Texel (T) and Suffolk (S) sired ram, wether and ewe lambs (no = 180) were kept at grass with their dams until weaned at 20 weeks and then taken to slaughter weights of 40, 44 and 48 kg, also at grass. Sex type, but not sire breed, affected lamb growth rate (rams 293, wethers 253, ewes 224 g/day; P < 0·001). Both T and S ewe lambs were fatter at slaughter in all fat depots compared with their ram and wether siblings such that rams could be slaughtered at a calculated 7·7 kg greater live weight than ewes at equal carcass fat cover. T carcasses were greater than S carcasses in eye-muscle area (793 v. 732 mm2; P < 0·001) and killing-out proportion (481 v. 476 g/kg; P < 0·05) but not in any fat measurement. Slaughter weight influenced killing-out proportion (P < 0·001) and all fat measurements (P < 0·01) but did not significantly affect eye-muscle area.
Mean serum insulin-like growth factor-1 (IGF-1) concentrations (μig/l), determined during weeks 8,11, 14, 17 and 20 in a balanced subset of 84 lambs, were higher in T than in S lambs (P < 0·05), in singles than in twins (P < 0·01; rams and wethers only) and in rams than in either wethers or ewes (P < 0·001). Mean serum IGF-1 concentration decreased between week 8 and week 20 with a greater rate of decline in singles than in twins (P < 0·001). IGF-1 was more strongly correlated with live weight at 8 weeks (r = 0·629; P< 0·001) than at 20 weeks (r = 0·293; P < 0·05). Mean IGF-1 (weeks 8 to 20) was correlated with rate of live-weight change (r = 0·576; P < 0·001). Significant differences for T and S lambs were found in the relationships between mean serum IGF-1 (weeks 8 to 20) and daily live-weight gain (weeks 8 to 20) and between mean IGF-1 and eye-muscle area at slaughter. Metabolite concentrations differed little between sire breeds and not at all between sex types.