Protein and body weight accretion of chicks on diets with widely varying contents of soyabean meal supplemented or unsupplemented with its limiting amino acids

1. Two studies were conducted to evaluate the effects of relative deficienc ies of methionine and threonine on the growth performance of 8- to 21-d-old chicks fed on isoenergetic diets containing a wide range of crude protein (CP) concentrations from dehulled soyabean meal (SBM). 2. Chicks fed on graded levels of SBM containing supplemental methionine an d threonine (BAL) accreted whole-body protein more efficiently (P < 0.05) t han those receiving graded levels of unsupplemented SBM (DEF), and superior (P < 0.05) growth performance was also obtained at lower CP levels when ch icks were fed on the BAL diets. 3. Voluntary food intake increased between 30 nd 220 g CP/kg in chicks fed on DEF diets, whereas food intake of chicks fed on BAL, diets increased onl y between 30 and 100 g CP/kg, after which it decreased between 100 and 220 CP/kg. 4. Protein efficiency ratio (g gain per g protein intake) decreased with ea ch incremental increase in CP between 30 and 260 g CP/kg, regardless of whe ther diets were BAL or DEE. 5. These data indicate that maintaining a balanced ratio of amino acids is a preferable approach when poultry producers are interested in employing lo w CP diets for economic, physiological or environmental reasons.