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 let el, 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 energ y content and differing markedly in their protein content, in which th e E:P ratios were lower and identical to the NRC (1994) recommendation s, were used in Experiment 1. Four diets, the combination of two energ y and two protein levels, in which the E:P ratios were lower and ident ical to the recommendations, were used in Experiment: 2. There were no differences in feed intake or weight gain until 42 d of age between b roilers fed Me diets with the low and those fed the recommended E:P ra tio. However, feed efficiency of the former was consistently and signi ficantly (P < 0.05) higher. The high-energy diets did not affect feed intake but significantly improved weight gain between 7 and 28 d of ag e 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, a t 57 d of age it was increased significantly by the low E:P ratio in E xperiment 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 i ncreased carcass yield only at 43 d of age, but on a factorial basis i ts effect was significant. Carcass yield was significantly (P < 0.001) higher at 57 than at 43 d of age. In both experiments, breast meat yi eld was negatively correlated with dietary E:P ratio, a significant fa ctorial effect. The difference in this variable between broilers fed t he diets containing the low and the recommended E:P ratio was signific ant, at both ages in Experiment 1, and only at 57 d of age in Experime nt 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. Rela tive abdominal fat pad weight was increased significantly by increasin g 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 tin most of the cases not sig nificantly) to be negative. However, this negative relationship was si gnificant, at both ages, when the analysis was carried out with the po oled 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.