©2013 Poultry Science Association, Inc.
Feed enzymes: The science, practice,
and metabolic realities1
V. Ravindran2
Institute of Veterinary, Animal and Biomedical Sciences,
Massey University, Palmerston North 4442, New Zealand
Primary Audience: Nutritionists, Researchers, Veterinarians
SUMMARY
The use of exogenous feed enzymes in poultry diets is becoming a norm to overcome the
adverse effects of antinutritional factors and improve digestion of dietary components and bird
performance. In this paper, an overview of the science behind the use of feed enzymes and the
current status of enzyme technology is provided. Responses to enzyme supplementation are
often variable and the reasons contributing to the observed variability are discussed. Though
there are opportunities to enhance nutrient utilization with enzyme supplementation, there will
be physiological limits to achievable responses. These limits are imposed by the pH and digesta
retention time within the digestive tract. Nutritional strategies to, at least partly, overcome these
limits need to be explored; potential approaches include feeding to restore functionality of crop
and gizzard as well as the use of unconventionally high doses of enzymes. Anticipated future
development of better forms of feed enzymes will also lower the physiological barriers.
Key words: feed enzyme, variable response, metabolic limit, nutrient utilization, poultry
2013 J. Appl. Poult. Res. 22:628–636
http://dx.doi.org/10.3382/japr.2013-00739
INTRODUCTION
Enzyme use in poultry diets has a long history, with the first report of an enzyme product,
known as Protozyme, being used in the 1920s
[1]. What is not widely appreciated is that most
of the early feed enzyme research originated
from the United States, with pioneering research demonstrating the value of the addition
of enzyme preparations to barley-based diets
reported during the 1950s and 1960s [2–4] and
early studies on the use of phytases in improving phosphorus availability from plant feed
ingredients conducted during the early 1970s
1
[5, 6]. The enzyme products evaluated in these
studies were crude enzymes and, importantly,
available only in small quantities. It was another
10 to 20 yr before the nonstarch polysaccharide
(NSP) enzymes and phytases became available in commercial quantities. During the past
3 decades, the chemistry of target substrates in
feed ingredients has been better understood and
it has become possible to fine-tune the production of enzymes that are specific for individual
substrates. Another development has occurred
in the area of biotechnology, specifically in fermentation and microbiological technologies and
molecular biology. As a result, it is now possible
Presented as a part of the Informal Nutrition Symposium “Metabolic Responses to Nutrition and Modifiers” at the Poultry
Science Association’s annual meeting in Athens, Georgia, July 9, 2012.
2
Corresponding author: [email protected]
Ravindran: INFORMAL NUTRITION SYMPOSIUM
to produce feed enzymes cheap enough to warrant their use in commercial diet formulations.
Other advances include the development of specific enzymes designed to function optimally in
the gastrointestinal tract of the animal and production technology to improve enzyme stability
during the processing of commercial feeds.
The feed enzyme market has grown rapidly,
notably during the past 5 yr, due largely in response to increasing raw material cost. Despite
wider acceptance, responses to enzyme supplementation are variable and many questions remain to be addressed as to how to use enzymes
to achieve consistent results. These variable responses, however, point to not only the limitations that exist, but also to the potential opportunities of enhancing the benefits of enzyme use.
Limitations in enzyme responses are inexorably
associated with 3 integral components, namely
the enzyme, substrate, and the bird. The objective of this paper is to provide an overview of
the science and practice of enzyme use in poultry diets and to highlight that there will be physiological limits to achievable enzyme responses.
THE SCIENCE
Characteristics of Enzymes
To better understand the limits of feed enzyme usage, the fundamental characteristics of
enzymes need to be considered first. Enzymes
are highly effective biological catalysts capable
of accelerating chemical reactions millions of
times over, in some cases [7]. Chemically, they
are proteins with a highly complex three-dimensional molecular structure. The protein nature
of enzymes has important implications for their
stability during high-temperature feed manufacture and transit through the gastrointestinal tract.
