1. Introduction
Antibiotic growth promoters have been used in livestock production systems for 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 the destruction of beneficial intestinal flora and the emergence of resistant bacteria, which are transmitted to humans through the food chain [2,3]. Therefore, the search for suitable alternatives has become 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 also have many advantages, such as the absence of pollution, drug resistance, and residues, as well as beneficial effects on health [8,9]. Dietary OAs promote the production of prebiotics and probiotic lactic acid bacteria [10]. OAs could potentially replace antibiotic growth promoters, with positive effects on the 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 can be divided into 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, which 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 an 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 feed commodities to minimize the impact of contamination, the growth of harmful bacteria and fungi, and deterioration, as well as to prolong their shelf life [23]. Therefore, they are known to be good feed preservatives. Acetic acid or benzoic acid and their sodium salts are represented as safe feed preservatives [11]. They could have similar functions to antibiotics [7]. For instance, OAs were added to poultry feed at rates of 0.5 kg/ton and 2.5–3.0 kg/ton to reduce mold and Salmonella growth, respectively [24]. In addition, dietary formic acid and propionic acid could reduce the bacterial load of Salmonella spp. in contaminated feed [18].
Poor hygienic conditions on livestock farms, such as increasing litter moisture and varying temperature, can enhance microbial growth and consequently reduce the nutritional content of proteins and carbohydrates. Supplementation with OAs could improve growth performance parameters and carcass traits [25,26,27,28], reduce the gut’s pH, enhance pepsin production, promote nutrient digestibility and utilization [29,30], boost the immune response [31], and suppress the growth of pathogenic bacteria [14,32,33,34,35,36]. In addition, Ma et al. [26] demonstrated the antioxidant capacity of OAs, as supplementing diets with mixed OAs increased the amount of serum superoxide dismutase and the catalase of 3- and 6-week-old broilers.
However, there are some limitations to using OAs in poultry nutrition, including a reduction in feed palatability, a corrosive effect on metallic equipment, the presence of acid-resistant bacteria that reduce the efficacy of OAs, and the buffering capacity of dietary ingredients.
Accordingly, this review article provides comprehensive insights into the role of using OAs in reducing microbial load, enhancing performance parameters in broilers and layers, improving gut health, and boosting the immune response.
2. The Different Effects of OAs Supplementation on Poultry
Table 1 and Figure 2 illustrate the different effects of OAs inoculation on poultry feed.
2.1. Antimicrobials
OAs are present in different forms; they can be solid in feed, sprayed 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 [104].
Dibner and Buttin [29] demonstrated that some OAs exhibit a narrow spectrum affecting bacteria (lactic acid) or fungi (sorbic acid), while others show a 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 [105]. 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 [106]. Under a low pH condition, the OAs become more available in a lipophilic dissociated form, easily diffuse into the bacterial and fungal cell membranes, and consequently disrupt the enzymatic reaction and transport system [6]. Moreover, the low pH condition can disturb the generation of energy and inhibit bacterial cell proliferation and growth (bacteriostasis) [6,107]. 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 [107]. However, there is a discrepancy regarding the role of OAs in reducing the pH of the intestinal tract [108,109], and this may be due to the differences in acidifiers’ types and concentrations, experimental animals, acidifiers’ formulations and test sites, diet types and compositions, and other factors.
The cytoplasm of the bacterial cells contains both positively charged protons and negatively charged anions. The accumulation of protons in the cells leads to an increase in its acidity to an unbearable limit. Therefore, the bacterial cell depletes most of its energies to adjust its internal pH. This depletion may cause the inhibition of growth and multiplication and even death. Further, 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 causes cells’ death [110]. On the other hand, OAs could release proton ions in the cytoplasm.
