The Role of Acidifiers in Poultry Nutrition

Antibiotic growth promoters (AGP) are being used for the past five decades to improve the performance of poultry. Sub-therapeutic levels of antibiotics in poultry feed supplements have increased feed efficiency and growth. They do not improve performance in germ free animals, which indicates their action on microflora.

Role of GIT Microflora in Poultry Nutrition

The gastrointestinal microflora of chicken plays a significant role in poultry nutrition and growth. The competition offered by gut microflora for poultry nutrition is often managed by in part through the use of low levels of antibiotics. This option is rapidly disappearing. As a result, there is a need to understand the role played by microflora in order to manage its effect on poultry nutrition, growth, health and disease by suitable replacements for antibiotics.

In chicken, the gut microflora plays a minor role in digestion of feed. However, the microbes play an important role in absorption of nutrients.

Factors affecting intestinal micro-environment

1.      Substrate availability

2.      pH

3.      Redox potential

4.      Toxins

5.      Antibodies

6.      Other bacteria

GIT microbes can be beneficial or harmful. The beneficial bacteria can inhibit the growth of pathogenic bacteria and prevent some specific intestinal diseases such as Clostridium perfringens, E. coli, Salmonella etc.

 Any disruption in this balance may cause the proliferation of pathogenic or performance hindering microflora.

The multiplication of harmful bacteria may start from the crop itself. Food stays in the crop for a longer time and the presence of moisture, temperature and time to multiply (depends on food retention time in crop), favors the multiplication of microflora. If acid-producing microbes reduce pH, multiplication of harmful bacteria is reduced.

Need for Alternatives to AGP?

Antibiotic growth promoters in animal nutrition have undoubtedly improved performance and health status. It is apparent that antibiotics function by modifying the intestinal microflora. The microbes can develop resistance to these antibiotics and when transferred to human beings, may pose a problem because of the resistance to these antibiotics. This has attracted global attention.

Many antibiotics in livestock and poultry production as growth promoters are banned in several countries. This ban on antibiotics in animal nutrition throws spotlight on alternatives to antibiotics. There is also a demand for alternatives to antibiotics in an increasingly health conscious market.

The use of some poultry feed supplements and additives are being reviewed here

What are Feed Acidifiers Poultry feed supplement?

Feed Acidifiers are acids included in poultry feed supplement in order to lower the pH of the feed, gut, and microbial cytoplasm thereby inhibiting the growth of pathogenic intestinal microflora. This inhibition reduces the microflora competing for the host nutrients and results in better growth and performance of the chicken. They also act as mold inhibitors. They are added upto 0.25% of the diet

Most acids are efficacious, and their effect remains as long as the acid is not volatilized.

Organic acids have been used extensively for more than 25 years in swine production and more recently in poultry nutrition

The antimicrobial effect of organic acid ions in controlling bacterial populations in the upper intestinal tract leads to beneficial effects. Inorganic acids such as HCl and H3PO4 though pH reducing are ineffective.

Chemistry of Organic Acids

Organic acids are organic carboxylic acids, including fatty acids and amino acids, of the general structure R-COOH. The short chain acids (C1-C7) are associated with antimicrobial activity. They are either

·         Simple monocarboxylic acids such as formic, acetic, propionic and butyric acids or

·         Carboxylic acids with hydroxyl group such as lactic, malic, tartaric and citric acids or

·         " Short chain carboxylic acids containing double bonds like fumaric and sorbic acids

Organic acids are weak acids and are only partly dissociated. Most organic acids with antimicrobial activity have a pKa - the pH at which the acid is half dissociated between C3 and C5.

Functions of Organic Acids

1.      To help maintain an optimum pH in the stomach, allowing correct activation and function of proteolytic enzymes.

2.      Total protein digestion in the stomach

3.      To stimulate feed consumption.

4.      To inhibit the growth of pathogenic bacteria

5.      Improves protein and energy digestibility by reducing microbial competition with host nutrients and endogenous nitrogen losses.

