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Organic Acids as Antibiotics Alternatives in Poultry Field: Recent Advances

This version is not peer-reviewed.

Submitted:

23 August 2024

Posted:

26 August 2024

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Abstract
Feed additive antibiotics have been used for many decades as growth promotors or antibacterial substances world-wide. However, the adverse impacts of using antibiotics in animal or poultry feeds were informed. Therefore, searching for alternatives such as probiotics, prebiotics, phytobiotics, post-biotics, bacteriophages, enzymes, essential oils, or organic acids (OAs) became urgent. The OAs are produced by beneficial intestinal bacteria through the fermentation process of carbohydrates. The OAs and their salts are still used as feed preservatives. They have been long added to feed in order to minimize contamination and growth of harmful bacteria and fungi, reduce the deterioration, as well as prolong the shelf life of feed commodities. Moreover, they have been mostly added to poultry feed as a blend to obtain a maximum beneficial effects. The supplementation of poultry with OAs could improve the growth performance parameters and carcass traits, promote utilization of nutrients, boost the immune response, and inhibit the growth of pathogenic bacteria. Therefore, this review article provides valuable insights into the potential benefits of using OAs-antibiotics alternative in reducing the microbial load, enhancing the performance parameters in broilers and layers, improving the gut heath, as well as boosting of the immune response.
Keywords: 
Subject: 
Biology and Life Sciences  -   Animal Science, Veterinary Science and Zoology

1. Introduction

Antibiotic growth promoters have been used in the livestock production systems since several years [1]. However, in 2006, the European Union prohibited the administration of these growth promoters due to the continuous development of antibiotic resistance. The hazardous use of antibiotics leads to destruction of beneficial intestinal flora and emergence of resistant bacteria which transmitted to humans through the food chains [2,3]. Therefore, the search for suitable alternatives becomes an urgent issue, especially for the poultry production system [4,5,6]. The European Union permitted the use of acidifiers or organic acids (OAs) and their salts in poultry production due to their safety [7]. They have also many advantages such as absence of pollution, drug resistance, and residues, as well as their beneficial effects on the health [8,9]. Dietary OAs promote the production of prebiotics and probiotic lactic acid bacteria [10]. The OAs could potentially replace antibiotic growth promoters with positive effects on performance and gut health of livestock [11] and poultry production [12,13,14,15].
There are two types of acids; organic and inorganic (Figure 1). The majority of feed additive OAs could be termed as volatile short chain fatty acids (e.g., propionic, acetic, fumaric, lactic, or butyric acids), medium chain fatty acids, and long-chain fatty acids [8]. Propionic acid, acetic acid, and butyric acid are produced by beneficial intestinal bacteria through the fermentation process of carbohydrates [16]. The organic carboxylic acid contains a generic structure of carboxyl “R-COOH” is regarded as an organic acid (including fatty acids and amino acids) [17]. Also, formic acid, propionic acid, citric acid, acetic acid, etc. are partially dissociated weak acids that are composed of saturated straight-chain monocarboxylic acids such as amino acids and fatty acids with R-COOH constituent [18]. They are present in the form of salts such as sodium, potassium, and calcium with variable physical and chemical properties. The solubility and acid-binding capacity of water [19] and feed ingredients [20,21] can affect the efficacy of OAs. The beneficial effects of OAs could be enhanced by using blends rather than a single acid treatment [22].
OAs have been added to minimize contamination, growth of harmful bacteria and fungi, and deterioration, as well as prolong the shelf life of feed commodities [23]. Therefore, they are known to be used as good feed preservatives. Acetic acid or benzoic acid as well as their sodium salts are represented as safe feed preservatives [11]. They could act similar functions as antibiotics [7]. For instance, OAs were added to poultry feed in rates of 0.5 kg/ ton and 2.5-3.0 kg/ton to reduce mold and Salmonella growths, respectively [24]. In addition, dietary formic acid and propionic acid could reduce the bacterial load with Salmonella spp. in the contaminated feed [18].
The bad hygienic conditions in livestock farms such as increasing litter moisture and worm temperature variables can enhance the microbial growth and consequently reduce the nutritional content of proteins and carbohydrates. So, the supplementation with OAs could improve the growth performance, parameters and carcass traits [25,26,27,28], reduce the guts’ pH, enhance pepsin production, promote nutrients digestibility and utilization [29,30], boost the immune response [31], and suppress the growth of pathogenic bacteria [14,32,33,34,35,36]. Besides, Ma et al. [26] proved the antioxidant capacity of OAs as supplementing diets mixed OAs increased the amount of serum superoxide dismutase and catalase of 3 and 6 week old broilers.
Accordingly, this review article provides a comprehensive insights into the role of using OAs-antibiotics alternative in reducing the microbial load, enhancing the performance parameters in broilers and layers, improving the gut heath, as well as boosting of the immune response.

