Dietary fibre in laying hens: a review of effects on performance, gut health and feather pecking

SUMMARY

Dietary fibre has traditionally been considered an energy-diluting component of laying hen diets. With low energetic values, and sometimes negative impacts on digesta viscosity and gut function, formulations have often aimed to minimise crude fibre contents. Increasing fibre contents can mean that the level of fat required to meet nutritional standards must be increased to account for the decrease in energy, with consequential impacts on feed handling. By contrast, recent research has shown that some dietary fibres can have beneficial effects on laying performance, driven by changes in digestibility, gut structure and function, as well as shifts in the gut microbiota. However, there is often a lack of clarity as to the quantity and type of fibre required to yield production benefits. Broadly, soluble fibres – while largely detrimental to nutrient digestibility – can act as prebiotics, favouring beneficial intestinal microbiomes, while insoluble fibres stimulate intestinal development and may have some preventative effects on feather pecking and cannibalism.

KEYWORDS:

• Fibre
• laying hens
• gut health
• behaviour
• feather pecking
• cannibalism

Introduction

Eggs represent one of the most abundant and affordable sources of animal protein and are generally accepted by most cultures and religions (Bain, Nys, and Dunn 2016). The poultry industry has selectively bred laying hens to be capable of laying over 300 eggs a year. However, modern production practices pose numerous challenges for optimal bird health and productive output. Furthermore, the 2006 EU ban on non-therapeutic antibiotics adds to the challenge of maintaining flock health and welfare. The inclusion of additional dietary fibre is a strategy that has the potential to support multiple aspects of laying production. Historically, the inclusion of dietary fibre in poultry diets was considered to have a diluting effect on nutrient content, thought to cause poor nutrient utilisation (Jha et al. 2019). However, in recent years, there has been a change in attitude towards the industry recognising the value of certain types and quantities of fibre in poultry diets, and the use of dietary fibre has been revisited due to its potential benefits on gut health, gastrointestinal development, performance and bird welfare. Therefore, this review aims to summarise modern thinking on the use of dietary fibre in laying hens.

What is fibre?

Physiologically, dietary fibre can be defined as: the consumable component of plants or analogous carbohydrates (polysaccharides, oligosaccharides, lignin, and associated plant substances) that are resistant to digestion and absorption in the small intestine with complete or partial fermentation in the large intestine (AACC 2001). There are a number of types of fibre, as shown in Table 1, all of which can possess different functional properties – exerting positive, negative or neutral effects on the host’s digestive function (Jha et al. 2019). Approximate soluble and insoluble fibre contents of common feed ingredients used in reviewed studies is shown in Table 2.

Table 1. Types of fibre (adapted from The British Nutrition Foundation, 2020)

Type of fibre	Description	Dietary sources in livestock feed
Cellulose	Insoluble fibre – primarily plant cell wall components. When eaten cellulose passes through the tract relatively intact. Forms a small part of the total non-starch polysaccharides content of fibre	Cereal grains
Lignin	A non-carbohydrate component associated with plant cell walls – Insoluble	Outer cell walls of cereal grains
Pectin	Non-starch polysaccharide, common to plant cell walls. Soluble fibre source, generally well metabolised by gut bacteria, reduce cholesterol by flushing fatty acids. Gelling quality can increase digesta viscosity. Forms a small part of the total non-starch polysaccharides fraction of fibre.	Grains and legumes
Hemicellulose	Polysaccharides and major component of plant cell wall. Gel forming type of soluble fibre, includes beta glucans, xyloglucans, mannans, arabinoxylans etc. – can increase digesta viscosity, potentially prebiotic qualities, but also ANF depending on quantity and type. Hemicellulose forms a major part of the total non-starch polysaccharides present in fibre – Insoluble	Cereal grains and legumes
Resistant starch	Soluble to some extent, beneficial to immune system, provides feeling of fullness. Resistant starch has characteristics of both insoluble and soluble fibres. LIKE INSOLUBLE FIBRE IT IS RESISTANT TO THE DIGESTIVE PROCESS, HOWEVER, ONCE IN THE COLON IT CAN BE FERMENTED BY BACTERIA
Oligosaccharides	Short chain carbohydrates of 3–9 monomers	Xylan in cereal grains

Table 2. Approximate insoluble and soluble fibre content of common feed materials

	Approximate insoluble fibre content (%)	Approximate soluble fibre content (%)	Reference
Alfalfa meal	46.7	7.9	Bailoni et al. 2005
Barley hulls	20.3	9.8	NRC 2001
Cellulose	97	2.3	Swanson et al. 2001
DDGS	25.5	3.4	Jaworski et al. 2015
Oat Hulls	83.3	1.7	Podolske 2013.
Pectin	0	65.4	Swanson et al. 2001
Straw			Podolske 2013.
Sugar beet pulp	1.9–3	28	Fischer et al. 2012
Rice hulls	87.3	2.7	Podolske 2013.
Wheat bran	44.9	7.6	Fischer et al. 2012
Wheat	9.3	1.9	Jaworski et al. 2015

Fibre composition and chemistry have been thoroughly reviewed elsewhere (Yangilar 2013; Fuller et al. 2016), and this is not the scope of this paper. Briefly, components of fibre can be either soluble (SF) or insoluble (IF). Soluble fibres can be readily fermented by gut microbes in the hind gut; however, they have also been shown to increase the viscosity of digesta (Cameron-Smith, Collier, and O’dea 1994; Choct 1997). Increased viscosity reduces passage rate due to increased satiety and ultimately reduces overall feed intake (Jha et al. 2019). Reduction of viscosity caused by dietary soluble fibre is the traditional reason for application of xylanases to poultry diets (Choct et al. 2004; Kiarie, Romero, and Ravindran 2014). Insoluble fibre is poorly fermented and passes through the tract mainly undigested. This increases the passage rate and faecal bulking and can result in increased feed intake by the host (Jha and Berrocoso 2015).

Fibre as a nutrient is difficult to define and there are many terms used in animal nutrition to describe and measure fibre components. Crude fibre quantification came into existence following the development of proximate analysis in Germany in the 19th century. This method separates carbohydrates into two sections of either more digestible (nitrogen free extract) or less digestible (crude fibre) (Henneberg and Stohmann 1859, 1860). Crude fibre is defined as the organic remnants of feed following immersion in hot sulphuric acid and sodium hydroxide and is composed of some insoluble non-starch polysaccharides (NSP), cellulose and some hemi-cellulose (Figure 1). Whilst this method has been in use for many years, it has innate flaws, such as not accounting for any of the soluble NSP content (Choct 2015). Due to this flaw, the neutral detergent (NDF) and acid detergent fibre methods (ADF) were developed (Van Soest 1963). This method is generally agreed to well quantify the levels of NSP in cereal-based raw materials; however, there is a lack of consensus for oilseeds and legumes, due to the levels of soluble NSP, such as arabinose, galacturonans, and arabinogalactans, that the NDF analysis fails to account for (Choct, 2015). Despite the values of this method, from a chemistry perspective, it fails to determine any specific fibre components.

