Dietary protein level and essential amino acid content: influence upon female broiler performance during the grower period

Three experiments were conducted to determine effects of dietary CP level and essential amino acid (EAA) content upon female broiler performance during the grower period (21 to 42 days). In each experiment, the design included a diet containing a conventional CP level (17.8 to 19.0%) and a low CP diet (15.9 to 16.7%) supplemented with several EAA. Regardless of CP content, all diets were formulated to meet National Research Council requirements in 1984 of EAA and to be isocaloric. As additional treatments, modifications of the low CP diet were made in an attempt to improve broiler performance. In Experiments 1 and 2, body weight gain (BWG) and feed efficiency (FE) were significantly impaired by feeding the low CP diet despite various combinations of increased levels of methionine, lysine, arginine, threonine, isoleucine, and tryptophan. In Experiment 3, BWG and FE were not significantly influenced by dietary CP level. Feed intake was unaffected by treatment except when a high level of glutamic acid (GLU) was added; this lowered the intake. The low CP diets increased abdominal fat deposition, which was prevented by 5% GLU supplementation. Breast yield was not affected by diet composition. Plasma free amino acids responded, in some cases, to changes in dietary EAA content.