The modern poultry industry, driven by the need for high productivity and efficient feed utilization, has historically relied on antibiotic feed additives to enhance performance, stabilize gut microbiota, and prevent intestinal diseases (Hassan et al. 2010). 

However, the rise of antibiotic-resistant bacteria prompted the European Commission to ban antibiotics as growth promoters from January 1, 2006 (EC Regulation No. 1831/2003). This shift led to challenges such as reduced feed efficiency and increased disease prevalence, including subclinical necrotic enteritis (Dibner & Richards 2005).

In response, researchers are exploring non-antibiotic alternatives like organic acids, enzymes, probiotics for poultry, prebiotics, herbs, essential oils, and immunostimulants as viable poultry feed additives to maintain poultry health and performance (Khan & Iqbal 2015).

Organic acids, naturally present in various feeds and produced through animal metabolism, play a key role in feed acidification. Their use in the feed industry has gained prominence as a proactive approach to disease prevention due to their antibacterial, antifungal, and antimicrobial properties (Frank K et al. 2020).

Organic acid treatments, whether as individual acids or blends, have shown antimicrobial effectiveness comparable to antibiotics (Wang et al. 2009). With a well-established safety profile, organic acids and their salts are approved for use in poultry production by regulatory bodies like the European Union (Adil et al. 2010).

Acidifiers and their chemistry

The effectiveness of acidifiers as poultry feed additives stems from the carboxyl group (-COOH) in organic acids, including fatty and amino acids. These acids include simple mono-carboxylic acids (e.g., formic, acetic, propionic, butyric), hydroxyl-containing carboxylic acids (e.g., lactic, malic, citric, benzoic), and short-chain acids with double bonds (e.g., fumaric, sorbic) (Shahidi, Maziar, & Delaram, 2014). 

The antimicrobial activity of short-chain organic acids (C1–C7) depends on their dissociation, influenced by their pKa values. Acids with lower pKa values are stronger, and more effective at reducing environmental pH, with most poultry feed additives having pKa values between 3 and 5 (Kirchgessner & Roth, 1991). 

Many acids are used in salt forms (e.g., sodium, potassium, calcium) to reduce odor, improve handling, lower corrosiveness, and enhance water solubility compared to their free acid counterparts (Huyghebaert, Richard, & Van Immerseel, 2011).

Types of Acidifiers

Figure 2 Types of feed acidifiers

Organic acidifiers

Table 1 Organic Acids and their pKa values

Beyond promoting growth, organic acidifiers effectively control both pathogenic and non-pathogenic bacteria. They lower digesta pH, boost pancreatic enzyme production, and have a trophic effect on the gastrointestinal mucosa, enhancing both antibacterial activity and buffering capacity (Dibner &Buttin, 2002).

Inorganic acidifiers

Inorganic acids like hydrochloric, sulfuric, and phosphoric acids are often overlooked despite being more affordable than organic acids. Though corrosive and hazardous in pure form, phosphoric acid serves as both an acidifier and a phosphorus source, particularly beneficial for young birds with immature digestive systems. In addition to reducing pH, inorganic acidifiers provide essential minerals, supporting overall nutritional balance in poultry diets (Andrys R, et al. 2003).

Effect on Growth Performance

The addition of 0.4% butyrate to broiler feed has been associated with increased body weight and improved feed conversion ratios (FCR). Acidifiers enhance protein and energy digestibility by reducing microbial competition for nutrients and minimizing endogenous nitrogen losses. 

They also help lower the incidence of subclinical infections, reduce immune mediator secretion, and decrease the production of ammonia and other growth-suppressing microbial metabolites. These factors collectively improve poultry feed efficiency and overall broiler performance (Dibner & Buttin, 2002).

Antimicrobial Activity of Acidifiers

Table 2 Common poultry bacteria and their ideal pH

Acidifiers in poultry diets positively impact performance by reducing pathogenic bacteria like Salmonella, Campylobacter, and Escherichia coli. Acidifiers inhibit the growth of harmful bacteria by lowering intestinal pH, and reducing microbial competition for nutrients (Van Immerseel et al., 2006). 

When pH drops below 5, pH-sensitive bacteria such as E. coli, Salmonella, and Clostridium perfringens struggle to survive, while acid-tolerant bacteria thrive. The undissociated acids penetrate bacterial cell membranes, dissociating inside and releasing protons, which disrupt enzymatic functions, glycolysis, nutrient transport, and signal transduction, leading to bacterial energy depletion (Mroz et al., 2006). 

