Influence on performance of three to six-week-old broilers of varying dietary protein contents with supplementation of essential amino acid requirements.

Effects were evaluated of feeding diets differing in CP content but formulated to be adequate in essential amino acids (EAA) on performance and certain plasma metabolites of female broilers from 21 to 42 days of age. In Experiment 1, isocaloric diets containing either 22.6, 18.2, 15.7, 14.2, or 12.3% CP were used. Experiment 2 was similar to Experiment 1 but included three additional diets, which consisted of the three lowest CP diets supplemented with L-glutamic acid (GLU) to increase formulated CP levels to 18%. Protein efficiency ratio and abdominal fat deposition (AFD) decreased linearly as CP increased in both experiments. Increasing CP increased plasma thyroxine in Experiment 1 but not in Experiment 2. Plasma triiodothyronine and triglycerides were not significantly influenced by treatment. A positive curvilinear relationship existed between CP level and body weight gain in both experiments. The calculated CP requirement for growth 95% of maximum ranged from 16.0 +/- 1.8 (Experiment 1) to 17.5 +/- 1.8% (Experiment 2). Feed efficiency (FE) increased in a curvilinear manner as CP increased in Experiment 1, and the calculated CP requirement for FE 95% of maximum was 18.9 +/- 2.5%. In Experiment 2, however, improvements in FE were obtained with each increase in CP. The calculated requirement levels of CP for optimal growth were low compared to conventionally fed levels. However, obtaining optimal growth consistently with the former levels appeared unlikely because of the magnitude of associated standard errors. The CP level required for optimal FE was clearly higher than that for optimal growth.