Sustainable pig and poultry nutrition by improvement of nutrient utilisation – A review

Most industrial by-products from flour, biofuel, or vegetable oil production contain a high amount of dietary fibre. Knowledge about the physiological action of the different fibre sources, in combination with the correct information on the available nutrient contents, enables the inclusion of higher amounts of such feedstuffs in diets for pig and poultry (Kluth and Rodehutscord, 2006; Schedle et al., 2010a; b; 2012).

Dietary fibre represents a group of feed constituents originating from plants. In addition to lignin, they consist of non-starch polysaccharides (NSP) and so-called resistant oligosaccharides, analogous carbohydrates and substances associated with the NSP and lignin complex in plants. One of the most prominent properties of fibre is that it is not hydrolysable by digestive enzymes released from the organism into the gut lumen. Consequently, the so-called resistant starch also has to be classified as a dietary fibre (AACC, 2001). Prebiotic carbohydrates are defined as food ingredients that selectively stimulate the growth and activity of bifidobacteria and lactobacilli in the gut. Therefore, they have health benefits (Gibson and Roberfroid, 1995; Cummings and Macfarlane, 2007). Furthermore, prebiotic carbohydrates may also have additional, less specific benefits because they are fermented in the large intestine (Cummings and Macfarlane, 2007). Nevertheless, from a physiological point of view, prebiotic carbohydrates can be classified as dietary fibre.

Dietary fibre varies widely with regards to different botanical origins, as well as within plant and cell compartments, and physical properties, such as the surface of the particles. Furthermore, their fermentation suitability may additionally affect the living conditions of intestinal microbiota and hence the entire digestive process to a significant extent (Knudsen, 1997; Wenk, 2001; Montagne et al., 2003). Consequently, many studies addressing the term fibre in general may produce different and sometimes contradictory results (Montagne et al., 2003). In this context, a more clear description of fibre is necessary. Figure 1 and Table 1 show a classification of the different dietary fibre constituents, as well as a scheme of carbohydrate fractions. Although dietary fibre is not considered a nutrient perse, it shows secondary actions on the digestion process in different parts of the organism (Bach Knudsen, 2001; Wenk, 2001; Montagne et al., 2003). Action starts with passive parameters like feed intake, and continues to affect gut microbiology and its metabolites, tissue morphology, immunology, and finally zootechnical performance (Wenk, 2001; Montagne et al., 2003; Schedle et al., 2008b). In general, dietary metabolisable energy content decreases with higher contents of dietary fibre (Hador and Wenk, 1996). Pigs utilise diets containing high amounts of dietary fibre to a greater extent than poultry (Sauvant et al., 2004), because the larger digestive tract of pigs in relation to body weight results in both a much higher presence of intestinal microbiota and increased fermentation capacity of dietary fibre (Bedford and Schulze, 1998; Schedle et al., 2008a; Humer et al., 2014). Therefore, commercial diets for poultry need to be highly digestible and low in dietary fibre.

Figure 1

Intensive research dealing with animal nutrition and dietary fibre started in the middle of the last century. Nevertheless, recent research has evaluated the effects of dietary fibre on the development of the gastrointestinal tract (GIT), nutrient retention and growth performance in depth (González-Alvarado et al., 2008; Schedle et al., 2008a; b; Amerah et al., 2009; Metzler-Zebeli et al., 2010; Schedle et al., 2012; Schedle et al., 2014). Furthermore, there is ongoing research into evaluating the mode of action of dietary fibre.

