Feed Enzyme Technology: Present Status and Future Developments
Exogenous enzymes are now well accepted as a class of feed additives in diet formulations for poultry and pigs to overcome the negative effects of anti-nutritional factors, and to improve digestion of dietary components and animal performance. An overview of the current status of feed enzyme technology, including the different type of enzymes and modes of action, is provided. Variable response to enzyme supplementation is an important reason limiting the widespread acceptance of feed enzymes. The major reasons contributing to these variable responses are discussed. Main features of the next generation of feed enzymes and the various trends that will drive the future use of enzymes are highlighted. The use of feed enzymes in poultry and pig feed formulations is expected to increase in the future and this will be driven by on-going changes in the world animal production. Aquaculture and ruminant industries are emerging markets for exogenous feed enzymes. The article presents some promising patents on feed enzyme technology.
The potential usefulness of exogenous enzyme preparations to improve nutrient utilisation and performance in poultry has been known for many years. But only during the past two 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 the substrate. Another development had been in the area of biotechnology, specifically in fermentation and microbiological technologies. As a result, it has now become possible to produce feed enzymes cheaply enough to warrant their use in commercial diet formulations. Other advances include development of specific enzymes designed to function optimally in the gastrointestinal tract of the animal and production technology to improve enzyme stability during processing of commercial feeds. The poultry industry is the largest user of feed enzymes. The highly integrated nature of the poultry sector has enabled the faster uptake of these new technologies, and exogenous enzymes are now well accepted as a class of feed enhancer to improve nutrient digestibility and efficiency of utilisation. There is also an increasing trend for the pig industry to use feed enzymes, especially the phytatedegrading enzymes. The benefits of using enzymes in diets for young pigs are also increasingly recognised. However, the commercial usage of feed enzymes in animal diets, other than in poultry diets, remains limited by the variability of responses observed among studies and the relatively high cost of enzymes.
Over 90% of all broiler diets in some countries (e.g. United Kingdom, Australia, New Zealand, Canada) contain feed enzymes, and on a worldwide basis as much as 70% of wheat and barley-based poultry feeds are supplemented with glycanases (xylanases and -glucanases). During the past decade, the use of microbial phytase in poultry and pig diets has increased, in response to concerns over phosphorus (P) pollution from effluents from intensive animal operations. The ban on the use of meat and bone meal, a major source of P, in the European Union is another factor that has accelerated the use of microbial phytases. As a result, in recent years, microbial phytase has overtaken the glycanases as the primary feed enzyme worldwide.
It is not the intention of this chapter to review the available literature on the influence of xylanases, -glucanases, microbial phytases on the performance and nutrient utilisation in poultry and pigs, since comprehensive reviews are available on these topics [1-5]. The aim instead is to provide an overview of the current status and future prospects of feed enzymes for use in poultry and pig diet formulations. The first part of this review will cover general aspects of feed enzymes, including the type of enzymes, potential benefits, modes of action and reasons for variable responses. The second part will give some insight into the main features of next generation of feed enzymes and the various trends that are expected to drive the future use of enzymes.
COMMERCIALLY AVAILABLE FEED ENZYMES
The enzymes widely used by the industry are the glycanases that cleave the non-starch polysaccharides (NSP) in viscous cereals (wheat, barley and triticale) and microbial phytases that target the phytate-complexes in plant ingredients. Several enzyme preparations, designed towards improving utilisation of protein, starch, lipids or fibre components in specific ingredients, are also available Table 1. While technically successful, not all enzymes are as yet totally cost effective in practical situations.
The potential nutritive value of raw materials is often not realised at the animal level because of the limitations imposed by the presence of a range of anti-nutritional factors and the lack (or insufficiency) of digestive enzymes to break down specific chemical linkages that bind and prevent the release of nutrients. The need to efficiently utilise the nutrients is the principal rationale for the use of feed enzymes in diets for monogastric animals. The ultimate aim of using feed enzymes is to improve animal performance through improvements in feed intake, weight gain and feed efficiency. But there are many other reasons for the attraction of using feed enzymes.
1. Improved efficiency of utilisation of dietary components (protein, amino acids, starch, lipids and energy) in feedstuffs
2. Increased flexibility and increase in the range of feedstuffs that can be used in feed formulations. Feed enzymes remove the constraint on the inclusion limits of poorly digested ingredients.
3. Reduced variability in the nutritive value of the ingredient. Enzyme supplementation reduces the variation between good and poor quality samples of a given ingredient and this, in turn, improves the degree of precision of feed formulation nutrient matrix. It is recognised that lower the ingredient quality, greater will be the enzyme response.
