A divergent selection experiment in a terminal sire breed of sheep was started in 1985 to examine the differences between genetically lean and fat animals, which were derived from the same population. Responses in performance and physiological traits measured on the selection lines were reported at the BSAP 1991 Winter Meeting. Responses In slaughter and carcass traits, measured on crossbred progeny of the terminal sires, are presented in this study.

The ram selection index was -0.995 FATD + 0.206 WT20, where FATD and WT20 are the ultrasonic backfat depth and liveweight at 20 weeks of age, with both traits standardised to have zero mean and unit phenotyplc variance. A total of 32 and 34 rams from the high (lean) and low (fat) selection lines were performance tested in 1988, and 11 rams were selected from the high line and 12 rams from the low line for mating to crossbred ewes for the progeny test, with a selection differential of 0.85 Index units. There were 8 and 11 rams from the high and low lines with progeny slaughtered at fixed weight, such that the selection differential was 1.0 Index units. A total of 329 crossbred lambs were slaughtered at 16 weeks of age and 250 lambs were slaughtered at fixed weight, 39±1 kg for castrates and 37±1 kg for ewe lambs.