2. The Different Effects of OAs Supplementation for Poultry

The different effects of OAs inoculation in the feed of poultry are illustrated in Table 1 and Figure 2.

2.1. Antimicrobials

The different forms of OAs include solid in feed, spray on the litter, or added to the water (Figure 3). The antimicrobial efficacy of OAs is still not fully investigated. The different mechanisms of actions of OAs as antimicrobials are illustrated in Figure 4. The positive influences of their antibacterial capacity are associated with the physical chemistry of the used acid, special characteristics of dissociation, composition and pH of media, animal species, type of organism, growth conditions, exact location in the intestines, and buffering capacity [18,29]. Additionally, the efficiency of OAs relies on the acid molecular weight, dissociation constant, and antimicrobial activity [37].
Dibner and Buttin [29] demonstrated that some OAs are of narrow spectrum which affect bacteria (lactic acid) or fungi (sorbic acid), while others are of broad spectrum against bacteria and fungi (formic acid and propionic acid). As short-chain fatty acids, both butyric acid and valeric acid have antibacterial effects against Gram-negative or Gram-positive bacteria [38]. However, formic acid and acetic acid can directly control pathogens by acting upon the cell wall of Gram-negative bacteria [9].
The concentrations and the pH of OAs affect their antimicrobial power [39]. Under low pH condition, the OAs become more available in a lipophilic dissociated form, easily diffuse into the bacterial and fungal cell membranes, and consequently cause disruption of the enzymatic reaction and transport system [6]. Moreover, the low pH condition can disturb the generation of energy and inhibit the bacterial cell proliferation and growth (bacteriostasis) [6,40]. In the upper digestive tract, the low pH enhances the antimicrobial effects of the OAs and helps their absorption by diffusion in the epithelia [6], while in the lower part of the intestine, the OAs decrease the hosts competition with the natural microflora resulting in improved digestion [40]. However, there is a discrepancy regarding the role of OAs in reducing the pH of the intestinal tract [41,42] and this may be due to the differences in acidifiers types and concentrations, experimental animals, acidifiers formulations and test sites, diets type and compositions, and other factors.
The cytoplasm of the bacterial cells contains both positive charged protons and the negatively charged anions. The accumulation of proton in the cells leads to an increase in its acidity to un-bearable limit. Therefore, the bacterial cell depletes from most of energies to adjust its internal pH. This depletion may cause inhibition of growth and multiplication and even death. Besides, the accumulation of anions in the bacterial cells disturbs the DNA copying and cells multiplication, increases the level of the internal osmotic pressure, and consequently causing cells deaths [43]. On the other hand, OAs could release proton irons in the cytoplasm.
OAs have bactericidal and bacteriostatic characteristics [44]. They diffuse into the bacterial cell membrane and dissolve in anions and protons of the cytoplasm [45] with a subsequent expulsion of protons outside the bacterial cells [46]. This process reduces the energy supply and ends by cell death [47]. The un-dissociated forms of OAs can enter the bacterial cell membrane where they are dissociated, produce H+ ions, and rise the pH acidity of the cytoplasm [48]. Then, pH-sensitive bacteria are forced to discard the redundant proton irons via the H+-adenosin triphosphatase pump which causes impeding of bacterial cells proliferation [9]. However, the bacterial cell use energy to restore the basic nature of cytoplasm. So, once the OAs enter the cell, where the pH is about 7, the acids are dissociated and suppress the bacterial cell enzymes such as decarboxylases and catalases and the nutrient transport systems [49]. Moreover, the dissociated OAs produce anions (RCOO−) to disturb the protein synthesis and unable the bacterial cells to replicate. The OAs may also affect the microbial cell membranes integrity or may interfere with the nutrient transport and energy metabolism causing bacterial cells deaths [18]. They can penetrate the bacterial membrane, inhibit the synthesis of adenosine triphosphate, disturb the bacterial membrane, and denaturant the DNA [50]. In addition, OAs can prevent the release of toxic compounds following bacterial colonization, thus averts the damage of the intestinal epithelial cell and improves the villus height [23]. Moreover, they can enhance the beneficial microbiota populations and thus creating eubiotic intestinal environment [51,52].
The more efficient release of OAs compounds could be achieved via the microencapsulation process [53]. OAs could be metabolized and rapidly absorbed from the upper segments of digestive tract (proventriculus, gizzard, and duodenum), but not from the lower parts [54]. The reduction of gut’s pH limits the pathogenic bacterial growth especially those which are less tolerant to the acidic pH [25,55]. However, others decrease the pH of the bacteria after dissociation causing death [13]. The orthophosphoric acid can lower the pH of the digesta resulting in more levels of the un-dissociated form of acids [29]. Moreover, carboxylic acid in citric acid, lactic acid, tartaric acid, and malic acid, as well as monocarboxylic acid in propionic acid, acetic acid, butyric acid, and formic acid have a pKa value in between 3 and 5 and consequently antimicrobial properties [56]. It has been demonstrated that acids are able to reduce the total intestinal microbial load and the subsequent infection rate leading to an enhancement of digestibility and reduction of the energy demand by the gut-associated tissue [57].
Some pathogenic intestinal pathogens such as Salmonella spp. [56,58,59,60], Campylobacter jejuni (C. jejuni) [29,61], pathogenic Escherichia coli (E. coli) [62,63,64], and Clostridium perfringens (C. perfringens) [65] or coccidia spp. [31,66] could be drastically affected by using OAs. However, the growth of beneficial gut microflora such as Lactobacillus spp. could be improved following the OAs treatment [14]. So, the reduction of intestinal bacterial load along with the enhancement of natural flora resulting in an improvement of the nutrients utilization and consequently the growth performance [3,17,67]. The drinking water acidification could diminish the clinical signs of Campylobacter infection in the gut [39]. Moreover, citric acid lowered the growth of Listeria monocytogenes on chicken’s thighs at 4°C for 8 days [68]. On the other side, different OAs can flourish the growth of beneficial bacteria such as Lactobacillus spp. [69,70,71]. For instance, the dietary citric acid/and or avilamycin enhanced the development of Lactobacillus spp., but inhibited the growth and proliferation of pathogenic Salmonella spp. and E. coli via activation of proteolytic enzymes, absorption of minerals, decreasing ammonia, depressing microbial metabolites, and stimulation of feed intake (FI) [72]. In addition, the by-product of wheat milling ‘’wheat bran” showed efficacy against Salmonella spp. in terms of percent and particle size. It has been proven that the rapid fermentation of butyric acid downregulated Salmonella spp. gene expression [6,73] and inhibited the bacterial cecal colonization due improvement of intestinal barrier function [74].

