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Digital Re-print - January| February 2011 The impact of feed enzymes on diet digestibility

and animal profitability

www.gfmt.co.uk

With increasing prices of key raw materials, there is now a great deal of pressure on feed

manufacturers to maximise the efficiency of nutrient utilisation. Much of the cost pressure has been on two nutrients in particular, namely energy and phosphorus (P).

This is likely a result of the surge in requirements for cereals for biofuels and in requirements for phosphates for fertiliz-ers, respectively. The net result is that the increase in shadow prices in least cost formulations (LCF) for energy and P has driven feed manufacturers to consider use of enzymes for releasing more energy from corn. They may also consider using phytase at considerably higher dosages than cur-rently employed. Not only does this increase P release from phytate, but it also spares energy as a result. The principal issues aris-ing from use of each of these enzymes is discussed below.

NSP enzymes for corn based dietsNon-starch polysaccharide- (NSP) or

fibre-degrading enzymes were first devel-oped in the 1980s for use in barley and then subsequently wheat-based diets. Their use coincided with an almost immediate and visible improvement in litter quality and an equally evident improvement in perform-ance (Elwinger & Teglof, 1991). Use of such enzymes in corn-based diets is not associated with improved litter quality, sim-ply because there are few such problems associated with corn. In addition, the scale of animal performance response is somewhat muted compared with wheat and particularly barley based diets, and as a result the rate of uptake of such products in the market has been relatively slow.

Recently, however, the dramatic increase in the price of dietary energy has meant that many feed manufacturers have taken a renewed interest in such enzymes due to the huge potential savings they offer.

Mode of actionIt is thought that NSP enzymes function

through a composite of three separate actions, the contribution of each action varying with ingredients and individual birds. These activi-ties include plant (cereal) cell wall destruction, reduction of digesta viscosity and stimulation of beneficial bacteria in the animal gut (Bedford & Schulze, 1998b).

The cell walls of the starchy endosperm of maize are constructed of a small amount of cellulose encrusted with hemicellulose, the bulk of which is arabinoxylan with minor ß-glucan components and lesser contents of mannans (Stone, 2004). Since monogastrics do not pos-sess the necessary enzymes to degrade plant cell walls, the contents of any cells which remain intact following milling, processing and the grind-ing action of the teeth or gizzard effectively bypass digestion. Microscopy has shown that there can be an appreciable amount of such material and thus an opportunity for cell wall degrading enzymes to improve on the digestive process (Tervila-Wilo et al., 1996; Parkkonen et al., 1997). Effective cell wall degradation requires the addition of sufficient amounts of the appropriate enzyme activity such that “holes” are created in the cell wall which are large enough to allow entry of pancreatic proteases and amylases. Xylanases, and to a lesser extent cellulases (ß 1-4 glucanases), have proven most effective in the field (Zanella et al., 1999; Zanella et al., 2004; Leslie et al., 2007). Mannanases and pectinases target the soybean fraction of the feed more so than the corn, but with the same endpoint in mind (Jackson et al., 2004). Many

studies have shown improvements in starch and to a lesser extent protein digestibility, which is indicative of activity of the enzyme towards corn endosperm cell walls.

The second mechanism relevant for NSP-enzymes is that of viscosity reduction. A por-tion of the hemicellulose may be soluble and of sufficient chain length to create a gel in the intestinal aqueous phase. The greater the chain length and quantity of this material, the more viscous the gel created in the gut (Bedford & Classen, 1992). Viscous gels reduce the rate of diffusion of all solutes (and thus with it the rate of digestion) with the effect being proportional to the viscosity of the solution. In the case of digestion, fat micelles are the largest solutes in the gut and would be most influenced by a viscous intestinal tract (Danicke et al., 2000). Viscosity, however, is most relevant for rye and barley based diets for poultry, to a lesser extent wheat, and to a minor extent to corn (Bedford & Schulze, 1998a). This is due to the fact that the content and chain length of the soluble viscous fibres is much lower in corn than in other grains, and for this reason viscosity is not so relevant in corn-soy diets. Nevertheless, viscosity varies with variety, climatic conditions during cereal growth, postharvest handling (e.g. drying) and pelleting/extrusion. It is possible, therefore, under the right set of circumstances, for viscosity to play a role even in a corn-soy diet, but the frequency of such events is probably low. Provided an enzyme is used which can attack the soluble fibres, the rate of viscosity reduction can be rapid as only a few, well-targeted attacks towards the middle of the chain are required to dramatically reduce average chain length and hence viscosity.

