Capping dietary starch: Protein ratios enhances performance of broiler chickens offered reduced-crude protein, maize-based diets

The objective of this study was to determine if capping dietary starch:protein ratios in reduced-crude protein diets enhances growth performance of broiler chickens offered maize-based diets. A total of 288 off-sex, male Ross 308 chickens were offered a 2 × 3 factorial array of dietary treatments from 7 to 35 days post-hatch. Dietary starch:protein ratios were either standard (mean: 2.47) or capped (mean: 2.12) with three dietary crude protein concentrations of 195, 185 and 175 g/kg. Dietary starch:protein ratios were capped by substituting soybean meal with full-fat soy; sourced from the same supplier. The parameters assessed included growth performance, relative abdominal fat-pad weights, nutrient utilisation, excreta nitrogen concentrations, distal jejunal and distal ileal digestibility coefficients and disappearance rates of starch and protein, jejunal and ileal starch:protein disappearance rate ratios, distal jejunal and distal ileal apparent amino acid digestibility coefficients, distal ileal amino acid disappearance rates and free amino acid concentrations in systemic plasma. Capping dietary starch:protein ratios significantly reduced ileal starch:protein disappearance rate ratios in birds offered 195, 185 and 175 g/kg crude protein by 14.6% (2.40–2.05), 13.8% (2.61–2.25) and 29.3% (3.75–2.65), respectively. Capping starch:protein ratios in 175 g/kg crude protein diets significantly improved weight gain by 3.45% (2398 versus 2318 g/bird) and feed conversion ratio by 3.75% (1.360 versus 1.413) and relative abdominal fat-pad weights were numerically lighter by 10.3% (11.47 versus 12.78 g/kg). Condensing ileal starch:protein disappearance rate ratios were quadratically related (P < 0.001) to improvements in weight gain (r = 0.740), feed conversion ratio (r = 0.786) and lighter fat-pad weights (r = 0.650). Reducing dietary crude protein concentrations depressed apparent ileal digestibility coefficients of 15 of the 16 amino acids assessed (P = 0.021 to < 0.001); average coefficients were reduced by 6.74% (0.803 versus 0.861). These compromised amino acid digestibilities probably contributed to the 6.43% inferior weight gains (2358 versus 2520 g/bird; P < 0.001) and 3.05% inferior feed conversion ratio (1.387 versus 1.346; P < 0.001) following the dietary crude protein reduction from 195 to 175 g/kg. A better comprehension of the impacts of reduced crude protein diets on endogenous amino acid flows and accumulations of microbial amino acids in distal ileal digesta needs to be achieved as both factors will influence apparent amino acid digestibility coefficients. Finally, capping dietary starch:protein ratios in 175 g/kg crude protein diets showed distinct promise in this study as both weight gain and FCR were significantly enhanced.