Fibre fermentation in the chicken: An important opportunity to extract extra value from feed ingredients

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.

Potential benefits

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.