OAs have bactericidal and bacteriostatic characteristics [111]. They diffuse into the bacterial cell membrane and dissolve in anions and protons of the cytoplasm [112] with a subsequent expulsion of protons outside the bacterial cells [113]. This process reduces the energy supply and ends by cell death [114]. The undissociated forms of OAs can enter the bacterial cell membrane where they are dissociated, produce H+ ions, and raise the pH acidity of the cytoplasm [115]. Then, pH-sensitive bacteria are forced to discard the redundant proton ions via the H+-adenosine triphosphatase pump, which causes the impeding of bacterial cells’ proliferation [9]. However, the bacterial cell uses energy to restore the basic nature of the 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 [116]. Moreover, the dissociated OAs produce anions (RCOO−) to disturb the protein synthesis and prevent the bacterial cells from replicating. The OAs may also affect the microbial cell membranes’ integrity or may interfere with nutrient transport and energy metabolism, causing bacterial cells’ death [18]. They can penetrate the bacterial membrane, inhibit the synthesis of adenosine triphosphate, disturb the bacterial membrane, and denature the DNA [117]. In addition, OAs can prevent the release of toxic compounds following bacterial colonization, thus averting the damage to the intestinal epithelial cell and improving the villus height [23]. Moreover, they can enhance the beneficial microbiota populations, thus creating a eubiotic intestinal environment [118,119].
A more efficient release of OAs compounds could be achieved via the microencapsulation process [120]. OAs could be metabolized and rapidly absorbed from the upper segments of the digestive tract (proventriculus, gizzard, and duodenum), but not from the lower parts [121]. The reduction in the gut’s pH limits pathogenic bacterial growth, especially of those which are less tolerant to acidic pH [25,103]. 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 undissociated form of acids [29]. Moreover, carboxylic acids, such as citric acid, lactic acid, tartaric acid, and malic acid, as well as monocarboxylic acids, such as propionic acid, acetic acid, butyric acid, and formic acid, have a pKa value of between 3 and 5 and consequently antimicrobial properties [122]. It has been demonstrated that acids can reduce the total intestinal microbial load and the subsequent infection rate, leading to enhanced digestibility and reduced energy demand by the gut-associated tissue [123].
Some pathogenic intestinal pathogens, such as Salmonella spp. [80,122,124,125], Campylobacter jejuni (C. jejuni) [29,60], pathogenic Escherichia coli (E. coli) [75,98,126], and Clostridium perfringens (C. perfringens) [127] or coccidia spp. [31,128], could be drastically affected by using OAs. However, the growth of beneficial gut microflora, such as Lactobacillus spp., could be improved following the OA treatment [14]. So, the reduction in intestinal bacterial load along with the enhancement of natural flora results in an improvement in the nutrients’ utilization and consequently the growth performance [3,17,129]. Drinking water acidification could diminish the clinical signs of Campylobacter infection in the gut [106]. Moreover, citric acid lowered the growth of Listeria monocytogenes on chicken thighs at 4 °C for 8 days [130]. On the other hand, different OAs can promote the growth of beneficial bacteria, such as Lactobacillus spp. [78,102,131]. For instance, 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 the activation of proteolytic enzymes, absorption of minerals, decreasing ammonia, depressing microbial metabolites, and the stimulation of feed intake (FI) [50]. In addition, the by-product of wheat milling, “wheat bran”, showed efficacy against Salmonella spp. in terms of percentage and particle size. It has been proven that the rapid fermentation of butyric acid downregulated Salmonella spp. gene expression [6,77] and inhibited bacterial cecal colonization due an improvement in intestinal barrier function [69].
2.2. Performance Parameters
The blends of OAs can improve FI and nutrient utilization, so they can enhance the body weight gain (BWG) and the feed conversion ratio (FCR) of poultry [7,15,78,85,102,127,131,132,133,134]. 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 [135].
