Starch is the main source of energy in sorghum-based diets but starch/energy utilisation by broiler chickens offered these diets may be substandard. Both in vitro and in vivo data indicate that the digestibility of sorghum starch is inferior to that of other feed grains, especially maize. Three 'starch-extrinsic' factors in grain sorghum, namely 'non-tannin' phenolic compounds, kafirin and phytate may negatively influence starch/energy utilisation in sorghum-based broiler diets. To test this hypothesis, concentrations of polyphenols, free, bound and conjugated phenolic acids, kafirin and phytate were quantified in six diverse 'tannin-free' (Type I) grain sorghum varieties. These sorghums were incorporated into nutritionally equivalent diets at 620 g/kg and offered to male broiler chickens from 7 to 28 days post-hatch. Growth performance, nutrient utilisation (AME, ME : GE ratios, N retention, AMEn) and starch and protein (N) digestibility coefficients and disappearance rates in four small intestinal segments were determined. Numerous relationships that were either significant (P < 0.05), or approached significance (P < 0.10), were detected that indicated various 'non-tannin' phenolic compounds, kafirin and phytate in sorghums negatively influenced nutrient utilisation parameters in broiler chickens. ME : GE ratios are sensitive indicators of efficiency of energy utilisation and were most negatively influenced by flavan-4-ols (r = –0.919; P < 0.015), which are polyphenolic compounds. Moreover, flavan-4-ols in tandem with conjugated vanillic acid negatively influenced (r = –0.993; P < 0.005) ME : GE ratios on the basis of a valid multiple linear regression. Similarly, conjugated vanillic and bound ferulic acids in tandem negatively influenced AME (r = –0.990; P < 0.005). N retention was most negatively influenced by kafirin (r = –0.887; P < 0.025). Thus, it appears that both phenolic compounds and kafirin may have deleterious effects on nutrient utilisation of sorghum-based broiler diets and recommendations are made that should enhance the quality of sorghum as a feedstuff for chicken-meat production based on these findings.
- condensed tannin
- conjugated and bound phenolic acids
- ferulic acid