As proteins, they can be denatured by heat and
pH and they can also be subject to proteolysis by
digestive enzymes.
A unique feature of enzymes is their high
substrate-specificity. Each enzyme breaks down
highly specific substrates at specific reaction
sites. Thus, to achieve maximal benefits from
enzyme addition, it is necessary to ensure that
the enzymes are chosen on the basis of substrates
in the ingredients used in feed formulations. In
addition, several reaction conditions needs to be
met for the enzyme to act, these include mois-
629
ture content, temperature, pH, enzyme concentration, and substrate concentration [7].
Moisture Content. Enzymes require an aqueous environment to act. Moisture is possibly essential for the mobility of the enzyme, solubility
of the substrate and enzyme, or both.
Temperature. In general, activity increases
up to 40°C and then sharply declines due to the
loss of structure through denaturing, which renders the enzyme inactive.
pH. Most enzymes are denatured at low- and
high-pH environments. In general, the optimum
pH for most enzymes is around 4 to 6.
Enzyme Concentration. In theory, the reaction rate is directly proportional to the concentration of enzyme. The reaction rate increases
with increasing enzyme concentration because
there are more active sites available and this
will continue until no more enzyme-substrate
complex can be formed. In practice, however,
because of other limitations within the digestive
tract of animals, this linear relationship does not
occur. The relationship will be nonlinear.
Substrate Concentration. In the presence
of adequate concentrations of the enzyme, the
rate of reaction increases with increasing substrate concentration until the maximum turnover is reached. This happens because there are
more substrates than the enzyme can handle. It
is noteworthy that enzymes vary considerably
in the reaction conditions needed depending on
their source (fungal vs. bacterial vs. yeast). The
source, therefore, has a major influence on how
closely particular enzymes are adapted to the
prevalent conditions in the digestive tract and
their effectiveness.
Substrates and Enzyme Types
Substrates in feed ingredients can be mainly
classified into 3 groups, namely those (1) for
which birds produce suitable enzymes in their
own digestive tract (e.g., starch, proteins, lipids), (2) for which enzymes are not produced by
the bird and not digested (e.g., cellulose), and
(3) for which enzymes are not produced by the
bird and, in addition to not being digested, have
antinutritive effects (e.g., β-glucans, pentosans,
phytate). Another point of note is that birds are
not fed substrates, but ingredients with substrates in complex matrices. For these reasons,
630
the potential nutritive value of ingredients is not
realized at the bird level and no common feed
ingredient is 100% digested. Even the digestion
of substrates (i.e., starch, proteins, and lipids)
for which birds produce sufficient amounts of
endogenous enzymes is incomplete, with 10 to
20% of these substrates being undigested and
excreted. The need to improve the digestion of
these undigested substrates is the principal rationale behind the use of exogenous enzymes.
In feed ingredients, the substrates (nutrients
and antinutrients) exist as complexes, limiting
the accessibility to enzymes. Despite advances,
the chemistry and structure of most target substrates are still poorly defined. For example, the
chemistry of NSP in different ingredients varies widely. Though basic quantitative information on the type of sugars making up the NSP is
available for cereals, corresponding information
for other ingredients is lacking. The efficacy of
enzymes can be greatly improved if the chemistry of the target substrates is more precisely
understood. Matching an enzyme, however,
does not guarantee the efficacy of the enzyme in
degrading the substrate, and affinity must also
be considered. Choct et al. [8], using 3 fungal
xylanases that are of different substrate affinities, demonstrated that substrate specificity is
dependent on the source of the enzyme.
For an enzyme to be effective, an adequate
enzyme to substrate ratio must be present in the
diet. A complicating factor is that a particular
substrate in one ingredient is not exactly the
same as the one found in another ingredient.
