt differences (3.7 versus 2.0 mm at 33 kg liveweight for rams, P < 0. 01) between the lines after 8 years of selection. The response to sele ction per unit of cumulative selection differential was generally high and apparently linear. A generation interval of 3.01 years was estima ted resulting in c. 2.7 generations of selection for 1986-born animals . Correlated responses in birth weight, pre-weaning growth, and post-w eaning growth were not significant. However, consistent correlated res ponses were observed in liveweight-adjusted height and length over the selection period with low-line animals being longer and taller. These responses imply a negative genetic correlation between these traits a nd LABF. It was concluded that divergent selection for LABF was effect ive, resulting in lines with significantly different backfat depth, he ight, and length at the same liveweight.