Directory UMM :Data Elmu:jurnal:A:Animal Feed Science and Technology:Vol86.Issue1-2.Jul2000:
Animal Feed Science and Technology
86 (2000) 1±13
Review article
Exogenous enzymes in monogastric nutrition Ð their
current value and future bene®ts$
Michael R. Bedford*
Finnfeeds, Marlborough, Wiltshire SN8 1XN, UK
Accepted 18 May 2000
Abstract
Exogenous enzymes which, for the purpose of this paper, include carbohydrases and phytase, are
now extensively used throughout the world as additives in non-ruminant diets. The chemical effects
of these enzymes are well understood, but the manner in which their bene®ts to the animal are
brought about is still under debate. Regardless, the overall effect of carbohydrase enzyme use is to
reduce the variation between good and bad samples of a target ingredient substantially. The net
bene®t is that the nutrient requirements of the animal are met more frequently, and with diets of
lower nutrient concentration. Variation in animal performance from ¯ock to ¯ock is also reduced.
Phytase, on the other hand, was originally used for one express purpose Ð to increase the
availability of plant phytate phosphorus, which reduces phosphorus pollution and allows reductions
in the amount of inorganic phosphate used. Further bene®ts of phytase utilisation on energy and
amino acid availability have recently been identi®ed which will, with appropriate dietary
modi®cations, allow for further improvements in resource utilisation. Current issues of concern for
all enzymes include variability in response. Substrate variability and interactive factors signi®cantly
in¯uence the response to exogenous enzymes. Currently, there are methods which take such factors
into account and allow for prediction of optimum dose of carbohydrase enzymes in wheat and
barley based diets and efforts are underway for maize based diets or for optimisation of the use of
phytase. Future research in these areas will allow for more ef®cient use of the current enzymes and
development of more ef®cient future products. Development of more thermotolerant enzymes will
also allow their use in diets where they currently cannot be applied. # 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Xylanase; b-glucanase; Carbohydrase; Amylase; Phytase; Enzymes; Monogastric
$
This paper is based on the material presented at the BSAS Conference in Scarborough, UK, 22±24 March
1999.
*
Tel.: 44-1672-517-777; fax: 44-1672-517-778.
E-mail address: mike.bedford@danisco.com (M.R. Bedford)
0377-8401/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 0 0 ) 0 0 1 5 5 - 3
2
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
1. Introduction
The use of enzymes in poultry diets in Europe is now almost universal. The reasons
why they are used are manifold and include:
To increase the feeding value of raw materials. Many publications have demonstrated
performance benefits of enzymes when added to barley (Classen et al., 1985; Elwinger
and Saterby, 1987; Broz and Frigg, 1990; Brenes et al., 1993; Marquardt et al., 1994),
wheat (Classen et al., 1995; Bedford and Morgan, 1996; Hughes and Zviedrans, 1999),
and more recently maize based diets (Wyatt et al., 1997a, 1999; Steenfeldt et al., 1998).
Phytases are routinely utilised particularly in environmentally sensitive areas of the
world due to their ability to increase the phosphorous availability from vegetable
ingredients (Simons et al., 1990; Jongbloed et al., 1997; Kornegay et al., 1997; Kemme
et al., 1999). The consequences of such observations are two-fold, the targeted
ingredient is used in greater abundance than would otherwise be the case, and
secondly, the costs of diet manufacture are reduced due to decreased utilisation of
scarce, high value ingredients such as fat and fishmeal.
To reduce the variation in nutrient quality of ingredients. The response to the use of
enzymes is greatest on the poorest quality raw materials (Classen et al., 1995; Scott
et al., 1995, 1998a; Bedford et al., 1998). As a result, variation in subsequent bird
performance is reduced which results in a more uniform flock but also more uniform
production from flock to flock. Such a benefit is considerable, given the losses incurred
by producers when growing to set weights and by feed compounders when attempting
to target a given diet nutrient density.