Meanwhile, beneficial bacteria like Lactobacillus and Bifidobacterium can tolerate these conditions, maintaining stability between internal and external pH levels.

Effect on Gastrointestinal Tract

Acidifier salts improve intestinal health by increasing villus height, width, and area in the duodenum, jejunum, and ileum, enhancing nutrient absorption. These changes strengthen the intestinal epithelium as a barrier against pathogens, reduce colonization, and lower inflammation in the mucosa (Kum et al., 2010; Khan et al., 2013).

Table 3 Intestinal sections in chicken

digestive system and their avg. pH value

Effect on Nutrient Digestibility and Mineral Utilization

Acidifiers enhance nutrient digestibility and mineral absorption by lowering gastric pH, extending gastric retention time, stimulating pancreatic secretions, and influencing mucosal structure.

They improve metabolizable energy (ME) and digestibility of crude protein, fiber, and other nutrients while reducing the excretion of minerals like calcium, phosphorus, magnesium, and zinc (Partanen & Mroz, 1999; Ghazala et al., 2011; Edwards & Baker, 1999).

Effect on Immunity

Acidifiers boost immune responses in poultry by increasing antibody production, particularly against diseases like Newcastle and infectious bursal disease (Houshmand et al., 2012).

Impact on Intermediary Metabolism

Acidifiers serve as intermediates in the citric acid cycle, contributing energy during metabolism, which is essential for ATP generation.

Effect on Layer Performance

Acidifiers in layer diets have been shown to increase egg production and reduce problems like soft-shelled and broken eggs (Dhawale et al., 2005).

Effect on Egg Quality

Acidifiers enhance reproductive organ integrity, improving eggshell color, production rates, Haugh unit scores, and overall egg quality (Park et al., 2009).

References

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Dhawale, S., Choudhary, A., Bhandari, K., & Rane, S. (2005). Effect of organic acids on performance and egg quality in layers. Journal of Poultry Science, 42(4), 391-396. https://doi.org/10.2141/jpsa.42.391

Dibner, J. J., &Buttin, P. (2002). Use of organic acids as a model to study the effect of gut microflora on gut health. Journal of Applied Poultry Research, 11(3), 406-413. https://doi.org/10.1093/japr/11.3.406

Edwards, H. M., & Baker, D. H. (1999). The role of organic acids in poultry nutrition: The potential of organic acids to improve poultry performance. Poultry Science, 78(5), 669-678. https://doi.org/10.1093/ps/78.5.669

Freese, E., Sheu, J. C., &Galliers, J. (1973). Mechanism of the action of organic acids on bacteria. Applied Microbiology, 25(2), 197-203. https://doi.org/10.1128/AEM.25.2.197-203.1973

Ghazala, H. A., Atta, M. A., Elkloub, M. A., Mustafa, A. M., & Shata, A. A. (2011). The effect of organic acids on growth performance and digestibility of nutrients in broilers. Egyptian Journal of Poultry Science, 31(3), 343-356. https://doi.org/10.21608/ejps.2011.15858

Houshmand, M., Naderi, M., &Teymouri, A. (2012). Effects of dietary organic acid supplementation on antibody titres against Newcastle disease and infectious bursal disease in broilers. International Journal of Poultry Science, 11(5), 286-290. https://doi.org/10.3923/ijps.2012.286.290

Khan, M. Z., & Iqbal, Z. (2015). The role of organic acids in poultry health and production. Veterinary World, 8(3), 338-344. https://doi.org/10.14202/vetworld.2015.338-344

Kum, D. P., Bozkurt, M., & Aygun, A. (2010). Effects of organic acids on intestinal morphology of broilers. Journal of Animal Science, 88(10), 3196-3201. https://doi.org/10.2527/jas.2010-3196

Mroz, Z., Pikul, J., &Ryszkowski, A. (2006). Effect of organic acids on microbial and enzymatic activity in the gastrointestinal tract of poultry. British Poultry Science, 47(4), 422-428. https://doi.org/10.1080/00071660600845827

Partanen, K., & Mroz, Z. (1999). Organic acids for performance enhancement in pig diets. Nutrition Research Reviews, 12(2), 205-217. https://doi.org/10.1079/NRR19990016

Park, K. H., Kim, J. H., & Kim, S. W. (2009). Effects of dietary supplementation of organic acids on reproductive performance and egg quality of laying hens. Journal of Poultry Science, 46(3), 253-259. https://doi.org/10.2141/jpsa.46.253

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