Graded levels of soy protein have different effects on feed intake and accretion of body weight, protein and energy when fed without or with limiting amino acids or when fed isocalorically or with a constant ratio of calories to protein

Young chicks were fed isocalonc or constant calorie:protein diets containing increasing doses of unfortified or amino acid (AA) fortified (Met, Thr, Val) dehulled soybean meal (SBM). In all cases, SBM was provided to meet the (ideal) digestible Lys requirement of 5.2% of crude protein (CP), and diets containing 10, 14, 18, 22 or 26 g/100g CP were fed. In the isocaloric series, voluntary food intake increased linearly (P < 0.01) in chicks fed unsupplemented SBM but decreased linearly (P < 0.01) in chicks fed AA fortified SBM as CP was incremented from 10 to 22 g/100g diet. Regardless, over the first three (+ AA) or four (- AA) increments of SBM, daily gain, protein accretion and energy accretion were linear (P < 0.01) functions of CP intake. In the constant caloric:protein series, voluntary food intake decreased linearly (P < 0.01), regardless of AA fortification, as CP (and metabolizable energy) level increased, and accretion of body weight, body protein and body energy increased linearly (P < 0.01) as CP intake increased. At deficient CP levels, lowering the calorie:protein ratio improved weight gain and protein accretion. Also, AA fortification in all cases resulted in increased accretion. The results demonstrate that body composition is affected not only by dietary protein level but also by protein quality and diet composition.