Responses of Cobb×Cobb 500 slow feathering broilers to feeding programs with increasing amino acid densities

An evaluation of the effects of feeding programs with three amino acid (AA) densities given to males and females from a fast growth broiler breed was conducted. A total of 1200 Cobb×Cobb 500 broilers were distributed across 6 treatments in a 2×3 factorial arrangement (Sex×AA Density) with 8 replications of 25 birds per pen. Maize-soybean meal, provided from 1 to 7, 8 to 21, 22 to 35 and 36 to 42 d of age, was formulated without crude protein restriction maintaining minimum ratios between essential AA and Lysine (Lys) in a true digestible (dig) basis as follows: total sulfur AA, 0.75; threonine, 0.65, valine, 0.75 (from 1 to 21 d) and 0.78 (from 22 to 42 d). Experimental treatments were Low (L), Moderate (M) and High (H) AA density diets with M having dig Lys of 1.25, 1.19, 1.09 and 1.05% in the pre-starter to withdrawal phases, respectively. Diets L and H had reductions and increases of 12% dig Lys in relation to the Moderate diet. There were no effects of treatments on mortality (P>0.05) and males had an overall better performance than females (P<0.001). No interactions were observed between Sex×AA density, with the exception of feed intake (1-42 days) and abdominal fat, which were further reduced with males as AA density increased than with females (P<0.01). Body weight gain was affected by AA density (P=0.0308) with birds fed L diets being lighter than those fed H and M diets. Feed conversion ratio (FCR) was improved (P<0.05) as AA density increased. Carcass and breast meat percentages were higher for females (P<0.01) and percent breast yield was highest for the H diet and lowest for the L diet, whereas the M diet was in between. Abdominal fat was reduced with H diet (P<0.05) for males but not for females. The decision making process to change AA density in the diet towards a higher density AA diet will depend on the price of feed ingredients and broiler meat.