Animal Feed Science and Technology
                                                 86 (2000) 1±13
                                                  Review article
              ExogenousenzymesinmonogastricnutritionÐtheir
                             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
              nowextensively 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: S0377-8401(00)00155-3
       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.
        Toreducetheincidence 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
                   downtherate 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.
         Theconsequences 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