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Factors influencing the total tract apparent digestibility of nutrients from wheat for the weanling pig

Kim, Jae Cheol (2003) Factors influencing the total tract apparent digestibility of nutrients from wheat for the weanling pig. PhD thesis, Murdoch University.

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The objectives of the studies conducted in the Part A of this thesis were to quantify the variation of digestible energy (DE) content of wheat and to document the responsible factors for the variation. The general hypothesis tested in Part A of this thesis was that the variety, growing region, growing season, supplementation of xylanase and postharvest grain storage (i.e., six months of storage at ambient temperature) will influence the DE content of wheat when fed to weaner pigs.

Experiment 1 (Chapter 3) examined the variation in the chemical composition of wheats as influenced by variety (Arrino, Westonia and Stiletto were selected), growing region (each wheat was selected from a high, medium and low rainfall region), season (1999 and 2000) and post harvest storage for 6 months. In this experiment, up to a 19% variation in the crude protein (CP) content (mean13.3, SD 2.59), up to a 5% variation in the starch content (mean 65.4, SD 3.56), and a 10-20% variation in the total (mean 9,2, SD 0.86), insoluble (mean 8.1, SD 0.84) and soluble (mean 1.1, SD 0.23) non-starch polysaccharides (NSP) contents were observed. The major experimental findings were:
* Variety of wheat influenced the variation of most carbohydrate components, including fast digestible starch (FDS) (P<0.001), acid detergent fibre (ADF) (P<0.01), total and insoluble NSP (P<0.05), and in vitro extract viscosity (P<0.05);
* The annual precipitation level in each region correlated to bushel weight (P<0.01), protein (P<0.01) and carbohydrate composition, including starch (P<0.05), soluble NSP (P<0.001), lignin (P<0.001) and free sugars (P<0.001), indicating the importance of the growing environment on the DE content of wheat;
* Growing season influenced bushel weight (P<0.001), CP (P<0.01), amylose and amylopectin (components of starch, P<0.05), ADF (P<0.001), lignin (P<0.001), soluble NSP (P<0.001) and free sugars contents (P<0.001), and the in vitro extract viscosity of wheat (P<0.05);
* Storage of wheat decreased ADF (P<0.05), lignin (P<0.01) and soluble NSP contents (P<0.01) and increased the free sugar content (<<0.001).

In Experiment 2 (Chapter 4), the effect of variety, growing region and growing season on the DE content of wheat in 5-week-old male weaner pigs was examined, and Summary III correlations between various chemical composition of wheat and DE content were established. The major findings were:
* The DE content of wheats varied by up to 1.3 MJ/kg (12.5 13.8; mean = 13.3) in 1999 and by up to 1.8 MJ/kg (12.6 14.4; mean = 13.7) in 2000;
* Both the variety and growing region significantly influenced (P<0.05 - P<0.001) the DE content of wheat;
* Generally, the wheat variety Westonia and wheats grown in the low rainfall region contained a higher DE content than other wheats and growing regions;
* In addition, growing season influenced (P<0.001) DE content of wheat;
* The mean DE content determined with weaner pigs in the current study was approximately 1 MJ/kg DM lower when compared to published estimates of the DE content of wheat determined with growing pigs;
* Prediction of in vivo DE content of wheat for weaner pigs using NIRS (Near Infra- Red Spectroscopy) was not successful.
* Significant inverse relationships between the DE content and xylose (P<0.05), NDF (P<0.01), total-P (P<0.01) and phytate-P content (P<0.05) of wheat were seen in 1999. However, such relationships were not significant in 2000, indicating a very strong seasonal influence on both chemical composition and DE content of wheat.

Experiment 3 (Chapter 5) examined the effects of a supplemental enzyme (xylanase) and storage for 6 months on the DE content of wheats. In 1999, use of the enzyme tended to increase the DE content from 13.3 to 13.6 MJ/kg (P=0.065) whereas in 2000, use of the enzyme caused no change in DE content (13.7 to 13.6 MJ/kg). However, the response to enzyme addition was much greater in some wheats grown in some regions than others, suggesting that the DE response to xylanase supplementation was not systematic but appeared to be associated with the grain structure, with the DE content consequently arising from an interaction between variety and growing conditions of the wheat. For wheats harvested in 1999, storage improved the E content from 13.3 to 4.0 MJ/kg (P<0.01), but for the 2000 wheats, storage for 6 months decreased DE content from 13.7 to 13.0 MJ/kg (P<0.001). However, the 2000 data need to be interpreted with caution because some of the wheats were infested with weevils.

The DE response to supplemental xylanase in 1999 was negatively correlated to the total and insoluble arabinose to xylose ratio (P<0.01), in vitro extract viscosity (P<0.01) and lignin (P<0.06), while FDS (P<0.06) and phytate-P (P<0.06) positively correlated to the DE response to xylanase.

