Influence of Non-Starch Polysaccharides Structure on the Metabolisable Energy of U.K. Wheat Fed to Poultry

Abstract

With the inclusion of wheat in European poultry diets at 600 g/kg, or more, there is increasing concern that its apparent metabolisable energy (AME) is more variable than would be predicted by conventional analysis. Twelve samples of wheat with a range of AME values (8·34–13·74 MJ/kg dry matter when fed to broiler chicks aged 11–14 d at 750 g/kg diet) were used to investigate the causes of this variability. AME was not correlated with the amount of total water-soluble non-starch polysaccharide (sNSP), soluble arabinoxylan (the major polysaccharide contributing to NSP) or (1→3, 1→4)-β-glucan released from the grain or with the viscosity of aqueous extracts. Surprisingly,in vitroviscosity was negatively related to soluble (r2=0·61) and total (r2=0·82) arabinoxylan. This was thought to be due to the slow, but cumulative, action of endogenous hydrolases in the stored grain. Soluble NSP from each wheat was characterised by measurement of molecular weight distribution and the structural features of arabinoxylan determined from the amount and nature of the oligosaccharides released following treatment with an endo-xylanase. Oligomer molecular weight was determined by matrix-assisted laser desorption/ionisation time of flight mass spectrometry and structure by NMR. Multivariate analysis of the 32 variables measured provided a three-term model able to explain approximately 0·80 of the variation between wheat samples: AME=8·07+11·16(XRAX)+30·67(AX-6)−0·355(sNSP) Two terms (XRAX, the proportion of arabinoxylan resistant to hydrolysis by xylanase and AX-6, the properties of branched six-sugar present in hydrolysates) reflected the degree of branching of arabinoxylan and were positively associated with AME while the third term, the amount of sNSP present, was negatively related.