Direct and correlated responses to selection for high or low ultrasonic backfat in depth in Southdown sheep

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Abstract
Direct selection for high or low liveweight-adjusted backfat depth (LABF), assessed by ultrasound at Position C over the longissimus dorsi muscle at the last rib, in Southdown sheep resulted in highly significant 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 selection per unit of cumulative selection differential was generally high and apparently linear. A generation interval of 3.01 years was estimated resulting in c. 2.7 generations of selection for 1986-born animals. Correlated responses in birth weight, pre-weaning growth, and post-weaning growth were not significant. However, consistent correlated responses 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 and LABF. It was concluded that divergent selection for LABF was effective, resulting in lines with significantly different backfat depth, height, and length at the same liveweight.