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The increased use of fiber-rich feedstuffs in pig and poultry diets requires an optimal utilization of these feed ingredients. Hence, the animal feed industry explores opportunities to improve degradability of these feedstuffs and maximize their inclusion levels in pig and poultry diets. Processing and enzyme technologies can modify the physicochemical characteristics of fiber fractions from feed ingredients, thereby affecting their degradability. In this way, fermentability of non-starch polysaccharides (NSP) and thus their potential energetic utilization might be enhanced. In addition, technologies can be aimed at alleviation of adverse effects on digestion and absorption of other nutrients, which might be particularly of interest for young pigs and poultry. However, to understand modifications that occur during processing detailed information on the composition of fiber structures is required.
This thesis aimed at identifying limiting factors in the degradability of fiber fractions in pigs and poultry and at development of technologies to improve their degradation. Focus was on recalcitrant fiber structures as found in in maize dried distillers grain with solubles (DDGS) and rapeseed meal (RSM). Fiber degradation in growing pigs and broilers was studied in detail and limiting structures in the degradation of NSP were identified (Chapter 5 to 7). Effects of processing and enzyme technologies on fiber-rich feedstuffs were evaluated based on literature and in vitro and in vivo studies in growing pigs and broilers (Chapter 2 to 7). In addition, marker methods to study digestibility of fiber-rich diets in broilers were discussed (Chapter 8). Furthermore, interactive effects between specific fermentable fiber sources and the digestive utilization of the diet were investigated (Chapter 9). In the final chapter, results of the thesis were summarized and synthesized. Methods to analyze fiber components and evaluate fiber degradation in vitro and in vivo were discussed, suggestions for future research were given, and implications of the results for feed formulation were addressed (Chapter 10).
Recalcitrant fiber fractions in DDGS and RSM
The fiber fraction of maize DDGS was found to consist of complex, highly substituted glucuronoarabinoxylans (GAX) that are cross-linked to or associated with cellulose and lignin within the cell wall matrix. In pigs, total tract degradation of non-glucosyl polysaccharides (NGP) from DDGS was between 51 and 62 %. Coumaric acid and ferulic acid associated (ester)-linkages were found to contribute to the recalcitrance of DDGS fiber to fermentation in the pig.The fiber fraction of RSM consists of pectic polysaccharides, xyloglucan, and cellulose that are linked via ester-linkages or H-bonds, forming a rigid cell wall matrix. This rigid matrix was found to hinder the complete degradation of NSP from RSM. In pigs, total tract degradation of NSP from RSM was ~70 %. Nearly 50 % of the unfermented carbohydrate structures in feces were tightly bound pectins (e.g. rhamnogalacturonan and arabinan), xyloglucan, and cellulose. The other half consisted of smaller uronyl-rich carbohydrates, presumably ester-linked or H-bound. In broilers, total tract degradation of NSP from RSM was ~24 %.
Processing and enzyme technologies
Common feed processing technologies may improve degradability of easily solubilizable NSP, but are not sufficient to affect rather recalcitrant fiber fractions, such as those found in DDGS and RSM. Particle size reduction, hydrothermal treatment with or without shear, acid hydrolysis, and cell wall degrading enzymes improved in vitro degradability barley (13-43 % units, P < 0.01), whereas only severe hydrothermal acid treatment increased in vitro degradability of fiber fractions from DDGS (30-60 % units,P < 0.01). In pigs, however, hydrothermal acid treatment did not improve degradability of NSP from DDGS, despite the increased solubility of the fiber fraction. Acid treatment shifted fermentation to more proximal gastrointestinal segments, but total extent of NSP degradation was not affected.Apparently, acid-extrusion accelerated degradation of NSP structures that are not resistant to degradation by microbial enzymes in the pigs’ gastrointestinal tract, whereas the most recalcitrant NSP structures were still not affected.Furthermore, acid treatment reduced feed intake, digestibility of crude protein (CP; 3 % units, P = 0.06) and starch (1 % unit, P = 0.10), and tended to reduce digestibility of crude fat (0.4 % units, P < 0.10). Degradability of NGP from rapeseed meal was found to be successfully improved by addition of pectolytic enzymes (9-20 % units, P < 0.01), due to increased degradation of branched water-soluble arabinans. This coincided with an increased NGP concentration in the ceca (4-7 g/g cobalt, P < 0.01), indicating that more NGP were solubilized such that they could enter the ceca and become available for fermentation. Particle size reduction, through wet milling and extrusion, facilitated solubilization of NSP, but solubilized structures could still not be degraded by the cecal microbiota. No interaction between processing technologies and enzyme addition was found. Apparently, the processing technologies studied were not facilitating accessibility of NSP to pectolytic enzymes added to the feed in vivo.