The supplementation of OAs can improve the performance parameters [12,15,103,136,137,138], which is probably due to the enhancement in the digestible energy and protein contents of the feed, reducing the intestinal bacterial colonization [8], increasing the proliferation of beneficial flora, modulating the anti-inflammatory immune response [114], and lowering the ammonia and other harmful metabolites [123]. The OAs work to improve the digestion of proteins, calcium, phosphorus, magnesium, zinc, and other nutrients which appear in the feed material of the small intestine [7]. In addition, the undissociated forms of OAs can penetrate the lipid layer of the bacterial and fungal cell membranes, causing the release of protons, accumulation of intracellular anions, a reduction in the intestinal pH, and then boosting the secretion of endogenous digestive enzymes [9,43]. Moreover, OAs can enhance the release of digestive enzymes, pancreatic secretion, the activity of microbial phytase, and the proliferation of intestinal cells [29]. A reduction in the pH of the crop, gizzard, and duodenum leads to an increase in the secretion of digestive enzymes, including pepsin, trypsin, chymotrypsin, proteinase, amylase, lipase, protein hydrolysate, and non-protease concentrations in the intestinal segment [99,139,140]. Furthermore, treatments with OAs can enhance the secretion of pepsin and chyme that reach the intestine to stimulate the decomposition and absorption of nutrients. This process plays a role in stimulating digestive system development, increasing amylase and lipase secretion, and consequently increasing the intestinal absorption capacity. The OAs slow the rate of digesta passage and thus enhance the absorption of the feed contents from the intestines [141].
The usage of OAs is also associated with the improvement in minerals’ digestibility [142]. The digestibility of minerals, particularly calcium and phosphorous, has been shown to improve, possibly due to the enhancement of digestive enzymes [143] or the effective role of Lactobacillus spp. in the gut [49,50].The mixing of OAs with essential oils can reduce the populations of pathogenic enteric bacteria and improve the growth of the beneficial gut microbiome, thus enhancing the intestinal health [14,15].
2.3. Carcass Traits
Treatments with different OAs can improve the meat quality of chickens’ carcasses [41]. Lee et al. [144] demonstrated that the pH of broiler thigh meat was increased by gallic acid and linoleic acid supplementations. Moreover, Fortuoso et al. [145] 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 [146] or the effect of the gut microbiota and their metabolites [99]. Dietary supplementation with benzoic acid or amylase improved antioxidant capacity, nutrient digestion, and meat quality [146]. The improved meat tenderness after dietary treatments with OAs is probably due to improved nutrient metabolism, reduced anaerobic digestion, and enhanced antioxidant capacity. During the carcass’s processing, the anaerobic conditions of protein breakdown may result in the accumulation of lactic acid, which affects the water holding capacity of meat [99].
2.4. Intestinal Health
The addition of OAs to the drinking water of birds resulted in an increase in the number of jejunal goblet cells, which led to the stimulation and production of the mucus layer [147] and improved the gut epithelial cells [12,106,148,149] and the duodenal villus height [150].
A decrease in the crypt depth and increase in the villus height-to-crypt depth ratio were also found [31]. Likewise, the results of García et al. [44], Kum et al. [151], and Islam et al. [15] showed an increased villus height and villus-to-crypt depth ratio, reduced lesion scores, and thus an improvement 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 [123]. The increase in the secreted pancreatic juice containing trypsin, amylase, protease, lipase, procarboxy peptidases, and chymotrypsinogen [7,103,152], as well as the enhancement in pepsin protein proteolysis activity, breaking down of proteins to simple peptides, and the releasing of gastrin and cholecystokinin hormones, have also been noticed following the addition of OAs to feed. Similarly, Ma et al. [26] reported that the supplementation of chicken diets with a mixture of OAs improved the pancreatic secretions and enhanced the expression of tight junction proteins, resulting in healthier broiler production. The acidic intestinal environment can reduce the bacterial metabolites, such as ammonia and amines [153], 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 [154]. Japanese quails that received a product containing acetic acid, formic acid, and butyric acid, as well as thymol, β-cymene, carvacrol, and borneol, showed an improvement in intestinal morphology, including in crypt depth, villus length and width, villus-to-crypt ratio, the thickness of the intestinal wall, goblet cell percentage, and the appearance of the intestinal surface area [95]. Adil et al. [155] demonstrated that a dietary 3% fumaric acid increased the villus height in all the segments of small intestines. Several studies showed that chickens which received butyrate have increased intestinal villus height, decreased crypt depth, and thereby an increased intestinal absorption surface [156,157,158]. It has been found that butyrate can regulate the gut barrier and plays an important role as an anti-inflammatory and immuno-regulatory substance to maintain gut homeostasis [105]. Butyric acid can promote the development of epithelial cells [159], preserve intestinal cells’ viability, and enhance the turnover of enterocytes, which may improve intestinal recovery. Similar results were obtained by Gao et al. [99] and Pham et al. [14]. Improved intestinal villi length and depth as well as increasing the number of goblet cells containing acidic mucins have also been reported in broilers fed on diets containing butyrate [160].
