Pattern of non-starch polysaccharide digestion along the gut of the pig: Contribution to available energy

The two wheats used in this study produced significantly different daily gains for the 14-day trial period and different feed intakes for the first seven days with pigs fed Wheat 1 being markedly lighter and eating significantly less than those fed Wheat 2.

The influence of wheat type on weaner growth rate is well known (Cadogan et al., 2003). In the current study, the effect of wheat type on growth performance was significant at day 7 of the experiment and then started to diminish over time, although there remained a strong trend for wheat type to influence growth rate after 21 days (day 7 of the experimental period). It is possible that older pigs deal with the anti-nutritive effect of NSP better than younger ones due to their better developed guts, and perhaps due to a more stable gut microflora in older pigs. Xylanase supplementation significantly improved FCR at day 14 of the experiment through a reduction in feed intake and an increase in weight gain, although such an effect on FCR was not apparent up until day 7 of the experiment. Interestingly, xylanase supplementation increased the daily gain of pigs fed Wheat 2 further without having a significant effect on feed intake, but only had minimal influence on weight gain of pigs fed Wheat 1. This was unexpected because poorer quality wheat samples usually respond better to xylanase supplementation than better quality samples (Cadogan et al., 2000). It is generally conceded that the NSP level of wheat contributes to the differences between wheat samples in terms of their nutritive value in young pigs (Choct et al., 1999b). Wheat 1, the low feed intake cultivar (Currawong) had higher levels of soluble and insoluble NSP and free sugars compared with Wheat 2, a normal Australian Standard White wheat cultivar (Whistler). Wheat 1, compared to Wheat 2 also contained a lower amount of starch (55.0% vs. 58.9%) and a higher amount of phytin phosphorus (0.28% vs. 0.22%). It is, thus, not likely to be the content of NSP or other nutrients per se that led to the different responses of the two wheats to xylanase supplementation. It is thought that there were significant differences in the cell wall architecture of the two wheats that elicited contrasting responses to xylanase supplementation. This argument is supported by our findings on the detailed composition of the NSP fractions and their pattern of degradation along the gut of weaner pigs. For instance, the duodenal digestibility values for free sugars, soluble NSP and insoluble NSP were −58.3, −8.8 and 7.3%, respectively for Wheat 1 without xylanase, whereas they were −10.0, −33.0 and −18.7%, respectively for Wheat 2 without xylanase. These figures show two things: firstly, in Wheat 1, much of the soluble NSP was degraded into low molecular weight carbohydrates in the stomach, yielding a large amount of free sugars in the duodenum, making the digestibility value highly negative. But in Wheat 2, a much smaller amount of free sugars and a markedly higher level of soluble NSP entered the duodenum, compared with Wheat 1, making their digestibility values less negative; secondly, the site for degradation of soluble NSP and solubilisation of insoluble NSP differed between the two wheats, where these occurred earlier in the digestive tract for Wheat 1 and for Wheat 2. The manifestation of these differences probably meant that digesta transit rates also differed between the two wheats, where the NSP in Wheat 1 were affected in the gut, moving the resulting digesta more quickly through the small intestine. The opposite seems to be true in the case of Wheat 2 as shown by the digestibility values for duodenal soluble and insoluble NSP. The highly negative values simply mean that the NSP were accumulated in the duodenum relative to the amounts present in the respective diets.

The different behaviours of the two wheats in terms of NSP digestibility may be related to the fact that NSP undergo changes within the gastrointestinal tract, especially under acidic conditions of the stomach (van der Meulen et al., 2001). In some cases, ester linkages may be cleaved, reducing originally soluble NSP into smaller fragments, including free sugars, and solubilizing some originally insoluble NSP (Choct and Cadogan, 2001). It appears that Wheat 1 was more susceptible to such changes in the stomach than Wheat 2. Increased amounts of osmotically active free sugars and viscosity-producing arabinoxylans in the stomach may be one of the factors that reduced feed intake of pigs fed Wheat 1. Indeed, soluble NSP have been shown to increase the viscosity of stomach contents in monogastric animals (Ellis et al., 1996). The pig has a sieving mechanism in the stomach in which large particles fall to the bottom of the stomach and undergo prolonged physical and chemical breakdown, and solubilised digesta flows into the duodenum (Ellis et al., 1996). An increase in digesta viscosity may reduce the sieving ability of the stomach and allow larger particles to stay suspended thereby reducing stomach digestion and flow of nutrients into the duodenum. The increase in particle size entering the duodenum would reduce diet digestibility. Furthermore, it is possible that the larger particle size may have diluted the digestible marker (as described above) and caused a temporary accumulation of particular dietary components. How the two wheats differed so much in their susceptibility stomach degradation is worthy of further investigation from their cell wall architecture.