Figure 1. Fibre composition. Adapted from Choct, 2015

In recent years, there has been a significant increase in research output surrounding fibre in poultry diets, and the postulation that dietary fibre may have beneficial effects. Both insoluble and soluble fibres have been shown to support various parameters of poultry production and improvements in performance, egg production, nutrient digestibility, behaviour and welfare have all been cited suggesting that fibre may be a viable, broad spectrum tool for successful production (Mateos, Lazaro, and Gracia 2002; Incharoen and Maneechote 2013). An overview of the general effects is shown in Table 3, and a summary of the literature and reported effects is found in Table 4.

Table 3. The effects of dietary soluble and insoluble fibre in the gastrointestinal tracts of laying hens

	Soluble	Insoluble	Reference
Passage rate	Reduces tract passage rate^1	Increases tract passage rate^2	^1Adibmoradi, Navidshad, and Jahromi 2016
	Can increase the viscosity of digesta^1		^2Chaplin 2003
Nutrient digestibility	Detrimental to fat, protein and starch digestibility^3	Improves starch digestibility^4	^3Sacranie et al. 2012
	Can bind nutrients, reducing digestibility^5	Increases dry matter content of the faeces^5	^5Hetland, Svihus, and Olaisen 2002
Fermentability	Mainly fermentable by gut bacteria^6	Not particularly fermentable by gut bacteria^7	^6Nabizadeh 2012 ^7Rezaei et al. 2018
Availability	Energy source for monogastric animals^8	No energy derived by monogastrics (particularly young animals)^9	^8Silva, Morita, and Bolei 2013 ^9Jimenez-Moreno et al. 2013
Gut function and development	Potential prebiotic effect^10	Can stimulate intestinal villi development^11 and organ weight/size^12	^10Tejeda and Kim 2021 ^11Gonzales-Alvarado et al. 2007 ^12Sozcu and Ipek 2020
Other		May help prevent cannibalism^13	^13van Krimpen et al. 2010)

Table 4. Summary of published literature assessing the effect of dietary fibre on the performance, gut health and feather pecking in laying hens

Author	Type of fibre	Effect
Performance and egg quality
Jimenez-Moreno et al. 2013	Oat hulls 5% or sugar beet pulp 5%	Improved ADG, FCR, Gizzard weight with fibre inclusion
Gonzalez-Alvarado et al. 2010	3% oat hulls or sugar beat	Oat hulls show sig increase BWG and FCR, Gizzard weight increase with oat hulls. Total tract dig increased significantly for all nutrient
Incharoen and Maneechote 2013	Rice hulls 3% + 6%	Linear increase in BW and FI. FCR lowest in the 3% group. Linear increase in egg production with fibre level.
Hassan, Morsy, and Hasan 2013	3,4,5,6% crude fibre (alfalfa meal) layers	3% crude fibre significantly increased egg production, however 6% significantly increased shell, albumin and yolk weights compared to the other inclusion rates.
Incharoen and Maneechote 2013	Rice hulls 3% + 6%	Linear increase in BW and FI. FCR lowest in the 3% group. Linear increase in egg production with fibre level
Walugembe et al. 2014	DDGS and Wheat bran 6%	No significant difference in FI or ADG compared to control
Panaite et al. 2015	Low, medium and high fibre (3.67–7.24% DDGS, rice bran, wheat bran and sunflower meal	No significant performance differences observed
Guzmán et al. 2015	2 or 4% cereal straw, sunflower hulls or sugar beet pulp, layers hatch to 5 wks	Fibre inclusion significantly increased ADG, ADFI and energy efficiency compared to no fibre. Sunflower hulls tended to havce higher ADG than sugarbeet pulp (straw intermediate)
Yokhana, Parkinson, and Frankel 2016	Commercial insoluble fibre product containing 65% crude fibre and 20% acid detergent lignin fed at 1%. 8 week old pullets	Sunflower hulls tended to havce higher ADG than sugarbeet pulp (straw intermediate)
De Arruda et al. 2018		Lower egge weights with increasing quantities of insoluble fibre
Röhe et al. 2019	10% lignocellulose, dual purpose birds	Significantly decreased BW compared to control, but significant improvement in number of eggs laid. Increased shell thickness and breaking strength.
Zdunczyk et al. 2019	Replaced soy beans with lupin seeds – 100, 200,300 g/kg 32 week layers	No significant differences
Kocer et al. 2020	Layer diets from 21 weeks of age, 3,4,5% high fibre sunflower meal. 24 week study	Linear increase with fibre inclusion rate for BW, egg production rate, and shell mass. Shell-less egg rate and cracked/broken egg rate numerically reduced.
Sozcu and Ipek 2020	Lignocellulose, 0,0.5, 1 and 2 kg/t Layers 18–38 weeks	0.5 and 1 kg/t significantly increased BW, FI and feed efficiency, Increased egg production compared to control. 0.5 and 1 kg/t resulted in increased shell breaking strength, egg shell thickness and yolk index. No benefit observed for 2 kg/t.
Digestibility
Jimenez-Moreno et al. 2009	3% oat or soy hulls, broilers	Improved total tract retention of nutrients and reduced gizzard pH
Biggs and Parsons 2009	10–20% whole wheat, broilers	Significantly improved AA digestibility
Hetland, Svihus, and Olaisen 2010	Whole wheat, oats and barley 300 g/kg, broilers	Reduced particle size in the duodenum with whole grain inclusion, significantly improved starch digestibility.
Gonzalez-Alvarado et al. 2010	3% oat hulls or sugar beat, broilers	Oat hulls show sig increase BWG and FCR, Gizzard weight increase with oat hulls. Total tract dig increased significantly for all nutrient
Jimenez-Moreno et al. 2013	0,25,50,75 g/kg oat hulls or sugar beet pulp, broilers	50 g/kg of oat hulls or sugar beet Improved ileal digestibility of all nutrients
Adibmoradi, Navidshad, and Jahromi 2016	0, 0.75 and 1.5% rice hulls, broilers	Increased protein digestibility (0.75 and 1.5%)
Yokhana, Parkinson, and Frankel 2016	1–5% insoluble fibre, 8 week old layers	Increased activity of pepsin, trypsin, chymotrypsin
Gut development
Gonzalez-Alvarado et al. 2010	3% oat hulls or sugar beat	Chromium inoculated at cloaca, reverse peristalsis demonstrated by the presence of chromium in the gizzard, increased gizzard weight, content and reduce pH
Sacranie et al. 2012	15% oat and barley hulls	Increased gizzard weight and reduced gizzard pH
Sadeghi, Toghyani, and Gheisari 2015	3% sugar beet pulp, rice hulls or a 3% combination of both, broilers	3% sugar beet significantly reduced villi height (duodenum and ileum)
Yokhana, Parkinson, and Frankel 2016	1–5% commercial insoluble fibre product, 8 week mould layers	Increased gizzard, liver and small intestine weight compared to control
Röhe et al. 2020	10% lignocellulose	Increased gizzard weight, longer villi and shorter crypts.
Sozcu and Ipek 2020	Lignocellulose, 0,0.5, 1 and 2 kg/t Layers 18–38 weeks	2 kg/t resulted in longer jejunal villi
Gonzalez-Alvarado et al. 2010	3% oat hulls or sugar beat	Chromium inoculated at cloaca, reverse peristalsis demonstrated by the presence of chromium in the gizzard, increased gizzard weight, content and reduce pH
Microbiome
Biggs and Parsons 2009	Whole soy and corn, layer chicks	Decreased gut permeability
Shakouri et al. 2006	0, 3% methylated citrus pectin, broilers	Increased colonisation of Enterobacteriacaeca spp. In the caeca.
Baurhoo et al. 2007	2.5% purified lignin	Inhibition of beneficial bacteria such as Lactobacilli and Bifidobacteria in the caeca.
Wanzenböck et al. 2020	Native or fermented wheat bran, layers	No significant effects on microbiome
Excreta consistency and litter quality
Rezaei, Karimi Torshizi, and Rouzbehan 2011	0, 0.3, 0.4 and 0.5% of micronised insoluble fibre	Improved excreta consistency and litter quality
Farran, Pietsch, and Chabrillat 2013	0.8% wheat bran or lignocellulose	Litter moisture reduced by 10%
Kheravii et al. 2017	1 and 2% lignocellulose	Significantly reduced litter moisture
Feather pecking and cannibalism
Bearse, Miller, and McClary 1940	3, 12% crude fibre	Reduced integument injuries with high fibre
Van Krimpen et al. 2009	high or normal NSP levels (13.3 or 19.5%) combined with normal and low energy diets (2540 or 2825 kcal ME/kg)	No significant differences between treatments
van Krimpen et al. 2010	Normal or 15% diluted diets during rearing and production periods	Significant reduction in feather damage −20% compared to control
Qaisrani, Van Krimpen, and Kwakkel 2013	Normal, −7.5% or −15% diluted diets wither either sunflower or oat hulls	Significant reduction in feather dame in line with increased dilution rate, oat hulls most effective
Kalmendal and Bessei, 2012	8% spelt hulls	Reduced feather pecking

