Performance and body composition of broilers fed different protein and amino acid levels or free-choice diets

An experiment was carried out to evaluate the effect of diets with different protein levels and energy:protein ratio and amino acids on the performance and body composition of broilers (300 males and 300 females from 1 to 49 days of age), as well as the effect of these factors on the nutrients utilization efficiency. Three isoenergy experimental diets (D) were used (3000 and 3150kcal ME/kg for starter and growing phase, respectively) varying the levels of crude protein (CP) and the first 4 essential amino acids (AA) (lysine, methionine+cysteine, threonine and tryptophan): D1, control, with 22%CP; D2: 24%CP and 20% more AA than D1 (HighCP+20%AA) and D3, 20% CP and 12% less AA than D1 (LowCP-12%AA). On the growing phase the CP were 20, 22 and 18% for D1, D2 and D3, respectively, keeping the same AA proportion. A fourth diet was defined by free-choice between diets D2 and D3. Birds were alloted in a completely randomized design, in a 2 x 4 factorial (sex x diets) scheme and 5 replications of 15 birds each per treatment. Comparative slaughter technique was used for body fat and protein gain. Birds fed LowCP-12%AA diets showed reduced feed conversion, gain and body protein gain, but better protein intake-to-gain ratio. Broilers fed HighCP+20%AA diets had worse protein intake-to-gain ratio, but similar weight gain, cuts or carcass weight to control or free-choice birds. Fat and protein gain were similar among those 3 treatments. Free-choice birds showed the same performance and body composition of the control broilers. LowCP-12%AA diet was worse for broilers in starter phase than in growing phase. Male broilers were more efficient than females using nutrients, specially in the growing phase.