Effects of dietary extrusion on the performance and apparent ileal digestion of broilers: a meta-analysis

Abstract The present research aimed to evaluate the effects of extruded feed ingredients on the growth performance and apparent ileal digestion of broilers using a meta-analysis. The database was developed based on 30 studies comprising 150 data points. The data were analysed using the OpenMEE software, considering the extrusion of feed ingredients as fixed effects and individual studies as random effects. Results showed that the extruded feed ingredients enhanced weight gain (p < 0.01) and enhanced the feed conversion ratio (p < 0.001) of broiler chickens as compared to the un-extruded feeds, but no effect was observed for the feed intake parameter. The digestibility of dry matter, crude protein, and crude fat were significantly higher for the extruded feeds (p < 0.05). In terms of amino acid digestibility, the current meta-analysis also demonstrated that extruded feed ingredients had significantly higher (p < 0.05) digestibility in Ala, Leu, Met, Phe, Thr, Try, Tyr, and Val than those of the un-extruded feeds. In conclusion, extrusion can improve the production performance of broilers, and it increases some nutrient and amino acid digestibility. HIGHLIGHTS Extrusion of feed ingredients increased weight gain and enhanced the feed conversion ratio of broilers. Digestibility of dry matter, crude protein and crude fat was higher for the extruded than that of the un-extruded feed. The extruded feed ingredients had higher digestibility amino acids in Ala, Leu, Met, Phe, Thr, Try, Tyr and Val than those of the un-extruded feeds.


Introduction
The feed costs are highly variable and may constitute up to 70 percent of the poultry production costs.The high price of broiler feed ingredients requires efforts to find alternative feed ingredients that are cheaper but have comparable nutritional content.For example, as a source of protein, soybean meal may be replaced by faba bean meal (Hejdysz et al. 2019) or flaxseed (Avazkhanloo et al. 2020).Another example using nonconventional ingredients is palm kernel cake in poultry feed formulation (Faridah et al. 2020;Hakim et al. 2020).However, the utilisation of those alternative ingredients in poultry diets must be limited, due to their high concentration of resistant starch and the presence of anti-nutritional factors (e.g.tannins, phytic phosphorus (phytic P), raffinose family oligosaccharides (RFO) and non-starch polysaccharides) (Diaz et al. 2006;Hejdysz et al. 2017).These components may cause detrimental effects on both broiler chickens' performance and nutrient digestibility.Therefore, it is necessary to apply a certain feed technology that is able to overcome the high concentration of resistant starch and anti-nutrients, and one of the technologies is the extrusion process.
Extrusion technology follows a pattern where agglomeration along with the application of heat, pressure, shear forces, and water, is involved in all of them.The extrusion process is often referred to as the high-temperature and short-time (HTST) process.It is considered an effective strategy for hygienising the feed contaminated with non-spore-forming bacteria.However, the extrusion process requires higher energy and cost to produce feeds.Therefore, it is important to ensure that the extrusion process can significantly improve the performance of broiler chickens.Some research demonstrated the increase of nutritive values of feed ingredients for broilers by the extrusion process (Brenes et al. 2008;Jahanian and Rasouli, 2016).On the other hand, there had been studies stating that the extrusion process did not improve the performance and digestibility of broiler chickens (Anjum et al. 2013;Ramiah et al. 2019).In terms of protein digestibility, extrusion was able to increase its digestibility (Ahmed et al. 2014;Brenes et al. 2008), but contrasting results were demonstrated by other authors (Liermann et al. 2019;Smulikowska et al. 2006;Son and Ravindran, 2012).In addition, based on available experimental evidence, there is no clear indication of the beneficial impact of hydrothermal on protein digestibility through protein denaturation.Furthermore, the supposed damaging impact of extrusion on amino acid digestibility of poultry feed through amino acid degradation and Maillard reaction seems to be not relatable (Goodarzi et al. 2016).
Although a number of experiments have been carried out on the effect of extruded feed in broilers, no meta-analysis study has attempted to quantitatively summarise such a relationship.A meta-analysis is a statistical technique that aggregates the results of scientific reports.Meta-analysis is able to calculate the effect size that is concerned with different studies and then combine all the studies into a single analysis (Sauvant et al. 2008;St-Pierre 2001).Therefore, the aim of this work was to evaluate the effects of extruded feed ingredients on growth performance and nutrient digestibility of broilers chicken by using the meta-analysis method.