Effects of dietary fibre on laying hen performance and egg quality

The performance effects of fibre inclusion have been varied and not always consistent. This is likely due to the varying quantities and types of fibre investigated, making comparisons difficult.

During the pullet stage (8–18 weeks) the use of insoluble fibre has been shown to be beneficial for GIT development and subsequent performance (Abdallah and Beshara 2015; Panaite et al. 2016). Panaite et al. (2016) studied the effect of pelleted alfalfa at 0%, 4%, 8% and 19.44% on Lohmann Brown hybrid pullets from 9 to 16 weeks of age. Results showed that pullets supplemented with 8% alfalfa had significantly increased liveweights compared to the control at the end of the study. Furthermore, the 8% treatment group achieved liveweights that were 6.73% higher than the Lohmann Brown management guidelines. Abdallah and Beshara (2015) found similar results. Supplementing pullets with sunflower meal (7% and 14%), olive cake (5% and 10%) and a combination of sunflower and olive cake (7% sunflower and 14% olive cake) compared to a control (3.5% CF) from 11 to 19 weeks, followed by feeding all birds a control diet from week 20 to 40. The results showed that increasing CF by up to 6% during the pullet phase resulted in significantly improved liveweight and FCR compared to the control. CF digestibility was also significantly increased for all fibre treatments compared to the control. The study found that increasing the CF significantly increased the gizzard and caeca weights compared to the control, suggesting that improved gizzard development and function may be a possible mechanism for improved growth. This study went on to look at the subsequent egg production performance and found that increasing the level of fibre in the diet significantly increased egg numbers, laying rate % and egg mass compared to the control birds, potentially suggesting ongoing benefits of fibre in the growing phase diets.

Guzmán et al. (2015) investigated the effect of supplementing laying pullets with sunflower hulls, cereal straw or sugar beet pulp at either 0%, 2% or 4% from hatching to 5 weeks of age, and demonstrated significant improvements in ADG, FI and energy efficiency. Furthermore, FCR was significantly improved when fibre was supplemented at 2% rather than 0% or 4%, suggesting that there may be a maximum inclusion rate to gain benefits from supplemental fibre. Yokhana, Parkinson, and Frankel (2016) found that a commercial insoluble fibre, fed at 1% to 8-week-old pullets, increased body weight gain compared to the control. Sozcu and Ipek (2020) investigated the effect of lignocellulose at 0, 0.5, 1 and 2 kg/t feed on performance, and showed that 0.5 and 1 kg/t of lignocellulose significantly increased bodyweight, feed intake and feed efficiency compared to the control at all sampling points between 18 and 38 weeks of age. Interestingly, supplementation at 2 kg/t had no further beneficial effect; this is similar to the findings of Guzmán et al. (2015), though the optimal dose differs between studies.

However, other studies have found either no effect or detrimental effects on hen bodyweight when supplementing dietary fibre. Panaite et al. (2015) trialled numerous sources of fibre, including DDGS, rice bran wheat bran and sunflower meal, comparing the inclusion of these fibre sources at both a low (3.67% starter, 3.86% grower and 5.39% finisher) and high (4% starter, 5% grower, 7.24% finisher) levels. Inclusion of these fibres had no significant impact, indicating that there were no detrimental effects of feeding up to 7% of dietary fibre. Similarly, Walugembe, Rothschild, and Persia (2014) found no significant differences in the feed intake or average daily gain of layer chicks supplemented with a 60 g/kg (6%) mix of DDGS and wheat bran compared to a control. Zdunczyk et al. (2019) saw no effect of increasing fibre levels when substituting lupins for soybeans in isocalorific diets, and Röhe et al. (2019) report that a 10% addition of lignocellulose resulted in significantly decreased bodyweight in dual-purpose birds compared to a standard commercial diet. However, this study also revealed a significant improvement in the number of eggs laid, which was also correlated with significantly improved feed conversion efficiency.

Increased egg production following fibre supplementation was also seen with lignocellulose by Sozcu and Ipek (2020), where 0.5 and 1 kg/t of lignocellulose (IF, NDF 60–90%) resulted in significantly increased egg production; 2 kg/t was unable to change performance versus the unsupplemented control. Röhe et al. (2020) also reported some egg production benefits when supplementing dual-purpose birds (Lohmann Dual) with 10% lignocellulose (IF, NDF 60–90%) and showed that the addition of lignocellulose resulted in significantly increased egg production. Likewise, shell breaking strength and thickness were numerically increased in the treatment group. However, the eggs produced by the control were significantly larger and heavier than the treated group. The authors suggest that the results seen may be related to bird weight, as the treatment group weighed significantly less than the control and it has been demonstrated that egg weight is directly related to bird weight. It is of note that the experimental birds were dual purpose, and therefore ingested energy was divided between muscle/fat deposition and egg production. Modern laying strains have been heavily selected over long periods of time to achieve maximum laying potential; it is plausible that the use of a dual-purpose bird complicates the laying response to fibre.

