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

Published on: 11/4/2016
Author/s : David J. Cadogan 1 and Mingan Choct 2. / 1 Feedworks Pty Ltd, Lancefield 3435, Australia; 2 School of Environmental and Rural Science, University of New England, Armidale 2351, Australia.
Summary

We investigated the pattern of non-starch polysaccharide (NSP) digestion along the gut of pigs fed two different wheats, which were offered with or without xylanase supplementation. The two wheats used were pre-characterised before the experiment on the basis of low and normal feed intake of young pigs. Wheat type significantly influenced feed intake and growth rate in the first 7 days, however, by day 14 the only significant effect of wheat type was on growth rate. Xylanase supplementation increased the growth performance of pigs fed the poor quality wheat to a level similar to those fed the normal wheat. It also increased the daily gain of pigs fed the normal wheat. Wheat type had no significant effect on the digestibility of dry matter (DM), energy, free sugars or the different fractions of NSP in the duodenum, ileum or in the faeces. The duodenal gross energy digestibility values for the low and high performance diets were −27.4 and −47.5%, respectively, and xylanase supplementation significantly increased the digestibility of energy back to positive levels. Dry matter digestibility values followed a similar pattern. In the duodenum, xylanase increased (P < 0.05) the digestibility values of both soluble and insoluble NSP, whereas in the ileum, xylanase had a significant effect only on the digestibility of the soluble NSP fraction. Xylanase did not affect free sugar digestibility. The reduction in soluble NSP level coincided with a marked reduction in the amount of fucose, a prominent component of mucosal polysaccharides. This suggests that soluble NSP substantially increase endogenous losses. The absence of differences in the digestibility of the measured NSP between the two wheat samples suggests that the structures of the NSP, rather than just their amount and solubility, are important for the anti-nutritional properties of NSP in pig diets.

Keywords

  • Wheat
  • Non-starch polysaccharides
  • Xylanase
  • Pigs

1. Introduction

Wheat is one of the most variable in composition among cereal grains (Choct et al., 1999a). These differences are exacerbated by the environmental conditions under which the grains are grown (Longstaff and MacNab, 1986). A review by van Barneveld (1997) using past data for more than 70 cultivars of wheat found differences of up to 3.7 MJ/kg dry matter (DM) in digestible energy (DE) content. The highest DE reported in Australia was at 17.0 MJ/kg DM (Kopinski, 1997), and the lowest was 13.8 MJ/kg DM (King, 1976). Batterham et al. (1980) found that the hemicellulose (mainly arabinoxylan) component of wheat correlated closely with the DE value in wheat. The relationship between DE and more specific components of wheat NSP has been variable (van Barneveld, 1997), although Zijlstra et al., 1988 found a strong correlation between xylose level and DE content in wheat.

The majority of research on wheat in pigs has evaluated the variation in energy content (Kopinski, 1997 and van Barneveld, 1997) and devised methods to improve the digestibility of cereal grains (Wiseman, 1997). However, faecal DM digestibility and DE content of cereals are poorly related to pig growth performance compared to ileal DM digestibility measurements (van Barneveld et al., 2001 and Cadogan et al., 2003). It appears that growth, feed intake and health of pigs are affected by wheat type but not by the energy and protein levels of wheat (Cadogan et al., 1999, Choct et al., 1999b, Simmins et al., 2001 and Caine et al., 1997). The majority of variation in pig growth performance produced by different wheats has been attributed to the content of non-starch carbohydrates (NSC), which includes NSP and free sugars, such as mono- and oligosaccharides up to 10 sugar units, present in feed ingredients (Cadogan, 1999). This observation is strongly supported by the significantly enhanced growth performance of pigs fed diets based on low quality wheats supplemented with xylanase (Choct et al., 1999b, Partridge et al., 1999 and Simmins et al., 2001).

There have been many studies investigating the effect of NSP on human health and animal performance but their detailed modes of action in pig diets are largely speculative (Choct and Cadogan, 2001 and Partridge, 2001). Numerous studies (Bach Knudsen and Hansen, 1991, Yin et al., 2000, van Barneveld et al., 2001, Bartelt et al., 2002 and Pederson et al., 2012) have reported the digestion pattern of wheat NSP fractions at the ileal level. There is, however, limited information on the digestion of NSP from different wheat types and of the modifying effects of exogenous xylanase on wheat fibre in the small intestine of the pig. The hypothesis is that NSP fractions of pre-characterised high and normal feed intake wheats behave differently in the various sections of the gastrointestinal tract (GI tract) of the pig. It is expected that xylanase supplementation will eliminate the differences between the two wheats.