The substrates differ and the same substrate in
different ingredients may respond differently to
the enzyme. Such differences arise from the location of the substrate in the ingredient matrix,
the presence of other limiting factors, and differences in accessibility or solubility. This has
been well illustrated in the case of phytate by
Leske and Coon [9]. Those researchers showed
that phytate from different ingredients are not
similarly susceptible to dephosphorylation and
that the reactive, and not total, phytate content is
critical in determining the responses to supplemental phytase. It was found, for example, that
canola meal contained a relatively high level of
total phytate, but less reactive phytate, and does
not respond well to added phytase.
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THE PRACTICE
Feed enzymes are the most researched feed
additive. Providing an overview of the current
usage of feed enzymes is a daunting task, given the number of feed enzymes (Table 1) and,
within each enzyme, the large number of commercial products available—varying in their
source, enzyme activities, and characteristics.
There are 4 distinct categories of enzyme products currently commercially available for use
by the feed industry: (1) microbial phytases,
(2) glycanases targeting viscous cereals (e.g.,
wheat, barley), (3) enzymes targeting nonviscous cereals (e.g., corn, sorghum), and (4) enzymes targeting noncereals (e.g., soybean meal,
grain legumes). With the exception of microbial
phytases, most other enzyme products contain
a mixture of enzymes that may be produced by
one or more organisms. There is evidence to
suggest that preparations with multiple enzyme
activities may provide a competitive strategy to
improve nutrient utilization in poultry diets [10,
11]. The combined application of enzymes may
result in additive, subadditive, or synergistic effects on nutrient utilization and animal performance [12–14]. Such enzyme cocktails, rather
than pure single enzymes, represent the next
generation of feed enzymes, because feed ingredients are exceedingly structurally complex. In
the native stage, nutrients in raw materials are
not isolated entities, but exist as complexes with
various linkages to protein, fat, fiber, and other
complex carbohydrates. For example, in wheatbased diets, merely targeting the arabinoxylans
with xylanases may not provide the full benefits.
The benefits of simultaneous inclusions of a carbohydrase enzyme with predominantly xylanase
activity and a microbial phytase in wheat-based
broiler diets, in terms of both protein and energy
utilization and growth performance, have been
reported [12, 15–17]. It appears that the activity of one type of feed enzyme may be facilitated by the other, possibly in a reciprocal fashion, by providing greater substrate access, and
also by reducing the antinutritive effects of the
substrates (NSP and phytate) on nutrient utilization. The simultaneous inclusion of phytase with
α-galactosidases, protease, β-glucanase, and xylanase in corn-, barley-, or wheat-based diets
Ravindran: INFORMAL NUTRITION SYMPOSIUM
631
Table 1. Type of commercial feed enzymes and target substrates
Enzyme
Target substrate
Target feedstuff
Phytases
β-Glucanases
Xylanases
α-Galactosidases
Proteases
Amylase
Lipases
Mannanases, cellulases,
hemicellulasespectinases
Phytic acid
β-Glucan
Arabinoxylans
Oligosaccharides
Proteins
Starch
Lipids
Cell wall matrix
(fiber components)
All plant-derived ingredients
Barley, oats, and rye
Wheat, rye, triticale, barley, fibrous plant materials
Soybean meal, grain legumes
All plant protein sources
Cereal grains, grain legumes
Lipids in feed ingredients
Plant-derived ingredients, fibrous plant materials
has been investigated [13, 14] and it was found
that phytase in combination with carbohydrase
and protease has additive effects in nutritionally
marginal broiler diets.
