Digestion Rates of Starch but not Protein Vary in Common Cereal Grains Used in Broiler Diets

Both glucose and amino acids are essential for muscle protein deposition and feed conversion efficiency. Total tract nitrogen retention was reported to be influenced by protein and starch digestion in broiler chickens (Liu et al., 2013). Embracing the concept of digestive dynamics and applying it in practical diet formulation requires understanding of the variations of protein and starch digestion rates in different ingredients. This present study evaluated protein and starch digestion rates of 18 cereal grains, including sorghum (7), wheat (4), corn (2), barley (3), and triticale (2), in male broiler chickens from 21 – 28 days post-hatch. Experimental diets included the test grain and soybean meal and were formulated to be iso-energetic and iso-nitrogenous but without synthetic amino acids. Grains were hammer-milled through 4.0 mm screen before mixing and cold-pelleting. On day 28, a total of 648 Ross 308 male chicks (6 cages per treatment, 6 birds per cage) were euthanised (intra-venous injection of sodium pentobarbitone), and samples of digesta were taken from the proximal jejunum (PJ), distal jejunum (DJ), proximal ileum (PI) and distal ileum (DI) and pooled for each cage. The calculations of apparent digestibilities, mean retention time and digestion rates of protein and starch were conducted as described previously (Liu et al., 2013). There were more variations in protein digestibilities in the jejunum than ileum in broiler chickens. The CV of protein digestibilities among different ingredients were 8, 6, 2 and 2 in PJ, DJ, PI and DI, respectively; whereas the CV of starch digestibilities among different ingredients were 7, 7, 6 and 6 in PJ, DJ, PI and DI, respectively (Figure 1). There were no significant differences between protein digestion rates among the 18 cereal grains; however, the starch digestion rates varied by student t-test (P = 0.048). On average, wheat had the highest starch digestion rate (0.118 min^-1), followed by barley (0.104 min^-1), triticale (0.093 min^-1), corn (0.087 min^-1) and sorghum (0.075 min^-1). The synchrony between glucose and amino acid absorption is essential for optimal feed conversion and nutrient utilisation, especially in reduced protein diets with high inclusions of crystalline amino acids. The present study showed the variations in starch digestion rates in different feed grains. Future consideration needs to take into account the differences of protein digestion rates in common protein ingredients.