WHEAT: COMPOSITION AND FEEDING VALUE FOR POULTRY - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 252

1996). In addition, relative weights of digestive organs increase with higher digesta viscosity
(Choct, 1997) which may increase the metabolic cost of maintaining the gut. Although most
of the research supports the viscosity mechanism and it has been discussed extensively in the
literature, it is reasonable to suggest that both mechanisms are likely to be involved (Bedford,
2006; Cowieson et al. , 2006). A summary of proposed effects of soluble NSP on gut and
metabolic parameters of birds fed wheat-based diets is shown in Figure 1.
The effects of insoluble NSP in wheat-based poultry diets, however, are not well
understood. In general, inclusion of moderate levels of insoluble NSPs had no effect on
digesta viscosity (Svihus and Hetland, 2001) but resulted in improved starch digestibility
(Rogel et al ., 1987; Hetland and Svihus, 2001) and feed per gain (Hetland et al ., 2003). The
suggested mechanisms by which the birds maintain normal weight gain when fed wheat-
based diets diluted with insoluble NSPe are either by increasing capacity of the digestive
system and/or faster passage through the digestive tract (Hetland and Svihus, 2001).
U SE OF E XOGENOUS X YLANASES
The effectiveness of exogenous xylanases in improving bird performance and nutrient
digestibility of wheats is well documented. Currently, exogenous xylanase preparations are
routinely used to mitigate the adverse effects of NSP and to minimize the variation in AME
and performance of poultry fed wheat-based diets.
Xylanases have the ability to degrade either the soluble or insoluble arabinoxylans. In
general, commercial xylanase preparations have broad spectrum activity on both soluble and
insoluble arabinoxylans in the feed. It has been found that xylanases obtained from the same
organism differ markedly in their catalytic activities on various xylans (Bedford and Schulze,
1998). Choct et al . (2004) compared three xylanase products, namely, xylanase A with
affinity for both soluble and insoluble NSP, xylanase B is known to solubilise insoluble NSP
and xylanase C with affinity for soluble NSP only. It was found that xylanase A increased the
soluble NSP levels in the jejunum, but had no effect on digesta viscosity which suggested that
while the enzyme attacked the insoluble cell walls to release soluble NSP into the digesta, it
also hydrolysed the released soluble NSP. Xylanase B, increased the soluble NSP levels in
both the jejunum and ileum and elevated digesta viscosity while xylanase C reduced digesta
viscosity.
The proposed mechanisms by which enzymes improve energy and nutrient utilization of
wheat based diets include degradation of NSP in the cell wall matrix and the release of
encapsulated nutrients, lowering of digesta viscosity in the intestinal tract, increased
accessibility of nutrients to endogenous digestive enzymes, stimulation of intestinal motility
and improved feed passage rate or indirectly by enhancing cell wall breakdown through
stimulation of the gizzard (Bedford and Schulze, 1998). As a result, enzyme addition has been
shown to improve performance and nutrient digestibility, reduce the incidence of sticky
droppings and modify the microflora of the distal gut (Cowieson et al ., 2006). Broiler
responses to xylanase supplementation, however, have not been consistent. Factors that may
cause variation in response include the type of xylanase, quality of wheat and breed and age
of birds (Bedford, 1997). Some studies have shown that differences in the response of enzyme
supplementation might be related to hindgut microflora (Choct et al ., 1996; 2006). Other