4. Reduced excreta moisture content, and lowered incidence of wet litter, vent plugging, hock lesions, carcass downgrading and dirty eggs. This is particularly relevant in the case of viscous grains and grain legumes.
5. Alteration of gut flora towards favourable bacterial species, which improves gut health and provides a protective effect on the overall health of the animal. This protective effect arises, in part, from the influence of flora on immune function.
6. Improved intestinal morphology, resulting in enhanced digestion and absorption of dietary components.
7. Lowered manure output in terms of the amount and nutrient load (excreta nitrogen and phosphorus levels). The environmental impact of these benefits has become relevant to intensive animal operations world-wide.
8. Better uniformity of animals at market time, by uplifting the growth of poorly performing animals.
MODE OF ACTION OF FEED ENZYMES
It must be noted that different feed enzymes will have different modes of action. Despite their increasing acceptance use as a feed additive, the exact mode(s) of action of feed enzymes remains to be elucidated. The consensus among researchers is that one or more of the following mechanisms are responsible for the observed improvements [1, 2, 5].
1. Degradation of specific bonds in ingredients not usually hydrolysed by endogenous digestive enzymes,
2. Degradation of anti-nutritional factors that lower nutrient digestion and/or increase digesta viscosity,
3. Disruption of endosperm cell wall integrity and the release of nutrients that are bound to or entrapped by the cell wall,
4. Shift of nutrient digestion to more efficient digestion sites,
5. Reductions in endogenous secretions and protein losses from the gut, resulting in reduced maintenance requirements [6-9],
6. Reduction in the weight of intestinal tract and changes in the intestinal morphology ,
7. Changes in the microflora profile in the small and large intestine. As enzymes influence the amounts and form of substrate present within the gut, their use has a direct impact on the bacteria that make up the microfloral populations [11, 12], and/or
8. Augmentation of endogenous digestive enzymes, which are either insufficient or absent in the animal, resulting in improved digestion. This is especially true for young animals with immature digestive systems.
VARIATION IN RESPONSES TO FEED ENZYMES
It is well recognised that animal responses to feed enzyme addition are not entirely predictable. The responses are variable and these inconsistencies could be attributed inter alia to enzyme type, amount of enzyme applied, side enzyme activities present, diet composition and animal variation. Some key dietary and animal factors are discussed below.
1. 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 manipulated and conditions have to be created to ensure maximum responses to added enzymes. For example, the use of phytase enzyme is potentially useful and appropriate only for diets with available P contents below the requirements and containing significant levels of plant-derived ingredients.
2. Species Differences
In general, the benefits of feed enzymes seem to be limited in pigs compared to poultry. The exception is young pigs, which are reported to benefit from enzyme supplementation. Differences in gastrointestinal anatomy, transit time and capacity of digestive system appear to contribute to the lower responses in pigs. Pigs have a longer transit time than poultry that gives them more time to extract the nutrients. Also the larger capacity of the gut means that pigs are less affected by the digesta viscosity characteristics. This is the reason why pigs do not suffer from digestive inefficiencies attributed to dietary non-starch polysaccharides in wheat and barley. In poultry, the presence of crop provides a balanced environment for some enzyme activation before the feed reaches the acidic environment of the proventriculus. Some evidence suggests that the presence of a welldeveloped gizzard may improve enzyme responses and give an edge to poultry.
Growth performance experiments in pigs pose even greater challenges than those in poultry since feed intake is dictated by digestible energy content for the former, rather than by digestive capacity as is true for the latter. Thus, with pigs, enzymes can be expected to slightly reduce feed intake and slightly improve performance although to a degree that is difficult to demonstrate statistically. Growth performance is also much more variable in pigs, since typically fewer animals are used than in poultry experiments.
3. Age of Animals
In general, the potential benefits from feed enzyme additions will be greater when the digestive system is simpler, as found in young animals. Young animals have an underdeveloped digestive enzyme capacity compared to adult animals. Thus young animals can 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. Studies with both pigs and poultry have shown that the benefits of adding enzymes generally, but not always, diminish with age. Responses are usually greater during broiler starter phase compared to finisher phase and young pigs respond better than growingfinishing pigs.
