GROWTH AND CARCASS CHARACTERISTICS OF HEAVY SLAUGHTER WEIGHT LAMBS - EFFECTS OF SIRE BREED AND SEX OF LAMB AND RELATIONSHIPS TO SERUM METABOLITES AND IGF-1

Texel (T) and Suffolk (S) sired ram, wether and ewe lambs (no = 180) w ere kept at grass with their dams until weaned at 20 weeks and then ta ken to slaughter weights of 40, 44 and 48 kg, also at grass. Sex type, but not sire breed, affected lamb growth rate (rains 293, wethers 253 , ewes 224 g/day; P < 0.001). Both T and S ewe lambs were fatter at sl aughter in all fat depots compared with their ram and wether siblings such that rams could be slaughtered at it calculated 7.7 kg greater li ve weight than ewes at equal carcass fat cover. T carcasses were great er than S carcasses in eye-muscle area (793 v. 732 mm(2); P < 0.001) a nd 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 significantl y affect eye-muscle area. Mean serum insulin-like growth factor-1 (IGF -1) concentrations (mu g/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 TP < 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 I GF-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 move 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) wa s correlated with rate of live-weight change (r = 0.576; P < 0.0011. S ignificant differences for T and S lambs were found in the relationshi ps 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 al ea at slaugh ter. Metabolite concentrations differed little between sire breeds and not at all between sex types.