Search strategy, inclusion criteria and data extraction
A comprehensive search of the literature published in English was conducted to identify experiments involving broiler chicken diets in either extruded or not extruded feed ingredients.The literature search was carried out using the Scopus (https://www.scopus.com/) databases.The search was conducted between February and March of 2022 using terms with a set of the following keywords in all searches: "extrusion", "feed", "poultry" and/or "broiler".
These initial searches resulted in 2029 potential references.Subsequently, the following criteria were used for literature selection: (1) published in English as full-text articles, (2) published in peer-reviewed journals, (3) a direct comparison between extruded and not extruded feed ingredients, and (4) comparison of weight gain, feed intake, Feed Conversion Ratio (FCR) and digestibility, including dry matter, crude protein, crude fat and amino acid both essential and nonessential.
After the preliminary title screening, 1888 references were eliminated because their topic was not relevant to our research.After reviewing the abstracts, 141 documents were assessed.Subsequently, 111 articles were eliminated due to a lack of comparison of interest (55 documents), irrelevant parameters (32 documents), insufficient data for statistical meta-analysis (19 articles) and not meeting any inclusion criteria (5 documents).Ultimately, the screening yielded 30 articles for use in subsequent data coding and statistical data analysis.Details of the selection process are presented in the PRISMA-P flowchart in Figure 1, and studies included in this meta-analysis are listed in Table 1.
Relevant data (Table 1) from each study were extracted into a spreadsheet using predefined criteria, including study type (randomized controlled studies), extruded ingredient feed, based diet, experimental extrusion temperature parameters, and broiler ages.

Statistical analysis
The effect size as Hedges'd was applied to quantify the parameter distance between extruded and unextruded feed products.This method was selected for its ability to calculate the effect size regardless of the heterogeneity in sample size, measurement unit, and statistical test results, as well as its suitability for estimating the effect of paired treatments (Sanchez-Meca and Marin-Martinez, 2010).The unextruded group was pooled into a control group (C) and the extruded group was combined into an experimental group (E).The effect size (d) was calculated as follows: where X E is the mean value from the experimental group and X C is the mean value of the control group.Therefore, a positive effect size indicates that the observed parameter is greater in the unextruded group and vice versa.J is the correction factor for small sample size, i.e.: and S is the pooled standard deviation, defined as: where N E is the sample size of the experimental group, N c is the sample size of the control group, S E is the standard deviation of the experimental group and S C is the standard deviation of the control group.The variance of Hedges' d (v d ) is described as follows: The cumulative effect size (dþþ) was formulated as follows: where w i is the inverse of the sampling variance: w i ¼ 1/v d .The precision of the effect size was described using a 95% confidence interval (CI), i.e. d±(1.96Â sd).All the above equations were derived from the study of Sanchez-Meca and Marin-Martinez (2010).The calculated effect size was statistically significant if CI did not reach a null effect size.A fail-safe number (N fs ) was calculated to identify publication bias caused by non-significant studies which were not included in the analysis.Nfs (fail-safe number), which is more than five times of sample size (N) and plus ten, was considered to provide evidence of a robust meta-analysis model.Nfs (fail-safe number) was calculated using the method of Rosenthal et al. (1979).The smallest sample size from individual studies was applied as N. Cohen's benchmarks were used as standard judgement borders for effect size assessment.These benchmarks were: 0.2 for small, 0.5 for medium and 0.8 for large effect size.All of the above effect size-related calculations were performed using OpenMEE 2.0.

