The effect of minimizing amino acid excesses in broiler diets

A series of experiments were conducted to evaluate the performance of 7- to 21-day-old broilers fed diets in which excesses of essential amino acids (EAA) were minimized. A 23% protein diet in which all EAA except TSAA were in excess was reduced in protein in a stepwise manner, keeping the corn; soybean meal ratio constant, to the point where all EAA were at minimum requirement level based on the 1984 National Research Council (NRC) requirements. Crystalline EAA supplemented those EAA that became deficient as dietary protein was reduced. Other test diets included one containing an additional 10% of the particular EAA in question and one with nonessential amino acids (NEAA) added to make the low-protein diet isonitrogenous with the 23% control. Performance of birds fed the low-protein test diets was not significantly different from that of the control birds. All EAA appeared to be adequate at levels recommended by the NRC in 1984 except Trp, which required .25% for optimal performance. A growth response was observed when Gly was used as the NEAA source; however, no response was noted when mixtures of NEAA were used as the NEAA source, suggesting that Gly may have been limiting in these earlier treatments. Total carcass protein of birds fed the low-protein test diets in which all EAA were minimized was equal to that of the control birds. In a final experiment utilizing the EAA-balanced, low-protein diet, dietary energy was allowed to vary by 15%. The EAA intake was constant, indicating that birds were eating to satisfy EAA requirements rather than energy requirements. Increased carcass fat deposition paralleled dietary energy increases.