Influence of conditioning temperatures on amino acid digestibility coefficients at four small intestinal sites and their dynamics with starch and nitrogen digestion in sorghum-based broiler diets

Abstract

The influence of conditioning temperatures on apparent digestibility coefficients of amino acids at proximal jejunum, distal jejunum, proximal ileum and distal ileum were investigated in this study. Broilers were fed sorghum-based diets steam-pelleted at three conditioning temperatures (65, 80, 95°C) from 7 to 28days post-hatch. Digesta samples were collected at day 28 and freeze-dried and weighed for determination of amino acid digestibilities and mean retention time in each small intestinal segment. The digestion kinetics of amino acids were determined using an exponential mathematical model to relate digestion coefficients with mean retention times in each small intestinal segment. Increasing conditioning temperatures from 65 to 95°C significantly increased apparent digestibility coefficients of phenylalanine at proximal jejunum, apparent digestibility coefficients of aspartic acid and proline at proximal ileum and apparent digestibility coefficients of arginine, lysine, aspartic acid and glycine at distal ileum. Conditioning temperatures linearly increased retention times in the distal ileum (P<0.05). There were no significant differences in digestion rates of amino acids, starch and nitrogen between treatments. Nitrogen retention was positively correlated (P<0.05) with amino acid digestibility coefficients at the proximal jejunum, but not at the other three small intestinal sites. Interestingly, relative gizzard weights were positively correlated with amino acid digestibility coefficients at the proximal ileum (P<0.05) and distal ileum (P<0.05). Starch digestion rate was positively correlated with digestion rates of 8 amino acids (P<0.05). Percentage responses of amino acid digestibilities to increased conditioning temperatures were negatively correlated with their initial digestibility at 65°C (P<0.001). Increasing conditioning temperatures from 65 to 95°C improved the average digestibility coefficients of amino acid by 29.8% at the proximal jejunum, 4.8% at the proximal ileum and 4.0% at the distal ileum. These results indicate that increasing conditioning temperatures generally enhanced amino acid digestibilities. These increases may have been generated by reversible protein denaturation that facilitated proteolysis, by heavier and more functional gizzards promoting protein digestion and, to a lesser extent, by inactivation of protease inhibitors inherent in the diet