Lamb growth, carcass and pelt measurements are reported from 5775 lambs born to Border Leicester × Merino (BLM) and Hyfer (Dorset × Merino composite) ewes joined to Dorset, Suffolk and Hyfer rams in three lamb-production systems at Cowra and Wagga Wagga over 5 years. The production systems involved lambings at different seasons of the year and increasing levels of intensification from autumn annual joining, spring joining with a backup mating for non-pregnant ewes, to an accelerated 8-monthly lambing system. Lambs from BLM ewes were 0.7 kg heavier at birth and grew 3% faster and reached slaughter, at 18 kg carcass weight, 1–2 weeks earlier than did lambs from Hyfer ewes (P < 0.01). The advantage in growth rate of lambs from BLM compared with Hyfer ewes was greater for lambs born in November. These lambs had poorer growing conditions over the summer months, with 22% lower growth rate and taking 5 weeks longer to reach slaughter than did lambs born in March or August. There was no difference between Dorset and Suffolk sire breeds for any lamb growth traits, with lambs from Hyfer sires growing 7% slower. Ram and cryptorchid lambs reached slaughter 1 week earlier than did wethers, with ewes a further 9 days later (P < 0.01). Sex differences were maximised when lamb growth was greatest. There were no differences in carcass fat measurements between ram and cryptorchid lambs, although both were considerably leaner than wethers (2.2–3.0 mm fat at the GR site), which were 1.3–2.1 mm leaner than ewes at 18-kg carcass weight. The range in average growth rate of progeny of the BLM ewes from the 12 different source flocks at Cowra and Wagga Wagga was 10–14% of the mean which was twice the difference in average growth rate of progeny from the BLM and Hyfer dams. There was also significant variation among the BLM source flocks for carcass fat measurements. Lamb progeny from the different sire- and dam-breed combinations had varying levels of heterozygosity. There appears to be little loss of heterosis or hybrid vigour for lamb growth, although the sire breed × dam breed interaction was significant (P < 0.01) for age at slaughter at Wagga Wagga, in which the ranking of the lamb types was consistent with the levels of heterozygosity. The estimates of between-lambing repeatability for the ewes were highest for birthweight (0.35 Cowra and 0.27 Wagga Wagga) and declined at later ages (0.26–0.17), with lower estimates for carcass traits.
