The diets of pigs and poultry are a complex matrix of nutrients variably contaminated with anti-nutrients and diluents. The digestibility of nutrients, such as fat, starch and protein, is generally high (that is, over 80%) but this can and does vary depending on a host of diet, animal and environmental factors.

1. Digital Re-print - September | October 2010
The law of diminishing returns: consequences for feed enzyme strategy
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2. Enzyme Feature Feature Enzyme
general principle that is true for xylanase The difficulty with
cannot be transferred to phytase. simply adding various
Thus, when the effect of phytase on ileal supplements to a ration
amino acid digestibility is plotted as a propor- and ascribing them the
tion of the undigested fraction the conclu- suppliers recommen-
The law of diminishing returns:
consequences for feed enzyme strategy
sions are quite different to that of xylanase dations is that in most
(see Figure 4). Indeed, those amino acids instances the magnitude
which benefit most from phytase addition of effect on for exam-
are those amino acids which are present at ple, energetic efficiency
high concentrations in endogenous proteins has been assessed inde-
such as mucins and pepsin. pendent from other
The net effect of this is that when phytase additives. The matrix Figure 2: The effect of xylanase on ileal amino
acid digestibility (IAAD) coefficients from 19 peer-
is present in a diet it improves the digest- values for phytases and
reviewed papers published between 1998 and 2009
ibility of some nutrients more than others. xylanases (see Table 1),
by Dr A J Cowieson and Dr M R Bedford, AB Vista Feed Ingredients, United Kingdom As the response to xylanase is largely driven for example, are gener-
by the digestibility of the diet to which it ally established by feeding diets containing these Total lysine = 1.23%
is added the efficacy of xylanase will be enzymes independent of other feed additives Digestible lysine = 1.15%
T
he diets of pigs and poultry are fully additive with that of another unless the ibility of starch and fat where between reduced in the presence of phytase, but not and measuring some response criteria such as Undigested lysine = 0.08%
a complex matrix of nutrients substrates and, importantly, the correspond- 15-20 percent of the undigested fraction is to the same extent for all nutrients. weight gain, conversion, bone ash, or digestibility Phytase: 0.017%
variably contaminated with ing nutrients which are released, do not rendered digestible with xylanase. (total tract or ileal). Xylanase: 0.024%
anti-nutrients and diluents. The digesti- overlap at all. Thus, the magnitude of response is largely The law of diminishing return One practical illustration of this is in the Protease: 0.048%
bility of nutrients, such as fat, starch It is the purpose of the current article to dictated by the inherent digestibility prior to Due to upwards pressure on the price of application of amino acid matrices for feed Using phytase, xylanase and protease
and protein, is generally high (that is, summarise these principles and suggest how xylanase addition but a similar proportion of many commonly used feed ingredients such enzymes used in combination. in combination = 0.089% improvement in
over 80%) but this can and does vary they be applied to the strategic selection of the undigested fraction is captured regardless as corn, fat and protein sources, the use of Taking lysine as an example and applying digestible lysine (i.e. from 1.15% to 1.24%).
depending on a host of diet, animal feed enzymes, with particular emphasis on of the nutrient in question. several feed additives can be, at least at face real published nutrient release values for vari- In the above example the use of sup-
and environmental factors. admixtures of xylanase and phytase. Since xylanase improves the digestibility value, attractive for poultry producers. ous commercially available products: plier’s recommendations for lysine results in
of nutrients based largely on the quantity of
Though feed enzymes are purported to Non-starch polysaccharide- the undigested fraction, any additive which
improve the digestibility of various dietary degrading enzymes improves digestibility (reducing the undigest-
nutrients, this response will vary depending The beneficial effect of carbohydrases ed fraction) will by definition mute xylanase
largely on the inherent digestibility of the diet on performance and nutrient digestibility is efficacy.
prior to enzyme intervention. thought to be due to reduced intestinal vis-
Thus, in diets with a relatively poor starch, cosity, reduced integrity of cell wall architec- Phytase
protein, fat or mineral digestibility there is a ture, improved microbial balance via removal The mechanism of action of exogenous
greater potential for feed enzymes to elicit a of fermentable starch and protein and provi- phytases are quite different to xylanases.
beneficial response. sion of oligomers from fibre digestion which Phytate represents a significant antinutri-
A good example of this relationship is the prove pre-biotic in their
effect of xylanases on ileal amino acid digest- nature.
ibility. Amino acid digestibility averages 80-85 Interestingly, recent
percent but this is highly dependent on the evidence suggests that
amino acid in question with methionine, a reduced intestinal vis-
arginine and glutamic acid being considerably cosity may be the most
more readily digested than cysteine, threo- immediately relevant, even
nine, serine or proline. in diets based on corn.
The consequence of this is that feed According to almost 20
enzymes will improve the digestibility of peer-reviewed papers
cysteine and threonine to a much greater (see Figures 2 and 3)
extent than they will for methionine. which report the effect of
So, as with many of life’s principles, the xylanase on ileal protein
Figure 1: Illustration of the relationship between
value of feed enzymes follows a distinct digestibility, a consistent inherent ileal nutrient digestibility and the
law of diminishing return (see Figure 1) as 15 percent improvement magnitude of response to feed enzymes
diet nutritional value improves. Thus, when in the undigested fraction
considering mixtures of enzymes, only the is apparent.
first can carry its full matrix and all subse- This consistency may only be explained ent in the diets of poultry and this antinutri-
quent additives must have their contribu- by a mass average mechanism such as tive effect is expressed through a physi-
tions truncated by the contributions of the improved diffusion of nutrients in the lumen ological cascade involving changes in pepsin,
incumbents. as no preference is evident for nutrients of mucin and NaHCO3 production.
By definition this suggests that the effect endogenous or exogenous origin. These As the effect of phytate and phytase is
of one feed enzyme is highly unlikely to be principles also hold true for the ileal digest- quite specific (rather than mass average) the
16 | September - october 2010 Grain &feed millinG technoloGy Grain &feed millinG technoloGy PREVIOUS PAGE NEXT PAGE September - october 2010 | 17