By contrast, De Arruda et al. (2018) reported lower egg weights with increasing levels of insoluble fibre, while Kocer et al. (2021) reported mixed findings when supplementing laying diets (from 21 weeks of age) with 3%, 4%, or 5% fibre (high fibre sunflower meal). In a 24-week study, the authors report that body weight, egg production rate, and shell mass were all linearly significantly increased by increasing fibre inclusion. Furthermore, the shell-less egg rate and cracked/broken egg rate were numerically reduced linearly with increased fibre inclusion. A typical layer requires 2.2 g of calcium for each egg laid – more than 60% of this is supplied in the diet, but the remainder is obtained via mobilisation of calcium from the medullary bone (Bouvarel 2011). Insufficient calcium absorption from the diet can contribute to the onset of osteoporosis. Therefore, provision and bioavailability of calcium are vital for both shell strength and bird welfare. Van der Aar et al. (1983) reported that the inclusion of 8% lignin (IF, NDF 60–90%) resulted in reduced mineral absorption, consequently reducing cation-binding protein at the brush border of the small intestine. However, there is also evidence that the reduced intestinal pH often observed with increased dietary fibre, can result in increased mineral digestibility, possibly due to increased mineral solubility (Engberg et al. 2004; Yokhana, Parkinson, and Frankel 2016; Kim et al. 2019). The increase in shell mass described above by Kocer et al. (2020), as well as the increase in shell breaking strength described by Röhe et al. (2018), seem to refute the earlier findings of Van der Aar et al. (1983); however, as the fibre sources and inclusion rates were different between studies, it is hard to compare fairly.

Sozcu and Ipek (2020) also looked at egg quality parameters and found that once again the supplementation of 1 kg/t of lignocellulose resulted in significantly increased eggshell breaking strength, eggshell thickness, increased yolk index percentage and increased yolk ratio percentage compared to the other treatments. As with the previous results of this study, the 0.5 kg/t treatment demonstrated significant improvements compared to the control, but to a lesser degree. However, the 2 kg/t treatment in general only delivered statistically similar results to the 0.5 kg/t group and a linear dose response was not observed. Hassan, Morsy, and Hasan (2013) investigated egg yolk cholesterol and production performance of layers fed 3%, 4%, 5% or 6% crude fibre in the form of alfalfa meal. This study found that layers fed 3% fibre laid significantly more eggs than the higher inclusion rates; however, the inclusion of 6% fibre resulted in significantly increased shell, albumin, and yolk weights compared to the other inclusion rates. Likewise, the cholesterol per yolk was significantly decreased linearly in line with increased fibre inclusion. This may indicate that including increased levels of dietary fibre may help to reduce the cholesterol in eggs, consequently improving the nutritional profile of eggs for the consumer.

There is a paucity of information on the effect of fibre on flock uniformity. Incharoen and Maneechote (2013) looked at the effect of rice hulls at 0%, 3% and 6% as a form of insoluble fibre on flock uniformity of laying hens. A 28-week study found that rice hulls significantly improved flock uniformity. Conversely, Asensio et al. (2020) found no significant effect of increasing fibre (Wheat bran) from 3% to 8.8% and reducing AMEn by 15% on flock uniformity. Whilst no detrimental effects were found, clearly more information is required to better understand this parameter.

Effects of dietary fibre on nutrient digestibility

Whilst it has been reported that nutrient digestibility is inversely proportional to the quantity of dietary fibre included in the ration (Jha et al. 2019), a number of studies have demonstrated positive effects when changing from diets lacking in fibre to formulations with significant fibre content. In some cases, the addition of fibre can result in improved AME (Hetland and Svihus 2001; Gonzales-Alvarado et al. 2007; Biggs and Parsons 2009). Likewise, Biggs and Parsons (2009) report that feeding broilers 10–20% whole wheat resulted in improved AA digestibility. A number of studies have reported that the inclusion of 3–10% oat hulls results in increased digestibility of faecal starch, protein, and fat, as well as ileal starch (Rogel, Annison, and Bryden 1987; Hetland and Svihus 2001; Jimenez-Moreno et al. 2009). Conversely, Singh et al. (2014) investigated the effect of replacing ground maize with 150, 300, 450 or 600 g/kg of coarse maize (10 mm sieve). The only effect observed between treatments was a significant increase in feed intake, and no differences in energy utilisation were seen.

The reasons for the improved utilisation of nutrients seen with the addition of insoluble fibre are likely to be multifactorial. It has been demonstrated that the inclusion of insoluble fibre decreases the particle size of digesta entering the small intestine. Hetland, Svihus, and Olaisen (2010) investigated the effect of feeding whole grains (wheat, oats and barley) to broilers. The study measured duodenal particle size by laser diffraction and found that the moderate inclusion of whole cereals (300 g/kg) reduced the particle size in the duodenum significantly compared to the ground cereal control. Likewise, the high inclusion of whole grains at 440 g/kg reduced the particle size still further. This ultimately resulted in a significant increase in starch digestibility for the inclusion of both whole wheat and barley. The authors suggested a hypothesis for this is that the gizzard grinds diet components to a critical particle size before releasing them into the duodenum. It has also been suggested that some constituents of the diet may be encapsulated within large particles and, therefore, harder to degrade sufficiently for absorption by the bird. This theory is supported by the work of Péron et al. (2007), who used scanning electron microscopy to elucidate the digestive process. This study was able to show starch granules trapped within larger particles in the small intestine. In contrast, another study by Péron et al. (2005) demonstrated that fine grinding of a hard wheat variety that had generally shown low starch digestibility, resulted in increased starch digestibility. This may suggest that the process is more complicated than simply providing structural fibre to increase gizzard efficiency and that rather the provision of a ground diet with a degree of structural components may provide more benefit to digestibility.

Adibmoradi, Navidshad, and Jahromi (2016) reported that the inclusion of rice hulls (IF87.3%, SF 2.7%, NDF ~60%) at both 0.75% and 1.5% significantly increased crude protein digestibility; improved gizzard function and pepsin secretion as a result of increased HCL production may be the mechanism for this improvement. This is in agreement with the results of Gonzales-Alvarado et al. (2007), and Amerah, Ravindran, and Lentle (2009). Similar results have been reported in pigs (Kirchgessner et al. 1994), where improvements were assigned to changes in the microbiome favouring N utilisation in the hind gut and thus shifting from urine N excretion to bacterial protein, lowering N volatilisation in the manure and improving environmental outcomes in both pigs and laying hens (Canh et al. 1998; Roberts et al. 2007). It is noted that much of the literature reviewed here is from broilers due to the paucity of layer data. Therefore, some discrepancies in the effect of adding fibre to layer diets may exist due to the variable composition and form of broiler diets.

