WHEAT: COMPOSITION AND FEEDING VALUE FOR POULTRY - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 249

The AME value of a wheat sample depends both on the gross content and digestibility of
starch, protein and lipids. Carré et al. (2007) estimated that the digestibility variations
between broiler diets based on 50% wheat can be as high as 15, 10 and 3% for lipids, starch
and protein, respectively. These digestibility variations can result in 300 kcal/kg variation in
the AME of wheat-based broiler diets. Steenfeldt (2001), however, studied the effect of
variability in chemical composition of 16 wheat cultivars on the performance of broilers and
reported poor correlations between performance and AME. Therefore care should be taken
when using AMEn values alone to predict the nutritive value of wheat and wheat-based diets.
Some wheat samples are known to suffer from very low AME values (less than 3100
kcal/kg), termed as the 'low AME wheat' syndrome. Low AME wheats have been identified
with high grain pentosan contents by Australian workers (Annison, 1993; Choct et al. 1995).
When these wheats are included at levels above 50% in broiler diets, the birds have sticky and
watery droppings accompanied by poor growth and feed efficiency. Another unique feature
associated with wheat is the 'new season grain' phenomenon, where the AME is low at the
harvest time and improves with post-harvest storage. The use of newly harvested wheats as
the main energy source in poultry diets is more problematic compared to stored grains (Choct
and Hughes, 1997). However, it must be noted that not all wheat samples exhibit this
phenomenon.
Starch Digestibility
The major constituent of wheat is starch, which is consequently the principal energy
yielding component. Therefore any factors which influence starch digestibility will have an
impact on AME.
The starches in cereal grains are generally considered to be highly digestible by the
amylolytic enzymes of poultry as they contain very little resistant starch. Longstaff and
McNab (1986) found that starch from wheat was almost completely digested by adult birds,
with a mean value observed of 99.7% across varieties, seasons and agronomic conditions.
However, in the studies of Mollah et al . (1983) and Rogel et al . (1987), when low-ME wheats
were fed to chicken, considerable amount of starch was detected in excreta suggesting
incomplete starch digestion. In these studies, the excreta digestibility of starch ranged from 80
to 99% and from 82 to 100%, respectively. Starch digestibility was found to be highly
correlated with the AME indicating that variation in starch digestibility was responsible to the
low-ME phenomenon. It was concluded that it is not that starch per se is poorly utilised in
some samples of wheat but that other factors within the wheat are responsible for lowering
the digestion of starch. The hypothesis of an access problem should be considered, but this
has not been demonstrated in cereals.
Based on the strong positive relationship between starch digestibility and AME, an in
vitro system has been recently proposed (Wiseman et al ., 2000) to separate low and high ME
wheats. The results indicate that the rate of starch hydrolysis varies widely in different
wheats, with hydrolysis of low-ME wheat being markedly slower.