It has been known that infection with Eimeria (E.) spp. is usually associated with gut health. Supplementation with OAs could be a suitable alternative for anticoccidial drugs due to their ability to improve the intestinal integrity that is damaged by such infection [31]. Acetic acid could decrease the cecal pH and consequently reduce the impact of oocysts that, in turn, decrease intestinal lesions. In broiler chickens, Abbas et al. [161] reported that acetic acid was effective against E. tenella infection, while Ali et al. [162] 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 reasons, as the main causes are unknown. However, several studies have proven the immuno-potentiating effects of OAs on poultry [6,50,85,163]. The weights of the immune organs of broiler chicks have been increased in response to OA supplementations [30]. Moreover, the levels of serum immunoglobulin (Ig) were elevated following the dietary feeding of layer chickens on OAs mixture and yeast culture [164]. For instance, chickens supplemented with OAs showed an improvement in immune response and an enhancement in antibody titer against Newcastle disease (ND) virus infection [59,165]. Moreover, Lee et al. [166] demonstrated that the percentages of cluster of differentiation (CD4+), CD25+, and T-cells were higher in broiler chickens that 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 in the ileal morphology and immunity [75]. Also, OAs showed the ability to reverse the detrimental effects of S. typhimurium and boost the immunological response in the challenged chickens [141]. Emami et al. [142] reported that broiler chickens that received a diet containing phytase and OAs showed high levels of IgG. It has been found that OA supplementation may increase trypsin and chymotrypsin production and consequently activate the digestive tract to secrete IgA in the ileal mucosa [77]. 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 [160]. The inclusion of butyric acid in the ration of broiler chickens was associated with a good cell-mediated immunity after the inoculation of phytohemagglutinin-P, improved humoral antibody production after vaccination with ND virus vaccine, and injection of sheep red blood cells, and increased thymus and spleen weights [160].
Increasing Lactobacillus spp. count in the gut [142], inhibiting nuclear factor kappa B activation [167], increasing tumor necrosis factor [31], improving the immunological features of blood and small intestine, and modulating the bacterial population of caecum [26] are possible causes of OAs’ immuno-potentiation effect. In the study of Rodríguez-Lecompte et al. [168], the treatment of broiler chickens with an OAs blend upregulated the interferon-γ in the cecal tonsils and interleukin 6 (IL-6) and IL-10 in the ileum. Similarly, Lee et al. [169] 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 an AI (H9N2) virus vaccine. Moreover, the gut-associated immunity produced by the lymphoid tissues was linked with the gut bacteria following treatment with OAs [75]. The immuno-protective effects of OAs against broilers’ coccidiosis were also reported [31,128]. However, Hedayati et al. [111] 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.
3. Conclusions
The supplementation of poultry feed with OAs can improve the performance of broilers and layers, carcass traits, gut health, the colonization of beneficial bacteria, and the immune response, but reduces the intestinal load of pathogenic bacteria. Therefore, they are highly valuable as they have contributed to improving birds’ performance and health and they can be used as an alternative to antibiotics in the poultry feed.
Not applicable.
Not applicable.
The author declares that there are no conflicts of interest regarding the publication of this paper.
Footnotes
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The effects of different OAs inoculations on poultry feed.