4. Quality of the Target Feedstuffs
In general, the quality of the target feedstuff is related to the level of anti-nutrient in the ingredient. e.g. soluble NSP levels in wheat, -glucans in barley. These levels can vary widely among batches of ingredient. Available research data suggest that enzyme responses are dependent, among others, on the quality of the ingredient. Lower the quality, greater will be the magnitude of improvements with added enzyme.
5. Inclusion Level of the Target Feedstuff
Higher the inclusion level of the target feedstuff, greater will be the enzyme response since it will proportionately increase the contents of the anti-nutrient that is causing the problem. e.g. xylanase and wheat fraction in diet formulations.
6. Performance Level of Control Animals
The degree of response is governed by 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 with enzyme addition. In this context, the enzymes act in a manner similar to that of in-feed antibiotics. This is more than a coincidence and is related to the effect of enzymes on gut flora and gut health.
MAJOR GROUPS OF ENZYMES IN CURRENT USE
There are four distinct categories of enzymes currently commercially available for use by the feed industry.
1 Enzymes targeting viscous cereals
2 Enzymes targeting non-viscous cereals
3 Enzymes targeting non-cereals
4 Microbial phytases
As noted previously, the first and last group of enzymes are the major ones; the other two groups represent emerging enzyme types.
ENZYMES TARGETING VISCOUS CEREAL GRAINS
Examples of this group include xylanases used in wheatbased diets and -glucanases used in barley-based diets. There have been a large volume of publications on the efficacy of these enzymes [1, 2, 4], but still there little agreement as to how these enzymes work. This lack of agreement is related largely to the inconsistency in responses reported to xylanase supplementation, which (as noted earlier) could vary widely and appears due partly to variations in the quality of the raw material.
It is generally accepted that the poor nutritional value of wheat and barley is related to their soluble NSP contents. These components lead to increased digesta viscosity, which impacts the digestion of nutrients by reducing the contact between digestive enzymes and target substrates, and diffusion of digestion end-products. An increased viscosity also decreases feed passage rate, thereby facilitating the overgrowth of bacteria in the upper segments of intestine. This has two major effects; first, bacterial deconjugation of bile salts, which is necessary for the emulsification of fats, resulting in lowered fat digestion, and second, the negative effect on general gut health [1,2].
The apparent metabolisable energy (AME) data from poultry studies suggest that the responses to supplemental xylanase are dependent, to a great extent, on the initial AME of the wheat sample. Large variability, ranging from 9.2 to 16.0 MJ/kg dry matter, in the AME of wheat samples has been reported in the literature samples [13-17]. Wheat samples of poor quality, assaying less than 13.0MJ/kg AME, have been shown to respond remarkably to enzyme supplementation, whilst the bulk of wheats yield a small to moderate responses, with good quality wheats showing practically no increment in AME Table 2. Corresponding information on the responses in protein/ amino acid digestibility to glycanase supplementation is lacking . Although, wide variability in the protein content (8.9 to 18.3%) ileal protein digestibility (72 to 85%) of wheat is known to exist , the variability in responses in protein/ amino acid digestibility to enzyme addition is not known.
ENZYMES TARGETING NON-VISCOUS GRAINS
Historically, utilisation by poultry of nutrients and energy in diets based on non-viscous grains such as maize and sorghum has been considered to be high, but recent data suggests that this is not the case . It is now known that considerable variability exists in the AME and starch digestibility of these non-viscous grains and that there is room for improvement by supplementation of enzymes that contain significant activity of amylase. For example, the AME of maize samples from a single harvest season in Canada has been shown to vary by as much as 1.85 MJ/kg . The reasons for the variability in maize AME are unclear, but appear to be related to differences in oil content and starch digestibility. Among the possible reasons for the variability in starch digestibility, starch characteristics (amylopectin: amylose ratio) and structural changes in starch-protein matrix during harvesting (drying and storage) and feed processing may be important. Some commercial enzyme preparations specifically developed for maize and sorghum-based diets are available. Consistency in enzyme responses in these diets was initially poor, but more effective enzymes are now becoming available as shown in several recent studies [22-24]. The potential for feed enzyme addition to diets based on non-viscous cereals has been highlighted in recent reviews by Brufau et al.  and
Cowieson et al. .