The largest user of these feed enzymes is the
poultry industry. The highly integrated nature
of the poultry sector has enabled the faster uptake of these new technologies, and inclusion of
exogenous enzymes has now become the norm
to improve the digestibility and efficiency of
utilization of nutrients. Almost all wheat- and
barley-based broiler diets worldwide are now
supplemented with glycanases (xylanases and
β-glucanases). During the past decade, the use
of microbial phytase in poultry diets has increased in response to concerns over phosphorus
pollution of effluents from intensive animal operations and the skyrocketing price of inorganic
phosphates. As a result, in recent years, microbial phytase has overtaken the glycanases as the
primary feed enzyme worldwide. Availability
of enzyme cocktails containing amylases, xylanases, lipases, and proteases, targeting corn-soy
diets, is another recent development, and the use
of such cocktails is receiving increasing attention. It is not the intention of this paper to review
the available literature on the influence of these
feed enzymes on the performance and nutrient
utilization in poultry, as exhaustive reviews on
these topics are available [10, 11, 18–21].
The Benefits
The ultimate aim of adding enzymes is to improve bird performance and profitability through
enhanced digestion of dietary components (protein, amino acids, starch, lipids, and energy) in
ingredients. There are, however, many other reasons for the wider acceptance of feed enzymes
and these will become more relevant in future
production systems.
1. Increase in the range of feedstuffs that
can be used and increased flexibility in
feed formulations by reducing or removing the constraint on the inclusion limit
of poorly digested ingredients.
2. Reduced variability in the nutritive value
between batches of ingredients. Enzyme
supplementation uplifts the value of
poor samples and reduces the variation
between good and poor quality samples
of a given ingredient. This effect, in turn,
improves the degree of precision of feed
formulation.
3. Reduced excreta moisture content and
lowered incidence of wet litter in birds
fed diets containing high levels of NSP.
Poor litter quality can lead, inter alia, to
several welfare issues, including foot
lesions, hock burns, and carcass downgrading [22].
4. Because of improved digestion and
fewer amounts of undigested nutrients
reaching the lower gut, as well as a shift
in gut flora toward favorable bacterial
species, gut health is improved [23]. A
related outcome is the protective effect
on the overall health of the bird, due in
part to the influence of flora on immune
function [24].
5. Improved intestinal morphology [25,
26] and integrity resulting in enhanced
digestion and absorption of dietary components.
6. Lowered manure output in terms of the
amount and nutrient load (excreta nitrogen and phosphorus levels), which are
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632
results of better utilization of these nutrients. The environmental effect of these
benefits is relevant to intensive bird operations worldwide.
7. Better uniformity of animals at market
time by uplifting the growth of poorly
performing animals.
In summary, the benefits of exogenous enzymes go far beyond just improving nutrient
digestion and have implications in the ongoing
changes in global poultry production in terms of
environment, gut health, bird welfare, and sustainability.
Mode(s) of Action
It must be recognized that different feed enzymes (Table 1) will have different modes of action. Despite their increasing acceptance as feed
additives, the exact mode(s) of action of feed
enzymes remains to be elucidated. The general
consensus is that one or more of the following
mechanisms are responsible for the observed
benefits [18, 20].
1. Degradation of specific bonds in ingredients that are not usually hydrolyzed by
endogenous digestive enzymes.
2. Degradation of antinutritional factors
that limit nutrient digestion directly, increase intestinal digesta viscosity indirectly, or both.
3. Disruption of endosperm integrity and
the release of nutrients that are bound to
or entrapped by the cell wall.
4. Shift of digestion to more efficient digestion sites.
5. Reductions in endogenous secretions
and protein losses from the gut resulting
in reduced maintenance requirements
[27, 28].
6. Reduction in the weight of the intestinal
tract and changes in the intestinal morphology [26, 29].
7. Changes in the microflora profile in the
small intestine. As enzymes influence
the amounts and form of substrate present within the gut, their use has a direct
effect on the bacteria that make up the
microfloral populations [23, 30, 31].
8. Augmentation of endogenous digestive
enzymes, which are either insufficient or
absent in the bird, resulting in improved
digestion. This will be especially true for
newly hatched chicks with immature digestive systems.