2.2. Performance Parameters

The blends of OAs could improve the FI and nutrient utilization, so they able to enhance the body weight gain (BWG) and the feed conversion ratio (FCR) of poultry [7,15,65,69,70,71,75,76,77,78]. The OAs treatment also showed reduced intestinal lesion scores and improved the gut health of broiler chickens with necrotic enteritis [14,15]. Moreover, under Eimeria challenge, a blend of benzoic acid and essential oils enhanced the growth performance in broilers [79].
The supplementation of OAs could improve the performance parameters [12,15,55,80,81,82] which is probably due to the enhancement of digestible energy and protein contents of the feed, reducing the intestinal bacterial colonization [8], increasing the proliferation of beneficial flora, modulation of the anti-inflammatory immune response [47], and lowering the ammonia and other harmful metabolites [57]. The OAs work to improve the digestion of proteins, calcium, phosphorus, magnesium, zinc, and other nutrients which present in the feed material of the small intestine [7]. In addition, the un-dissociated forms of OAs are able to penetrate the lipid layer of the bacterial and fungal cell membranes causing release of proton, accumulation of intracellular anion, reduction of the intestinal pH, and then boosting the secretion of endogenous digestible enzymes [9,83]. Moreover, OAs could enhance the release of digestive enzymes, pancreatic secretion, activity of microbial phytase, and proliferation of intestinal cells [29]. Reducing in the pH of the crop, gizzard, and duodenum leads to increasing the secretion of digestible enzymes, pepsin, trypsin, chymotrypsin, proteinase, amylase, lipase, protein hydrolysate, and non-protease concentrations in the intestinal segment [84,85,86]. Besides, the treatments with OAs could enhance the secretion of pepsin and chyme which reach the intestine to stimulate the decomposition and absorption of nutrients. This process plays a role in stimulating the digestive system development, increasing amylase and lipase secretion, and consequently increasing the intestinal absorption capacity. The OAs slower the rate of digesta passage and thus enhance the absorption of the feed contents from the intestines [87].
The usage of OAs is also associated with the improvement in minerals digestibility [88]. The digestibility of minerals, particularly calcium and phosphorous, has been improved possibly due to the enhancing of digestible enzymes [89] or the effective role of Lactobacillus spp. in the gut [72,90]. Mixing of OAs with essential oils could reduce the populations of pathogenic enteric bacteria, while improve the growth of beneficial gut microbiome, and so, enhance the intestinal health [14,15].