To reduce the incidence of wet litter. Feeding diets rich in barley, rye, oats, triticale and
to a lesser extent wheat, often results in the production of a viscous, wet manure
(Classen et al., 1985; Elwinger and Teglof, 1991; Newman et al., 1992; Carre et al.,
1994; Bedford and Morgan, 1996).
Evidently these bene®ts are realised by the poultry industry and the consumer. This
paper will separate the enzymes currently being utilised into three distinctive categories:
1. Viscous grain targeted, i.e. rye, wheat, oats, triticale and barley.
2. Non-viscous grain targeted, i.e. corn and sorghum.
3. Phytase.
Categories 1 and 2 are generally carbohydrase based products and will be dealt with
separately from phytase.
2. Carbohydrases
1. Viscous grain enzymes. Of the non-starch polysaccharide (NSP) enzymes, these have
received the most attention. It is not the remit of this paper to discuss mechanistic
theories in detail, the reader is referred to more detailed reviews for such information
(Campbell and Bedford, 1992; Jeroch et al., 1995; Simon, 1998). In short, viscous
grains induce a condition of increased intestinal viscosity, which effectively slows
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
3
down the rate of digestion. The physical structure of the endosperm cell walls of these
grains may also impede access to their contents by digestive enzymes. Addition of the
appropriate enzyme diminishes these constraints and allows digestion to occur more
rapidly and completely.
2. Non-viscous grain enzymes. Maize variability has recently been demonstrated to be as
great as that observed for wheat and barley (Leeson et al., 1993; Collins et al., 1998).
Whilst enzymes can reduce this variation and accelerate the rate of digestion of maize
and sorghum based diets (Wyatt et al., 1997a,b, 1999; Pack et al., 1998a), the exact
mechanism of action is yet to be con®rmed, although several are offered.
Taking both classes of cereals as one, regardless of mechanism of action, the result of
enzyme use is an increase in the rate of nutrient digestibility. This is important since it
moves the site of digestion and absorption of starch and protein to a more anterior site
wherein the bird has a greater competitive edge over its resident micro¯ora. This is more
the case as the bird ages and its intestinal tract matures and becomes more heavily
populated, and is most signi®cant when antibiotics are not utilised. Fig. 1 illustrates the
case in discussion.
As feed passes through the proventriculus/gizzard, it is largely sterilised by the
extremes of pH and activity of pepsin. In addition, as it enters the duodenum it is exposed
to a rapid and signi®cant pH shift towards neutral which further stresses any bacterial
survivors of gastric transit. Large in¯uxes of digestive enzymes, bile acids, lecithin and
lysozyme further test the surviving bacteria such that the duodenum is largely devoid of
bacteria. In the upper regions of the gut, digestive ef®ciency is maximal due to the high
concentrations of pancreatic enzymes and ef®cient and highly active absorptive
enterocytes (Uni et al., 1999). As feed passes though the small intestine, there is a
progressive decline in digestive enzyme and bile acid concentration as these are either
Fig. 1. Relationship between the rate of digestion of a diet and microbial population density. A rapidly
digestible ration supports fewer microbes.
4
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
catabolised and/or absorbed (Campbell et al., 1983; Schneeman and Gallaher, 1985; Noy
and Sklan, 1994; Raul and Schleiffer, 1996; Taranto et al., 1997). As a result, the
environment of the small intestine becomes increasingly hospitable to bacterial
colonisation.
If the diet being fed is highly digestible then the majority of nutrients are digested and
absorbed prior to the establishment of an environment favourable to bacterial growth. As
a result, the populations of the lower small intestine are kept to a minimum essentially
through substrate limitation. With a poorly digested diet, however, nutrients evade
digestion and absorption by the bird and as a result enter the mid-lower small intestine
where the bacterial populations are able to make good use of such substrate, and ¯ourish
as has been shown when comparing rye (poorly digested) with corn (well digested) based
diets (Wagner and Thomas, 1987). In stimulation of bacterial growth, there are inevitably
species which are able to colonise the anterior reaches of the intestine by production of
enzymes which actively degrade the very antimicrobials the bird produces, such as bile
acids (Christl et al., 1997; Taranto et al., 1997). Through deconjugation and
dehydroxylation, these compounds lose their antibacterial effect and as a result the
sensitive bacteria are able to thrive. Elimination of these active compounds also results in
impaired fat digestion since bile acids are essential for ef®cient micelle formation
(Campbell et al., 1983).
