The five fattest and five leanest rams were selected on the basis of their liveweight-adjusted ultrasonic backfat thickness from a flock of 100 Coopworths at around 6 months of age. A total of 110 male progeny of these 10 rams were slaughtered between January and April at a mean hot carcass weight (HCW) of 17.13 kg. There were no significant differences between sire classes (i.e., lean, fat) in the weights of the progeny (HCW of 16.95 and 17.31 kg, SED #xB1;0.621). However, there were very marked differences in the regression slopes for the allometric relationships between several carcass variables and HCW, with the lean class having significantly lower slopes for carcass chemical fat content and three subcutaneous fat depths than the fat class. The estimated heritability for backfat thickness (progeny-sire regression) was 0.25. The results indicate that significant progress in terms of carcass leanness should be achievable through the use of suitable ultrasonic devices for measuring backfat thickness in the live animal.
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