6.      Lowers the incidence of sub clinical infections and secretions of immune mediators.

7.      Reduces the production of ammonia and other growth depressing microbial metabolites

8.      Increased pancreatic secretion and trophic effects on gastrointestinal mucosa.

Factors Influencing the Efficacy

Ø  pKa-value

Ø  Chemical form (acid, salt, coated or not),

Ø  Molecular weight

Ø  MIC-value of the acid

Ø  Kind of micro-organism

Ø  Animal species

Ø  Site and location in the GIT

Ø  Buffering capacity of poultry feed supplement

Mode of Action

The antibacterial action of organic acids depends on whether the bacteria are pH sensitive or not. Only Certain types of bacteria are sensitive to pH (ex.: E. coli, Salmonella sp., L. monocytogenes, C. perfringens) while other types of bacteria are not sensitive (Bifidobacteriumsps., Lactobacillus sps).

·         For pH sensitive bacteria:

The mode of action in pH sensitive bacteria is shown in Figure 3. Organic acids in undissociated (non-ionized, more lipophilic) state penetrate the semipermeable membrane of bacteria cell wall and enter cytoplasm.

At the internal pH of bacteria (7.0), the undissociated organic acids dissociate, releasing H+ and anions (A-). The internal pH of bacteria decreases. The pH sensitive bacteria are unable to tolerate a large spread between the internal and the external pH. A specific H+ -ATPase pump acts to bring the pH inside the bacteria to a normal level. This phenomenon consumes energy and eventually can stop the growth of the bacteria or even kill it.

The lowering of pH also suppresses the enzymes (e.g. decarboxylases and catalyses), inhibit glycolysis, prevent active transport and interfere with signal transduction. The anionic (A-) part of the acid trapped inside the bacteria (it can diffuse freely through the cell wall only in its non-dissociated form), becomes toxic involving anionic imbalance leading to internal osmotic problems for the bacteria.

Thus in animal nutrition, the antibacterial effect of organic acids is by

·         Modification of bacteria’s internal pH

·         Inhibition of bacteriaísfundamental metabolic functions

·         Accumulation of toxic anions in bacteria and

·         Disruption of bacteria’s cellular membrane

B. For non-pH sensitive bacteria:

The non-pH sensitive bacteria tolerate a larger differential between internal and external pH. At a low internal pH, organic acids re-appear in a non dissociated form.. Equilibrium is created and the bacteria do not suffer (Figure 3 (B)).

Site of Action

In animal nutrition Organic acids exert their antimicrobial action in the feed and in the GI-tract.

The antibacterial effect of dietary organic acids in chickens is believed to occur in the upper part of the digestive tract (crop and gizzard). Following the addition of a combination of formic and propionic acid, high concentrations of these acids could only be recovered from crop and gizzard.

Feed Acidification Strategies

Form of Organic acids incorporated

·         Free acid form (powder or liquid) or

·         As salts form

o    Free form or

o   Protected / Coated salts.

Inclusion Levels of Organic acids

·         At 0.5 kg / Ton of poultry feed  to control molds and

·         At 2.5 to 3.0 kg / Ton of poultry feed  to reduce pH and help in control of Salmonella.

Impact of Organic Acids on Broiler Performance

Organic acids as poultry feed supplement are beneficial in practical studies. The efficacy of poultry digestion depends on microorganisms, which live naturally in the digestive tract. Inclusion of formic and propionic acids reduced pH in crop and gizzard but not in intestinal tract. Organic acids in crop reduce salmonella populations. Organic acids reduce production of toxic components by bacteria and a change in the morphology of the intestinal wall and reduce colonization of pathogens on the intestinal wall, thus preventing damage to the epithelial cells.

Various studies in the field of animal nutrition reveal that body weight gain, poultry feed intake,  feed conversion rate, carcass weight, abdominal fat weight, abdominal fat percentage, intestinal weight improved significantly

Selection/ Screening of organic acids

Comparative studies of six organic acids showed that the inhibiting effect of the acids was more pronounced in stomach contents than in content from the small intestine, probably due to the lower pH in the stomach content. The bactericidal effect of the organic acids is: benzoic acid >fumaric acid > lactic acid > butyric acid > formic acid > propionic acid

Poultry feeds usually have high alkalinity characteristics: (very rich in protein and mineral substances). Vegetal protein and calcium carbonate meals in feeds have a strong buffer function .