In conclusion, both processing and enzyme technologies can be effective in solubilizing NSP from DDGS and RSM, but in vivo research demonstrated the limited potential to improve the degradation, and thus feeding value, of recalcitrant fiber fractions. Future research should aim at targeted degradation of recalcitrant NSP structures only, while minimizing the effects on relatively easy degradable NSP and other nutrients. Enzyme technologies, targeting specific structures, seem to provide more perspective than more rigorous processing technologies.In DDGS and RSM, ester-linkages or H-bonds seem to be involved in the recalcitrance of the fiber fraction to degradation in the animal, presumably due to anchorage of NSP in the rigid cellulose-lignin matrix. Hence, technologies that degrade such linkages, as alkali treatments and especially esterases could be of interest for future research.
Digestibility measurements are a crucial tool in the evaluation of the nutritive value of feedstuffs. The marker method, where digestibility is estimated from the ratio between an indigestible marker and the nutrient of interest in feed and digesta or excreta, is commonly used as alternative for the laborious total collection method. In broilers, separation of marker and specific digesta fractions occurs, and especially when degradation of fiber fractions is the matter of interest, the marker method has limitations. When estimating apparent ileal digestibility (AID), separation of marker and digesta resulted in unrealistic high estimates for the digestibility of non-glucosyl polysaccharides (54-66 %), exceeding ATTD values by 16-42 % units. Moreover, the effect of pectolytic enzyme addition on the AID of non-glucosyl polysaccharides was in opposite direction when compared with total collection.These data illustrate that fractionation of digesta, particularly in high-fiber diets, complicates accurate ileal digestibility measurements in broilers, regardless the choice of markers used. It is recommended to add a soluble marker when fiber degradation is of interest, even though it does not allow quantifying fermentative degradation of nutrients.
Interactions between fiber and digestive utilization of the diet
In current feed formulation systems interactions between feed ingredients are assumed to be absent. This assumption can be challenged as interactions between specific feed components, such as various types of fiber, and the digestive utilization of the diet exist. Although the effects of fiber inclusion on the digestive utilization of the diet are complex, specific properties can be ascribed to certain fiber types. β-Glucan, a rapidly fermentable, viscous, fiber source, enhanced the degradation of xyloglucan from RSM (ATTD of NGP increased by 6 % units, P < 0.001) but did not seem to affect the recalcitrant fiber fraction of DDGS. Furthermore, β-glucan decreased enzymatic digestion of CP and starch in the small intestine. In contrast, resistant starch (RS), a more slowly, but-well fermentable fiber, decreased degradation of fiber-fractions from DDGS as well as RSM (> 10 % units, P < 0.01). These results clearly show the interactive effects between specific fiber fractions in the diet and the degradation of NSP and other nutrients. It is suggested to include effects of individual feed ingredients on the physicochemical properties of the chyme, such as viscosity and water binding capacity, and retention times in various segments of the gastro-intestinal tract in feed formulation, to more accurately predict the nutritive value of diets.
|Qualification||Doctor of Philosophy|
|Award date||20 Jun 2014|
|Place of Publication||Wageningen|
|Publication status||Published - 2014|
- animal nutrition
- nutrition physiology