Variability in Responses
A dilemma faced by the users of exogenous
enzymes is that bird responses to feed enzyme
addition are variable and not entirely predictable. The factors contributing to these inconsistencies are complex, involving enzyme (enzyme
source, enzyme dose, side activities present),
diet (ingredient quality, diet composition, diet
form, particle size), and bird (species, age, sex,
individual variation) factors and their interaction. Selected dietary and animal factors of importance are briefly highlighted below.
Dietary Nutrient Density. In most cases, the
use of exogenous enzymes can be beneficial
only if the dietary nutrient density is marginal.
Diet formulation has to be adjusted and conditions have to be created to ensure maximum
responses to added enzymes. Clearly, responses
will be minimal if there is an oversupply of nutrients (energy, amino acids, phosphorus) in the
diet. A simple example is dietary phosphorus
level and microbial phytases; if the diets contain
excess amounts of phosphorus, then the chance
of any animal response to phytase addition will
be low. The use of microbial phytase is potentially useful and appropriate only for diets with
suboptimal phosphorus levels and containing
significant levels of plant-derived ingredients.
Age of Birds. In theory, the potential benefits from feed enzyme additions will be greater when the digestive system is simpler, as in
young birds. Young birds, especially during the
first few weeks of life, have an underdeveloped
digestive enzyme capacity compared with adult
birds [32]. Thus, young birds may benefit from
a wide spectrum of enzymes, such as lipase, proteases, and amylases, in addition to those feed
enzymes that are added to diets based on certain ingredients. Researchers have shown that
the benefits of enzymes generally diminish with
age and responses are usually greater during the
broiler starter phase compared with the finisher
phase. This belief, however, is not always true;
Ravindran: INFORMAL NUTRITION SYMPOSIUM
sometimes responses may be greater during the
finisher phase. The effect of bird age on recommendations of enzyme doses is a related issue,
as it has not been fully elucidated [10].
Quality of the Target Ingredient. The term
ingredient quality is not easy to define. In general, the quality of an ingredient is related to the
concentration of antinutrient (e.g., phytate contents, soluble NSP contents in wheat) or the content of available nutrients or energy, all of which
can vary widely among batches of an ingredient.
Based on the available data, it is possible that
enzyme responses are dependent on the quality of the ingredient. The lower the ingredient
quality, the greater will be the magnitude of improvements with added enzyme. For example,
apparently the responses of wheat AME to supplemental xylanase are determined, to a great
extent, by the initial AME of the wheat sample.
Large variability, ranging from 2,200 to 3,820
kcal/kg of DM, in the AME of wheat samples
has been reported in the literature [33–38]. Low
quality wheat samples, assaying less than 2,870
kcal/kg of AME, have been shown to respond
remarkably to enzyme supplementation, whereas good quality wheat samples show practically
no increment in AME (Table 2).
Inclusion Level of the Target Feedstuff. The
higher the inclusion level of the target ingredient, the greater the enzyme response will be, as it
will proportionately increase the contents of the
substrate for enzyme action or the antinutrient(s)
that is causing the problem (e.g., xylanase and
wheat inclusion level) in diet formulations.
Performance Level of Control Birds. The
degree of response is also governed by the existing level of performance in animals fed the
unsupplemented diet. If the performance is poor,
whether due to poor husbandry, marginal nutrition, or stress, then responses will be greater
Table 2. Influence of initial AME of wheat on responses
to xylanase addition [39]
Initial AME,
kcal/kg
<2,870
2,870 to 3,350
>3,350
1
No. of assays
Improvement over
the unsupplemented
control,1 %
12
18
15
11 (5–22)
5 (0–9)
1 (0–3)
Values in parentheses refer to range of responses recorded
in different assays.
633
with enzyme addition. In this context, it is noteworthy that the enzymes act in a manner similar
to that of in-feed antibiotics. This is more than
a coincidence, and is likely to be related to the
effect of enzymes on gut microflora and gut
health.