Evidently, the consequences of bacterial overgrowth are manifold, not least since the
presence of a greater population will demand a greater energy and protein requirement
from the diet which is ultimately taken at the expense of the host.
The consequences of reduced diet digestibility, therefore, need to be assessed from two
viewpoints if the bene®ts of cereal targeted enzymes are to be correctly assessed. The ®rst
is the direct effects of a poorly digested diet on the nutrient assimilation rate of the host
and the second is the rami®cations that such an increase in substrate delivery will have on
micro¯oral populations inhabiting both the small intestine and the caeca. The former will
of course limit the growth rate of the animal and the latter may result in a less ef®cient
utilisation of digested and/or utilised nutrients through competition for substrates and
interactions with the health status of the animal.
Enzymes have clearly been demonstrated to increase the digestibility of poorly
digested cereals to a much greater extent than well digested cereals (Classen et al., 1995;
Scott et al., 1998a,b). There are two consequences of such an effect of enzyme addition as
far as the feed compounder is concerned:
1. Variation between the best and worst samples of a given grain is reduced.
2. In practice, the average nutrient content of the cereal is greater in the presence of
enzyme than in the absence. As a result, addition of an enzyme allows feed
formulation nutrient matrix values to be elevated.
Fig. 2 demonstrates these bene®ts clearly. The response to enzyme addition is thus
mediated through improvements in nutrient extraction in the small intestine by the host
through accelerated digestion, and reduced microbial activity as a result of substrate
limitation in the ileum (Fig. 3). The two consequences are evidently bene®cial to bird
performance but recently it has emerged that there is likely a third mechanism which
needs consideration, namely active feeding of speci®c bacterial species. This paper will
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
5
Fig. 2. In¯uence of variety of barley on AME and subsequent response to enzyme addition (Scott et al., 1998b).
not go into details here but refers the reader to the following references (Apajalahti et al.,
1995; Apajalahti and Bedford, 1999). Essentially, the activity of the enzyme on viscous
polymers and cell wall carbohydrates produces sugars and oligomers, which are utilised
preferentially by certain ileal and caecal bacterial species. These ¯ourish at the expense
of other, possibly detrimental species as far as optimal growth or health of the animal is
concerned (Apajalahti and Bedford, 1999).
It is useful at this point to note that the consequences of a poorly digested diet, and
hence the bene®ts of enzyme addition to such a diet, is more apparent in conventional
compared with germ free chicks (Langhout, 1998; Schutte and Langhout, 1999). Since
the micro¯ora increase in numbers as the animal progresses from essentially germ free to
fully populated some 3 weeks later, it is likely that the bene®t of added enzymes is
mediated through the micro¯oral route in older birds, whereas in young animals which
have a poorly developed digestive system (Kirjavainen and Gibson, 1999), the effect is
probably more direct. It is suggested that until the bird reaches 8 days of age, the output
of pancreatic enzymes may well limit digestion (Lindemann et al., 1986; Krogdahl and
Sell, 1989; Nitsan et al., 1991; Dunnington and Siegel, 1995). The addition of a relevant
Fig. 3. In¯uence of enzyme addition on total ileal microbial counts (Apajalahti and Bedford, 1999). LAB: lactic
acid bacteria.
6
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
exogenous enzyme is, therefore, likely to supplement the digestive capacity of the
younger bird, and again the response to such will be more apparent with poorly digested
compared with highly digestible diets. It is likely that, as birds age and their digestive
ability increases, as does their micro¯oral population, the effect of exogenous enzymes is
more and more mediated through the micro¯oral route.