Profile of selected studies
Due to conflicting research findings and small sample size, not all results can be considered reliable due to publication bias.Briefly, the fail-safe number (Nfs) indicates which studies are suitable to be included into the final robust conclusions.This number expresses how many sample study sizes should be added in order to change the initial effect size into a negligible variable.If Nfs > 5 N þ 10, where N is the study effect size used to calculate the initial effect size, then the result can be considered as the final robust conclusion (Rosenthal 1979).According to these fail-safe number rules, robust parameters included were: weight gain, feed intake, feed conversion ratio (FCR), dry matter digestibility, crude protein digestibility, crude fat digestibility and metabolise energy.In term of amino acid digestibility, robust parameters include Ala, Asp, Ile, Leu, Met, Phe, Thr and Tyr.Table 2 shows the detailed meta-analysis results tested according to Cohen's methodology for performance (three parameters), digestibility of nutrients (four parameters) and amino acid digestibility (19 parameters).

Effect of extruded and unextruded feed ingredients on performance, nutrient digestibility, and apparent ileal amino acid digestibility in broiler
Cumulative of effect size (dþþ) are represented in forest plot of Figure 2(a, b and c).If the horizontal  .On the other hand, no significant effects were observed between the birds fed extruded and normal feed ingredients/feed on digestibility of Arg, Asp, Cys, Glu, Gly, His, Ile, Iso, Lys, Pro, and Ser.Based on Cohen's benchmarks, all growth parameters are in the small effect size category.Meanwhile, all digestibility parameters are in the strong effect size category.In terms of amino acids digestibility, the small effect sizes are Gly and His.medium effect sizes are Arg, Cys, Glu, Ile, Iso, Lys, Pro and Ser. while the others are included in the large category.

Effect of extrusion on the performance of broilers
The development of feed production is possible by improving product quality and reducing feed production costs.The quality can be improved by using the extrusion method, which increases the digestibility and nutritional value of the feed.The advantages of extruded include a longer feed storage time considering the use of high temperatures causing harmful microorganisms to be destroyed and water vapour to evaporate (Fattakhova et al. 2020;Tangjaidee et al. 2019).Starch, the major carbohydrate source in commercial poultry diets, is the most abundant storage polysaccharide in plants.Native starches differ in granule size, amylose to amylopectin ratio, and amylose structure.Simply put, extrusion is the process of cooking under pressure, moisture, and high temperatures (Abd El-Khalek and Janssens, 2010).This meta-analysis review evaluated the influence of extruded and un-extruded process on the performance.The current meta-analysis also shows that the higher body weight was achieved in birds fed the extruded feed, with a large effect size.This provides evidence that, although the feed ingredient and also the process parameters of extruded feed vary among studies, it seems sufficient to guarantee that broiler weight gain of extruded feed is higher level than on raw feed.The pronounced effect of extrusion on broiler performance in the current meta-analysis was well in agreement with the results reported previously that extrusion increases weight gain and reduces feed conversion ratio (FCR) (Arija et al. 2006;Avazkhanloo et al. 2020).On the other hand, several research shows that feed intake, body weight gain, and feed conversion ratio were not affected (De Vries et al. 2014) and (Khalique et al. 2004).The evidence seems able to answer hesitancy among the feed manufacturers in the extruded feed that, although extrusion is a high investment cost, extrusion drives to produce an enhanced growth performance of broiler.It appears that the most plausible reason for this evidence is the difference in nutrition content between the extruded and un-extruded feed.The quality can be improved by using the extrusion method, which increases the digestibility and nutritional value of the feed such as corn, soybean meal, fish meal, rapeseed, canola meal, and rice bran is very attractive for animal producers because these feed ingredients are one of the most important sources of energy and protein feed ingredients, in addition to other nutrients.The extrusion process is a method to increase the usability value of starch in energy and protein sources, such as enzyme susceptibility and digestion.Starch, the major source of carbohydrates in commercial poultry/broiler diets, is the most abundant storage polysaccharide in plants.
Native starches differ in granule size, amylose to amylopectin ratio, and amylose structure.Extrusion, in simple terms, is a process of cooking under pressure, moisture, and elevated temperature.There are several effects of extrusion processing on protein, lipid, and starch by changing physical, chemical, and nutritional properties.Yet, starch plays a key role during extrusion and undergoes several significant structural changes, which include starch gelatinisation, melting, and fragmentation.Related literature gives no consensus on the impact of extrusion processing; hence this review will give an overview of factors affecting the gelatinisation degree of starch and the effects of extrusion processing on performance in poultry.Extrusion treatment improved energy utilisation and palatability of feed.In addition, not only was the availability of starch was more in extruded corn, but also DM ileal digestibility coefficient was higher for extruded corn than that for infra-red corn.Studies report that extruded corn-fed breeding pigs showed inconsistent results in growth performance and feed efficiency (Hongtrakul et al. 1998;Matoba and Nair 1985;Van Der Poel et al. 1989).
The extrusion process produces high-quality products because the cooking process involves high temperatures and short temperatures (HTST) so that sensitive feed ingredient components are not damaged and produce perfect gelatinisation at certain moisture content.In addition, extrusion is environmentally friendly because this process has low water content and does not produce waste.The extrusion process breaks down the bonds a À 1: 4 but does not change the amylopectin chain.
The increase in growth performance by extruded treatment is due to the increased digestibility of vegetable protein materials, especially CHO, from higher starch gelatinisation in feed.Another thing is likely that feed intake is higher for animals fed with extrusion treatment, resulting in better growth performance.Broilers fed extruded feeds have a higher consumption so that the livestock will grow faster, as they consume significantly more of the total feed mass.In addition, the compact feed composition reduces feed waste (Barrows et al. 2007) Poultry body fat continues to increase as the feeding rate increases, and the metabolic demands for growth can lead to more fat accumulation.
The extruded feed has better pellet durability and contributes to a lower P release.Extruded feed results in better growth and FCR, likely due to increased digestibility of nutrients associated with extrusion conditions.Extrusion pellets can increase faecal particle size and durability in water, both of which have the potential to improve waste collection and reduce pollution in waste.

