Summary

The performance of broiler chickens offered sorghum-based diets is inconsistent, or even sub-optimal, in comparison to wheat-based diets. Typically in such diets, sorghum provides approximately 30% of the protein content but 65% of the energy density, most of which is derived from starch. However, the efficiency of feed energy utilisation in sorghum-based diets is inferior to that for wheat (Black et al., 2005). Therefore, the objective of this project is to investigate the inherent factors in grain sorghum that limit starch utilisation. The intention is to use both dehulled and whole sorghum as a means to vary analysed concentrations of phenolic compounds and phytate due to their peripheral location in grain sorghum. This is because it is likely that both phenolics and phytate impede starch digestion. This project should also settle the debate as to whether contemporary Australian sorghum cultivars contain condensed tannin or are, indeed, ‘tannin-free’. Additional factors that merit investigation because of their likely influence on sorghum starch digestibility include (i) grain particle size, (ii) starch:protein ratios, (iii) kafirin proportions of sorghum protein, (iv) amylose:amylopectin ratios in sorghum starch and (v) starch gelatinisation following steam-pelleting of sorghum-based diets. Inclusions of exogenous enzymes (protease, phytase) and reducing agents (sodium bisulphite) in sorghum-based diets subjected to different steam-pelleting temperatures will also prove instructive. This project has the potential to identify the important factors limiting sorghum starch utilisation in broiler chickens.

Program

Chicken Meat

Research Organisation

The University of Sydney

Objective Summary

The prime objective of the project is to identify factors inherent in sorghum grain that limit starch digestibility in broiler chickens, which will be delivered by six lines of investigation. (1) To assess the impact of phenolic and phytate concentrations in sorghum on starch digestibility by using sorghum grain, dehulled sorghum and sorghum bran. Dehulled sorghum should contain substantially lower phenolic and phytate concentrations due to their peripheral location in sorghum grain. (2) To assess the impact of grain particle size, as dictated by sieve sizes in the hammer-mill prior to the incorporation of sorghum into steam-pelleted diets, on starch digestibility coefficients along the small intestine. (3) To assess the impact of starch:protein ratios on starch digestibility by determining the starch and protein concentrations in sorghum. It is believed that kafirin (protein bodies) and/or glutelin (protein matrix) may impede digestion of starch (starch granules) due to their close physical proximity in sorghum endosperm. (4) To assess the proportion of kafirin in sorghum protein by 2D-gel electrophoresis as an extension to the previous point. (5) To determine amylose and amylopectin concentrations in sorghum starch and their relationship to starch digestibility. (6) To determine the extents of starch gelatinisation starch granule disruption (assessed by light microscopy and/or differential scanning calorimetry)
following steam-pelleting of sorghum per se and sorghum-based diets in relation to starch digestibility.