The chemical identity of the viscous carbohydrates, the cell wall structures and the
indigestible starch/protein complexes which are the focus of the enzymes discussed to
this point is heterogenous in the extreme. The enzymes employed are, thus, wide ranging
in their target substrate preference in the sense that the molecular weight of the substrate
may vary several orders of magnitude as may the complexity and sugar identity of side
chain adjuncts on the targeted backbone. As a result, bene®ts can be achieved by use of
any one of a wide variety of enzymes, from divergent source organisms, which often have
different mechanisms of action. As discussed later, this provides an avenue for
improvements in ef®cacy as more is understood of the chemical nature of the most
important antinutrients.
3. Phytase
There are many reviews recently published describing the use and effects of phytase in
animal feeds. Phytate (Fig. 4), the target substrate of this enzyme, unlike that of the
carbohydrases discussed in the previous section, is a ®xed chemical entity. It is presumed
to be the plant storage form of phosphate which also happens to have considerable
antinutritive effects for most animals. Phytate itself is not a good source of phosphorus for
non-ruminants, particularly younger animals as they appear to lack a meaningful ability
to utilise this compound, even though phytases have been isolated from the small
intestines of broilers and laying hens (Maenz et al., 1997; Maenz and Classen, 1998).
Phosphorus de®ciency would, therefore, often result if inorganic sources were not
routinely added to most non-ruminant diets. As a result of uncertainty with regards to the
availability of plant phosphorus sources, formulations routinely rely on added phosphates
to supply most of the animals needs. Consequently, most non-ruminant diets contain far
more phosphorus than the animal actually needs. The excess to requirement is simply
excreted. Pressures to reduce phosphorus pollution levels in several parts of Europe and
Fig. 4. Phytic acid.
7
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
Table 1
Phytate phosphorus content of various feed ingredients according to several sources
Ingredient
Matthaus
(1997)
McKnight
(1997)
Maize
Wheat
Barley
Soybeans
Rapeseed
Soyabean meal
Rapeseed meal
0.265
0.235
0.223
0.196
0.255
0.230
Lantzsch
et al. (1992)
NRC
(1994)
MeanS.D.
0.20
0.24
0.19
0.2200.038
0.2430.010
0.2140.021
1.2000.030
1.3500.101
0.3060.020
0.87
1.200.03
1.350.10
0.372
0.870
0.40
0.87
more recently the USA has created a market opportunity for the introduction of
exogenous phytases. The application of this enzyme allows the animal to access much of
the plant phytate phosphorus, and thus, reduce reliance on inorganic phosphate sources.
Phosphorus pollution as a result has been signi®cantly reduced.
Phytase activity is not entirely predictable, however. The bene®t achieved is known to
depend upon several factors, including the raw materials used, the source of the phytase,
the age of the animals, dietary content of calcium, phosphorus and Vitamin D, and the
level of phytase activity present in the ingredients used. Each will be discussed in more
detail below.
Phytate contents of many ingredients varies considerably as shown in Table 1 and as
reported recently; the availability of phytate to exogenous phytase hydrolysis varies from
ingredient to ingredient (Ravindran et al., 1999). The consequence of such an observation
is that for practical purposes, a signi®cant safety margin needs to be employed in
estimation of the phosphorus contribution as a result of phytate hydrolysis. Indeed, the
consequences of overestimating the bene®t of the enzyme are dramatic. Initial estimates
of the P release as a result of use of phytase proved over-optimistic when translated into
commercial terms with the result that savings are less than ®rst envisaged (van Tuijl,
1998). Nevertheless, the bene®ts in reducing pollution and feed formulation costs are
clear and as a result phytase is now a ubiquitous feed additive.
