Variability in the chemical composition of wheat and its utilisation by young poultry

Abstract

The summary and overall conclusions from the programme, to include the relevance to the industry, are presented below.

Importance of wheat to the UK poultry industry

Wheat may supply up to 0.7 of the dietary energy requirements of poultry, principally through the starch content.
Its price makes it an extremely cost effective raw material
Variations in nutritional value (apparent metabolisable energy - AME - which is closely linked to starch digestibility) continue to be a major cause for concern, leading to reduced performance and litter problems which adversely influence carcass quality and waste disposal.
Variability in AME values / starch digestibility of wheat fed to poultry

A wide range of AME values / starch digestibilities was obtained.
The influence of site of growth and harvest year is important but the complex factors involved remain to be quantified.
Varietal effects are inconsistent, and may interact with environmental conditions.
The young bird is unable to utilise high rates of inclusion of wheat which is associated with the production of copious amounts of creamy excreta, although this is not linked to any evident pathological condition.
Individual bird variability is important, the more so with wheats of low nutritional value. Poor nutritional value is associated with increased feed intake.
The nutritional value, of low AME wheats improves considerably with dry storage at ambient temperature, either in the ground form or as whole grains, between 6-10 months after harvest.
Investigations into starch digestibility

Significant amounts of undigested starch granules were. found in the digesta of birds fed low AME wheats.
Differences in granule surface characteristics were not evident, although some evidence was obtained suggesting that low AME wheats have a higher proportion of smaller 'B' granules which, although in principle might be more susceptible to enzyme attack, might create a more viscous environment.
Starch isolates were uniformly well-digested suggesting that it is not the starch per se that is poorly digested in low AME wheats but factor(s) associated with it, although both chemical and physical properties of starch can influence the degree to which it is hydrolysed.
Starch hydrolysis in vitro was slower in low AME wheats
Relationship between digesta viscosity and nutritional value.

Starch digestibility in vitro was slower in the presence of aqueous extracts from low AME wheats; this was also observed when aqueous extracts from digesta taken from birds fed low AME wheats were employed and thus these samples had lower amylase activity.
The relationship between low AME wheat samples and digesta viscosity in the 'foregut', the 'hindgut' and also in the excreta was examined. High digesta viscosity (principally in the 'foregut') was associated with lower nutritional value and the fact that variations in excreta viscosity was also observed indicate that the factor(s) involved are not digested.
Relationship between nutritional value and chemical composition

No single chemical component was responsible for the range of nutritional values observed.
Detailed investigations of the non starch polysaccharide (NSP) fraction revealed that the relationship with AME 1 starch digestibility is not simple. NSP content (total, soluble or insoluble NSP) and the ratio between soluble and insoluble NSP were not correlated with nutritional value.
The soluble NSP fraction may be associated with reduced starch digestibility and a tenuous link between this and the arabinose:xylose ratio within the soluble NSP was occasionally observed. Investigations into the structure (molecular weight, degree of branching) of arabinoxylans, which are heterogenous, are warranted.
An in vitro test to predict the nutritional value of wheat for broilers.

Two laboratory procedures have emerged from this programme which are of potential value in the screening of wheat samples for both the broiler industry and plant breeders:

Viscosity of aqueous extracts of wheat
Rate of digestion of starch in vitro