It is well known that additional insoluble dietary fibre (in nutritive amounts) tends to increase voluntary feed intake of pigs (Jørgensen et al., 1996; Schedle et al., 2008b). Fibre addition raises gut fill and in the case of viscosity increases fibre gut volume, which in turn is probably associated with an overall increase of viscera organ weight (Jørgensen et al., 1996; McDonald et al., 2001). Consequently, fibre intake may raise total dry matter (DM) consumption capacity and thus indirectly body weight gain. However, study designs showing a balanced metabolisable energy content and an increased fibre content between treatments, did not observe differences in feed intake or viscera organ weight (Widmer et al., 2008; Schedle et al., 2010b; 2012; Kraler et al., 2015). Regarding feed intake, similar results were observed for poultry (Vranjes and Wenk, 1995; Hador and Wenk, 1996; Schedle et al., 2010; 2014). Hence, observed differences in feed intake can be associated with differences in metabolisable energy contents or variation in digesta viscosity of diets. In general, dietary fibre affects the digestive process due to different modes of action (Wenk, 2001; Montagne et al., 2003). In this context, we must distinguish between soluble and insoluble fibre. Within soluble fibre, further classification separates fibre sources which increase viscosity of the intestinal contents (e.g. pectins in sugar beet pulp) from those with no effect on physicochemical and rheological properties (e.g. inulin as oligofructose). The insoluble category can be divided into insoluble and slowly fermented fibres (wheat bran, soya bean hulls) or insoluble and unfermentable ones without effects on physicochemical properties, with the exception of increasing the faecal bulk volume (maize cobs, linseed hulls) (Van Nevel et al., 2006).

Viscous dietary fibre delays stomach emptying, satiety, and increases transit time. In the small intestine, this type of fibre creates a “cage effect” upon other dietary particles possibly containing valuable nutrients like proteins or fat, thus impairing access to digestive enzymes and causing an overall decrease in digestibility (Montagne et al., 2003). This kind of dietary fibre is considered to reduce villi length in the jejunum and ileum, which is positively correlated with germ counts of potential pathogens (Drochner et al., 2004).

Dietary composition may influence the carbohydrate structures of the mucosal and mucin glycoconjugates, with marked consequences for the adherent microflora (Kelly et al., 1994). Supplementation of insoluble dietary fibre may reduce transit time and vulnerability of the intestinal tissue to attacks from potential pathogens (Montagne et al., 2003). A possible reason for this action seems to be a decrease in adherence possibilities for pathogen germs due to the alteration of the glycoproteins in the mucosa (Drochner et al., 2004). Schedle et al. (2008a; b) reported from a reduction of pathogen germs affected by insoluble dietary fibre, which was also confirmed by the decrease in pro-inflammatory marker genes in the hindgut. In contrast to soluble dietary fibre, insoluble fibre may increase intestinal size and hence stimulate the release of brush border enzymes, as well as increasing adsorption surface (Pluske et al., 1997). In the hindgut, water holding capacity (WHC) will be maintained by insoluble dietary fibre. Hence, mostly insoluble dietary fibre may increase WHC and produce more faecal volume (Glitsø et al., 1998).

Dietary fibre addition may decrease apparent digestibility of DM and crude protein (Kreuzer et al., 1991; Sauer et al., 1991; Hansen et al., 2006), because in terms of bacterially fermentable substrates, the bacterial growth in the hindgut is promoted. Furthermore, it reflects the transformation of mainly NH₃, into bacterial protein and hence the increase in faecal N excretion (Kreuzer et al., 1991; Hansen et al., 2006). Such reduction in nutrient digestibility can also appear as a result of increasing insoluble dietary fibre supplementation (Wenk, 2001).

During the process of digestion in the small intestine, the content of highly digestible components (e.g. protein, fat, sugars, starch) decreases and the content of hardly digestible material, such as fibre, increases proportionally. This material, mixed with endogenous secretion, enters the hindgut and is substrate to a number and diverse range of microbes (Bach Knudsen and Hansen, 1991; Kreuzer et al., 1991).

Fermentation of fibre is much higher for cell material from non-lignified materials than for lignified materials (Bach Knudsen and Hansen, 1991; Wenk, 2001). Consequently, soluble dietary fibre represents the main source of fermentable material in the large intestine (Bach Knudsen and Hansen, 1991; Kreuzer et al., 1991). The most important products of microbial fermentation are SCFA, H₂O, various gases (CO₂, H₂, CH₄) and bacterial protein (Montagne et al., 2003). Between 95% and 99% of total SCFA produced in the GIT are absorbed before reaching the rectum (Van Engelhardt et al., 1989; Montagne et al., 2003). These SCFA still contain considerable amounts of energy providing up to 15-24% of maintenance requirements for pigs (Montagne et al., 2003). SCFA are an important source of energy to the colonocytes, with butyric acid being the primary source (Williams et al., 2001). Acetate acts as an energy substrate for muscle tissue and propionate may be transformed into glucose in the liver (Montagne et al., 2003). As carbohydrate sources become depleted due to microbial degradation and utilisation, fermentation becomes more and more proteolytic. This may lead to the formation of potentially toxic metabolites like NH₃ or biogenic amines (Williams et al., 2001). Biogenic amines can influence feed intake by triggering hypothalamic reactions along the serotonin pathway (Eder et al., 2003).