The third mechanism relates to the fact that as a result of the NSP enzymes breaking down cell walls or reducing chain length of viscous polymers, smaller fragments of cell

The impact of feed enzymes on diet digestibility

and animal profitabilityby Dr Michael Bedford, AB Vista Feed Ingredients, Marlborough, Wiltshire, United Kingdom

Grain&feed millinG technoloGy20 | January - february 2011

FEATURE

wall material are produced. At some point the fragments become small enough (i.e. oligosac-charides) and numerous enough to act as a substrate (pre-biotic) for bacterial fermenta-tion. Xylanases, mannanases and cellulases produce xylo-, manno- or gluco- oligosaccha-rides, respectively. The benefit of such end products depends upon the type and quantity of the oligosaccharides produced, with different enzymes producing different oligosaccharides. Many beneficial species of bacteria are able to utilise such products to varying degrees and in doing so produce volatile fatty acids (VFAs), which provide a source of energy and patho-gen control for the chick (Choct et al., 1999; Bedford & Apajalahti, 2001; Sohail et al., 2003). Care must be taken in selection of the enzyme, however, as some can, if overdosed, reduce the size of the oligosaccharides down to mon-osaccharides. If sufficient monosaccharide is produced, this may result in osmotic diarrhoea and/or poor performance (Schutte, 1990). Such a problem is most likely to occur with endo-xylanases, which is an issue with crude preparations which contain substantial amounts of exo- rather than endo-xylanase activity.

A consequence of increased production of VFAs in the large intestine is that butyric and propionic acids may play a role in stimu-lating hormonal responses which encourage enhanced gastric motility. In the pig and espe-cially the chicken, this may result in enhanced particle size reduction and endosperm cell wall

breakdown, to the extent that the contents of the cells are better exposed to gastric and small intestinal digestion. Ironically this may well explain the so called cell wall effect of NSP enzymes, since the amount of time that the diet spends in the stomach of chickens and to a lesser extent pigs, coupled with the extremes in pH, means that it is unlikely that significant cell wall damage will take place through direct exogenous enzyme attack. Thus, the use of an NSP enzyme may well result in significant cell wall destruction, but perhaps more through its effects on hind gut fermentation and the consequent hormonal responses rather than a direct destruction of cell walls in the gastric environment.

Value in practiceIn principal, corn-soy enzymes offer an

improvement in the average digestible energy content of the diet by markedly improving digestibility of the poorest quality corn samples whilst having minimal value on the best sam-ples. Unfortunately, with no method available to allow rapid determination of the quality of corn prior to diet manufacture, the only option is to use such an enzyme in all cases. Since use of the enzyme effectively increases the matrix value of corn, the result is that even when a good quality, un-responsive sample of corn is used in diet manufacture, value has been extracted through the savings created by the higher energy values used in the LCF. When

a poor sample of corn is utilised, the enzyme significantly enhances its digestibility such that the elevated matrix energy value used in the LCF is justified. The net result is consistent cost savings. There is also the added advantage of reduced variability in animal performance as a result of the enzyme reducing the differences in digestible energy content between good and bad samples of corn.

There are many different NSP enzymes on the market today, many of which dif-fer markedly from one another. Even within xylanases, there are enormous differences in pH profiles, end products produced and their ability to attach soluble and/or insoluble xylan structures. It is important when making a choice of enzyme that the decision is based on the biological performance of the product and not simple in vitro assays, none of which bear much relationship to the environment in which these products act.

PhytasesPhytases were originally employed in the

1990s in response to financial penalties for P pollution imposed on pig and poultry produc-ers, particularly in Benelux. Phytases degrade plant phytate P which would otherwise pass through to the manure intact. Without the economic penalties for waste disposal at that time, the use of phytase would not have been established since the cost savings in inorganic phosphate were offset by the cost of

Grain&feed millinG technoloGy January - february 2011 | 21

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if the 500 unit product had an AvP value of 0.1 percent AvP and the 1000 unit product a value of 0.13 percent, then the 500 to 1000 unit product would have a value of 0.03 per-cent AvP. Since the cost of the two products is exactly the same per gram, the second product with the lower matrix value would not be pulled into the LCF until the full 500 units of the former had been used. The error in the LCF for solutions between 500 and 1000 units of use would be markedly reduced as is evident from the proximity of the logarithmic curve to that of the 500 to 1000 units matrix line represented in red. Such approaches will help maximise the value that a feed producer can extract from this enzyme, particularly when high ingredient prices justify much greater inclu-sion levels of phytase.

Enzyme delivery to the animalOne source of variability which must be

addressed by the enzyme manufacturer is that of consistent delivery of the enzyme to the bird. This means that the enzyme must be able to survive or bypass the thermal stresses of the feed manufacturing process, act under the rigours of the intestinal tract, and be simple to assay in premix, mash and pelleted feed for quality control purposes. Various solutions to this are offered, ranging from truly thermostable enzymes with no susceptibility to problems associated with the assay (binding to fibres of the feed; inhibition by xylanase inhibitors found in wheat, barley and triticale, for example), to use of coated enzymes and post-pelleting liquid application. Intrinsically thermostable enzymes are clearly the best solu-tion, with compromises arising from the use of either coating or post-pelleting application which vary with each product.