 

 

2. Materials and methods

2.1. Pig husbandry and bioassay

The animal protocol was approved by the Ethics Committee of Rivalea Australia Pty Ltd (formerly QAF Meat Industries), and followed principles established by the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes (National Health and Medical Research Council: Canberra, 1997). Sixty male pigs used in the experiments were from Large White × Landrace cross dams crossed with a synthetic terminal sire (Primegro Genetics, Corowa, NSW Australia). The pigs were weaned at 25 days of age and then put into individual cages with wire floors and sides and offered a commercial nursery diet for three days until the experiment commenced. The temperature was kept at a constant 28 °C. Water and feed were available ad libitum. Pigs were selected with a live weight range of 6.5 to 7.5 kg. After three days, pigs were re-weighed and randomly allocated to treatment groups of equal mean weight (12 pigs per treatment). Each pen had an area of 0.5 × 0.9 × 0.5 m and was equipped with a conventional dry feeder and a single drinking nipple. The drinking water was medicated with Apralan (Elanco Animal Health, Greenfield, IN, USA) to reduce potential of Escherichiacoli scours on a daily basis throughout each experiment.

Weight gain and feed intake of each individual pig were taken on a weekly basis (0 to 7, 7 to 14, and 14 to 21 days). From these measurements daily feed intake, feed conversion ratio (FCR) and rate of gain values were calculated on an individual pig basis.

2.2. Wheat pre-charactisation and experimental diets

Ten newly harvested wheats were sourced from the eastern and central Riverina districts of New South Wales, as well as the north eastern, central north and Wimmera regions of Victoria, Australia. From this ten, two wheats were selected, one low quality and one normal quality, based on feed intake of young pigs. Briefly, the low quality one was a Currawong cultivar (Wheat 1), and produced 23% (90 g/d) and 26% (83 g/d) lower feed intake and daily gain, respectively, compared with a Whistler cultivar (Wheat 2), which represented the average performance expected of the genotype of pigs used in the study. The total NSP contents of both Wheat 1 and Wheat 2 were similar at 9.49 and 9.75%, respectively, however wheat 1 contained a higher level of soluble NSP (1.67% versus 0.99%) but a lower level of insoluble NSP (11.39% versus 9.75%) compared with wheat 2. The protein and starch content of wheat 1 was 12.56 and 55.0%, respectively, with crude protein and starch in wheat 2 was measured at 11.81 and 58.9%.

Both wheats were hammer-milled through a 3.2 mm screen. A basal diet containing 70% wheat was formulated to have a DE of 14.5 MJ/kg and an available lysine content of 0.90 g/MJ DE for both wheats (Table 1). The dietary essential amino acid contents were formulated to 115% of requirements, which had been pre-determined for the pig genotype within Rivalea Australia Pty Ltd. The 15% excess was based on the differences in the protein contents of the wheats in order to maintain an adequate level of protein in all diets. A commercially produced xylanase from Thermomyces spp. (Ronozyme WX; DSM Nutritional Products, Wagga Wagga, NSW, Australia), was added to the experimental diets at 0 or 300 mg/kg. An alkane digestibility marker, hexatriacontane (C36H74), was added to the diets at a level of 150 mg/kg.

2.3. Measurements

After 21 days, 8 randomly selected pigs were slaughtered, and digesta samples were obtained from the first 80 cm of the small intestine (duodenum and upper jejunum), from the terminal ileum and from the rectum. These were frozen immediately; freeze-dried after 4 days and stored for digesta (alkene) marker and nutrient analyses. Gross energy (GE) was analysed by complete combustion in a DDS CP500 isoperibol calorimeter (Digital Datasystems, Johannesburg). Free sugars, soluble and insoluble NSP were measured using the alditol acetate method of Englyst and Hudson, 1987. Dry matter (DM) was determined gravimetrically following drying at 105°C for 24 h. The alkane marker, hexatriacontane (C36H74), was determined as follows: from (100 to 500 mg) freeze-dried samples, an appropriate amount (50 to 200 mg) of internal standard (C34H70 in dodecane) was added. The samples were then subjected to 1.5 M ethanolic KOH in a heating-block at 90°C for 1 h with stirring. After cooling, the alkanes were extracted in n-hexane several times, filtered, purified and quantified by gas chromatography. Dry matter and energy digestibility were calculated using the alkane (C36H74) marker in both the diets and gut contents. The two pre-characterised wheats were analysed for various fractions of NSP.

2.4. Digestibility calculation

The digestibility coefficient (DC) of nutrients or energy was calculated using the following equation:

2.5. Statistical analyses

Multifactorial analyses of variance were carried out to determine statistical significance for the performance and digestibility data generated from each experiment. The Fisher's Least Significant Difference (LSD) procedure was carried out to provide multiple comparisons to ascertain significant differences between treatment means. The software package used was Statistica v. 4.5, supplied by Tulsa, OK, USA.