Special kinds of dietary fibre appear to increase protection against enteric infections with pathogenic germs, and/or infections and subsequent diseases in monogastric mammals (Montagne et al., 2003). In the case of insoluble dietary fibre, a reduced proliferation of E.coli was reported post-weaning (Bertschinger et al., 1979). Hence, dietary fibre seems to exert a protective effect against major problems of intensive pig production, like diarrhoea or ulcers (Sanders et al., 2012; Schedle et al. 2008; 2012).

In summary, in a nutritionally fibre-rich balanced diet with low viscosity-increasing effect, no decrease in performance can be expected for pigs and broilers (Schedle et al., 2010; 2012; 2014; Kraler et al., 2015). Furthermore, moderate amounts of insoluble dietary fibre can improve the performance and health status of pigs and poultry (Montagne et al., 2003; Schedle et al., 2008a; González-Alvarado et al., 2010). From a sustainability perspective, using high amounts of such fibre-rich feedstuffs would be desirable, in order to reduce the strong competition for food between humans and monogastric livestock.

As shown in Figure 1 and Table 1, NSP fraction constitutes the majority of dietary fibre. Hence, the possible modes of action, as well as positive or negative effects on the gastrointestinal physiology, are explained in Section 2.1. dietary fibre. Generally, the overall effect of NSP-hydrolysing enzymes should result in an improvement of performance parameters, a reduced occurrence of sticky faeces, as well as an increase of metabolisable energy, by abolishing the viscosity-increasing effect of β-glucans and arabinoxylans and eliminating the nutrient encapsulating effect of cell walls (Bedford and Schulze, 1998; O’Neill et al., 2014). According to O’Neill et al., (2014), NSP-hydrolysing enzymes can be divided in three categories:

single-component products

Single-component products are marketed by emphasising their main products like β-glucanase or arabinoxylanase. Nevertheless, the final product of single component products will include some enzyme side activities, which may or may not be promoted by the manufacturer. However, in modern products the target enzyme activity is both over-expressed and has characteristics such as thermostability, which the side activities do not possess, rendering the product virtually a single-component product in a heat treated feed. Blended products contain two or three pronounced activities, which are produced individually and in a blended manner. Therefore, they may have two or three primary activities which, individually, are subject to the same considerations as single-component products. Enzyme cocktails are the products of a single fermentation that produces a multitude of activities. It is unlikely that all are quality controlled or promoted by the manufacturer (O’Neill et al., 2014). The modes of action of the different NSPases are well known and were reviewed by Bedford and Schulze (1998) and O’Neill et al. (2014), and will not be further mentioned in this review.

Reports about positive effects on zootechnical performance as a result of NSPases in fibre-rich or rather NSP-rich diets for pigs and poultry are contrasting (Bedford et al., 1992; Vranjes and Wenk, 1995; Emiola et al., 2009; Widyaratne et al., 2009; Boguhn and Rodehutscord, 2010; Ayoade et al, 2012; Schedle et al., 2012; 2014). A possible explanation for the absence of improving effects on zootechnical performance in the studies of Schedle et al. (2012; 2014), which were carried out with mostly insoluble fibre-rich/NSP-rich DDGS, could be the inclusion of similar NSPdegrading enzymes in the production process of the DDGS product which were used. Additional reasons for the absence thereof could be a lack of the hydrolysable substrate or in the case of feedstuffs high in β-glucans, such as barley, a greater extent of endogenous enzyme denaturation takes place at the ileum (Dierick and Decuypere, 1994; Högberg and Lindberg, 2004). In general, it has been observed that the activity of β-glucanases and other exogenous enzymes is lower in pigs than in poultry (Medel et al., 2002). Furthermore, the optimum dietary concentration of NSPases may depend on the variety of grain utilised in the animals’ diet (Bedford, 1995).

In summary, the application of NSP-hydrolysing enzymes in NSP-rich diets should be applied carefully, to make sure its application is successful and to ensure sustainable and efficient nutrition for pigs and poultry.