It has been widely demonstrated that fibre inclusion can reduce the pH in the GIT. Jimenez-Moreno et al. (2009) looked at the effect of cereal type on gizzard pH and nutrient utilisation in broilers. In this study, we compared the inclusion of 3% oat (IF83.3%, SF1.7%, NDF ~75%) or soy hulls (IF49.3%, SF13.3%, NDF 67%) to a control and showed that the inclusion of either oat or soy hulls improved the total tract apparent retention of nutrients and reduced the pH in the gizzard. The authors suggest that the reduced pH in the gizzard may be responsible for further degrading nutrients and to make them more available for subsequent absorption. Engberg et al. (2004) found similar results and suggested the same mechanism of increased nutrient degradation in the gizzard. The reduced pH in the gizzard is likely to have further benefits for nutrient digestibility. Pepsin activity is stimulated by low pH (Yokhana, Parkinson, and Frankel 2016). Therefore, if fibre inclusion is reducing intestinal pH, it is plausible that this may be the mechanism for increased pepsin activity observed by Yokhana, Parkinson, and Frankel (2016) in layers fed 1% insoluble fibre. Likewise, reduced gizzard pH has also been demonstrated to increase calcium digestibility (Kim et al. 2019). The reduction in pH improves the solubility of limestone in the diet, rendering increased calcium available for absorption. This mechanism may be beneficial for layer production, particularly in later lay when shell strength starts to diminish.

Svihus (2011) suggests that there is a link between the proventriculus and gizzard, whereby the functionality of the gizzard can control the preceding secretion of HCl and pepsinogen in the proventriculus. The author theorises that when the gizzard is stimulated by the presence of fibre, the proventriculus can respond by increasing gastric secretions. This may therefore aid the further degradation of nutrients, increasing the availability of absorption.

Fibre has also been shown to influence digesta retention time, and therefore digestibility (Hetland, Svihus, and Olaisen 2002; Tejeda and Kim 2021). Sacranie et al. (2012) demonstrated that the supplementation of 15% oat (IF 83.3%, SF 1.7%, NDF ~75%) and barley hulls (IF 20.3%, SF 9.8%, NDF 60–75%) resulted in a larger particle of dietary fibre leaving the gizzard. This reduced passage rate in the upper digestive tract and increased retro-peristalsis, therefore increasing exposure to HCl and digestive enzymes, and consequently increasing the digestibility of starch. However, Hetland et al. (2002) suggests that the accumulation of insoluble fibre in the gizzard may trigger temporary satiety and could therefore reduce feed intake and subsequent growth and/or performance.

Effects of dietary fibre on gut development and function

The beneficial effects of dietary fibre inclusion (particularly insoluble fibre) on the development and function of the GIT are well reported in poultry (Hetland, Svihus, and Krogdahl 2003; Hetland, Svihus, and Choct 2005; Sacranie et al. 2012). It is likely that this stems from the physical/mechanical demands of the consumption of fibre. Sacranie et al. (2012) reported that the typical gizzard content is higher in fibre than the contents of other parts of the intestinal tract, demonstrating that the fibre component of the diet is physically harder to grind. In the absence of teeth, poultry relies on the gizzard to grind digesta (Svihus 2011). The gizzard is comprised of myelinated smooth muscle and sits immediately after the proventriculus (glandular stomach). The body of the gizzard is composed of two thick, laterally opposed muscles and two smaller anterior and posterior muscles (Akester 1986). Together, these allow rotary and crushing movements upon contraction. Within the gizzard, the surface is coated in a thick layer of glycol-proteins that are hardened by the low pH environment that increase grinding ability (De Lima and Sasso 1985). The adequate function of the gizzard is essential as feed particles can only leave the gizzard when a critical size is reached (Clemens, Stevens, and Southworth 1975; Moore 1999). Likewise, feed must be adequately ground to optimise nutrient uptake, as unground feed particles will prevent enzyme access and subsequent degradation. However, increased particle size is only part of the answer. Whilst indeed, increasing particle size can increase gizzard size and function, it is the insolubility of the fibre that is the most active factor. Increasing pellet-size is likely to increase feed intake; however, if the pellet is mainly soluble, much of this structure will be lost, along with the potential beneficial effect during digestion. The entire gastrointestinal tract relies heavily on retro peristalsis and reflux. Due to the small size of the proventriculus (glandular stomach), feed retention tends to be low causing much of the gastric secretion activity to take place within the gizzard. The reflux effect allows insufficiently degraded feed to be refluxed back to the proventriculus for additional gastric secretions to be applied (Duke 1986). It is well established that feed particle size and structure have a significant effect on the development of both the proventriculus and the gizzard. Svihus (2011) states that gizzard size can increase by up to 100% its original size with the addition of insoluble fibre and structural diet components, such as whole grains and wood shavings. Interestingly, the degree of weight change in the gizzard content is often far greater than that of size change, with the content weight often increasing two-fold (Svihus 2011). It is generally considered that increased gizzard size allows for additional feed intake and throughput, and therefore may be beneficial for growth performance.

The work of Yokhana, Parkinson, and Frankel (2016) demonstrates this effect: when supplementing 8-week-old layers with 1% insoluble fibre (Commercial source), birds fed insoluble fibre had significantly increased organ weights for liver, gizzard, and small intestine compared to the control. GIT enzyme activities were also analysed, and again revealed that the inclusion of insoluble fibre resulted in significantly increased pepsin, trypsin, and chymotrypsin activities compared to the control. Similarly, Röhe et al. (2020) saw increased gizzard weights following supplementation of 10% lignocellulose and Sacranie et al. (2012) showed that the addition of 15% oat and barley hulls either coarsely or finely ground significantly increased gizzard weight and significantly reduced gizzard pH. Whilst both fine and coarse hull inclusions significantly increased gizzard weight compared to the control, the effect was not as pronounced for the fine ground treatment. This could suggest that the increased particle size is partly responsible for the benefits derived. This study revealed no significant differences in gut motility between treatments but did confirm complete reflux with the inert marker applied to the cloaca being detected in all parts of the GIT.

The lack of fibre provision may also be detrimental to bird health and downstream productivity. As already explained, it has been widely demonstrated that the provision of fibre improves the development and function of the proventriculus and gizzard. However, if fibre is not provided to serve this function, the proventriculus can be poorly developed affecting functionality in terms of gastric secretions (Gonzales-Alvarado et al. 2008; Jimenez-Moreno et al. 2009; Mateos et al. 2012) resulting in widening of the gastric isthmus and subsequent rupture of the proventriculus during processing, resulting in increased condemnations (birds used for meat products, stock and soups) and financial loss (Dormitorio, Giambrone, and Hoerr 2007; Svihus 2011; Mateos et al. 2012).

