The influence of non-bound amino acid inclusions and starch-protein digestive dynamics on growth performance of broiler chickens offered wheat-based diets with two crude protein concentrations

The primary objective of this study was to investigate the influence of high and low inclusions of non-bound amino acid (NBAA) in standard and reduced-crude protein (CP), wheat-based diets on growth performance in broiler chickens. Dietary treatments were formulated to either 210 or 180 g/kg CP. The 210 g/kg CP diets contained either 12.1 or 21.1 g/kg NBAA and 180 g/kg CP diets contained either 44.0 or 55.5 g/kg NBAA. The formulations also generated different dietary starch:protein ratios which impacted on starch-protein digestive dynamics. Each of the four dietary treatments were offered to 7 replicates of 15 birds housed in floor pens from 14 to 35 days post-hatch or a total of 420 male Ross 308 chickens. Growth performance, relative abdominal fat-pad weights, breast muscle and leg shank yields were determined. Ileal starch and protein (N) digestibility coefficients, disappearance rates and starch:protein disappearance rate ratios were defined. Apparent ileal digestibility coefficients and disappearance rates of 16 amino acids were determined at 35 days post-hatch and free concentrations of 20 amino acids in systemic plasma were determined at 34 days post-hatch. The transition from 210 to 180 g/kg CP diets depressed weight gain by 11.3% (1742 versus 1964 g/bird) and FCR by 10.4% (1.606 versus 1.455), although both parameters were subject to treatment interactions. The treatment interaction (P < 0.001) observed for FCR was because high NBAA inclusions significantly improved FCR by 4.17% (1.424 versus 1.486) in birds offered 210 g/kg CP diets, but significantly depressed FCR by 3.36% (1.632 versus 1.579) in 180 g/kg CP diets. A quadratic relationship (r = 0.860; P < 0.001) between dietary NBAA inclusions and FCR was detected, which indicated that when NBAA inclusions exceed 18.5 g/kg efficiency of feed conversion deteriorated. However, a multiple linear regression (r = 0.913; P < 0.001) was detected for FCR where both NBAA inclusions and analysed dietary starch:protein ratios were significantly (P < 0.001) related to FCR. This relationship indicates that growth performance of broiler chickens offered wheat-based diets is strongly influenced by dietary NBAA inclusions coupled with dietary starch:protein ratios and consideration is given to the possible underlying mechanisms.