In contrast to NSP-hydrolysing enzymes, results with phytase are not subject to such extreme variations. Nevertheless, they are inconsistent with respect to the degree to which P can be replaced by this enzyme (Bedford and Schulze, 1998). The majority of P in plant-derived feedstuffs occurs in the form of phytic acid (myo-inositol-1,2,3,4,5,6-hexakisphosphate). Based on the negative charge of this inositol phosphoric ester, phytic acid forms a complex salt called phytate, with divalent metal cations such as calcium, iron, zinc, magnesium and manganese. Therefore, it inhibits the absorption of these minerals (Kornegay, 2001). The enzyme phytase catalyses the stepwise hydrolysis of phytate. With respect to livestock nutrition, there are four possible sources of this enzyme available for the animals: endogenous mucosal phytase, gut microfloral phytase, plant phytase and exogenous microbial phytase. As endogenous mucosal phytase in monogastric organisms appears incapable of hydrolysing sufficient amounts of phytate-bound P, the common method to improve P digestibility in plant-derived feedstuffs for pigs and poultry is supplementation with exogenous phytase (Selle and Ravindran, 2008). The mode of action of phytases was recently reviewed by Humer et al. (2015b).

Improved P digestibility reduces the requirement for Psupplementation in swine and poultry diets (Humer et al., 2013; 2015b), which lowers costs and saves the natural resource P. Furthermore, the amount of P excreted by the animal is reduced (Leytem and Thacker, 2010; Humer et al., 2013). Both reduced use of P in animals’ diets and the lower output as a result of phytase supplementation significantly contributes to sustainable monogastric animal and poultry nutrition.

In addition to the supplementation of microbial phytase, processing techniques (see chapter 2.3 for details) are alternative approaches to reducing phytate content. Thus, techniques such as germination, soaking or fermentation enable activation of naturally occurring plant phytase. Lactic acid bacteria obviously produce microbial phytases during fermentation (Lopez et al., 2000). Thus, they are able to promote phytate degradation (DeAngelis, 2003). Furthermore, the acidification of the diet can lead to activation of endogenous plant phytases (Kozlowska et al., 1996).

According to Parker (1974), probiotics are defined as organisms and substances which contribute to intestinal microbial balance. Fuller (1989) improved Parker’s definition of probiotics with the following distinction: “A live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance.” The most commonly used probiotics are yeasts and bacteria targeting the hindgut (Chaucheyras-Durand & Durand 2010). Kabir (2009) reviewed the mode of action of probiotics as follows:

maintaining normal intestinal microflora through competitive exclusion and antagonism;

Considerable research efforts have been devoted to the determination of effects of probiotics on growth performance, as well as their potential mode of action (Huang et al., 2004; Canibe et al., 2008; Chaucheyras-Durand and Durand, 2010; Mair et al., 2010a; b). To a great extent, the effects of probiotics are influenced indirectly by alterations in microbial fermentation patterns in the GIT and by the strains themselves (Domeneghini et al., 2006). This includes modifications of the intestinal microbial population, the morphology and transport properties of the in testinal mucosa. In the special case of poultry, a reduction of colonisation and shedding of Salmonella and Campylobacter was observed when a number of probiotics were applied (Nisbet, 1998; Netherwood et al., 1999; Fritts et al., 2000). Also, in diets for weaning piglets, a reduction in the incidence of post weaning diarrhoea was reported (Simon, 2010). Furthermore, modifications of the immune system are discussed. The hypothesis that probiotics affect gut morphology and immune parameters could be supported by Mair et al. (2010b). Hence, the effects observed occurred along the entire GIT with no restriction to the large intestine.

Nevertheless, regarding zootechnical performance in pigs, Simon (2010) concluded from both their own data and the published data, that the effects of probiotics on performance are rarely significant. More promising effects of probiotics on performance were reported for poultry (Mountzouris et al., 2007; 2010; Kabir, 2009).