ConclusionsRecent changes in the cost-structure of the

feed industry have significantly altered the value of P and energy. Enzymes which influence the utilisation of these nutrients are now being con-sidered in situations which would have been uneconomic in the past, and in some cases at inclusion levels which would previously never have been contemplated. Such changes have forced the feed enzyme industry to review how it should support such products, and to consider how to provide products better suited to the task. The combination of better products and more considered application advice should help mitigate at least some of the financial burden being placed on the feed industry by current world events.

References available on request.

the above has significantly improved the ability of such products to reduce feed costs.

Value in practicePhytase is quite unlike the NSP enzymes

for corn soy diets in that the dosage used in practice, even that of the E. coli phytases, is well below that of the biologi-cal optimum. In fact the benefit of this enzyme is linearly relat-ed to logarith-mic increments in dose – i.e. improvement in P digestibility is doubled with a 10 fold incre-

ment in dose. Despite the extension of the nutritional matrices of most phytases into energy and amino acids, the economic incen-tive for feed manufacturers to increase the dos-age of phytase has not been sufficiently obvi-ous. Clearly, with increasing ingredient costs, phytase dose rate should be reconsidered. A clear problem in use of phytase in LCF is that the benefit of the enzyme increases in logarith-mic fashion while the LCF linearly relates dose to benefit. For example, if a phytase is included as an ingredient with a nutrient matrix for 500 units and the LCF selects only 250 units of phytase, it will assume that the phytase has provided only 50 percent of the given matrix, whereas in reality the actual value is closer to 75 percent for such a dose. This relationship is shown in figure 1.

The feed formulator is therefore faced with an economical problem. If shadow prices encourage greater use of phytase, then the traditional approach would be to replace the 500 units matrix with a 1000 units matrix. This 1000 unit matrix has a lesser nutrient value per unit of activity but a maximum inclusion rate which is double that of the 500 units. At the maximum inclusion rates, the 1000 units deliver 30 percent more nutrients than the 500 units. These two matrices are represented by the straight regression lines which pass through the origin in figure 1. The problem is that the actual value of the enzyme is represented by the log curve in figure 1. Use of any dose below the product maximum results in the LCF assum-ing that the enzyme delivers less value than it actually does in vivo. This loss is represented as the difference between the curve and either of the straight regression lines. As a result the true optimum will not be found and a large part of the value of the enzyme is lost.

A solution under these circumstances is to have two ingredients in the LCF. One would be the standard 500 unit product with its given matrix, the second would be a new 500 to 1000 unit product which would have a matrix defined as the difference between the 500 and 1000 unit matrix products. For example,

the enzyme. With time, however, the cost of the enzyme reduced and that of the nutrients spared by use of this enzyme increased (for various reasons, e.g. the ban on meat and bone meal in EU countries), such that its use spread through much of the EU in the mid

and late 1990s. As understanding of its mode of action improved, and with the realisation that this enzyme may spare more than P, its use spread further, and at the present day it is now the most commonly used feed enzyme in the world.

Mode of actionPhytases release P from phytate, and as

a result enable the feed producer to reduce the use of inorganic phosphates in the ration. As more P is released from phytate, the less able it is to bind or chelate minerals, starch or proteins either directly or via ionic bridges (Selle & Ravindran, 2007). As a result, use of phytase may directly improve the digestibil-ity of P, divalent cations such as calcium (Ca), magnesium (Mg) and zinc (Zn), and energy and protein. Phytate has also been shown to be an active antinutrient, interacting with the gut in such a way as to stimulate the small intestinal immune system and increase production and losses of mucin proteins (Cowieson et al., 2004). Destruction of phytate reduces this anti-nutritive effect in a directly proportional manner, and as a result energy and amino acids that would have been used in a maintenance activity (immune surveillance and intestinal turnover) can instead be directed towards pro-ductive energy. It must be noted that this effect of phytase is mostly a post-adsorptive effect, and as a result the value of this activity is not captured in simple AME or even TME assays.

The recent development of E. coli derived phytases has significantly improved the value of this enzyme for broilers in particular. An equiva-lent 500 unit dose from a second-generation E. coli source delivers 20-30 percent more nutri-ents than the same dose of a first-generation Aspergillus phytase (Augspurger et al., 2003), and its effect is more consistent due to its more suitable pH profile and greater stability towards pepsin digestion in the intestinal tract (Igbasan et al., 2000). Some E. coli phytases have been further enhanced either by genetic modification or coating to make them even more stable through the feed manufacturing process. All of

Figure 1: Relationship between dose of phytase and expected response

More inforMation:AB VistaWoodstock Court, Blenheim Road Marlborough Business Park, Marlborough, Wiltshire SN8 4AN, United Kingdom

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