Responses of Cobb × Cobb 500 broilers to dietary amino acid density regimens

Dietary amino acid density responses are fairly well documented for Ross broilers but are sparse for the Cobb bird. Cobb 500 fast-feathering chicks were obtained from a commercial hatchery and fed either high, moderate, or low dietary amino acid density diets through a starter (0 to 14 d, in crumbles), grower (14 to 28 d, in pellets), and finisher (28 to 42 d, in pellets) phase program, for a total of 17 possible treatment combinations at d 42. Before formulation of the experimental diets, samples of the protein-contributing ingredients (corn, soybean meal, and Pro-Plus) were analyzed for total amino acid concentration. Live performance, carcass traits, breast and thigh meat composition, and feed cost efficiency served as evaluation criteria for this study. Body weight gain (P < 0.05) and feed conversion (P < 0.001) were improved when broilers were fed the high diets through most of their life, but mortality and BW uniformity remained unaltered (P > 0.05). Carcass yield was unaffected (P > 0.05) by dietary treatments. Breast meat yield was maximized (P < 0.05) in birds fed the high diets, whereas abdominal fat percentage (P < 0.01) was increased in birds fed the low diets. Breast meat protein and fat content were unaffected (P > 0.05) by diet, but protein concentration of thigh meat increased (P < 0.10) in birds fed the high diets compared with those fed the low diets, and birds fed the moderate diets had intermediate responses. The benefits observed when feeding high amino acid density diets were not necessarily the most economical when feeding these throughout the entire grow-out period. The Cobb 500 male showed an ability to respond to increasing amino acid density as the bird aged, and any beneficial effects of feeding high amino acid density diets early in life may not be warranted for this strain.