3. Enzyme Feature
a new digestible lysine which is in excess ofTotal lysine = 1.23%
total lysine. Digestible lysine = 1.15%
Clearly this cannot possibly be the caseUndigested lysine = 0.08%
Phytase: 0.017%
and the error is in assumed additivity of matrix
New undigested lysine = 0.063%
values across all additives. In truth only the
New xylanase effect: (16%
first additive will carry its full matrix and sub-
sequent products will have their effect mutedof 0.063%) = 0.010
as a consequence of the improvements con- New undigested lysine = 0.053%
ferred by the current incumbent. New protease effect: (max 30% of
Thus, it is more appropriate to assign remaining undigested) = 0.016%
Using phytase, xylanase and protease
nutrient matrices as a proportion of the undi- Figure 3: The effect of exogenous xylanase
in combination = 0.043% improvement
gested fraction as this automatically accounts on ileal amino acid digestibility expressed as
a proportion of the undigested fraction. The
in digestible lysine (i.e. from 1.15% to
for any reduction in the undigested fraction
undigested fraction ranges from 12% for
1.19%).
associated with the use of other feed addi- methionine to 28% for cysteine but xylanase
tives. For example, in the above example: In this second example some delivers around 15-16% of this fraction regardless
acknowledgement of the
Table 1: Example* phytase1, xylanase2 and improved digestibility of
phytase+xylanase nutrient release values the diet is made when
demonstrating sub-additivity in response considering the likely
Econase Quantum effect of the next addi-
Quantum tive.
Nutrient (16,000 +
(500FTU)
U/Kg) Econase The author submits
that this process is more
logical than simply adding
AvP, % 0.130 0.000 0.130
supplier’s recommenda- Figure 4: The effect of exogenous phytase
Ca, % 0.130 0.000 0.130 on ileal amino acid digestibility expressed
tions together and takes
ME Kcal/Kg 45.000 100.000 116.000 into account that the as a proportion of the undigested fraction.
The undigested fraction ranges from 12%
Protein, % 0.365 0.374 0.592 opportunity for further for methionine to 28% for cysteine. Unlike
Cys, % 0.027 0.027 0.043 improvement in digest- xylanase, phytase delivers between 7 and
ibility declines as each 17% improvement in this fraction, depending
Met, % 0.006 0.011 0.014
new product is added. on the amino acid
Thr, % 0.029 0.025 0.043
The order in which prod-
Lys, % 0.015 0.019 0.027 ucts are added to the that realistic assumptions are made about the
Ile, % 0.022 0.019 0.033 diet will to an extent dictate extent of improved digestibility of this fraction.
Val, % 0.020 0.018 0.030 their value (the first product Achieving ileal starch, protein and fat digest-
Gly, % 0.023 0.027 0.041
being more valuable than ibility of 100 percent is extremely unlikely and
subsequent). the literature would suggest that only up to
Asp, % 0.024 0.024 0.038
However, this approach around 30 percent of the undigested frac-
Ser, % 0.026 0.018 0.036 reduces the likelihood of tion may be rendered digestible by enzyme
Ala, % 0.017 0.020 0.030 overvaluing combinations intervention. With these principals in mind it
Pro, % 0.016 0.023 0.032 of additives whose indi- is possible to assign enzyme matrices that are
His, % 0.021 0.024 0.036 vidual contributions have dynamic and consider both the sub-additive
been assessed independent incremental advantage of each new additive
Tyr, % 0.013 0.017 0.024
of one another but whose as well as the quantity of substrate remaining.
Trp, % 0.017 0.015 0.026 combined effect is less than Further research is warranted to explore
Phe, % 0.018 0.018 0.028 the sum of the various parts. the less tangible effects of the ingestion
Leu, % 0.018 0.015 0.026 of enzymes such as physiological changes,
Glu, % 0.015 0.012 0.022 Conclusion secretory and absorptive function differences
Substantial confusion per- and ultimately the effect of enzymes on
Arg, % 0.011 0.013 0.020
sists regarding the additivity nutrient requirement and the net value of
Na, % 0.030 0.000 0.030
of matrix values for feed addi- energy and other nutrients.
*
Note that phytase and xylanase matrices are not constant tives.
and will vary depending on the nature of the diet which is
As there is a law of dimin- More inforMation:
fed including e.g. dietary phytate-P concentration and the
quality of the corn. Thus these matrices are for illustrative ishing return for the incre-
Dr A J Cowieson and Dr M R Bedford
purposes only and should not be considered to be absolute mental addition of each new AB Vista Feed Ingredients
for all diets. additive it is prudent to cal- 3 Woodstock Court, Blenheim Road
1 Quantum phytase (500 FTU/kg) culate the undigested fraction Marlborough Business Park
of the diet at the level of the Marlborough, Wiltshire, SN8 4AN
2 Econase XT xylanase (16,000 BXU/kg)
terminal ileum and ensure United Kingdom
18 | September - october 2010 Grain &feed millinG technoloGy

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