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Digital Re-print - September | October 2010 The law of diminishing returns: consequences for feed enzyme strategy

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The diets of pigs and poultry are a complex matrix of nutrients variably contaminated with

anti-nutrients and diluents. The digesti-bility 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.

Though feed enzymes are purported to improve the digestibility of various dietary nutrients, this response will vary depending largely on the inherent digestibility of the diet prior to enzyme intervention.

Thus, in diets with a relatively poor starch, protein, fat or mineral digestibility there is a greater potential for feed enzymes to elicit a beneficial response.

A good example of this relationship is the effect of xylanases on ileal amino acid digest-ibility. Amino acid digestibility averages 80-85 percent but this is highly dependent on the amino acid in question with methionine, arginine and glutamic acid being considerably more readily digested than cysteine, threo-nine, serine or proline.

The consequence of this is that feed enzymes will improve the digestibility of cysteine and threonine to a much greater extent than they will for methionine.

So, as with many of life’s principles, the value of feed enzymes follows a distinct law of diminishing return (see Figure 1) as diet nutritional value improves. Thus, when considering mixtures of enzymes, only the first can carry its full matrix and all subse-quent additives must have their contribu-tions truncated by the contributions of the incumbents.

By definition this suggests that the effect of one feed enzyme is highly unlikely to be

fully additive with that of another unless the substrates and, importantly, the correspond-ing nutrients which are released, do not overlap at all.

It is the purpose of the current article to summarise these principles and suggest how they be applied to the strategic selection of feed enzymes, with particular emphasis on admixtures of xylanase and phytase.

Non-starch polysaccharide-degrading enzymes

The beneficial effect of carbohydrases on performance and nutrient digestibility is thought to be due to reduced intestinal vis-cosity, reduced integrity of cell wall architec-ture, improved microbial balance via removal of fermentable starch and protein and provi-sion of oligomers from fibre digestion which prove pre-biotic in their nature.

Interestingly, recent evidence suggests that a reduced intestinal vis-cosity may be the most immediately relevant, even in diets based on corn. According to almost 20 peer-reviewed papers (see Figures 2 and 3) which report the effect of xylanase on ileal protein digestibility, a consistent 15 percent improvement in the undigested fraction is apparent.

This consistency may only be explained by a mass average mechanism such as improved diffusion of nutrients in the lumen as no preference is evident for nutrients of endogenous or exogenous origin. These principles also hold true for the ileal digest-

ibility of starch and fat where between 15-20 percent of the undigested fraction is rendered digestible with xylanase.

Thus, the magnitude of response is largely dictated by the inherent digestibility prior to xylanase addition but a similar proportion of the undigested fraction is captured regardless of the nutrient in question.

Since xylanase improves the digestibility of nutrients based largely on the quantity of the undigested fraction, any additive which improves digestibility (reducing the undigest-ed fraction) will by definition mute xylanase efficacy.

PhytaseThe mechanism of action of exogenous

phytases are quite different to xylanases. Phytate represents a significant antinutri-

ent in the diets of poultry and this antinutri-tive effect is expressed through a physi-ological cascade involving changes in pepsin, mucin and NaHCO3 production.

As the effect of phytate and phytase is quite specific (rather than mass average) the

by Dr A J Cowieson and Dr M R Bedford, AB Vista Feed Ingredients, United Kingdom

The law of diminishing returns:consequences for feed enzyme strategy

Figure 1: Illustration of the relationship between inherent ileal nutrient digestibility and the

magnitude of response to feed enzymes

Grain&feed millinG technoloGy16 | September - october 2010

FeatureEnzyme

general principle that is true for xylanase cannot be transferred to phytase.

Thus, when the effect of phytase on ileal amino acid digestibility is plotted as a propor-tion of the undigested fraction the conclu-sions are quite different to that of xylanase (see Figure 4). Indeed, those amino acids which benefit most from phytase addition are those amino acids which are present at high concentrations in endogenous proteins such as mucins and pepsin.

The net effect of this is that when phytase is present in a diet it improves the digest-ibility of some nutrients more than others. As the response to xylanase is largely driven by the digestibility of the diet to which it is added the efficacy of xylanase will be reduced in the presence of phytase, but not to the same extent for all nutrients.

The law of diminishing returnDue to upwards pressure on the price of

many commonly used feed ingredients such as corn, fat and protein sources, the use of several feed additives can be, at least at face value, attractive for poultry producers.

The difficulty with simply adding various supplements to a ration and ascribing them the suppliers recommen-dations is that in most instances the magnitude of effect on for exam-ple, energetic efficiency has been assessed inde-pendent from other additives. The matrix values for phytases and xylanases (see Table 1), for example, are gener-ally established by feeding diets containing these enzymes independent of other feed additives and measuring some response criteria such as weight gain, conversion, bone ash, or digestibility (total tract or ileal).

One practical illustration of this is in the application of amino acid matrices for feed enzymes used in combination.

Taking lysine as an example and applying real published nutrient release values for vari-ous commercially available products:

Total lysine = 1.23% Digestible lysine = 1.15% Undigested lysine = 0.08% Phytase: 0.017% Xylanase: 0.024% Protease: 0.048%

Using phytase, xylanase and protease in combination = 0.089% improvement in digestible lysine (i.e. from 1.15% to 1.24%).