2.3. Carcass Traits

The treatments with different OAs could improve the meat quality of chickens’ carcasses [91]. Lee et al. [92] demonstrated that the pH of broiler thigh meat was increased by gallic acid and linoleic acid supplementations. Moreover, Fortuoso et al. [93] showed that a dose of 300 mg/kg glycerol monolaurate improved the nutritional quality of meat. The decrease in the muscle pH of broilers supplied by OAs may be related to the increase in the antioxidant activity in meat [94] or the affection of the gut microbiota and their metabolites [86]. The dietary supplementation with benzoic acid or amylase improved the antioxidant capacity, nutrient digestion, and the meat quality [94]. The improved meat tenderness after dietary treatments with OAs may be due to the improving nutrients metabolism, reducing anaerobic digestion, and enhancing antioxidant capacity. During the carcass’s processing, the anaerobic conditions with the protein breakdown may result in accumulation of lactic acid which affects the water holding capacity of meat [86].

2.4. Intestinal Health

The addition of OAs to the drinking water of birds resulted in increasing the number of jejunal goblet cells which lead to stimulation and production of the mucus layer [95] and improving the gut epithelial cells [12,39,96,97,98] and the duodenal villus height [99].
Decreasing in the crypt depth and increasing in the villus height: crypt depth ratio were also found [31]. Likewise, the results of García et al. [100], Kum et al. [101], and Islam et al. [15] showed increasing the villus height and villus: crypt depth ratio, reducing the lesion scores, and thus improving in intestinal integrity following the dietary supplementation with OAs.
The treatments with OAs may reduce the pH of digesta and raise the gastric proteolytic activity [67]. The increase in the secreted pancreatic juice containing trypsin, amylase, protease, lipase, procarboxy peptidases, and chymotrypsinogen [7,55,102] as well as the enhancement of pepsin protein proteolysis activity, broken down of proteins to simple peptides, and releasing of gastrin and cholecystokinin hormones have been also noticed following addition of OAs to feed. Similarly, Ma et al. [26] reported that supplementation of chickens diets with a mixture of OAs improved the pancreatic secretions and enhanced the expression of tight junction proteins, resulting in a healthier broiler production. The acidic intestinal environment can reduce the bacterial metabolites such as ammonia and amines [103] which consequently may improve the digestion process.
The fermentation process of some OAs such as acetate, propionate, and butyrate could enhance the intestinal morphology, tight junctions, and immunological status of birds [104]. Japanese quails received a product contains acetic acid, formic acid, and butyric acid, as well as thymol, β-cymene, carvacrol, and borneol showed an improvement of the intestinal morphology including crypt depth, villus length and width, villus/crypt ratio, thickness of the intestinal wall, goblet cell percentage, and appearance of the intestinal surface area [105]. Adil et al. [106] demonstrated that a dietary 3% fumaric acid increased the villus height in all the segments of small intestines. Several studies showed that chickens received butyrate have increased intestinal villus height, decreased crypt depth, and thereby increased intestinal absorption surface [107,108,109]. It has been found that butyrate can regulate the gut barrier and plays an important role as anti-inflammatory and immuno-regulatory substance to maintain the gut homeostasis [38]. Butyric acid can promote the development of epithelial cells [110], preserve the intestinal cells viability, and enhance the turnover of enterocytes which may improve intestinal recovery. Similar results were obtained by Gao et al. [86] and Pham et al. [14]. Improved intestinal villi length and depth as well as increasing the number of goblet cells containing acidic mucins have been also reported in broilers fed on diets containing butyrate [111].
It has been known that the infection with Eimeria (E) spp. is usually associated with the gut health. The supplementation with OAs could be a suitable alternative for anticoccidial due to their ability to improve the intestinal integrity that is damaged by such infection [31]. Acetic acid could decrease the caecal pH and consequently reduces the impact of oocysts that eventually lower the intestinal lesions. In broiler chickens, Abbas et al. [112] reported that acetic acid was effective against E. tenella infection and Ali et al. [113] showed that the dietary inclusion of butyric acid glycerides reduced the intestinal lesion score produced by E. maxima.