| Organic Acid(s) | Effects | Reference |
|---|---|---|
| Dietary ascorbic acid, malic acid, and tartaric acid | ↑ BWG and feed efficiency | [ |
| 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 | [ |
| 0.5–1% fumaric acid | Improved metabolizable energy | [ |
| 0.16% butyric acid | ↓ Salmonella count in caecum | [ |
| 0.2% butyric acid | ↑ Carcass weight, breast muscles yield, and dressing % | [ |
| Dietary citric acid | ↑ FI | [ |
| 5 and 10 g/kg formic acid | Improved ileal nutrient digestibility | [ |
| 5000 and 10,000 ppm formic acid | ↑ Growth | [ |
| 0.05% sodium butyrate | ↓ Lactobacilli and E. coli | [ |
| Butyric acid, 285 mg/kg of feed | ↑ Eggshell strength | [ |
| A combination of acetic acid, citric acid, and lactic acid | ↑ BW | [ |
| A dietary mixture of formic (70%) and propionic acid (30%) | Improved FI in a quadratic form | [ |
| Dietary citric acid and phytase | ↑ Specific gravity and eggshell thickness | [ |
| 0.5% citric acid or avilamycin and their combination | ↑ FI, growth, carcass yield, and bone ash | [ |
| 0.09% free or protected sodium butyrate | ↓ S. enteritidis in crop, cecum, and liver | [ |
| Dietary citric acid | ↑ Lymphocyte number in lymphoid organs | [ |
| 0.45% of potassium diformate | ↓ Reduced necrotic enteritis-related mortality and amount of C. perfringens in the jejunum | [ |
| Dietary 0.4% butyric acid | ↑ BWG and FCR | [ |
| Dietary 3% citric acid | ↓ Ileal coliform contents | [ |
| Formic acid in the drinking water | No effect on the counts of total organisms and E. coli in intestine | [ |
| 3% butyric acid | ↓ Crop pH and cecal coliform count | [ |
| 0.50% formic acid, 0.50% fumaric acid, 0.25% acetic acid, and 2.0% citric acid | ↑ Villus height in duodenum | [ |
| 250–7000 mg/kg N-butyric acid | ↓ S. Typhimurium or C. perfringens colonization | [ |
| Dietary 0.15% blend of OAs for broilers | ↑ Antibody titers against ND at 21 days old | [ |
| 1% 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 | [ |
| 1% formic acid in feed for 5 days | ↓ Salmonella count | [ |
| 3% butyric acid, 3% fumaric acid, and 3% lactic acid in the drinking water of broilers | ↑ BW | [ |
| 0.1% butyric acid | ↓ Salmonella count in caecum | [ |
| 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 | [ |
| 0.075% 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 | [ |
| 0.4% formic acid, propionic acid | Improved villus height-to-crypt depth ratio | [ |
| 1% fumaric acid in diets | ↑ BWG | [ |
| 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 | [ |
| 0.05% encapsulated butyrate | ↑ Intestinal weight and epithelial cell area | [ |
| 2 g/kg organic oil blend | Villus height in ileum | [ |
| 0.02%, 0.03%, and 0.04% protected calcium butyrate | ↑ BWG | [ |
| 2% citric acid | ↑ Epithelial cell proliferation and villi height of gastrointestinal tract | [ |
| 5 g/kg formic acid | ↑ BWG, dressing % | [ |
| 3 kg/ton commercial acidifier | ↑ Average daily gain | [ |
| 0.1%, 0.02%, and 0.04% of formic and propionic acids | ↑ Beneficial intestinal bacterial flora load | [ |
| 0.1% and 0.3% formic acid and citric acid for ducklings | ↑ BW, BWG, and FCR | [ |
| 0.05 or 0.1% encapsulated sodium butyrate | ↑ Ileal digestibility energy coefficient | [ |
| 0.2% mixture of 32% fumaric acid, 3% formic acid, 13% lactic acid, 3% propionic acid, and 1% citric acid | ↑ Villus height and crypt depth | [ |
| 800 mg/kg micro-encapsulated sodium butyrate | ↑ BW, daily gain, and FCR | [ |
| 0.1% fermented fatty acids of wheat bran | ↓ Salmonella count | [ |
| 1% formic acid in water of S. typhimurium-infected broilers | ↓ BW | [ |
| 0.05% encapsulated butyric acid | ↑ Lactobacilli and Bifidobacterium | [ |
| Protected or unprotected 0.1% butyrate | No effect on gut weight, retention time, dry matter, organic matter, nitrogen, and non-protein nitrogen | [ |
