Identification of phenolic compounds and their relationships to in-vitro digestibility of sorghum leaves from bird-resistant and non-bird-resistant varieties

Soluble phenolics in leaf blades and sheaths from the crop residues of 24 sorghum varieties were studied. Apigenin, luteolin, apigenin and luteolin 7-O-glucosides, p-coumaric acid, butin and apigeninidin were identified. This is the first report of butin in sorghum tissues. Derivatives of the following compounds were also detected but not characterised further: apigeninidin, luteolinidin, chalcone, flavanone and/or dihydroflavonol, cinnamic acid, apigenin and luteolin. The composition of phenolics was clearly different between leaf blades (LB) and sheaths (LS), and also between leaf sheaths of bird-resistant (BR) and non-bird-resistant (NBR) varieties. Routes of flavonoid biosynthesis in BR and NBR varieties appear to diverge at the flavanone/dihydroflavonol level. Several negative correlations were found between HPLC peaks and in-vitro digestibilities, ie true dry matter and neutral-detergent fibre digestibility. These were highly significant with butin and significant with several luteolin derivatives but only with one apigenin derivative. Butin in turn was highly negatively correlated with colorimetric measurements of 3-desoxyanthocyanidins. This may suggest that butin—rather than the 3-desoxyanthocyanidins as previously reported—is implicated in reducing dry matter digestibility. Selecting varieties low in luteolin derivatives and butin and rich in apigenin should enhance the digestibilities of sorghum crop residues.