2.5. Immune Response

The modulation of immune response in hosts fed on OAs may be due to different speculations as the main causes are unknown. However, several studies have proven the immuno-potentiating effects of OAs for poultry [6,72,76,114]. The weights of immune organs of broiler chicks have been increased in response to OAs supplementations [30]. Moreover, the levels of serum immunoglobulin (Ig) were elevated following dietary feeding of layer chickens on OAs mixture and yeast culture [115]. For instance, chickens supplemented OAs showed an improvement in immune response and enhancement of antibody titer against Newcastle disease (ND) virus infection [116,117]. Moreover, Lee et al. [118] demonstrated that the percentages of cluster of differentiation (CD4+), CD25+, and T-cells were higher in broiler chickens received avian influenza (AI) (H9N2) virus vaccine along with a diet containing OAs. The influence of three OAs on the immunity and intestinal morphology of E. coli (K88) challenged broiler chickens was investigated and the results revealed an improvement of the ileal morphology and immunity [63]. Also, OAs showed the ability to reverse the detrimental effects of S. typhimurium and boost the immunological response in the challenged chickens [87]. Emami et al. [88] reported that broiler chickens received a diet containing phytase and OAs showed high levels of IgG. It has been found that OAs supplementation may increase trypsin and chymotrypsin production and consequently activate the digestive tract to secrete IgA in the ileal mucosa [73]. Butyric acid has a positive impact on the birds’ immunity through the improvement of gut eubiosis and pH, increasing the number of beneficial bacteria and limiting the colonization of pathogens [111]. The inclusion of butyric acid in the ration of broiler chickens was associated with a good cell-mediated immunity after inoculation of phytohemagglutinin-P, improved humoral antibody production after vaccination with ND virus vaccine and injection of sheep red blood cells, and increased the thymus and spleen weights [111].
Increasing Lactobacillus spp. count in the gut [88], inhibiting nuclear factor kappa B activation [119], increasing tumor necrotizing factor [31], improving immunological features of blood and small intestine, and modulating bacterial population of caecum [26] are possible causes of OAs immuno-potentiation effect. In the study of Rodríguez-Lecompte et al. [120], the treatment of broiler chickens with OAs blend up-regulated the interferon-γ in the caecal tonsils and the interleukin (IL-6) and IL-10 in the ileum. Similarly, Lee et al. [121] reported that the dietary addition of OAs activated the regulatory T cells and reduced the inflammatory response signal (α 1-acid glycoprotein) in broilers following vaccination with AI (H9N2) virus vaccine. Moreover, the gut associated immunity produced by the lymphoid tissues was linked with the gut bacteria following the treatment with OAs [63]. The immuno-protective effects of OAs against broilers coccidiosis were also reported [31,66]. However, Hedayati et al. [44] found no significant difference among the dietary treatments with blends of different OAs and the antibody titers against ND, infectious bursal disease, and AI viruses in broilers.
Table 1. The effects of different OAs inoculation in the feed of poultry.
Table 1. The effects of different OAs inoculation in the feed of poultry.
Organic acid (s) Effects Reference
Dietary ascorbic acid, malic acid, and tartaric acid ↑ BWG and feed efficiency [122]
0.5, 1, 2, 4, and 6% of acetic acid, citric acid, lactic acid, malic acid, mandelic acid, propionic acid, or tartaric acid, respectively ↓ S. typhimurium colonization count [123]
0.5-1% fumeric acid Improved metabolizable energy [124]
0.16% butyric acid ↓ Salmonella count in caecum [125]
0.2% butyric acid ↑ Carcass weight, breast muscles yield, and dressing %