The use of diets in animal nutrition is characterized by such a high buffer capacity that it can compromise the intestine capability to keep an acidity level that can support growth and in some cases, maintain beneficial intestinal microflora.

Many harmful bacterial species found in poultry feed supplement have an optimal pH for their growth around 7, whereas useful bacterial species such as lactobacillus and enterococcus have their best growth at pH around 6.

Poultry intestinal tract acidification allows modulation of the intestinal bacterial flora in a positive and natural way and, at the same time, it works against the multiplication of that bacterial flora that besides being harmful and dangerous for the animal nutrition industry.

The buffering capacity of poultry feed supplement particularly of protein and mineral sources is high Buffering capacity or B-value  is often expressed as meq of 1.0 M HCl required to acidify 1 kg of material (poultry feed supplement or feed  ingredient) to pH 3 -5. Usually, the amount of 0.1 M HCl required to reduce the pH to 5 of 10 g of poultry feed supplement in 90 ml distilled water is represented as buffering capacity.

Note

·         pH of ingredients in water slurry and B-value are not related in poultry nutrition.

·         pH of  additives are variable but B-values are much more variable in poultry nutrition.

·         Cereal has low B-values.

·         Protein sources have high B values.

·         Mineral sources such as DCP have high B-value.

·         Limestone has a very high B value.

·         B-values can be additive if same end point is used: say pH 5.0

·         B-values of different batches of a poultry feed supplement or an ingredient may vary

·         It is not easy to calculate B value of a final poultry feed supplement from B value of constituent ingredients.

In stomach (in proventriculus) in birds, gastric juice is secreted. This lowers pH in between 2.0 & 4.0. A low gastric pH is important to activate pepsin from pepsinogen, which digests protein. Fermentation of undigested protein by microbes leads to formation of toxic biogenic amines. Low gastric pH controls bacterial population. In the acid environment, pathogenic bacteria such as E.coli and Klebsiella spa as well as Bacteroides diminish. Beneficial bacteria, such as Bifido and Lactobacilli sps are more tolerant towards low pH values.

In young animals, capacity to secrete gastric juice is limited. High B-value may pose problems. Pathogenic bacteria multiply in the digestive tract. The recommended B-value in poultry nutrition is about1-10 for 1-10 days age and 10-20 for 10-30 days age.

It may not be possible to reduce B-value of  sufficient low. High protein and mineral is necessary in broilers and layers. Organic acids as poultry feed supplement  are of value in controlling the pathogenic bacteria.

New Developments

In animal nutrition industry Organic acids can be mixed with fatty acids, mono- and diglycerides to form microgranules. Organic acid is released slowly from these microgranules. Medium chain fatty acids (chain length: 6 to12 C) with a lower absorption rate by the host may improve the efficacy of the short chain fatty acids. Acids produced by fermentation with microbes (Pediococcusacidilactici) may be less expensive and equally effective.

Limitations

·         Palatability may be diminished

·         Organic acids are corrosive to metallic poultry equipment

·         Bacteria are known to develop acid resistance when exposed to acidic environments for over long term

·         Presence of other antimicrobial compounds can reduce its efficiency

·         Cleanliness of the production environment

·         Buffering capacity of dietary ingredients

Conclusion

Prevention of infections, good nutritional balance and better performance is of paramount importance in poultry production. The use of alternatives to antibiotic growth promoter in specific the use of organic acids as poultry feed supplement is receiving greater attention.

In the absence of antibiotic growth promoters, and feeding strategies must supplement and not be a substitute for good management in animal nutrition . The use of acidifiers in poultry nutrition and diets is beneficial for the poultry sector. Combination of different acids seems to lead the way to greater efficacy.