Effect of extrusion on nutrient digestibility of broiler
Extrusion affects the digestibility of crude proteins, crude fats, and dry matter.The current meta-analysis also shows that extruded feed increases digestibility more than un-extruded feed.The higher digestibility of crude protein in extruded feed is also been reported (Ahmed et al. 2014;Arija et al. 2006;Faridah et al. 2020, andHejdysz et al. 2017).In terms of crude fat digestibility, some researchers state that the extrusion process is able to increase digestibility (Hejdysz et al. 2018;Oryschak et al. 2010 andRutkowski et al. 2016).In contrary, Goodarzi et al. (2016) reviewed that there is not a positive relationship between processing of a poultry feed and ileal fat and protein digestibility.Experiment research about rapeseed meal as feed state that total tract apparent digestibility of CP and crude fat were not affected by extrusion processing technologies De Vries et al. (2014) because of that, the meta-analysis has the main role in answering the scepticism effect of extrusion to broiler nutrient digestibility.
The occurrence of higher crude protein digestibility may be initiated by the breaking down of high-energy covalent bonds, such as disulphide, along with relatively weak non-covalent bonds such as hydrophobic bonds (Bhattacharya and Hanna, 1988;Hayakawa et al. 1996).In addition, increased protein digestibility by extrusion could be due to the exposure to high temperature and pressure, which breakdown the protein bind to the fibre component allowing the better enzyme to digest the protein molecules (Abd El-Khalek and Janssen,s 2010).In the study conducted by Rutkowski et al. (2016), the use of extrusion improved the crude fat digestibility and nitrogen retention in broiler chickens containing legume seeds.Based on the report Hejdysz et al. (2019) in the extrusion process of faba beans, higher diet nutrient digestibility was probably caused by the destruction of NDF, ADF, RS and phytic P. It can be an extrusion of foodstuffs that can affect the chemical composition as well as the viscosity of the resulting product.It is known that the beneficial effects resulting from extrusion cooking include increasing the concentration of soluble fibre, starch digestibility, and viscosity.