According to Schrezenmeir et al. (2001), synbiotics refer to products that contain both probiotics and prebiotics. Because the word alludes to synergism, this term should be reserved for products in which the prebiotic compound selectively favours the probiotic compound. However, the studies of Mair et al. (2010a; b) showed that several investigated parameters like gut morphology, immune parameters or pH-value showed no synbiotic effect, but there were distinct individual effects of inulin used as a prebiotic and a multispecies probiotic formulation. Hence, interactions between several experimental factors were observed, showing the antagonistic effects of the synbiotic group (Mair et al., 2010a; b). This indicates that either inulin or the probiotics showed distinct individual effects, which could not be used to predict the results in the synbiotic group. The results from Mair et al. (2010a; b) are supported by Roller et al. (2004). However, further understanding of the underlying mechanisms of prebiotics and/or probiotics, in particular, their interactions with different feed matrixes is still required, in order to significantly improve the animals’ performance and hence their sustainability.

The class of phytogenic feed additives refers to plant-derived products used in animal feeding to improve the performance of agricultural livestock. These products have recently gained increasing popularity, especially for use in singlestomached animals. Hence, numerous mixtures of plants and their extracts have lately entered the commercial swine and poultry feeding sector. Their potential mode of action was thoroughly reviewed by Windisch et al. (2008). Several of these phytogenic blends or substances have emerged as showing promise in improving productivity, either through their antioxidant and antimicrobial properties, improvement in diet palatability, improvement of gut function (digestive secretion, absorptive capabilities, or changes to barrier functionality), suppression of pathogen virulence and/or tissue recovery after damage (Windisch et al., 2008). Nevertheless, contrasting results regarding performance show the necessity for greater knowledge and more mechanistic approaches to this subject to fully understand where and how exactly plant-derived feed additives act in monogastric livestock and poultry. Reports from literature show a similar mode of primary action compared to antibiotic growth promoters in piglets, namely, an overall antimicrobial effect in the GIT (Cho et al., 2006; Kroismayr et al., 2008a; b; Fang et al., 2009). The investigations of Humer et al. (2015a) showed less pronounced antibiotic effects. Possible explanations could be that the positive effects of the tested dosages of the phytogenic feed additive would have been more pronounced under less hygienic environmental conditions, higher dosages of the product or when using a less digestible diet (Amad et al., 2011; Humer et al., 2015a). Also, the reports about an immune-stimulating effect affected by such phytogenic substances are contrasting (Böhmer et al., 2009; Hoffmann-Pennesi and Wu, 2010; Najafi and Torki 2010, Mueller et al., 2013; Humer et al., 2015a). Hence, the conflicting results regarding different modes of action in recently published studies shows the need for further investigations to ensure a precise use of these kinds of feed additives in monogastric animal and poultry nutrition.

Organic acids (C1-C7) are natural constituents of some feeds which occur in the body metabolism and the GIT. They show a similar mode of action in the intestine as antibiotic growth promoters (Roth and Kirchgeßner, 1998; Partanen and Mroz, 1999). Furthermore, it is generally considered that dietary organic acids or their salts

lower gastric pH;

Organic acids may also positively influence gut morphology and therefore absorptive capacity, which could contribute to improved protein, energy and/or mineral absorption. Organic acids are also known to be effective preservatives which protect stored foods and feeds against undesirable bacterial or fungal growth (Roth and Kirchgeßner, 1998; Partanen and Mroz, 1999). The growthpromoting and hence the efficiency-increasing effects of organic acids may be due to increased nutrient and energy digestibility as well as retention, alteration of the intestinal microflora and metabolites in the GIT. In addition to their well-established growth-promoting and energy-delivering effects, special kinds of organic acids like benzoic acid have the potential to reduce NH₃ emission from excrement by reducing urinary pH, without harming the animal (Partanen and Mroz, 1999; Bühler et al., 2006; Plitzner et al., 2006). This is explained by the fact that urease oxidises free NH₃ only at high pH values (Pommerening-Röser and Koops, 2005). This is of particular importance in sustainable animal nutrition, as the negative effects of NH₃ on animals and the environment can be reduced.

In summary, most of the substances or additives explained in Chapter 2.2. showed a positive effect on performance in the postnatal and juvenile phase of the animals (Overland et al., 2000; Plitzner et al., 2006; Awad et al., 2008). Furthermore, poor hygienic environmental conditions can enhance the alleged effects of the additives used (Kommera et al., 2006; Humer et al., 2015a). This is very important for their application under practical conditions, because the high standards of hygienic status and feed formulation at research facilities are well known.