By Rob AHM ten Doeschate and MR Bedford, AB Vista
For monogastrics, and especially poultry, fibre has traditionally been seen as a nutrient with little relevance, something of a diluent (or even an antinutrient in the case of viscous fibres) which should be held below a threshold so as not to cause problems, rather than something to consider as a positive element of the diet: Because chickens are not cows! Over recent years this attitude has been changing, with fibre nutrition in the chicken being studied in greater detail as its importance has increased with the removal of prophylactic antibiotics.
This paper intends to give an overview of the subject area, focussing on how modulation of fibre fermentation in the chicken can help improve performance and intestinal resilience.
Fibre definition and analysis
When talking about fibre, there are various challenges with respect to definition and analysis which are partly the reason why, historically, fibre may not have received the attention it deserves. Most people have a conceptual understanding of what fibre is, but a detailed and precise definition and analysis is more difficult to achieve.
The definition of the term ‘fibre’ varies, depending on the author as well as the purpose of the definition. A conceptual, physiological definition would be something like “The fraction of the diet not digested by the animal’s digestive enzymes but can potentially be fermented by the microflora” (Bach Knudsen, 2014) or “Dietary fibre is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the (…) small intestine with complete or partial fermentation in the large intestine” (American Association of Cereal Chemists, 2001).
The problem with this is that while this covers the concept of fibre, it doesn’t match analytical procedures.
Methods to analyse fibre
In most countries, feed materials are still analysed to get a crude fibre (CF) measure for labelling purposes. The CF method (Henneberg and Stohmann, 1859) measures the residue after solubilising the sample in strong acid and alkali solutions, and weighing the residue. This provided nutritionists with a vague idea of the quantity of the fibre components, cellulose and lignin. As conditions used in this method are very aggressive, the true fibre content is underestimated, and it also does not serve as a way to identify the fibre structure or composition.
A more advanced method was developed by Van Soest et al. (1963) using graded solubilisation of the sample in a series of neutral/acid solutions, then drying and weighing the sample and, lately, discounting ash content. This method effectively measures what is insoluble in every step of the method, giving neutral detergent fibre (NDF) as a measure for hemicellulose, cellulose and lignin; acid detergent fibre (ADF) as a measure for cellulose and lignin; and acid detergent lignin (ADL) as a measure for lignin.
This detergent fibre method is commonly used in ruminant and pig nutrition, but the method has not been widely used in poultry nutrition. While these parameters give a better estimate of fibre content, it doesn’t help in determining the functional effect of the fibre and also underestimates total fibre as soluble fractions are drastically underestimated.
Measuring constituent sugars
To improve the understanding of fibre, the analytical method of Englyst et al. (1994) is based on measuring the constituent sugars from both soluble and insoluble fibre components, thus providing a more complete understanding of fibre composition and structural content in feed materials. This method focusses on the understanding of non-starch polysaccharide (NSP) content which, combined with lignin, gives a value for total dietary fibre (TDF).
NSPs are composed of hemicellulose, cellulose and pectin and vary in their monomeric sugar compositions. Having a greater understanding of fibre’s components will help us to understand how to optimise its use. For example, hemicellulose is a combination of arabinoxylans (AX), β-glucans, xyloglucans, mannans and fructans. The AX backbones within hemicellulose are, however, composed of arabinose and xylose and can vary in their substitutions along the xylan backbone.
In cereals, the vast majority of the arabinose and xylose sugars determined in this assay are derived from arabinoxylans. These two sugars can be used to determine the degree of substitution which is important, as the higher the substitution the more challenging it is for enzymes to hydrolyse the xylan backbone and generate smaller arabino-xylooligosaccharides (AXOS).
While a better understanding of the constituents of NSPs can benefit in the long run the improvement of the performance and/or health of animals, we still do not know exactly which components of fibre are most important for intestinal health. For example, the size, solubility, sugar composition and degree of lignification are all topics which need further attention. But using dietary fibre analysis on a routine basis in research and development will eventually give us the opportunity to link performance results with analytical parameters.
Soluble or insoluble fibres
Fibre has historically been considered at best a diluent or even an anti-nutritional factor (ANF), which meant controlling maximum levels in diets rather than looking for opportunities to utilise fibre as a source of useful nutrients. In some respect this attitude is correct. For example, it is well known that soluble NSPs (β-glucans and AX) can influence viscosity in the small intestine (Choct et al., 1996; Bach Knudsen, 2014) leading to negative effects in animal performance.
In poultry, increasing viscosity is reported to raise the incidence of wet litter, dirty eggs and foot-pad lesions, and reduce nutrient availability. Across the cereals, AX is the main NSP component followed by cellulose and β-glucans. From the A:X ratios, it is also apparent that there are big variations in the structural features of AX molecules caused by the type of grains and the relative proportions of the different tissues in the grains. Today, however, using enzymes, we are able to break these longer chain NSPs into smaller fragments, reducing this effect of viscosity (Bedford, 1996).
As mentioned by Bach Knudsen (2014), both soluble AX and β-glucan can increase viscosity. The viscosity of both polymers is directly related to the molecular structure (molecular conformation, weight and weight distribution) and the concentration of the polymer.
One thing to take into consideration is that in vitro and in vivo viscosities may not correlate. For example, the molecular weight of β-glucan is higher than that of AX, which causes higher in vitro viscosity for the former. However, AX is more resistant to degradation than β-glucans, and once it enters the gut (especially under gastric conditions), the viscosity caused by β-glucans will largely reduce while the viscosity from AX will be more resistant.