ENZYMES TARGETING NON-CEREAL FEEDSTUFFS
With the ban on the use of meat and bone meal in Europe, there is the realisation that a bigger market will be created for soybean meal and alternative plant-based protein ingredients. Such ingredients include a range of oilseed meals (e.g. canola meal, sunflower meal, palm kernel meal) and grain legumes (e.g. peas, lupins). Most of these ingredients have obvious potential, but their increased utilisation is constrained by several nutritional limitations including high fibre or NSP levels and the resulting poor nutrient digestibility. The NSP in these ingredients vary widely in terms of chemical structure and type of component sugars, and for most of these feedstuffs, data on these aspects are lacking. This is largely responsible for the ambiguous results reported in studies conducted to degrade the NSP present in legumes and oilseed meals. Overall, the efficacy of current enzymes in poultry diets based on legumes and non-conventional oilseed meals is limited [27-30]. Given the importance of these ingredients in strengthening the feed base for the future, one can expect considerable research input into the development of suitable enzyme preparations specifically targeting them.
The substrate for microbial phytases is phytic acid [myoinositol 1,2,3,4,5,6-hexakis dihydrogen phosphate], which is the major storage form of P in plant-derived ingredients. Phytate-bound P in plant ingredients, however, is poorly available to monogastric animals  and this is reflected by the excretion of large amounts of phosphorus in effluent from intensive animal operations. These environmental problems have prompted the development and acceptance of microbial phytase feed enzymes in diets for monogastric animals. Microbial phytases entered the feed market in the early 90’s in Europe. After some initial reluctance and concerns regarding efficacy, it has now become the most accepted feed enzyme.
The effectiveness of phytase in releasing phytate-bound P in diets based on a range of plant derived feedstuffs for utilisation by poultry and pigs is now well documented [5, 32] and phytase equivalency values to replace 0.1% available P in diets for different classes of animals are established. Recommended equivalency values will vary depending on the phytase brand used. With the introduction of new types of phytases (such as bacterial phytases), there is expectation that these values may be further lowered.
Of the six P units in the phytic acid molecule, only two or three are released by the action of microbial phytases within the digestive tract of poultry and pigs for utilisation by the animals. While complete dephosphorylation (i.e. removal of all six P units) is possible under in vitro conditions, this will not happen in the gut of the animal because of the transit time and pH limitations. Poultry and pig studies report improvements in P availability ranging from 20 to 45%, with the amount of phytate P released being dependent on several factors: (i) level of added phytase, (ii) dietary level of nonphytate P, (iii) dietary Ca level and, in particular, Ca: P ratio, (iv) source of phytase (v) source of phytate (ingredients used), (vi) various additives (vitamin D, organic acids), and (vii) phytate content of the diet [5,32,33].
Understandably, the initial focus of research on microbial phytase had been towards improving P utilisation because phytic acid was considered primarily as a factor limiting P availability from plant-derived feedstuffs. Current evidence, however, suggests that the deleterious influence of phytic acid in the nutrition of monogastric animals goes beyond its effects on P availability. In its natural state, phytic acid is present as a complex with various minerals, protein and starch, lowering the digestion and utilisation of these nutrients. Consequently, supplemental phytase, during the process of hydrolysing the phytate, can also be expected to release and improve the availability of these nutrients.
In particular, the influence of microbial phytase on protein digestion and utilisation is a topic of current interest as evidenced by an upsurge in research reports [5, 32, 34, 35]. In some studies, however, phytase had no beneficial effects on the digestibility of amino acids highlighting the variability in amino acid responses to phytase addition. Factors influencing the magnitude of amino acid responses are probably complex and future studies are needed to identify and quantify these factors. On the other hand, published data on the effects of phytase on the digestibility of amino acids for pigs is limited and controversial, and the protein responses of the enzyme are generally less consistent than in poultry.
Often associated with the protein response to added phytase is improvements in energy utilisation. That added phytase can improve the AME of poultry diets based on maize, wheat and sorghum has been demonstrated in several studies . Depending on the diet type, improvements in AME ranging from 25 to 140 kcal/kg dry matter have been reported. It appears, however, similar energy responses may not be seen in pigs . While the basis for the observed energy effect is not well understood, it would appear that the response might be multifaceted resulting from small, and possibly additive, improvements in protein, fat and carbohydrate digestion.
Recent studies have shown that bacterial phytases liberated more P in broilers and pigs than fungal phytases [5,32], owing to their relative resistance to pepsin activity and high relative activity over a broader pH range. In the future, 'second-generation' phytases with a greater capacity to hydrolyse dietary phytate will be developed. This would further reduce P excretion and generate greater amino acid/energy responses, and phytase use will become more cost effective.
FEED ENZYMES - NEXT GENERATION
Although feed enzymes have had a significant impact on the poultry industry, we are yet to fully harness the full potential of enzymes and expand their use to other farmed animals. Several issues need to be addressed before their full potential is reached and these will lead future developments in enzyme technology.
