Reductions in dietary CP through the supplementation of feed-grade amino acids will allow for decreased dietary inclusion of soybean meal but previous experimentation has shown that the efficacy of reduced CP diets varies dependent upon diet composition. Therefore, two Experiments (Exp) were conducted to determine the efficacy of including wheat and corn at a 1:1 ratio in reduced CP diets. A High and Low diet was formulated to contain 23.5, 21.5, and 19.5 and 19.5, 17.5, and 15.5% CP in the starter [2,940 kcal/kg; 1.28% digestible lysine (dLys)], grower (3,083 kcal/kg; 1.15% dLys), and finisher diets (3,130 kcal/kg; 1.02% dLys), respectively. The High and Low diets were then blended to produce a Medium diet containing 21.5, 19.5, and 17.5% CP in the starter, grower, and finisher diets. Starter, grower, and finisher diets were fed from 0 to 11, 12 to 27, and 28 to 41 d of age in Exp1 (816 Cobb 500 broilers; 8 replicates; 34 birds/pen) and the grower diet was fed from d 8 to 28 in Exp 2 (108 Ross 308 broilers; 6 replicates; 6 birds/cage). Body weight gain, feed intake, and feed conversion ratio were determined for each experimental period in both Exp and diet AME, N retention, AMEn, and N digestibility were determined for an 8 to 28 d grower period in Exp 2. Data were subjected to a one-way ANOV A analysis and statistical significance was considered at P≤0.05. No effects (P>0.05) of reduced CP were found on final BW, BW gain, or feed intake for any time point in Exp 1 but feed conversion was significantly increased (P<0.05) by approximately 3.5 and 4.4 points for broilers fed the Low diet compared to broilers fed the High and Medium diets for the 0 to 27 and 0 to 41 d periods, respectively. In E 2, broilers fed the Low diet displayed 0.166, 0.166, and 0.063 kg lower (P<0.05) final BW, BW gain, and feed intake, respectively, than those fed the High and Medium diets. Feed conversion increased (P<0.05) in a stepwise manner by 2.5 and 9.8 points as CP was reduced. Nutrient utilization assays found stepwise (P<0.05) increases in AMEn as dietary CP level was reduced and increased (P<0.05) N digestibility for broilers fed the Medium and Low diets compared to those fed the High. Live performance was sustained in the floor pen environment of Exp 1 with the exception of feed conversion. Nutrient utilization data from Exp 2 indicate that while gross nutrient utilization increases, these increases may be brought about by diet composition instead of as an internal regulation of metabolism to cope with reduced CP levels.
These data indicate that diets using a 1:1 wheat and corn blend as the starch supplying grain allow for adequate performance in reduced CP diets when broilers are reared in floor pens.