Moderate excess of dietary protein increases breast meat yield of broiler chicks

Two factorial experiments were carried out to evaluate the effects of dietary protein level, expressed as energy to protein (E:P) ratio, on yields of carcass and breast and on abdominal fat pad weight of male broiler chicks at 43 and 57 d of age. Two diets similar in their energy content and differing markedly in their protein content, in which the E:P ratios were lower and identical to the NRC (1994) recommendations, were used in Experiment 1. Four diets, the combination of two energy and two protein levels, in which the E:P ratios were lower and identical to the recommendations, were used in Experiment 2. There were no differences in feed intake or weight gain until 42 d of age between broilers fed the diets with the low and those fed the recommended E:P ratio. However, feed efficiency of the former was consistently and significantly (P < 0.05) higher. The high-energy diets did not affect feed intake but significantly improved weight gain between 7 and 28 d of age and feed efficiency until 42 d of age. In both experiments, carcass yield was not affected by dietary E:P ratio at 43 d of age; however, at 57 d of age it was increased significantly by the low E:P ratio in Experiment 1 and in birds fed the high energy diet in Experiment 2. On a factorial basis, the effect of the low E:P ratio on increasing yield was significant only in Experiment 1. The high dietary energy level increased carcass yield only at 43 d of age, but on a factorial basis its effect was significant. Carcass yield was significantly (P < 0.001) higher at 57 than at 43 d of age. In both experiments, breast meat yield was negatively correlated with dietary E:P ratio, a significant factorial effect. The difference in this variable between broilers fed the diets containing the low and the recommended E:P ratio was significant, at both ages in Experiment 1, and only at 57 d of age in Experiment 2. Breast meat yield was not affected by dietary energy level, and it was significantly (P < 0.001) higher at 57 than at 43 d of age. Relative abdominal fat pad weight was increased significantly by increasing dietary E:P ratio. Dietary energy level and age did not affect this variable. The relationship between abdominal fat pad weight and breast meat yield within the treatments tended (in most of the cases not significantly) to be negative. However, this negative relationship was significant, at both ages, when the analysis was carried out with the pooled data at each age. It is concluded that the optimal E:P ratio for maximal breast meat yield for broiler chicks, particularly at 57 d of age, may be below the NRC (1994) recommendations.