Enzymes that Match Substrate Chemistry and Structure
Variable responses in current enzymes are related, partly, to poor understanding of the substrate chemistry and structure. In most cases, the target substrates are poorly defined. The efficacy of enzymes can be greatly improved if the chemistry of the target substrates is more precisely understood. Then better methods of degrading these substrates and appropriate enzyme(s) can be developed. e.g. new appropriate enzyme(s) can be developed. e.g. new etc) targeting NSP and oligosaccharides, amylases and glucanases targeting starch, and proteases targeting specific
New Forms of Enzymes
Better forms of enzymes will be developed in the future. A perfect enzyme must have high specific catalytic activity (per unit of protein), good thermostability, high activity under wide ranges of gut pH, resistance to proteolysis and good stability under ambient temperatures. Current feed enzymes are produced mainly from fungi. In the future, the developments will include production and/or expression of enzymes in other forms of microorganisms, such as bacteria and yeast. These new enzymes will clearly improve the efficacy of the enzymes within the gut. Such a development is already taking place in the case of a number of exogenous enzymes [36-38].
Currently, available exogenous enzymes lose significant amounts of activity when processing temperatures exceed 70°C. As a result, enzymes are usually included as liquids, via post-pelleting application systems, to avoid thermostability problems at high pelleting temperatures. Applying liquid enzymes accurately after pelleting can be a complex and costly procedure. In this context, development of thermostable enzymes will simplify the pre-pelleting application of dry product and promote the use of the enzyme in pelleted diets. Three different approaches are being used to resolve the issue of enzyme survival during the pelleting process . These include, (i) coating of the dry enzyme products with a coating which can withstand the heat and moisture employed in feed manufacture [40-44], (ii) genetically manipulating the enzyme product so that it is more inherently thermostable [45, 46], and (iii) discovery of intrinsically thermostable enzymes [47, 48]. There is some evidence, however, suggesting that the coating of enzymes may reduce the efficacy of the product, compared to an uncoated version of the same product .
Enzyme Preparations with Multiple Activities
To achieve maximal benefits from feed enzyme addition, it is necessary to ensure that the enzymes are chosen on the basis of composition of ingredients used in feed formulations. In other words, we need to match the enzyme with the substrates in the feed. There is recent evidence suggesting that preparations with multiple enzyme activities may provide a competitive strategy to improve nutrient utilization in poultry diets. The combined application of enzymes may result in additive, sub-additive or synergistic effects on nutrient utilisation and animal performance [50, 51]. Such enzyme cocktails, rather pure single enzymes, represent the next generation of feed enzymes. This is because feed ingredients are structurally exceedingly complex. In the 'native' stage, nutrients in raw materials are not isolated entities but exist as complexes with various linkages to protein, fat, fibre and other complex carbohydrates. For example, in wheat-based 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 utilisation and growth performance have been reported [52-54]. It appears that the activity of one type of feed enzyme appears to be facilitated by the other, possibly in a reciprocal fashion, by providing greater substrate access, and also by reducing the anti-nutritive effects of the substrates (NSP and phytate) on nutrient utilisation [26, 55, 56].
FEED ENZYMES - FUTURE PERSPECTIVES
Over the past two decades, feed enzymes have graduated from an undefined entity to a well-accepted tool to improve nutrient utilisation. This is a testament for their efficacy in improving nutrient utilisation and animal performance. The use of feed enzymes in poultry and pig feeds is expected to increase in the future and this will be driven by on-going changes in world animal production. These changes will add 'value' to feed enzymes and lower their opportunity cost. Some key drivers of this change are highlighted below:
Ban on Antibiotic Growth Promoters
The poultry and pig industries are facing a future without the benefit of the most effective feed additive: in-feed antibiotics. The ban in the European Union and different degrees of voluntary withdrawal in other parts of the world on the use of in-feed antibiotics will put extra pressure on the gut health and general health of animals. The nutrients for the multiplication and growth of bacteria in the intestinal tract are derived largely from dietary components, which are either not digested by digestive enzymes or absorbed so slowly so that the bacteria can compete for them. Since the substrate preference and nutrient requirement of different bacterial species differ, the nutrient composition of the digesta largely determines the profile and numbers of bacteria in both upper and lower gut. As a consequence, any dietary factor that could change digestive efficiency will have pronounced effects on the intestinal microflora.