↑ FCR
↓ Abdominal fat		 [91]
Dietary citric acid ↑ FI [126]
5 and 10 g/kg formic acid Improved ileal nutrient digestibility [83]
5000 and 10,000 ppm formic acid ↑ Growth
↑Apparent ileal digestibility		 [100]
0.05% sodium butyrate ↓ Lactobacilli and E. coli [127]
Butyric acid 285 mg/kg of feed ↑ Eggshell strength
↓ Mal-formed eggs		 [128]
A combination of acetic acid, citric acid, and lactic acid ↑ BW [129]
A dietary mixture of formic (70%) and propionic acid (30%) Improved FI in a quadratic form [130]
Dietary citric acid and phytase ↑ Specific gravity and eggshell thickness
↓ Egg weight		 [90]
0.5% citric acid or avilamycin, and their combination ↑ FI, growth, carcass yield, and bone ash
↑ Lactobacillus spp. development
↓ Growth and proliferation of Salmonella and E. coli
↑ Phosphorus utilization in intestine		 [72]
0.09% free or protected sodium butyrate ↓ S. enteritidis in crop, cecum, and liver [131]
Dietary citric acid ↑ Lymphocyte number in lymphoid organs [132]
0.45% of potassium diformate ↓ Reduced necrotic enteritis-related mortality and the amount of C. perfringens in the jejunum [133]
Dietary 0.4% butyric acid ↑ BWG and FCR [134]
Dietary 3% citric acid ↓ Ileal coliform contents [135]
Formic acid in the drinking water No effect on the counts of total organisms and E. coli in intestine [136]
3% butyric acid ↓ Crop pH and caecal coliform count
↑ Intestinal length		 [137]
0.50% formic acid, 0.50% fumaric acid, 0.25% acetic acid, and 2.0% citric acid ↑ Villus height in duodenum [30]
250–7,000mg/kg N-butyric acid ↓ S. Typhimurium or C. perfringens colonization [138]
Dietary 0.15% blend of OAs for broilers ↑ Antibody titers against ND at 21 days old [116]
1% a mixture of formic acid (32%), acetic acid (7%), ammonium format (20%), mono- and diglyceride of unsaturated fatty acids, and copper acetate in the drinking water of C. jejuni infected broilers ↑ FI
No effect on the BWG and FCR 		[61]
1% formic acid in feed for 5 days ↓ Salmonella count [139]
3% butyric acid, 3% fumaric acid, and 3% lactic acid in the drinking water of broilers ↑ BW
Improved FCR
No effect on the cumulative FI		 [140]
0.1% butyric acid ↓ Salmonella count in caecum [141]
Soft Acid S includes 60% formic acid, 20% propionic acid and 20% soft acid and Soft Acid P consists of 70% propionic acid, 5% citric acid and 25% soft acid (2.5 kg/ton of feed of layer chickens) ↑ Small intestinal villi
↓ The total bacteria, total yeast-fungi account, and sheep red blood cells levels
No effect on the FI, egg production, egg weight, and FCR
No effect on the shell stiffness, shape index, shell thickness, albumen index, yolk index, and Haugh Unit		 [142]
0.075% a blend of formic acid, acetic acid, propionic acid, and sorbic acid; medium-chain fatty acids combined with ammonium formate; and coconut/palm kernel fatty acid distillate in their water No growth-promoting effects [143]
0.4% formic acid, propionic acid Improved villus height: crypt depth ratio [144]
1% fumaric acid in diets ↑ BWG [145]
1-3 g/kg (0.1–0.3%) of a blend of formic acid, lactic acid, malic acid, tartaric acid, citric acid, and orthophosphoric acid in the drinking water ↑ The apparent metabolizable energies and total phosphorous ileal digestibility
↑ BW, average daily gain, and average daily FI
Negative impact on FCR 		[146]
0.05% encapsulated butyrate ↑ Intestinal weight and epithelial cell area [74]
2 g/kg organic oil blend Villus height in ileum [147]
0.02%, 0.03% and 0.04% protected calcium butyrate ↑ BWG
↑ Mucosa thickness, villus length, and crypt depth		 [148]
2% citric acid ↑ Epithelial cell proliferation and villi height of gastrointestinal tract [149]
5g/kg formic acid ↑ BWG, dressing %
↓ FCR		 [150]
3 kg/ton a commercial acidifier ↑ Average daily gain
↓ FCR		 [151]
0.1, 0.02, and 0.04% of formic and propionic acids ↑ Beneficial intestinal bacterial flora load
↓ E. coli (K:88)