Phytase delivers economic bene®ts through its ability to replace added inorganic
phosphorus. One issue that needs consideration is that the amount of phytase required to
replace 10 g added phosphate ranges from
86 (2000) 1±13
Review article
Exogenous enzymes in monogastric nutrition Ð their
current value and future bene®ts$
Michael R. Bedford*
Finnfeeds, Marlborough, Wiltshire SN8 1XN, UK
Accepted 18 May 2000
Abstract
Exogenous enzymes which, for the purpose of this paper, include carbohydrases and phytase, are
now extensively used throughout the world as additives in non-ruminant diets. The chemical effects
of these enzymes are well understood, but the manner in which their bene®ts to the animal are
brought about is still under debate. Regardless, the overall effect of carbohydrase enzyme use is to
reduce the variation between good and bad samples of a target ingredient substantially. The net
bene®t is that the nutrient requirements of the animal are met more frequently, and with diets of
lower nutrient concentration. Variation in animal performance from ¯ock to ¯ock is also reduced.
Phytase, on the other hand, was originally used for one express purpose Ð to increase the
availability of plant phytate phosphorus, which reduces phosphorus pollution and allows reductions
in the amount of inorganic phosphate used. Further bene®ts of phytase utilisation on energy and
amino acid availability have recently been identi®ed which will, with appropriate dietary
modi®cations, allow for further improvements in resource utilisation. Current issues of concern for
all enzymes include variability in response. Substrate variability and interactive factors signi®cantly
in¯uence the response to exogenous enzymes. Currently, there are methods which take such factors
into account and allow for prediction of optimum dose of carbohydrase enzymes in wheat and
barley based diets and efforts are underway for maize based diets or for optimisation of the use of
phytase. Future research in these areas will allow for more ef®cient use of the current enzymes and
development of more ef®cient future products. Development of more thermotolerant enzymes will
also allow their use in diets where they currently cannot be applied. # 2000 Elsevier Science B.V.
All rights reserved.
Keywords: Xylanase; b-glucanase; Carbohydrase; Amylase; Phytase; Enzymes; Monogastric
$
This paper is based on the material presented at the BSAS Conference in Scarborough, UK, 22±24 March
1999.
*
Tel.: 44-1672-517-777; fax: 44-1672-517-778.
E-mail address: mike.bedford@danisco.com (M.R. Bedford)
0377-8401/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 0 0 ) 0 0 1 5 5 - 3
2
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
1. Introduction
The use of enzymes in poultry diets in Europe is now almost universal. The reasons
why they are used are manifold and include:
To increase the feeding value of raw materials. Many publications have demonstrated
performance benefits of enzymes when added to barley (Classen et al., 1985; Elwinger
and Saterby, 1987; Broz and Frigg, 1990; Brenes et al., 1993; Marquardt et al., 1994),
wheat (Classen et al., 1995; Bedford and Morgan, 1996; Hughes and Zviedrans, 1999),
and more recently maize based diets (Wyatt et al., 1997a, 1999; Steenfeldt et al., 1998).
Phytases are routinely utilised particularly in environmentally sensitive areas of the
world due to their ability to increase the phosphorous availability from vegetable
ingredients (Simons et al., 1990; Jongbloed et al., 1997; Kornegay et al., 1997; Kemme
et al., 1999). The consequences of such observations are two-fold, the targeted
ingredient is used in greater abundance than would otherwise be the case, and
secondly, the costs of diet manufacture are reduced due to decreased utilisation of
scarce, high value ingredients such as fat and fishmeal.
To reduce the variation in nutrient quality of ingredients. The response to the use of
enzymes is greatest on the poorest quality raw materials (Classen et al., 1995; Scott
et al., 1995, 1998a; Bedford et al., 1998). As a result, variation in subsequent bird
performance is reduced which results in a more uniform flock but also more uniform
production from flock to flock. Such a benefit is considerable, given the losses incurred
by producers when growing to set weights and by feed compounders when attempting
to target a given diet nutrient density.
