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 bacteriaÃsfundamental
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.