A progeny test was designed to test whether genetic superiority for lean growth in terminal sires is expressed in their crossbred progeny when reared in a different environment. In each of 1986, 1987 and 1988, 22 Suffolk rams were chosen at the conclusion of an indoor, intensive performance testing regime on an index score that rated their propensity for lean growth, while constraining fat growth, at 150 days of age. Half of these rams had high index scores and half had low index scores. In each year, around 400 crossbred ewes were mated and the resulting lambs were finished on grass to one of three target live weights (35·5, 41·5, and 47·0 kg). Shoulder joints were dissected on 1505 lambs whilst half carcasses were dissected on 372 lambs. Double sampling techniques were then used to combine the data from the shoulder and half carcass more precisely to predict the lean, fat and bone weight and content in the carcass.
With each increment in target live weight, the carcasses were heavier and had proportionally more fat. The progeny of high index rams consistently had 144 (s.e.d. 32) g more lean, 66 (s.e.d. 12) g more bone, and 186 (s.e.d. 32) g less fat in a 19·7 (s.e. 0·5) kg carcass than progeny of low index rams, from the double sampling procedure. This improved composition reflected a correlated response to ram selection on the index. One standard deviation increase in ram index score corresponded to 51 g more lean and 64 g less fat in the 20 kg carcass of their crossbred offspring. These results show that the use of rams with high lean index scores in a crossbreeding system will produce lambs with leaner carcasses. Visual appraisals of fat and conformation both increased as the weight and, consequently, the fatness of the carcass increased. Offspring of high index rams were consistently scored as less fat than offspring of low index rams. But, at the lighter weights (35·5 and 41·5 kg), they were also scored lower in conformation — in effect, a penalty for their higher genetic merit for lean growth.