In addition to the upper GIT, fibre may also influence the architecture of the small intestine. Sozcu and Ipek (2020) investigated the effect of lignocellulose (at 0, 0.5, 1 and 2 kg/t) on the histomorphological traits of layers in peak lay and showed that birds supplemented with 2 kg/t of lignocellulose had significant longer jejunal villi compared to other treatments. This result is interesting as the 1 kg/t treatment performed best in terms of performance, and 2 kg/t tended to cause negative or negligible results in comparison. Similarly, Röhe et al. (2020) saw that the addition of 10% lignocellulose had a larger villus absorptive area, longer villi and shorter crypts. Zdunczyk et al. (2019) saw that increased crude fibre from lupin inclusion resulted in increases in both duodenal and jejunal mucosa thickness, crypt depth and villus height. Conversely, Sadeghi, Toghyani, and Gheisari (2015) investigated the effect of 3% sugar beet pulp rice hulls or a 3% combination of both compared to a control in a broiler trial. This trial showed that the inclusion of sugar beet pulp significantly reduced duodenal and ileal villi heights compared to the control at 21 days of age. This may suggest that the type, inclusion rate and form of the fibre used may influence its effects on gut development and function. Interestingly, the inclusion of sugar beet pulp (Sadeghi, Toghyani, and Gheisari 2015) also significantly increased the antibody titre against Newcastle Disease.

Effects of dietary fibre on the intestinal microbiome

The microbiome can be affected by innumerable factors, including diet, breed, environment and genetics (Shang et al. 2018). However, diet is considered one of the most potent regulating factors (Mahmood and Guo 2020). Dietary fibres can contain varying quantities of non-starch polysaccharides (NSPs) which are complex carbohydrates derived from the structural components of plant cell walls (Mahmood and Guo 2020). NSPs have been widely shown to have detrimental effects on poultry performance, mainly associated with the increased viscosity of digesta as they absorb water and form viscous gels (Bedford 2018). Consequently, the use of NSP degrading exogenous enzymes is common in the industry. Degradation of NSPs is thought to depolymerise polysaccharide components into oligosaccharides (SF) which are considered prebiotics due to their ability to enhance beneficial bacteria in the GIT (Ding et al. 2018; Li et al. 2017). This prebiotic shift following supplementation with either oligosaccharides or NSP-degrading enzymes is well documented, and is shown to work through ‘cross-feeding’ mechanisms that favour mutualistic microbial populations and ultimately upregulate the production of butyrate and other SCFAs in the gut (De Maesschalck et al. 2015; Rivière et al. 2015).

Nutrients that remain undigested upon reaching the hind gut (distal small intestine and large intestine) can be utilised as a vital nutrient source for commensal bacteria. The commensal bacteria play a role in immune defence by stimulating the development of the intestinal mucus layer, epithelial monolayer and intestinal immune cells. Furthermore, the provision of fermentable substrate for the microbiota results in the production of microbial metabolites. Metabolites produced can have a variety of beneficial functions, such as antimicrobial and anti-inflammatory properties and the production of short-chain fatty acids (SCFAs), such as butyric and propionic acids (Shang et al. 2018). Butyrate is the primary energy source for enterocytes, allowing cells to be renewed and therefore helping to prevent intestinal permeability (leaky gut) (Jha et al. 2019). Likewise, butyrate encourages growth of beneficial bacteria, contributes towards optimal immune function and also plays a role in brain function (Belkaid and Hand 2014; Liu et al. 2018).

The huge variety of chemical structure of dietary fibre is likely to have equally variable effects on the resident bacteria: while Wanzenböck et al. (2020) saw no significant effects of native or fermented wheat bran on the microbiome of laying hens, Ding et al. (2018) studied the effects of dietary xylo-oligosaccharides (SF) (XOS, at 0, 0.01, 0.02, 0.03, 0.04 and 0.05%) on the gut microbiota of laying hens and saw a significant increase in the numbers of Bifidobacteria in the caeca (linear increase in line with inclusion rate). Furthermore, XOS inclusion significantly increased the digesta content of butyrate and tended to increase acetic acid in the caeca (again linear increase with inclusion rate). This may suggest that provision of only a very small insoluble fibre component may be beneficial.

Conversely, the provision of excess undigested feed can contribute to dysbiosis and the proliferation of pathogenic bacteria (Shang et al. 2018; Mahmood and Guo 2020). Dysbiosis has been reported to reduce intestinal barrier function (leaky gut), reduce nutrient digestibility and increased likelihood of systemic translocation of luminal bacteria (Shang et al. 2018). This may suggest that there is an optimal inclusion level of fibre to derive maximum benefits – as seen previously when assessing the effects of fibre on performance – and that the provision of excess fibre may be detrimental to the bird. Furthermore, the increased viscosity, which has been well described in the literature as a result of NSPs in the diet, has also been shown to have a detrimental effect on the proliferation of potentially pathogenic bacteria in the hind gut. Johnson and Gee (1981) state that the increased viscosity of digesta causes the thickness of the unstirred water layer that covers the mucosa cells, to increase. In turn, this limits contact between nutrients and gastrointestinal secretions (pancreatic enzymes and bile acids), resulting in increased quantities of undigested protein and starch reaching the hindgut and acting as a substrate for proliferation of potentially pathogenic bacteria (Bedford and Cowieson 2012). This effect has been described by Shakouri et al. (2006), who found that 3% supplementation of methylated citrus pectin resulted in increased colonisation of Enterobacteriacaeca spp. in the caeca of broilers. Baurhoo et al. (2007) found similar results demonstrating that supplementing purified lignin at 2.5% resulted in inhibition of beneficial bacteria, such as Lactobacilli and Bifidobacteria in the caeca. It is of note that neither of these studies used an NSP degrading enzyme, so it is unclear whether this may mitigate the problem.

Effects of dietary fibre on excreta consistency and litter quality

Due to the evolutionary adaptation of birds to excrete both solid and liquid waste in one combined movement, maintaining litter quality can be a difficult task. Poultry excreta can contain approximately 80% moisture (Henuk and Dingle 2003). In broiler production, this can become problematic, leading to wet litter and numerous problems, including pododermatitis, breast blisters/scald, hock burn, and poor air quality (Dunlop et al. 2016; Kheravii et al. 2018)

