Protein sources and starch-protein digestive dynamics manipulate growth performance in broiler chickens defined by an equilateral-triangle response surface design

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.

Identifying the shortfalls of crude protein-reduced, wheat-based broiler diets

The objective of this review is to identify the shortfalls of wheat-based, crude protein (CP)-reduced diets for broiler chickens as wheat is inferior to maize in this context but to inconsistent extents. Inherent factors in wheat may be compromising gut integrity; these include soluble non-starch polysaccharides (NSP), amylase trypsin inhibitors (ATI) and gluten. Soluble NSP in wheat induce increased gut viscosities, which can lead to compromised gut integrity, which is not entirely ameliorated by NSP-degrading feed enzymes. Wheat ATI probably compromise gut integrity and may also have the capacity to increase endogenous amino acid flows and decrease apparent starch and protein digestibilities. Gluten inclusions of 20 g/kg in a maize-soy diet depressed weight gain and feed intake and higher gluten inclusions have been shown to activate inflammatory cytokine-related genes in broiler chickens. Further research is required, perhaps particularly in relation to wheat ATI. The protein content of wheat is typically higher than maize; importantly, this results in higher inclusions of non-bound amino acids in CP-reduced broiler diets. These higher inclusions could trigger post-enteral amino acid imbalances, leading to the deamination of surplus amino acids and the generation of ammonia (NH₃) which, if not adequately detoxified, results in compromised growth performance from NH₃ overload. Thus, alternatives to non-bound amino acids to meet amino acid requirements in birds offered CP-reduced, wheat-based diets merit evaluation. The digestion of wheat starch is more rapid than that of maize starch which may be a disadvantage as the provision of some slowly digestible starch in broiler diets may enhance performance. Alternatively, slowly digestible starch may result in more de novo lipogenesis. Therefore, it may prove instructive to evaluate CP-reduced diets based on maize-wheat and/or sorghum–wheat blends rather than entirely wheat. This would reduce non-bound amino acid inclusions by lowering dietary CP derived from feed grains and may enhance starch digestive dynamics by retarding starch digestion rates. Also, the use of biomarkers to monitor gut integrity in broiler chickens is examined where calprotectin, ovotransferrin and possibly citrulline appear to hold promise, but their validation requires further research.