 

 

3. Results

3.1. Performance

At day 7, wheat type significantly affected (P < 0.05) feed intake and growth rate, but not FCR. Xylanase supplementation did not affect feed intake, weight gain or FCR. At day 14, wheat type did not affect feed intake, but significantly influenced (P < 0.05) growth rate, and tended to (P < 0.08) affect FCR. There was no wheat type × xylanase interaction either at day 7 or at day 14 ( Table 2).

3.2. Non-starch polysaccharide digestibility along the gut

Duodenum: wheat type had no significant effect on the digestibility of energy, free sugars or the different NSP fractions in the duodenum (Table 3). Xylanase supplementation improved (P < 0.008) the duodenal digestibility of soluble NSP (P = 0.008), insoluble NSP (P = 0.024) and total NSP (P = 0.012). There was a tendency for xylanase to increase duodenal digestibility for energy (P = 0.071) and DM (P < 0.082). Both wheat-based diets, without added xylanase, had negative digestibility values for energy and soluble NSP in the duodenum. The digestibility values for insoluble and total NSP of Wheat 2 were also negative in the absence of the enzyme.

All treatments showed that there was a negative digestibility value for free sugars in the duodenum. There was a trend (P = 0.069) for an interaction between wheat type and xylanase supplementation on free sugar digestibility. The enzyme improved the digestibility of the free sugars of Wheat 1, however, it had a negative effect on the free sugar digestibility of Wheat 2.

Ileum: the ileal digestibility results showed that wheat type had no significant impact on energy, free sugars or NSP digestion (Table 3). Xylanase supplementation increased the digestibility of DM (P < 0.036), energy (P = 0.045) and soluble NSP (P < 0.001). An interaction between wheat type and enzyme supplementation for soluble NSP digestibility tended to exist (P = 0.115) where the enzyme increased the digestibility of soluble NSP in Wheat 2 but not in Wheat 1. Xylanase had no significant effect on the digestibility of free sugars or insoluble NSP at the distal ileum.

Total tract: there was no significant effect of wheat or xylanase supplementation on the faecal digestibility of energy, free sugars and fractions of NSP. Xylanase supplementation, however, numerically reduced the faecal digestibility of insoluble NSP by 21.2 and 17.7%, respectively, for Wheat 1 and Wheat 2 (P = 0.093). Although the free sugar digestibility was above 99%, there was a strong trend for an interaction between enzyme addition and wheat type on free sugar digestion (P = 0.066). The enzyme improved free sugar digestibility in Wheat 1 but reduced the digestibility for Wheat 2.

3.3. Fucose content along the GI tract

Mucin polysaccharides are rich in fucose (Lien et al., 1997). Fucose was measured as an indicator of endogenous losses via mucus secretion in this experiment. There was no effect of wheat type on fucose level throughout the gut. The fucose content was very high in the duodenum without xylanase supplementation, but low in the ileum and very low in the faeces. Xylanase supplementation markedly (P < 0.01) decreased fucose level in the duodenum for both wheats and in the ileum for Wheat 2. There was no difference in fucose level in the faeces regardless of wheat type or enzyme supplementation ( Fig. 1).

 

 

4. Discussion

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., 1990Schulze, 1994Angkanaporn 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).

 

 

5. Conclusion

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.

Acknowledgements

The authors are grateful to the financial support of Novozymes A/S and DSM Nutritional Products that enabled the research to be undertaken. The animal research work was conducted at the Corowa facility of Rivalea Australia Pty Ltd (formerly QAF Meat Industries). We are thankful to Dr Dan Pettersson of Novozymes for his contribution to the experimental design and enthusiasm for research in general and to Ms Shuyu Song of the University of New England for her meticulous work for the carbohydrate analyses.

 
 
 
 
This article was originally published in Animal Nutrition, Volume 1, Issue 3, September 2015, Pgs 160–165. http://dx.doi.org/10.1016/j.aninu.2015.08.011. This is an Open Access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
 
 
 
 

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Peer review under responsibility of Chinese Association of Animal Science and Veterinary Medicine.

 
Author/s
Mingan is the Chief Executive Officer of the Poultry Cooperative Research Centre and a professor at the University of New England. Mingan's main areas of interest include carbohydrate chemistry and nutrition, feed enzymes, energy evaluation and nutrition x disease interaction in poultry. He has supervised more than 40 postgraduate students and published over 270 papers in journals and proceedings. He is an active member of the scientific community, serving as assistant editor and on editorial boards of many journals.
 
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