The ability of the enzyme to effectively depolymerise the NSP is clearly demonstrated inTable 3. In addition to the negative digestibility values for the NSP, the dry matter and energy digestibility values in the duodenum of pigs fed the control diets were also negative. For example, the duodenal energy digestibility values of diets based on Wheat 1 and Wheat 2 were −27.4 and −47.5%, respectively, and were increased the to positive values by xylanase supplementation. The DM digestibility followed a similar pattern. The negative digestibility values for NSP residues, as observed in the duodenum and ileum, have been recorded in the past (Pettersson et al., 1997). The values are mostly a reflection of the fact wheat NSP are not absorbed in the proximal GI tract and are consequently enriched relative to the content found in the diet, when calculated per gram of dried digesta. Negative DM and energy digestibility values, however, have not been reported in past literature, and at first the results seem confusing. The best explanation for the negative energy digestibility values is that endogenous secretions diluted the dietary nutrients and the digestibility marker. The results indicated that endogenous protein, fat and carbohydrate secretions from the mucosa, pancreas, and bile duct were substantially increased from non-hydrolysed soluble and insoluble NSP. The effect of NSP on endogenous secretions in rats, pigs and poultry is well documented (Ikegami et al., 1990, Schulze, 1994, Angkanaporn et al., 1994 and Dänicke et al., 2000). It appears that NSP, in particular, the soluble fraction, interact with the mucosal surface, perhaps leading to modification of peptide hormones in the gut. When the molecular structure of NSP are broken down by exogenous enzymes, their ability to increase secretions seems to be reduced (Dänicke et al., 2000). The negative digestibility of energy calculated in the duodenum does not necessarily suggest that net energy is lower for the control diets. The majority of endogenous secretions are re-absorbed in the small intestine, therefore absolute differences in net energy would be more accurate when measured at the terminal ileum. The presence of xylanase negated the negative effects on energy and soluble NSP digestibility and the results also showed that there is significant hydrolisation of soluble NSP by the enzyme, especially xylose and arabinose, in the proximal gastrointestinal tract.

Xylanase supplementation significantly increased ileal DM digestibility by 10 and 5.7%, respectively, for Wheats 1 and 2. There was, however, no significant difference between wheat type or enzyme supplementation on the faecal DM digestibility. Digestible energy was found to be unrelated to growth performance of young pigs offered diets containing different wheat types (Cadogan et al., 1999), even though there was a 45% difference in feed intake and growth rate. Nortey et al. (2008) observed a 2.2 and 2.8% increase in ileal and faecal dry matter digestibility, respectively, however growth performance was not reported. These observations highlight the limitations of faecal DM and gross energy digestibility measurements, and caution must be placed on the relationships between the DE of grains and pig growth performance.

The negative digestibility value for energy raises questions about an excessive endogenous loss in the duodenum and its impact on voluntary feed intake. Effects of NSP on endogenous secretions have been clearly demonstrated in broiler chickens (Angkanaporn et al., 1994 and Selle et al., 2003). It is proposed that NSP, especially the soluble fraction, places an increased pressure on pancreatic and mucosal secretions, which may lead to negative feedback on feed intake, perhaps through increased cholecystokinin (CCK) and gastric inhibitory polypeptide (GIP) production. Cholecystokinin and GIP are both involved in negative feedback mechanisms on pig satiety and thus feed intake in the pig (Ellis et al., 1996). The current study showed a high level of fucose present in the duodenum and ileum (Fig. 1), which was a markedly reduced by xylanase supplementation. It is known that mucin carbohydrates contain a considerable amount of fucose, with up to 74% of fucose in the ileal digesta of pigs arising from mucin (Lien et al., 1997). Turch et al. (1993) found that the fucose concentration in mucin was significantly affected by diet and age. In the current study, the elevated fucose level in the ileum suggests the extent of endogenous losses induced by the two wheats. Endogenous losses lead to loss of energy efficiency for maintenance and growth, and this effect is exacerbated as the pig gets older (Snyder and Walker, 1987).

The effect of wheat type on pig performance is manifested through massive variation in feed intake in young pigs. The NSP in wheat play a key role, but it is not just the content of NSP that affects feed intake, it appears that the cell wall architecture of wheat is important to determining where in the GI tract NSP are solubilised or degraded to low molecular weight carbohydrates and their subsequent effects on the movement of digesta. The use of an efficacious xylanase can alleviate most of the negative effects of NSP on feed intake.