Influence of white- and red-sorghum varieties and hydrothermal component of steam-pelleting on digestibility coefficients of amino acids and kinetics of amino acids, nitrogen and starch digestion in diets for broiler chickens

Two experiments were conducted to examine the influence of sorghum variety and the hydrothermal component of steam-pelleting diets on apparent amino acid digestibility coefficients and digestion kinetics of amino acids, nitrogen and starch in broiler chickens. In experiment one, 84 male Ross 308 chicks were offered steam-pelleted diets based on white (Liberty) and red (Buster) sorghum from 7 to 28 days post-hatch. In experiment two, the same red sorghum-based diets were offered to chicks from 7 to 28 days post-hatch as either unprocessed raw mash or reground mash following steam-pelleting at 95 °C. In both experiments, digesta samples from proximal jejunum, distal jejunum, proximal ileum and distal ileum were collected in their entirety at day 28. The samples were then freeze-dried and weighed for determination of mean retention time in each small intestinal segment. The digestion kinetics of amino acids, nitrogen and starch were determined using an exponential mathematical model to relate apparent digestibility coefficients with mean retention times in each small intestinal segment. In experiment one, white sorghum-based diets had significantly higher average amino acid digestibility coefficients than red sorghum in the proximal ileum and distal ileum (P<0.05). Nine from sixteen amino acids in white sorghum-based diets had significantly higher apparent digestibility coefficients in the proximal ileum and distal ileum. The potential digestible fraction of starch and rate of starch digestion in white sorghum-based diet were significantly higher than red sorghum (P<0.05). However, there were no significant differences in digestion rates of nitrogen and amino acids. However, pre-determined feed conversion ratios were strongly correlated with potential digestible starch (r = −0.840, P<0.001). The differences in white and red sorghum-based diets were more pronounced in starch digestion than nitrogen digestion and further research is required to identify the precise causes of deficient starch digestion in red sorghum. In experiment two, the hydrothermal component of the steam-pelleting process had a negligible impact on digestion of amino acids as there were no significant differences in the extent or rate of amino acid digestion in raw mash and reground mash diets. Therefore, the linear increases of amino acid digestibilities with increased conditioning temperatures observed in a previous study may be attributed to the feed form effect of intact pellets generating heavier relative gizzard weights; thereby enhancing the overall protein digestive process.