↑ Growth performance parameters
↑ IgG titer to sheep red blood cells and vaccination with infectious bursal disease and infectious bronchitis viruses		 [88]
0.1% and 0.3% formic acid and citric acid for ducklings ↑ BW, BWG, and FCR [152]
0.05 or 0.1% Encapsulated sodium butyrate ↑ Ileal energy digestible coefficient [72]
2 g/kg OAs combined with 2 g/kg probiotics ↑ Villus height and crypt depth [153]
800mg/kg micro encapsulated sodium butyrate ↑ BW, daily gain, and FCR [154]
0.1% fermented fatty acids of wheat bran ↓ Salmonella count [58]
1% formic acid in water of S. typhimurium infected broilers ↓ Decreased BW [155]
0.05 % encapsulated butyric acid ↑ Lactobacilli and Bifidobacterium
↓ Salmonella and coliform
No effect on amylase, protease, and lipase		 [156]
Protected or unprotected 0.1% butyrate No effect on gut weight, retention time, dry matter, organic matter, Nitrogen, and non-protein nitrogen [157]
0.2% mixture of 32% fumaric acid, 3% formic acid, 13% lactic acid, 3% propionic acid, and 1% citric acid ↑ The expression of tight junction proteins and performance [69]
0.1%, 0.15%, and 2% a blend of ortho phosphoric acid, formic acid, and propionic acid in the drinking water ↓ Growth performance parameters [158]
Dietary 0.30 g/ kg sorbic acid and fumaric acid ↑ Secretion of trypsin, lipase, and chymotrypsin in the intestine
↑ Spleen index
↑ Ig A in duodenal and ileal mucosa		 [75]
0.06% sodium butyrate ↑ Lactobacilli
↓ E. coli in Ileum		 [159]
A combination of sodium butyrate, citric acid, phosphoric acid, acetic acid, propionic acid, formic acid, and lactic acid ↑ Growth performance parameters [160]
3 g/kg organic acid blend in Japanese quails ↑ Villus height and width in jejunum and dudenum [161]
A blend of OAs (0.1%) in the drinking water of broiler chickens orally challenged with (109 CFU/mL) C. jejuni ↓ C. jejuni counts [162]
0.9% formic acid and sodium format ↓ S. typhimurium colonization
↑ Growth performance parameters		 [163]
Dietary fumaric acid ↑Erythrocyte counts, hemoglobin concentration, and the serum total protein, albumin, globulin, total cholesterol, high-density lipoprotein cholesterol [164]
0.1% (formic acid, acetic acid, and ammonium formate) in drinking water of broilers ↑ Growth performances
↑ Actinobacteria count
↓ Proteobacteria, Verrucomicrobia, and Cyanobacteria count
The relative abundance of the Bacteroidetes, Firmicutes, and the Firmicutes/Bacteroidetes ratio were not affected		 [165]
0.6 and 1.2g/kg Sodium butyrate ↑ Average daily gain and FCR [166]
3% fumaric acid in a diet ↓ Cholesterol and total lipids [167]
Encapsulated organic acids of formic acid, acetic acid, and butyric acid, besides, essential oils thymol, carvacrol,β-cymene, borneol and myrcene coated with a matrix of triglyceride ↑ Epithelium thickness and surface area [105]
0.5 kg/ton feed formic acid with cinnamaldehyde ↓ Proliferation of C. coli
No effect on the cecal and carcass surface loads		 [168]
0.2% butyric acid No significant effect on dry matter, crude protein, ether extract, calcium, phosphorus, and apparent metabolized energy [169]
0.2% a mixture of 32% fumaric acid, 3% formic acid, 13% lactic acid, 3% propionic acid, and 1% citric acid ↓ E. coli population
↑ Lactobacillus spp. and E. coli ratio in the ileum and caecum 		[64]
0.3% a blend of acetic acid, propionic acid, formic acid, and ammonium formate ↑ Villus height [4]
Dietary supplementation of phosphoric acid (0.1, 0.2, and 0.3/kg) and lactic acid (0.3 g/kg). ↑ Feed-to-gain ratio
↑ Trypsin, chymotrypsin, and lipase secretion in the duodenum
↑ Breast and thigh muscle pH value
↓ Cooking loss and meat tenderness
↓ Abundance of E. coli and Salmonella
↑ Villus height of the duodenum		 [86]
1 g/kg of diet a mixture of formic acid 40%, formate 40%, and sodium 20% ↑ Serum glucose level [170]
0.5-2.5 g/kg feed short and medium chain fatty acids ↓ C. perfringens shedding in the caecum [171]