It is well known that high level of inclusion of cereals that contain soluble NSP could significantly alter the profile and activity of gut flora, by increasing digesta viscosity and adversely affecting the digestion [11, 12]. The reduced rate of digestion results in greater amounts of undigested materials consisting largely of carbohydrates reaching the lower gut and providing substrate for bacterial growth. A similar effect could be seen when diets containing poorly digested protein ingredients are fed, but with significant amounts of undigested nitrogen reaching the lower gut. In this context, feed enzymes, through their effects on improving digestibility and reducing the amount of undigested nutrients reaching the hindgut, will play a key role in lowering proliferation of harmful species in the gut environment [57, 58]. Thus, it is clear that exogenous feed enzymes must be part of the 'package' being considered in any decision replacing antibiotic growth promoters.
With increasing public interest over environment, the reduction of nutrient excretion in effluents from intensive animal operations has now become a major issue. Without question, animal agriculture must achieve the goal of sustainability since environmental concerns will have a big influence on its future growth and expansion. As public attention is increasingly focused on the environment, feeding regimens that are sustainable need to be considered. Among the many possibilities to overcome the nutrient pollution problem, the use of feed enzymes to improve efficiency of nutrient utilisation is perhaps most promising.
Alternative Feed Ingredients
It is projected that the global demand for pig and poultry continue to increase over the next decade and much of this increase will be in developing countries. Such a growth will have a profound effect on demand for feed and raw materials. It is also becoming clear that the requirements for traditional raw materials, both energy and protein sources, cannot be met even with optimistic forecasts, and that new, alternative ingredients will become critical to support this growing appetite for raw materials. A good example of new ingredient entering the feed market is dried distillers grains with solubles. Feed enzymes will need to play a key role in maximising the use of these ingredients. It will be necessary to develop ‘enzyme cocktails’ that can target more than one substrate at a given time. Given that high fibre and NSP levels may be limiting nutrient availability in these ingredients, an especially crucial components of the cocktail will be the enzymes that will target the fibre fraction such as appropriate mannanases, cellulases etc.
Nutrition of Young Animals
It is recognised that young animals, for example, may be limited in the types and amounts of endogenous enzymes needed to utilise nutrients, including lipids, starch, protein and minerals. Given that the digestion of nutrients may be limiting during early life [59, 60], there may be opportunities to improve digestibility by the use of exogenous enzymes sources to complement the animal’s digestive system. The enzymes of interest will be amylases, lipases and proteases, most likely in combination rather than individually.
Enzymes for Fish and Ruminants
Although feed enzymes have not achieved significant penetration of feed sectors other than poultry and pigs, there is increasing interest in recent years in aquaculture and ruminant industries. The use of exogenous enzymes in ruminant diets has been limited because of the view that fibrolytic activity within the rumen environment is normally very high and it is assumed that exogenous enzymes would not survive proteolysis in the rumen. A growing body of evidence during the past decade, however, has shown that enzyme preparations can be effective in enhancing lactation and growth performance in cattle.
In the aquaculture industry, there is an increasing interest in the search for alternative protein sources for fish meal. The major concerns with many available alternative feedstuffs are their relatively low nutrient digestibility and the presence of an array of anti-nutritional factors. These constraints could be overcome, in part, by the addition of exogenous enzymes, provided optimal means of application are found.
Excellent reviews on the value of feed enzymes for ruminants [4, 61, 62] and on the use of phytases in aquafeeds [63, 64] are available. While positive effects have been observed with enzyme supplementation in these species, the magnitude of responses has been variable. For enzyme supplementation of ruminant and fish diets to gain acceptance, it is important that the variability of results is reduced.
CURRENT AND FUTURE DEVELOPMENTS
Future directions in animal production will be driven by on-going changes in world agriculture and by societal issues. Future animal production systems will be subject to more and more government restrictions, and will be in public scrutiny. Decisions in animal agriculture will be increasingly influenced by consumer views, environmental issues and public health concerns. On the basis of expected changes in animal production highlighted in this review, exogenous feed enzymes are expected to play key roles in the future. Feed enzyme technology is an active area of research and development, and one can be certain that better forms of enzymes will be developed in the future. In particular, the development of more thermostable enzymes will simplify the pre-pelleting application of dry product and promote the use of the enzyme in pelleted diets. It is also clear that the next generation of enzymes will be those with multiple enzyme activities rather than individual enzymes. These developments will improve the cost effectiveness of enzyme addition under practical situations.
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Article made possible through the contribution of Velmurugu Ravindran and Jang-Ho Son