However, in laying hens, excreta consistently is more problematic in terms of dirty eggshells and contaminated plumage. Dirty eggs are problematic for the laying industry due to downgrades at processing that can be costly to producers. The effect of fibre on excreta consistency seems to depend largely on the type and quantity of fibre used. A number of studies have reported that provision of dietary soluble NSPs can significantly increase the incidence of water and sticky droppings, due to the increased viscosity of content (Choct and Annison 1992; Jimenez-Moreno et al. 2013). However, other studies – particularly those looking at insoluble fibre – have shown that the provision of dietary fibre can significantly improve excreta consistency, consequently improving litter quality. Rezaei, Karimi Torshizi, and Rouzbehan (2011) investigated the effect of feeding 0%, 0.3%, 0.4% and 0.5% of micronised insoluble fibre (commercial source) on litter moisture in broilers; there was a linear reduction in litter moisture with the increased inclusion of micronised insoluble fibre. The improvements observed with insoluble fibre may be due either to an effect on the mucosal layer of the intestine, allowing increased absorption of nutrients and water from the digesta, or that the micronised insoluble fibre itself, as the material has a high water-holding capacity (up to 4–8 times its weight). Similarly, Farran, Pietsch, and Chabrillat (2013) found that litter moisture could be reduced by 10% with the addition of either 0.8% wheat bran or lignocellulose, while Kheravii et al. (2017) also found significantly reduced litter moisture with the supplementation of 1% and 2% lignocellulose. Kheravii et al. (2017) suggest two potential mechanisms for this: firstly the reduced litter moisture may be due to the increased water-holding capacity of the feed material, consequently reducing the moisture in the excreta, or alternatively, the increased production of SCFAs as a result of increased fermentation of fibre fractions in the hind gut may be increasing intestinal enterocyte proliferation, consequently increasing the surface area available for water resorption. However, as both lignin and cellulose are insoluble, the degree of fermentation may be questionable. Whilst this literature is mainly on broilers, the principle of improved excreta quality is likely to have beneficial effects on the occurrence of dirty shells. Higher moisture levels in the excreta can increase the propensity of plumage and nests becoming contaminated, thus increasing dirty shells. Soluble fibres such as β-glucan and xylan that have been shown to increase the viscosity of digesta, can actually have a negative impact on excreta consistency. Increased viscosity has been reported to increase thirst, consequently causing birds to increase their water intake, and moisture content of the excreta (Choct 1997Kalmendal and Wall 2012). However, the provision of insoluble fibres such as lignin and cellulose has been shown to increase the structure of manure (Kalmendal and Wall 2012; Johansson 2014).

Effects of dietary fibre on feather pecking and cannibalism

Feather pecking and cannibalism are abnormal behaviours of poultry that can cause significant welfare, health and productivity problems for the industry (Hartini et al. 2002; Van Krimpen et al. 2009). Nutritional approaches to preventing these issues have been extensively reviewed elsewhere (Rodenburg et al. 2013; Mens, Van Krimpen, and Kwakkel 2020) As far back as 1940, the use of dietary fibre was cited as an effective strategy for preventing cannibalistic behaviour (Bearse, Miller, and McClary 1940). However, there is also a significant weight of modern research to confirm this (Hartini et al. 2002; Van Krimpen et al. 2009). Feather pecking is generally considered a redirected foraging behaviour, antagonised by the lack of need to forage for food due to commercial feed being available ad libitum (Blokhuis 1986; Huber-Eicher and Wechsler 1997; Steenfeldt, Kjaer, and Engberg 2007; Albiker and Zweifel 2017). The provision of increased fibre (generally insoluble fibre) as a source of roughage has been widely shown to reduce feather pecking and/or cannibalistic behaviours (Bearse, Miller, and McClary 1940; Scott, Holm, and Reynolds 1954; Seemann 1982; Wahlström, Tauson, and Elwinger 1998;; Steenfeldt, Kjaer, and Engberg 2007; Albiker and Zweifel 2017). This effect is generally attributed to increased time spent eating, thus reducing redirected behaviours (Hartini et al. 2002; van Krimpen et al. 2008; Walser 1997). Furthermore, the act of actually reducing the metabolisable energy, by diluting the diet with fibre has been demonstrated to reduce stereotypic behaviours by encouraging birds to spend extra time eating in order to intake sufficient nutrients (Hartini et al. 2002; van Krimpen et al. 2008).

When considering the effect of various fibrous raw materials, it has been reported that the inclusion of oats seems to have more beneficial effects on cannibalistic behaviours than wheat (Bearse, Miller, and McClary 1940; Scott, Holm, and Reynolds 1954; Seemann 1982; Wahlström, Tauson, and Elwinger 1998). This may be due to the increased level of crude fibre present, as Abrahamsson et al. (1996) found that diets high in barley were more effective than diets high in wheat. However, this effect has also been reported to vary with strain in birds, with some studies finding no effect (Karlson 1996).

Bearse, Miller, and McClary (1940) investigated the effect of low (3% crude fibre) and high (11–12%) fibre diets on the respective damage to the integument and found that the birds receiving the lower fibre diets suffered a higher incidence of integument injury. The study went on to add oat hull water extract, oat hull ash or sodium silicate to layer diets to try and determine how the oat hulls were helping to prevent injury. However, this part of the study revealed no reduction in integument injuries, suggesting that the benefit is derived from fractions other than those investigated. The work of Qaisrani, Van Krimpen, and Kwakkel (2013) reports that the use of nutrient diluted diets (7.5% diluted sunflower seed extract (SF) and oat hulls (IF, NDF, NDF 75%) 15% diluted, sunflower seed extract and 15% diluted with oat hulls) resulted in a significant reduction in feather damage in line with the increased dilution percentage and that oat hulls were more effective in preventing feather damage than sunflower seed extract. Van Krimpen et al. (2009) are in agreement, finding that the use of diluted by 15% resulted in birds compensating for this by directly increasing feed intake. Consequently, a significant and sustained reduction in feather pecking behaviour was observed, although not completely mitigated.

In contrast, Van Krimpen et al. (2009), when supplementing ISA brown layers with either high or normal NSP levels (13.3% or 19.5%) combined with normal and low-energy diets (2540 or 2825 kcal ME/kg) found no significant differences in the feather scores or culling rates due to cannibalism between treatments. The results of these two studies would seem to indicate that the type of fibre used is vital for improvements to be observed. In general, it appears that insoluble fibre is more effective in this parameter.

The form in which the feed is provided may also be important. Bearse et al. (1940) observed that high fibre diets were more effective in reducing cannibalistic behaviours when fed as a mash rather than pelleted. It seems plausible that this is due to the increased time required to ingest mash as opposed to pellets.

The effect of fibre on layer mortality is generally associated with feather pecking and cannibalistic behaviours (Hartini et al. 2002; Wahlström, Tauson, and Elwinger 1998; Hartini and Choct 2011; Walugembe, Rothschild, and Persia 2014). Hartini et al. (2002) reported reduced rates of mortality when feeding high levels of soluble fibre (Barley); however, mortality was reduced still further when fibre was used in conjunction with beak trimming. Wahlström, Tauson, and Elwinger (1998) report that increasing the quantity of crude fibre in laying diets from 44 to 64 g/kg resulted in a 31% reduction in mortality. This reduction was also accompanied by a significant reduction in traumatic skin wounds and so is assumed to be related more to the effect of fibre on feather pecking behaviours, rather than a direct effect on mortality. Hartini and Choct (2011) found no significant difference in mortality between fibre sources in an experiment comparing wheat, guar gum and lucerne diets containing identical crude fibre percentages.

The effect of adding additional fibre to diet formulations

Typically, the addition of fibre to poultry diets has been thought to have detrimental effects on performance and the utilisation of nutrients, due to the low metabolisable energy content of ingredients with high fibre content (Lee et al. 2003). A traditional peak-lay (26–50 weeks) diet contains approximately 2–3% crude fibre. Additional fibre in the diet generally results in an energy deficit; this is often compensated by the inclusion of additional oil. Unfortunately, mash diets containing more than 4% oil are highly likely to have handling problems due to the high moisture content. An example of formulations containing increasing quantities of fibre is shown in Table 5.