A blend of formic acid, acetic acid, and ammonium formate (1.5 ml/L drinking water) + a blend of encapsulated butyrate, encapsulated multi-chain fatty acids, OAs mainly sorbic acid, and phenolic compound) was added to the basal diets at 0.15% and 0.1% in Eimeria spp. challenged broilers ↑ Average BW, average BWG, and FCR
↑ TNF-γ
↓ Intestinal crypt depth
↑ Villus-height: crypt depth ratio
↑ Intestinal goblet cells
↑ Lactobacillus reuteri, Cyanobacteria 		[31]
0.3% a mixture of 11% formic acid, 13% ammonium formate, 5.1% acetic acid, 10% propionic acid, 4.2% lactic acid, and 2% of other lower levels of OAs (sorbic acid and citric acid) (3000 mg/kg diet) ↑ Formic acid in cecal contents on day 21 and acetic acid, propionic acid, butyric acid, and the total volatile fatty acids in the cecal content on day 42.
↑ IgA, D-lactate, and IL-10
↓ pH value in duodenum
↑Amylase activity of the pancreas and the tight junction protein (mainly Claudin-1, Claudin-2, and ZO-1) in duodenum
↑ Villus: crypt ratio in ileum
Modulate s microbiota structure
↓ Abundance of E. coli 		[26]
Dietary fumaric acid (15 g/kg feed) in Japanese quails ↑ BW, BWG, and FCR [27]
0.1% organic acid ↑ villus height of jejunum [70]
A blend of formic acid (32%), acetic acid (7%), and ammonium formate (20%) Formic acid improved the physical growth, digestibility, immunity, and antimicrobial activity
Acetic acid showed anti-bacteria effect 		[3]
0, 1, 1.5 g/kg feed formic acid ↑ BW, BWG, and the amount of feed ingested
↓ Glucose, triglycerides, and cholesterol		 [28]
A mixture of formic acid (32%), acetic acid (7%), ammonium format (20%), mono- and diglyceride of unsaturated fatty acids, and copper acetate (Under high stocking density) ↓ Chyme pH value in the proventriculus, gizzard, and duodenum
↑ acetic acid, butyric acid, and isovaleric acid in cecal chyme
↓ Valeric acid in cecal chyme 		[54]
A combination of both OAs blend (formic acid, propionic acid, ammonium formate, and ammonium propionate) (200 mg/kg) and essential oils mixture (150mg/kg) Improve BWG and FCR
↑ Villus height
↓ Growth of C. perfringens, E. coli, and Salmonella
↓ Intestinal lesion score
↓ Serum level of calprotectin and liver enzymes 		[15]
↓= Decrease ↑= Increase.

3. Conclusion

The supplementation of poultry feed with OAs could improve performance of broilers and layers, carcass traits, gut health, colonization of beneficial bacteria, and the immune response, but reduced the intestinal load of pathogenic bacteria. Therefore, they were highly valuable that might have contributed to improve the birds’ performance and health and improved performance and they could be used as an alternative to antibiotics in the poultry feed.

Author Contributions

WAA collected the date: wrote, prepared, and submitted the original article draft, as well as revised and edited the article. The author has read and agreed to the published version of the manuscript.

Funding

This article received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

Not applicable.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Types of acids used in the field of poultry industry.
Figure 1. Types of acids used in the field of poultry industry.
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Figure 2. The different uses of OAs in poultry production system.
Figure 2. The different uses of OAs in poultry production system.
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Figure 3. The different forms of OAs in poultry production.
Figure 3. The different forms of OAs in poultry production.
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Figure 4. The different antimicrobial mechanisms of actions of OAs.
Figure 4. The different antimicrobial mechanisms of actions of OAs.
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Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
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