Xylanases alleviate viscosity in the animal by reducing AX molecular weight consequently allowing better diffusion of digestive enzymes and substrates thus improving nutrient digestion and absorption.
Fermentation in the lower gut
On the positive side, fibre can be seen as the main nutrient driving fermentation in the lower gut. Dietary fibre can be utilised by fibre fermenting microbes and be converted to a range of useful nutrients, supporting both microbial growth and the host organism.
As fibre fragments are depolymerised into smaller and smaller molecules, they are more rapidly attacked and even absorbed by some resident species of the microbiota. In this regard, NSPases can directly ‘feed’ the large intestinal microbiota by taking insoluble and poorly fermented, large molecular-weight, soluble fibre and converting it into fermentable smaller components.
The prebiotic concept envisaged the production of a series of oligosaccharides by NSPases which are quantitatively fermented to VFAs. More recent work has suggested, however, that while NSPase can produce oligosaccharides from some cereals, the quantity produced is limited and certainly not enough to sustain the incremental volatile fatty acid production noted in many studies.
Stimulating fibre fermentation
The use of xylanase has been shown to lead to a change in the ability of the caecal microbiome of broiler chickens to ferment NSP as evidenced by gas production in an ex-vivo model. When caecal content from broilers fed with a xylanase containing diet was incubated with a range of NSP sources, there was an increase in gas production compared to caecal content from broilers fed a control diet (Bedford and Apajalahti, 2018).
This effect, and the resultant increase in SCFA production, takes time to establish, as shown by Lee et al. (2017) where increased SCFA in caecal content resulting from xylanase in the diet became obvious at 42 days of age, whereas increases in fermentable sugar content in the caeca was already seen at 21 days of age.
The term ‘stimbiotic’ (STB) has been introduced recently and is defined as non-digestible but fermentable additive that stimulates fibre fermentability, but at a dose that is too low for the stimbiotic itself to contribute in a meaningful manner to volatile fatty acid (VFA) production (Cho et al., 2020). The idea is that an STB would accelerate the development of a fibre fermenting microbiome whereby a small amount of a target oligosaccharide acts not only as a substrate, but also as a signalling molecule to encourage the relevant bacteria to produce fibre-degrading enzymes to accelerate fermentation and increase SCFA production. Various trials have been carried out with this concept in both pigs and poultry.
Using an STB has been shown to result in upregulation of oligosaccharide transporters in caecal bacterial cell walls (Amir, 2021), suggesting an increased ability of the microbiome to utilise fibre.
The products of fibre fermentation can be beneficial to the chicken in various ways. Firstly SCFA, and especially butyrate, can be utilised by the chicken as a source of energy. Butyrate has been studied widely as a feed additive, but the potential yield of butyrate from fibre fermentation widely exceeds the typical supply of butyrate from butyrate-based feed additives, suggesting that stimulation of fibre fermentation may be more effective than butyrate addition to the diet.
Secondly, a shift towards fibre fermentation can reduce the putrefactive fermentation of protein (Apajalahti et al., 2015). As protein fermentation results in production of branched chain fatty acids (BCFA) this effect can be seen in an increased ratio of VFA:BCFA, which has been measured several times across a range of STB studies (AB Vista internal data, 2020).
Stimulation of a fibre fermenting microbiome has been demonstrated in a study where an STB product was compared to a commercial enzyme blend containing xylanase and beta-glucanase in a diet rich in β-glucans due to inclusion of 30% barley (Morgan et al., 2021). In this study, β-glucan levels measured at ileal level at 21 days of age were decreased either when the STB product was included or when the enzyme blend was used. This resulted in reduced viscosity and improved performance for both products.
Total caecal SCFAs, acetic acid and propionic acid were significantly increased in the STB-fed group at 21 and 35 days of age, but the enzyme blend only had a significant effect on these parameters t 35 days of age. This indicates that the objective of stimulating a fibre fermenting microbiome at an earlier age was achieved by the inclusion of the STB product.
A further hypothesis is that a fibre fermenting microbiome would be more resilient to challenge, and thus use of an STB would be more beneficial under challenging conditions. This was reported in piglets where, under poor sanitary conditions, the use of STB resulted in improved performance as well improvements in biomarkers indicating better gut health (Cho et al., 2020).
In broilers the use of STB or STB combined with a suite of additive products based on phytogenic, yeast (MOS, glucans), prebiotics and probiotics was tested in a necrotic enteritis challenge model (Lee et al., 2022). Challenged birds showed increased intestinal lesions, increased serum levels of TNF-α and endotoxin, resulting in impaired performance as well as increased foot pad dermatitis.
Use of the STB product reduced the impact of the challenge for all these parameters. There was also a shift in caecal bacteria, with more Lactobacillus and less Escherichia coli present when the STB product was fed. In challenged birds there was a clear reduction in Clostridium perfringens in STB fed birds.
Adding the suite of other additive products did improve the response of the birds (Lee et al., 2022). This study shows that stimulating fibre fermentation may result in birds that are more resilient to challenge, suggesting better results in commercial practice where a higher level of challenge than in typical research trials is to be expected.
Stimulation of fibre fermentation in the chicken can give substantial benefits, both in terms of productivity as well as resilience of the chicken.