LIMITS TO MAXIMIZING
ENZYME RESPONSES
Although feed enzymes have a significant effect in the poultry industry, their full potential is
yet to be fully harnessed. The biological reality
is that there are physiological limits, imposed by
the conditions in the digestive tract, to enzyme
responses. Even in highly digestible corn-soy
diets, only 85 to 90% of starch, protein, and lipids are digested. In diets containing poorly digested ingredients, the digestibility could be as
low as 75%, with greater potential to elicit better
responses. It must be noted, however, that it is
unrealistic to expect that enzymes can improve
the digestibility close to 100%, as achieving
such a target will be restricted by substrate, enzyme characteristics, and physiological limitations in the bird. Based on available literature, it
is reasonable to expect the digestion of possibly
25 to 35% of the undigested fraction by supplemental enzymes.
The effectiveness of a feed enzyme in the digestive tract of a bird depends on several prerequisites relating to enzymes (source, specific catalytic activity, resistance to pepsin’s proteolytic
action), substrate characteristics (concentration
and accessibility), and digestive tract conditions (moisture content, pH, temperature and
the time digesta spends in the tract—especially
in the early gastric phase where most of the enzyme action occurs). An aqueous environment is
needed for the enzyme to initiate their activity.
When the feed with exogenous enzyme is ingested, the condition for moisture is met quickly
and the feed becomes increasingly moistened as
it moves down the digestive tract [40]. Most enzymes are active between 40 and 60°C. Within
the bird body, the temperature requirement is
met and is not a limiting factor. But a range of
pH is encountered along the digestive tract and
pH becomes the first physiological limitation for
the activity and stability of enzymes. In addition, the feed spends only a relatively short time
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in the digestive tract of poultry (i.e., between
ingestion and the time it reaches lower ileum)
and this becomes the second physiological limitation. This complex matrix of conditions will
finally determine the magnitude and variation of
activity of an enzyme added to the diet and, thus,
its beneficial effects.
The pH of feed is typically close to neutral,
the crop of the chicken is mildly acidic, the proventriculus and gizzard are acidic, and the intestine is mildly acidic at the proximal end, becoming mildly alkaline to neutral toward the distal
part. Most exogenous enzymes have an optimum pH of between 4 and 6 [40], but considerable variation exists between different sources
of enzymes, which may result in catalytic activity of some sources at both lower and higher
pH. It is most likely that exogenous enzymes are
active and degrade their substrates in the forestomach (crop, proventriculus, gizzard) before
they are subjected to hydrolysis by endogenous,
proteolytic enzymes [11]. However, if the exogenous enzyme is resistant to the proteolytic action of pepsin, then it can remain active in the
small intestine and be more effective. For example, Escherichia coli phytases have been shown
to be refractory to pepsin and pancreatin and
to have high proteolytic stability [41], which
makes them favorable candidates to increase the
release of phytate-bound phosphorus.
The passage time of feed in the digestive tract
of poultry is relatively short (Table 3). The average retention time in the digestive tract, excluding the ceca, is probably around 3 to 4 h
[40]. Of this, digesta possibly spends only 60
to 90 min in the anterior digestive tract (crop,
proventriculus, and gizzard), which gives only
limited opportunity for enzyme action. The current practices of ad libitum feeding and use of
finely ground pelleted feeds do not support the
normal functions of crop and gizzard. Under
discontinuous feeding systems, the role of the
crop is as a storage organ, whereas in continuous
feeding systems this function appears to be lost.
Similarly, feeding finely ground feed does not
promote the development of gizzard. A less developed gizzard serves as a transit organ rather
than a grinding organ, with implications for reduced retention time. Manipulation of residence
time in the anterior digestive tract is a potential
strategy to overcome the physiological limita-
Table 3. Average transit time and pH in different
segments of the digestive tract of broiler chickens
pH
Transit time,
min
5.5
2.5 to 3.5
5 to 6
6.5 to 7.0
7.0 to 7.5
8.0
10 to 50
30 to 90
5 to 10
20 to 30
50 to 70
20 to 30
Segment
Crop
Proventriculus/gizzard
Duodenum
Jejunum
Ileum
Cecum/colon
tion of retention time and to further enhance the
efficacy of exogenous enzymes.