In the above example the use of sup-plier’s recommendations for lysine results in

Figure 2: The effect of xylanase on ileal amino acid digestibility (IAAD) coefficients from 19 peer-

reviewed papers published between 1998 and 2009

Grain&feed millinG technoloGy September - october 2010 | 17

Feature Enzyme

a new digestible lysine which is in excess of total lysine.

Clearly this cannot possibly be the case and the error is in assumed additivity of matrix values across all additives. In truth only the first additive will carry its full matrix and sub-sequent products will have their effect muted as a consequence of the improvements con-ferred by the current incumbent.

Thus, it is more appropriate to assign nutrient matrices as a proportion of the undi-gested fraction as this automatically accounts for any reduction in the undigested fraction associated with the use of other feed addi-tives. For example, in the above example:

Total lysine = 1.23% Digestible lysine = 1.15% Undigested lysine = 0.08% Phytase: 0.017% New undigested lysine = 0.063% New xylanase effect: (16%

of 0.063%) = 0.010 New undigested lysine = 0.053% New protease effect: (max 30% of

remaining undigested) = 0.016%Using phytase, xylanase and protease

in combination = 0.043% improvement in digestible lysine (i.e. from 1.15% to 1.19%).

In this second example some acknowledgement of the improved digestibility of the diet is made when considering the likely effect of the next addi-tive.

The author submits that this process is more logical than simply adding supplier’s recommenda-tions together and takes into account that the opportunity for further improvement in digest-ibility declines as each new product is added. The order in which prod-ucts are added to the diet will to an extent dictate their value (the first product being more valuable than subsequent).

However, this approach reduces the likelihood of overvaluing combinations of additives whose indi-vidual contributions have been assessed independent of one another but whose combined effect is less than the sum of the various parts.

ConclusionSubstantial confusion per-

sists regarding the additivity of matrix values for feed addi-tives.

As there is a law of dimin-ishing return for the incre-mental addition of each new additive it is prudent to cal-culate the undigested fraction of the diet at the level of the terminal ileum and ensure

that realistic assumptions are made about the extent of improved digestibility of this fraction.

Achieving ileal starch, protein and fat digest-ibility of 100 percent is extremely unlikely and the literature would suggest that only up to around 30 percent of the undigested frac-tion may be rendered digestible by enzyme intervention. With these principals in mind it is possible to assign enzyme matrices that are dynamic and consider both the sub-additive incremental advantage of each new additive as well as the quantity of substrate remaining.

Further research is warranted to explore the less tangible effects of the ingestion of enzymes such as physiological changes, secretory and absorptive function differences and ultimately the effect of enzymes on nutrient requirement and the net value of energy and other nutrients.

More inforMation:Dr A J Cowieson and Dr M R BedfordAB Vista Feed Ingredients3 Woodstock Court, Blenheim RoadMarlborough Business ParkMarlborough, Wiltshire, SN8 4ANUnited Kingdom

Table 1: Example* phytase1, xylanase2 and phytase+xylanase nutrient release values demonstrating sub-additivity in response

Nutrient Quantum (500FTU)

Econase (16,000 U/Kg)

Quantum +

Econase

AvP, % 0.130 0.000 0.130

Ca, % 0.130 0.000 0.130

ME Kcal/Kg 45.000 100.000 116.000

Protein, % 0.365 0.374 0.592

Cys, % 0.027 0.027 0.043

Met, % 0.006 0.011 0.014

Thr, % 0.029 0.025 0.043

Lys, % 0.015 0.019 0.027

Ile, % 0.022 0.019 0.033

Val, % 0.020 0.018 0.030

Gly, % 0.023 0.027 0.041

Asp, % 0.024 0.024 0.038

Ser, % 0.026 0.018 0.036

Ala, % 0.017 0.020 0.030

Pro, % 0.016 0.023 0.032

His, % 0.021 0.024 0.036

Tyr, % 0.013 0.017 0.024

Trp, % 0.017 0.015 0.026

Phe, % 0.018 0.018 0.028

Leu, % 0.018 0.015 0.026

Glu, % 0.015 0.012 0.022

Arg, % 0.011 0.013 0.020

Na, % 0.030 0.000 0.030*Note that phytase and xylanase matrices are not constant and will vary depending on the nature of the diet which is fed including e.g. dietary phytate-P concentration and the quality of the corn. Thus these matrices are for illustrative purposes only and should not be considered to be absolute for all diets.

1 Quantum phytase (500 FTU/kg)

2 Econase XT xylanase (16,000 BXU/kg)

Figure 3: The effect of exogenous xylanase on ileal amino acid digestibility expressed as a proportion of the undigested fraction. The

undigested fraction ranges from 12% for methionine to 28% for cysteine but xylanase

delivers around 15-16% of this fraction regardless

Figure 4: The effect of exogenous phytase on ileal amino acid digestibility expressed

as a proportion of the undigested fraction. The undigested fraction ranges from 12%

for methionine to 28% for cysteine. Unlike xylanase, phytase delivers between 7 and

17% improvement in this fraction, depending on the amino acid

Grain&feed millinG technoloGy18 | September - october 2010

FeatureEnzyme

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• The law of diminishing

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