To reduce the incidence of wet litter. Feeding diets rich in barley, rye, oats, triticale and
to a lesser extent wheat, often results in the production of a viscous, wet manure
(Classen et al., 1985; Elwinger and Teglof, 1991; Newman et al., 1992; Carre et al.,
1994; Bedford and Morgan, 1996).
Evidently these bene®ts are realised by the poultry industry and the consumer. This
paper will separate the enzymes currently being utilised into three distinctive categories:
1. Viscous grain targeted, i.e. rye, wheat, oats, triticale and barley.
2. Non-viscous grain targeted, i.e. corn and sorghum.
3. Phytase.
Categories 1 and 2 are generally carbohydrase based products and will be dealt with
separately from phytase.
2. Carbohydrases
1. Viscous grain enzymes. Of the non-starch polysaccharide (NSP) enzymes, these have
received the most attention. It is not the remit of this paper to discuss mechanistic
theories in detail, the reader is referred to more detailed reviews for such information
(Campbell and Bedford, 1992; Jeroch et al., 1995; Simon, 1998). In short, viscous
grains induce a condition of increased intestinal viscosity, which effectively slows
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
3
down the rate of digestion. The physical structure of the endosperm cell walls of these
grains may also impede access to their contents by digestive enzymes. Addition of the
appropriate enzyme diminishes these constraints and allows digestion to occur more
rapidly and completely.
2. Non-viscous grain enzymes. Maize variability has recently been demonstrated to be as
great as that observed for wheat and barley (Leeson et al., 1993; Collins et al., 1998).
Whilst enzymes can reduce this variation and accelerate the rate of digestion of maize
and sorghum based diets (Wyatt et al., 1997a,b, 1999; Pack et al., 1998a), the exact
mechanism of action is yet to be con®rmed, although several are offered.
Taking both classes of cereals as one, regardless of mechanism of action, the result of
enzyme use is an increase in the rate of nutrient digestibility. This is important since it
moves the site of digestion and absorption of starch and protein to a more anterior site
wherein the bird has a greater competitive edge over its resident micro¯ora. This is more
the case as the bird ages and its intestinal tract matures and becomes more heavily
populated, and is most signi®cant when antibiotics are not utilised. Fig. 1 illustrates the
case in discussion.
As feed passes through the proventriculus/gizzard, it is largely sterilised by the
extremes of pH and activity of pepsin. In addition, as it enters the duodenum it is exposed
to a rapid and signi®cant pH shift towards neutral which further stresses any bacterial
survivors of gastric transit. Large in¯uxes of digestive enzymes, bile acids, lecithin and
lysozyme further test the surviving bacteria such that the duodenum is largely devoid of
bacteria. In the upper regions of the gut, digestive ef®ciency is maximal due to the high
concentrations of pancreatic enzymes and ef®cient and highly active absorptive
enterocytes (Uni et al., 1999). As feed passes though the small intestine, there is a
progressive decline in digestive enzyme and bile acid concentration as these are either
Fig. 1. Relationship between the rate of digestion of a diet and microbial population density. A rapidly
digestible ration supports fewer microbes.
4
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
catabolised and/or absorbed (Campbell et al., 1983; Schneeman and Gallaher, 1985; Noy
and Sklan, 1994; Raul and Schleiffer, 1996; Taranto et al., 1997). As a result, the
environment of the small intestine becomes increasingly hospitable to bacterial
colonisation.
If the diet being fed is highly digestible then the majority of nutrients are digested and
absorbed prior to the establishment of an environment favourable to bacterial growth. As
a result, the populations of the lower small intestine are kept to a minimum essentially
through substrate limitation. With a poorly digested diet, however, nutrients evade
digestion and absorption by the bird and as a result enter the mid-lower small intestine
where the bacterial populations are able to make good use of such substrate, and ¯ourish
as has been shown when comparing rye (poorly digested) with corn (well digested) based
diets (Wagner and Thomas, 1987). In stimulation of bacterial growth, there are inevitably
species which are able to colonise the anterior reaches of the intestine by production of
enzymes which actively degrade the very antimicrobials the bird produces, such as bile
acids (Christl et al., 1997; Taranto et al., 1997). Through deconjugation and
dehydroxylation, these compounds lose their antibacterial effect and as a result the
sensitive bacteria are able to thrive. Elimination of these active compounds also results in
impaired fat digestion since bile acids are essential for ef®cient micelle formation
(Campbell et al., 1983).
