Genetically lean (LL) or fat (FL) male chickens were fed from 28 to 47 days of age on 5 experimental diets differing by their methionine+cystine content (5.4, 5.8, 6.2, 6.6 and 7.0 g/kg, respectively). 2. Growth rate of LL chickens was reduced by the lower sulphur-containing amino acid (SAA) concentrations whereas that of FL was not modified. 3. LL chickens exhibited a larger feather protein gain than FL, which was stimulated by SAA intake. 4. SAA retention, when plotted against SAA consumption, was always greater in LL than in FL. 5. Large differences were observed between genotypes for plasma-free amino acids. Lysine, glutamic acid, histidine and serine were found at significantly higher concentrations in LL birds. Branched amino acids, aromatic amino acids, SAA and arginine were found at higher concentrations in FL. No differences were observed for aspartic acid, glycine, alanine and total amino acids. Methionine supplementation decreased free amino acid concentrations, with the exceptions of arginine and leucine. 6. It is concluded that lean chickens require a higher dietary concentration of SAA than FL. This is mainly caused by their lower food consumption and their greater feather synthesis. However, LL use SAA more efficiently than FL.
