Causes for variation in digestibility of starch among feedstuffs

Reasons for starch digestibility variations are reviewed by considering starch granule structure, antinutritional factors and access problems in coarse particles. Feed technology treatments that can overcome low starch digestibilities are also reviewed. Before considering these factors, the particular features of the digestive system of chickens are briefly reviewed. The digestive system of chickens for starch hydrolysis is characterised by a high enzymatic potential. However, in contrast with mammals, the potential of chickens for completing starch digestion by microbial degradation in the distal part of the digestive tract is very low. Variations in starch granule structure especially concern legume seeds compared to cereals, with greater resistance observed for the former. Plant mutants may result in variations in amylose contents of starch granules. Those with high amylose content (70%) produce low starch digestibilities. Cereals with no amylose (waxy starch) do not show strong evidence of benefit despite lower resistance of starch granules. In practical conditions, antinutritional factors such as proteinaceous alpha-amylase inhibitors, condensed tannins and viscous water-soluble non-starch polysaccharides generally result in low effects on starch digestibility. However, viscosity may induce a noticeable effect with high viscosity barleys. There is much less evidence that viscosity can explain the low starch digestibilities (< 94%) which are observed with some wheat samples. Positive effects of enzyme additives on starch digestibility of cereals probably result from effects both on viscosity and on disruption of coarse particles. Access problems in coarse particles are responsible for low starch digestibilities observed with legume seeds. However, mechanical treatments such as fine grinding and pelleting can readily overcome this problem. Access problem in coarse particles is also probably the reason that explains low starch digestibilities in some cereal samples, as shown by negative relationships between hardness and starch digestibility of wheats. However, grinding and pelleting of cereals seem to be less efficient than with legume seeds. Interactions with feeding behaviour and genetic origin of birds have also to be considered for explaining variations in starch digestibility of cereals.