There is evidence that meal feeding, instead
of ad libitum access to feed, may markedly increase the retention time in the crop, together
with a rapid moisturization and a reduction in
pH to between 4 and 5. In a study by Svihus et
al. [42], broilers were trained to meal feeding on
a wheat-based diet containing phytase. It was
found that phytate was gradually dephosphorylated, with a 50% reduction in inositol-6-phosphate after 100 min of retention time. The use of
coarse particles or whole grains has been shown
to stimulate growth of the gizzard, resulting in
better grinding function, increased reverse peristalsis of digesta, and larger gizzard volume,
thus increasing the retention time [43, 44].
Some evidence indicates that gizzard development, possibly associated with increased retention time, may improve efficacy of exogenous
enzymes (Table 4). A comprehensive overview
of these 2 strategies as means of enhancing enzyme efficacy is available [40].
A recent strategy, which is becoming popular,
is the use of higher than recommended doses of
microbial phytase. The aim is to dephosphoryTable 4. Feed efficiency and relative gizzard weight
(g/kg of BW) of broilers as influenced by whole wheat
inclusion and xylanase supplementation [45]
Treatment1
Ground wheat diet
Ground wheat diet + xylanase
Whole wheat diet
Whole wheat diet + xylanase
Pooled SEM
1
Feed per
gain, g/g
Gizzard
weight
1.686
1.661
1.648
1.590
0.0149
9.9
10.0
13.5
15.2
0.60
10 and 20% whole wheat replaced ground wheat during d 1
to 21 and 22 to 35, respectively.
Ravindran: INFORMAL NUTRITION SYMPOSIUM
late the phytic acid as quickly as possible to
less-reactive inositol phosphate esters during the
early gastric phase of digestion and to reduce its
antinutritional effects. This strategy was tested
by Shirley and Edwards [46] a decade previous,
but has become a practical option only now because of decreasing enzyme cost and increasing
raw material costs. Those researchers investigated the responses of corn-soy broiler diets to
graded phytase inclusion levels to a maximum
of 12,000 FTU/kg and reported that increasing
phytase inclusions were associated with substantial increases in total tract phytate degradation ranging from 40.3 to 94.8%. Moreover,
phytate degradation was correlated with marked
improvements in bird performance, nutrient retention, tibia ash, and AME, and these increases
were most pronounced at the highest phytase inclusion rate. As shown by Cowieson et al. [47],
the beneficial effects of unconventionally high
doses of microbial phytase in broiler diets are
consistent and could be substantial. This approach, however, has not been tested for other
feed enzyme groups.
CONCLUSIONS AND APPLICATIONS
1. The potential benefits of exogenous feed
enzymes in improving nutrient digestion
and bird performance are well recognized. There is, however, a physiological
limit to the extent that feed enzymes can
improve digestion and these barriers relate to pH and retention time within the
digestive tract.
2. Even in highly digestible corn-soy diets,
ileal digestibility of starch, protein, and
lipids ranges between 85 and 90%, and
it is unrealistic to expect that exogenous
enzymes can improve the digestibility
of these nutrients to 100%. Based on the
literature, possibly only a third of these
undigested nutrients may be digested by
exogenous enzymes.
3. Strategies to increase the digestion of
undigested substrates need to be explored; potential approaches include
feeding practices to restore the functionality of crop and gizzard as well as the
use of higher than recommended doses
of enzymes.
635
4. Feed enzyme technology is an active
area of research and development, and
one can be certain that enzymes better
adapted to the conditions, which exist
in the digestive tract of animals, may be
developed in the future. These developments will further improve the effectiveness of enzyme addition under practical
situations.
REFERENCES AND NOTES
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