Effect of extrusion apparent ileal amino acid digestibility
Figures 2(c) show that the most coefficient of ileal apparent amino acid digestibility was improved significantly by extrusion process.According to Cowieson et al. (2006), phytic P has a negative effect on crude protein (CP) and amino acid digestibility.In this metaanalysis study, there was definited effect of extrusion on crude protein (CP) and most amino acid such effects were confirmed.Consistent with some researchers, the phytate mechanism action on amino acids can be described by the presence of proteinphytic complexes in feed.Selle et al. (2006) continue that the de novo formation of two-component and ternary protein-phytate complexes in the gastrointestinal tract not only constrains the action of proteolytic enzymes but also impairs losses of endogenous amino acids.Furthermore, a negative relationship between phytic P content in faba bean seeds and amino acids' apparent ileal digestibility (AID) was initiated (Rutkowski et al. 2016).Therefore, we can guess that the lower phytic P content in extruded faba beans was the main factor that influenced better amino acids apparent ileal digestibility (AID) and partially affected a reduction in FI and FCR.
Increasing the proportion of grains such as soybeans in the feed requires an important process to disable anti-nutritional factors while maintaining protein quality.According to Clarke and Wiseman (2007) Ground full-fat soybeans (FFSB) extruded using a Clextral twin-screw extruder produces different trypsin inhibitor activity.A statement different from the research conducted by Clarke and Wiseman (2007), the concentration of apparently digestible ileal lysine increased from 10-53 to 17-63 g/kg FFSB by increasing the extrusion temperature from 90 to 160 C. It must be coefficient of apparent ileal digestibility of lysine also improved from 0.58 to 0.86.Other amino acids showed similar improvements in the coefficient of apparent ileal digestibility concentrations.

Conclusions
The present meta-analysis results have demonstrated that the extrusion process on feed ingredients favourably affects the performance and digestion of broiler chickens.The extruded feed ingredients had positive effects on weight gain, FCR, and nutrient digestibility and led to increased growth performance.Interestingly, in terms of the digestion of amino acid parameters, there were not all amino acids increased by extruded feed.A further identical meta-analysis may be the best option to evaluate and summarise the effect of extrusion on other parameters such as meat quality and blood haematology or certain types of feedstuffs.
parameters crosses the zero vertical line then there is no significant difference between extrusion and non-extrusion.The extruded feed ingredients increased weight gain and enhanced FCR (p < 0.001) of broiler chickens compared to the unextruded feed ingredients (Figure2(a)).On the other hand, feed intake was similar between the birds fed extruded and unextruded/normal feed ingredients/feed.With regard to nutrient digestibility, the extruded feed increased crude protein, crude fat, and dry matter digestibility (p < 0.05) of broiler chickens compared to the unextruded feed ingredient (Figure2(b)).The extruded feed ingredients increased the digestibility of amino acids, including Ala, Leu, Met, Phe, Thr, Try, Tyr, and Val (p < 0.05) of broiler chickens compared to the unextruded feed ingredients (Figure2(c))

Figure 2 .
Figure 2. Forest plot of the effects extruded and unextruded feed ingredients on performance (a), nutrient digestibility, and amino acid digestibility (c) of broiler chickens.

Table 1 .
Figure 1.Flow chart of the literature selection process according to PRISMA protocols.Studies selected for the meta-analysis.

Table 2 .
Meta-analysis of the effects of feed ingredients extrusion on growth performance and nutrient digestibility of broiler.
Doc: document; N: number of data; Std.error: standard error; s 2 : variance of the effect size parameters across the study populations; Q: weighted sum of squared deviations; Het P-Value: P-value for heterogeneity; I 2 : heterogeneity level between studies; ADG: average daily gain; DMI: dry matter intake; DM digestion: dry matter digestion; OM digestion: organic matter digestion; CP digestion: crude protein digestion; NDF digestion: neutral detergent fibre digestion; ADF digestion: acid detergent fibre digestion; g/d: gram/d