Evidently, the consequences of bacterial overgrowth are manifold, not least since the
presence of a greater population will demand a greater energy and protein requirement
from the diet which is ultimately taken at the expense of the host.
The consequences of reduced diet digestibility, therefore, need to be assessed from two
viewpoints if the bene®ts of cereal targeted enzymes are to be correctly assessed. The ®rst
is the direct effects of a poorly digested diet on the nutrient assimilation rate of the host
and the second is the rami®cations that such an increase in substrate delivery will have on
micro¯oral populations inhabiting both the small intestine and the caeca. The former will
of course limit the growth rate of the animal and the latter may result in a less ef®cient
utilisation of digested and/or utilised nutrients through competition for substrates and
interactions with the health status of the animal.
Enzymes have clearly been demonstrated to increase the digestibility of poorly
digested cereals to a much greater extent than well digested cereals (Classen et al., 1995;
Scott et al., 1998a,b). There are two consequences of such an effect of enzyme addition as
far as the feed compounder is concerned:
1. Variation between the best and worst samples of a given grain is reduced.
2. In practice, the average nutrient content of the cereal is greater in the presence of
enzyme than in the absence. As a result, addition of an enzyme allows feed
formulation nutrient matrix values to be elevated.
Fig. 2 demonstrates these bene®ts clearly. The response to enzyme addition is thus
mediated through improvements in nutrient extraction in the small intestine by the host
through accelerated digestion, and reduced microbial activity as a result of substrate
limitation in the ileum (Fig. 3). The two consequences are evidently bene®cial to bird
performance but recently it has emerged that there is likely a third mechanism which
needs consideration, namely active feeding of speci®c bacterial species. This paper will
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
5
Fig. 2. In¯uence of variety of barley on AME and subsequent response to enzyme addition (Scott et al., 1998b).
not go into details here but refers the reader to the following references (Apajalahti et al.,
1995; Apajalahti and Bedford, 1999). Essentially, the activity of the enzyme on viscous
polymers and cell wall carbohydrates produces sugars and oligomers, which are utilised
preferentially by certain ileal and caecal bacterial species. These ¯ourish at the expense
of other, possibly detrimental species as far as optimal growth or health of the animal is
concerned (Apajalahti and Bedford, 1999).
It is useful at this point to note that the consequences of a poorly digested diet, and
hence the bene®ts of enzyme addition to such a diet, is more apparent in conventional
compared with germ free chicks (Langhout, 1998; Schutte and Langhout, 1999). Since
the micro¯ora increase in numbers as the animal progresses from essentially germ free to
fully populated some 3 weeks later, it is likely that the bene®t of added enzymes is
mediated through the micro¯oral route in older birds, whereas in young animals which
have a poorly developed digestive system (Kirjavainen and Gibson, 1999), the effect is
probably more direct. It is suggested that until the bird reaches 8 days of age, the output
of pancreatic enzymes may well limit digestion (Lindemann et al., 1986; Krogdahl and
Sell, 1989; Nitsan et al., 1991; Dunnington and Siegel, 1995). The addition of a relevant
Fig. 3. In¯uence of enzyme addition on total ileal microbial counts (Apajalahti and Bedford, 1999). LAB: lactic
acid bacteria.
6
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
exogenous enzyme is, therefore, likely to supplement the digestive capacity of the
younger bird, and again the response to such will be more apparent with poorly digested
compared with highly digestible diets. It is likely that, as birds age and their digestive
ability increases, as does their micro¯oral population, the effect of exogenous enzymes is
more and more mediated through the micro¯oral route.