| 0.1%, 0.15%, and 2% blend of ortho phosphoric acid, formic acid, and propionic acid in the drinking water | ↓ Growth performance parameters | [ |
| Dietary 0.30 g/kg sorbic acid and fumaric acid | ↑ Secretion of trypsin, lipase, and chymotrypsin in the intestine | [ |
| 0.06% sodium butyrate | ↑ Lactobacilli | [ |
| A combination of sodium butyrate, citric acid, phosphoric acid, acetic acid, propionic acid, formic acid, and lactic acid | ↑ Growth performance parameters | [ |
| 3 g/kg organic acid blend in Japanese quails | ↑ Villus height and width in jejunum and duodenum | [ |
| A blend of OAs (0.1%) in the drinking water of broiler chickens orally challenged with (109 CFU/mL) C. jejuni | ↓ C. jejuni counts | [ |
| 0.9% formic acid and sodium formate | ↓ S. typhimurium colonization | [ |
| Dietary fumaric acid | ↑ Erythrocyte counts, hemoglobin concentration, and total serum protein, albumin, globulin, total cholesterol, high-density lipoprotein cholesterol | [ |
| 0.1% formic acid, acetic acid, and ammonium formate in the drinking water of broilers | ↑ Growth performances | [ |
| 0.6 and 1.2 g/kg sodium butyrate | ↑ Average daily gain and FCR | [ |
| 3% fumaric acid in a diet | ↓ Cholesterol and total lipids | [ |
| Encapsulated organic acids of formic acid, acetic acid, and butyric acid, in addition to essential oils, like thymol, carvacrol, β-cymene, borneol, and myrcene coated with a matrix of triglyceride | ↑ Epithelium thickness and surface area | [ |
| 0.5 kg/ton feed formic acid with cinnamaldehyde | ↓ Proliferation of C. coli | [ |
| 0.2% butyric acid | No significant effect on dry matter, crude protein, ether extract, calcium, phosphorus, and apparent metabolized energy | [ |
| 0.2% mixture of 32% fumaric acid, 3% formic acid, 13% lactic acid, 3% propionic acid, and 1% citric acid | ↓ E. coli population | [ |
| 0.3% blend of acetic acid, propionic acid, formic acid, and ammonium formate | ↑ Villus height | [ |
| Dietary supplementation of phosphoric acid (0.1, 0.2, and 0.3 g/kg) and lactic acid (0.3 g/kg) | ↑ Feed-to-gain ratio | [ |
| 1 g/kg of a diet of a mixture of 40% formic acid, 40% formate, and 20% sodium | ↑ Serum glucose level | [ |
| 0.5–2.5 g/kg feed of short- and medium-chain fatty acids | ↓ C. perfringens shedding in the caecum | [ |
| 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, were added to the basal diets at 0.15% and 0.1% in Eimeria spp.-challenged broilers | ↑ Average BW, average BWG, and FCR | [ |
| 0.3% 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 | [ |
| Dietary fumaric acid (15 g/kg feed) in Japanese quails | ↑ BW, BWG, and FCR | [ |
| 0.1% organic acid | ↑ Villus height of jejunum | [ |
| A blend of formic acid (32%), acetic acid (7%), and ammonium formate (20%) | Formic acid improved the physical growth, digestibility, immunity, and antimicrobial activity | [ |
| 0, 1, 1.5 g/kg feed formic acid | ↑ BW, BWG, and the amount of feed ingested | [ |
| A mixture of formic acid (32%), acetic acid (7%), ammonium formate (20%), mono- and diglyceride of unsaturated fatty acids, and copper acetate (under high stocking density) | ↓ Chyme pH value in the proventriculus, gizzard, and duodenum | [ |
| A combination of both OA blend (formic acid, propionic acid, ammonium formate, and ammonium propionate) (200 mg/kg) and essential oils mixture (150 mg/kg) | Improves BWG and FCR | [ |
↑= increase; ↓ = decrease; BW = body weight; BWG = body weight gain; FI = feed intake; FCR = feed conversion ratio; Ig = immunoglobulin; IL = interleukin; TNF = tumor necrosis factor; ND = Newcastle disease; E. coli = Escherichia coli; C. perfringens = Clostridium perfringens; C. jejuni = Campylobacter jejuni; C. coli = Campylobacter coli; S. typhimurium = Salmonella typhimurium; S. enteritidis = Salmonella enteritidis.
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