Table 5. Effect of increasing the level of dietary fibre on example feed formulations for laying hens in peak lay

	Low fibre	Medium fibre	High fibre
Raw Materials, %
Corn	62.81	35.29	35.39
SBM (47% CP)	25.87	15.94	12.02
Wheat, soft	–	26.67	17.45
Sunflower meal	–	4.86	10.00
Rapeseed meal	–	5.00	5.00
Limestone	9.30	9.24	9.23
Wheat bran	–	–	6.42
Soybean oil	0.67	1.70	3.28
MCP	0.40	0.35	0.31
Sodium chloride	0.30	0.30	0.29
Layer premix	0.25	0.25	0.25
Phytase and carbohydrase^a	0.11	0.11	0.11
Carotenoids – red natural	0.05	0.05	0.05
Choline chloride	0.05	0.05	0.05
Sodium bicarbonate	0.02	0.01	0.01
L-Lysine HCl	–	0.06	0.11
L. Threonine	–	–	0.01
D.L. Methionine	0.17	0.12	0.11
Calculated Nutrient Composition, %
Dry matter	88.46	88.39	88.77
Crude protein	17.00	17.00	17.00
Crude fat	3.35	3.86	5.56
Crude fibre	2.24	3.40	4.50
NDF	8.19	10.16	12.92
NSP	13.58	14.26	16.94
Calcium	3.85	3.85	3.85
Phosphorus	0.42	0.45	0.50
Available Phosphorus	0.36	0.36	0.36
Na	0.16	0.16	0.16
C18:2	1.59	1.79	2.66
Digestible lysine	0.79	0.73	0.73
AME (CVB 2018), kcal/kg	2730	2730	2730

^aProviding 500 FTU/kg phytase, ########

It is of note that laying hens are generally regarded to have good capability to increase feed intake when diets are diluted; however, considerations have to be made for the potential extra costs of feed processing and transportation of diluted diets. To avoid the use of high quantities of soya oil, the increased use of oil seeds such as sunflower and rapeseed can be used. Whilst this solution still results in increased crude fat inclusion, the handleability of the diet is improved. Diets containing high levels of oil are thought to be typically well tolerated by layers (Kleyn 2013); however, interactions between fats and dietary fibre have been reported. Galbraith and Miller (1973) showed that lipids can reduce the microbial degradation of fibre by forming a layer on the surface of the fibre matrix. When the oil level is increased (particularly with soya oil) the linoleic acid level is also likely to increase; increased linoleic acid has been associated with increased egg weight (Grobas, Mateos, and Mendez 1999a), and at maximum levels of 3.45% are unlikely to be problematic to production.

Increasing the fibre content of diets may also be associated with an increase in NSPs (Table 4), which has been shown to increase the viscosity of digesta (Choct 1997). Increased digesta viscosity has been associated with reduced digestibility of various nutrients, including lipids (Smits 1996; Choct 1997). This indicates the need for an NSP degrading enzyme to be added to the diet to counteract the antinutritional factors associated with higher levels of NSPs. Adding NSP degrading enzymes may also have the benefit of reducing the amount of oil required in high fibre diets. The addition of a typical NSP degrading enzyme to wheat or barley can increase the AME by 3–6% (Lázaro et al. 2003; Mirzaie et al. 2012), thus reducing the energy deficit from adding fibre.

Summary

Despite the historical aversion to the inclusion of dietary fibre in poultry diets, there is increasing evidence that the use of some forms and quantities of fibre may be beneficial to the host. Primary benefits include enhanced nutrient digestibility, improved gut health and development, reduced cannibalistic behaviours, and increased hind gut fermentation of undigested substrate. However, the type, form and quantity of dietary fibre used appears to be a crucial factor, with excess amounts potentially counteracting any beneficial effects.

The main factor to determine is what effects are desired and, therefore, what type of fibre may induce this effect. In the literature reviewed, soluble fibre may have significant effects on the microbiome, resulting in increased SCFA production, potentially producing an energy sparing and antimicrobial effects. The inclusion of insoluble forms of fibre appears to have beneficial effects on excreta consistency, egg hygiene, litter quality, increased GIT passage rate and reduced feather pecking and cannibalism. Both soluble and insoluble forms of fibre seem to play a role in enhancing digestibility of nutrients. There is extensive evidence that the inclusion of insoluble fibre can help improve gizzard development and function, often resulting in reduced GIT pH, allowing increased degradation of nutrients, thereby making additional nutrients available for absorption. These changes suggest that the use of increased quantities of both insoluble and soluble fibre, may have the potential to support increased persistency of lay, where issues such as poor intestinal health, reduced eggshell strength and long-term welfare issue resulting from feather pecking and cannibalism combine to reduce the length of laying cycles.

There is a paucity of literature looking at the effect of fibre in free-range systems, so it remains unclear whether similar effects could be expected in that setting. Likewise, few studies looking at uniformity were found. As such, further research in these areas would be valuable to obtain a holistic view of the subject.

There does seem to be a clear maximum inclusion rate, with many studies reporting detrimental effects when fibre inclusion reaches levels over ~10%, with most benefits seeming to be found with inclusion rates of around 5–7%. With the continuing cost increases of raw materials and trend towards reduced meat prices, it is unsurprising that identifying opportunities to reduce production costs is an industry priority. The use of a limited quantity of fibre has been demonstrated to be beneficial to both bird health and production performance. Typically, sources of fibre are inexpensive and therefore the potential is there to reduce production costs. Likewise, the benefits of fibre inclusion appear to support the maintenance of a healthy microbiome. As such, there is additional potential for the use of increased fibre inclusion to support efficient and profitable production.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Alexandra Desbruslais

Alexandra Desbruslais got her PhD at Nottingham Trent University, where she focused on poultry nutrition and oxidative stress. Today, Alexandra's research interests are broad, including sustainable animal production, oxidative reactions in feed and production animals and fundamental monogastric nutrition.

Alexandra Wealleans

Alexandra Wealleans gained her PhD from the University of Reading, where she focused on the nutrition of pigs in alternative husbandry systems. Since then, her research focus has been primarily on improving nutrient digestion and absorption of monogastrics, and the ways in which this interacts with their health and performance.

David Gonzalez-Sanchez

David Gonzalez Sanchez received his degree in Agronomic engineering and MSc in poultry nutrition from the department of Animal Production of Polytechnic University of Madrid. He worked as poultry and livestock feed formulator, feed ingredient purchaser and nutritionist for a multinational feed company and later on being involved in technical support positions for several companies in the feed additive industry. His main research activities and interests have been oriented to the role of fiber in poultry diets, carbohydrase enzymes and feed formulation.

Mauro di Benedetto

Mauro Di Benedetto received his degree in Veterinary Medicine from the department of Animal Nutrition of Bologna University. He worked as veterinarian and nutritionist in livestock production primarily giving technical assistance in the animal nutrition and animal health area. In the last years his main research and application interest is dedicated to the link between nutrition and health and its reflection on animal welfare, both in poultry and swine.