The chemical identity of the viscous carbohydrates, the cell wall structures and the
indigestible starch/protein complexes which are the focus of the enzymes discussed to
this point is heterogenous in the extreme. The enzymes employed are, thus, wide ranging
in their target substrate preference in the sense that the molecular weight of the substrate
may vary several orders of magnitude as may the complexity and sugar identity of side
chain adjuncts on the targeted backbone. As a result, bene®ts can be achieved by use of
any one of a wide variety of enzymes, from divergent source organisms, which often have
different mechanisms of action. As discussed later, this provides an avenue for
improvements in ef®cacy as more is understood of the chemical nature of the most
important antinutrients.
3. Phytase
There are many reviews recently published describing the use and effects of phytase in
animal feeds. Phytate (Fig. 4), the target substrate of this enzyme, unlike that of the
carbohydrases discussed in the previous section, is a ®xed chemical entity. It is presumed
to be the plant storage form of phosphate which also happens to have considerable
antinutritive effects for most animals. Phytate itself is not a good source of phosphorus for
non-ruminants, particularly younger animals as they appear to lack a meaningful ability
to utilise this compound, even though phytases have been isolated from the small
intestines of broilers and laying hens (Maenz et al., 1997; Maenz and Classen, 1998).
Phosphorus de®ciency would, therefore, often result if inorganic sources were not
routinely added to most non-ruminant diets. As a result of uncertainty with regards to the
availability of plant phosphorus sources, formulations routinely rely on added phosphates
to supply most of the animals needs. Consequently, most non-ruminant diets contain far
more phosphorus than the animal actually needs. The excess to requirement is simply
excreted. Pressures to reduce phosphorus pollution levels in several parts of Europe and
Fig. 4. Phytic acid.
7
M.R. Bedford / Animal Feed Science and Technology 86 (2000) 1±13
Table 1
Phytate phosphorus content of various feed ingredients according to several sources
Ingredient
Matthaus
(1997)
McKnight
(1997)
Maize
Wheat
Barley
Soybeans
Rapeseed
Soyabean meal
Rapeseed meal
0.265
0.235
0.223
0.196
0.255
0.230
Lantzsch
et al. (1992)
NRC
(1994)
MeanS.D.
0.20
0.24
0.19
0.2200.038
0.2430.010
0.2140.021
1.2000.030
1.3500.101
0.3060.020
0.87
1.200.03
1.350.10
0.372
0.870
0.40
0.87
more recently the USA has created a market opportunity for the introduction of
exogenous phytases. The application of this enzyme allows the animal to access much of
the plant phytate phosphorus, and thus, reduce reliance on inorganic phosphate sources.
Phosphorus pollution as a result has been signi®cantly reduced.
Phytase activity is not entirely predictable, however. The bene®t achieved is known to
depend upon several factors, including the raw materials used, the source of the phytase,
the age of the animals, dietary content of calcium, phosphorus and Vitamin D, and the
level of phytase activity present in the ingredients used. Each will be discussed in more
detail below.
Phytate contents of many ingredients varies considerably as shown in Table 1 and as
reported recently; the availability of phytate to exogenous phytase hydrolysis varies from
ingredient to ingredient (Ravindran et al., 1999). The consequence of such an observation
is that for practical purposes, a signi®cant safety margin needs to be employed in
estimation of the phosphorus contribution as a result of phytate hydrolysis. Indeed, the
consequences of overestimating the bene®t of the enzyme are dramatic. Initial estimates
of the P release as a result of use of phytase proved over-optimistic when translated into
commercial terms with the result that savings are less than ®rst envisaged (van Tuijl,
1998). Nevertheless, the bene®ts in reducing pollution and feed formulation costs are
clear and as a result phytase is now a ubiquitous feed additive.
Phytase delivers economic bene®ts through its ability to replace added inorganic
phosphorus. One issue that needs consideration is that the amount of phytase required to
replace 10 g added phosphate ranges from