The DE response to storage in 1999 was positively correlated to the NDF (P<0.001) and xylose contents (P<0.01) of the wheats, which had negative influences on the DE content of non-stored wheats. The addition of xylanase in stored wheat did not improve the DE content, suggesting changes in chemical composition (eg, activation of xndogenous xylanases) during storage.

In Experiment 4 (Chapter 6), the same wheats were analysed or phosphorus (P) and hytate-P content, as the latter is considered to be anti-nutritive in pig diets. The total P and phytate-P content of all wheat samples ranged 2.6 (s.e. .64) g/kg and 1.8 (s.e. 0.13) /kg DM, respectively. Endogenous phytase activity was highly variable and averaged 63 (s.e. 29.6) FTU/kg between wheats. The variety, rowing region, season and storage of wheat did not influence the variation of P content. However, the precipitation level over two growing seasons was positively correlated to total-P (P<0.05) and hytate-P contents (P<0.05) of wheat. Phytate-P content of wheat can be predicted from total-P content (r=0.974, P<0.001).

In Part B, two experimental designs were generated based n the results of experiments described in Part A. From the previous experiment (Experiment 2, Chapter 4), it was evident that the structure of starch (i.e., amylose: mylopectin ratio) and phytate-P content of wheat were correlated to DE content of wheat. To reinforce the established correlations the following experiments were carried out, with the general hypothesis that the structure of starch (i.e., waxy vs. non-waxy wheat), particle size of ground wheat, and phytate-P content of wheat will influence the digestibility of nutrients (i.e., nitrogen, energy, minerals) in wheat-based weaner pig diets.

In Experiment 5, the same variety (Janz) of a waxy (98% of total starch was amylopectin) isoline and a non-waxy isoline (71% of total starch was amylopectin) wheat were ground through a hammermill fitted either with a 8.5 mm or 4.5 mm screen to achieve average particle sizes of 930 and 560 mcm, respectively. Diets were formulated and then fed to weaner pigs (3-5 weeks of age) with or without xylanase plus beta-glucancase. Digestibilities of starch, nitrogen, energy, and DE content of diets were measured at day 7 and day 21 to examine any age-related improvement of nutrient digestibilities during the first 3 weeks post-weaning. Waxy wheat improved total tract digestibility of starch (P<0.05) and CP (P<0.05), and the reduction of particle size improved total tract starch digestibility (P<0.001) at both 7 and 21 days after weaning.

However, energy digestibility and DE content of the diet were not influenced by either wheat type or particle size. Supplementation of xylanase plus ?-glucanase improved starch digestibility (P<0.01) and DE content (P<0.05) but not other nutrients, and this occurred especially in non-waxy (P=0.03) and larger particle-sized wheat (P=0.01).

Pigs increased ability to digest protein with age (P<0.05). However, the improvement in nutrient digestibilities in waxy and fine particle size wheat diets did not translate to improved pig performance.

In Experiment 6, the hypothesis tested was that the digestibility and pig performance responses to supplemental xylanase, phytase and xylanase plus phytase would differ in weaner pigs according to P content of wheat. To test this hypothesis, two wheats containing high and low levels of total P (2.52g vs. 3.76 g total-P/kg DM) were obtained and fed to weaner pigs. Diets either had no enzyme or were supplemented with xylanase, phytase, or a combination of xylanase plus phytase. The hypothesis was partly supported in this experiment, since P and Ca digestibility were influenced by a wheat content by enzyme ineraction. Overall improvements in macronutrient digestibility and pig performance by supplementation of various enzymes were not significant. Daily growth (P<0.05 - P<0.01) and FCR (P<0.05 - P<0.01) were improved by the supplemental enzyme only in the first week of the feeding trial, mainly due to the increased P and Ca availability induced by the enzymes. Amounts of P and Ca digested were below the recommended requirements by NRC in the first two weeks of the feeding trial. Supplementation of phytase generally improved P and Ca digestibility (P<0.05) in both low-P and high-P wheats. However, xylanase plus phytase supplementation did not produce synergistic effect for macronutrients, and mineral digestibilities, over single supplementation of xylanase or phytase.

From the results obtained in this thesis I propose that:
1) The DE content of wheat for weaner pigs is variable due mainly to the change of chemical composition mediated by variety, growing environment and storage after harvest, and efficacy of supplemental xylanase is dependent on the chemical structure of NSP present in cell walls of wheat;
2) Structure of starch, particle size, age after weaning, and P content of wheat are factors influencing nutrient digestibility in weaner pigs fed a wheat-based diet, and the efficacy of phytase on P and Ca digestibilities is dependent on the phytate-P content of wheat.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Pluske, John
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