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The use of exogenous fibrolytic enzymes to enhance microbial activities in the rumen and the performance of ruminant animals |
 
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Author: KARL A. DAWSON and JUAN M. TRICARICO (Courtesy of Alltech Inc.)
Slow or incomplete digestion of fibrous substrates often limits the overall digestive process in the rumen and can significantly influence animal performance in livestock production systems that use forages as a major component of the diet.
As a result, many strategies have been developed to stimulate the digestion of the fibrous components in ruminant feeds. These have included the use of specific nutrients which stimulate fiber digestion, processing feeds to increase the rate and extent of fiber digestion, and the use of direct-fed microbials and yeast culture to stimulate the natural ability of the microbial population in the rumen to digest fibrous substrates.
Recent advances in fermentation technology and biotechnology have allowed for the economic production of large quantities of biologically active enzymes that can also be used as livestock feed supplements. These technologies provide new possibilities for altering digestive processes in a wide variety of animals. Specific enzyme preparations with specific activities can be designed and added as feed supplement. Such preparations can be used to drive specific metabolic and digestive processes in the gastrointestinal tract and may augment natural digestive processes to increase nutrient availability and feed intake.
In the last decade, specific enzyme preparations have become valuable tools for economically manipulating digestive processes in monogastric animals and poultry (Johnson et al., 1993; Annison and Choct, 1993); but there has also been considerable interest in using enzymes as supplements for ruminant diets (Dawson, 1993; Feng et al., 1996; Lewis et al., 1996; Annison, 1997; Howes et al., 1998). Strategies that use supplemental enzyme activity in the rumen may be important since the digestibility of organic matter in the rumen does not reach 100% and even small changes in digestibility can influence the efficiency of ruminal fermentations.
While it is possible to define a number of enzyme systems or preparations that could be used to enhance digestive processes in the rumen, most enzyme applications in ruminants have focused on the use of polysaccharide-digesting enzymes to enhance the degradation of the slowly digested substrates in plant fiber. Two major types of enzyme activities are commonly considered in this respect. Cellulases are usually complex mixtures of proteins that act to hydrolyze ß1-4 linkages connecting the individual sugar molecules that make up the cellulose found in forages. In comparison, xylanases are enzymes that act on the xylans found in the hemicellulose fraction of many forages and in the fiber fractions of grains. Neither of these enzyme systems is found in the array of digestive proteins produced by animals and both are produced commercially using microbial fermentation processes.
In the past, the application of enzymes in ruminants has mainly focused on their use in young pre-ruminant animals or as a pretreatment for ruminant feed materials. Direct feed applications of exogenous enzymes to provide activities in the gastrointestinal tract of mature animals were not considered because of the potential destruction of the enzyme by the microbial population in the rumen. However, a number of recent microbiological studies and production trials have suggested that the addition of active enzyme preparations to ruminant diets prior to feeding may have a significant impact on microbial and digestive activities in the rumen. Enzyme-induced changes in digestive processes are reflected in both altered ruminal fermentation patterns and improved animal performance. This paper will review some of the recent data demonstrating the important effects of specific enzyme preparations on microbial activities in the rumen and discuss their impact on the performance of dairy cattle.
Effects of specific exogenous enzymes on disappearance of fibrous substrates
A number of in vitro studies have demonstrated that it is possible to use specific fibrolytic enzyme preparations to enhance the processes associated with fiber digestion in the rumen. This can take place at several levels but is often associated with an increase in the initial rate of fiber disappearance.
The response is generally measured as an increase in the initial rate of dry matter disappearance (Table 1) or as an increase in the rate of neutral detergent fiber (NDF) disappearance in batch cultures of ruminal bacteria that have been supplemented with enzyme preparations. This activity is believed to be the result of the ability of the enzyme preparation to render the fiber soluble or more available for microbial attack in the rumen. The most active period for enzyme effects appears to be during the first 6-12 hrs of the digestive process. In most instances, fibrolytic enzymes appear to have no significant effects on the total extent of fiber digestion after extended incubation periods.
The ability of specific exogenous enzyme systems to increase the initial rate of fiber digestion is important in many animal production systems because it influences the processes which control feed intake. In vitro digestion during the first 6-12 hrs of fermentation has been found to be closely correlated with voluntary dry matter intake in ruminant animals and is believed to be a measure that reflects the effects of plant fiber on the relative dry matter intake of ruminants fed forage based diets (Van Soest, 1994). As a result, the observed increases in the initial rate of fiber disappearance are consistent with the ability of certain fibrolytic enzyme systems to enhance feed intake in practical production systems.
However, most of the available data have indicated that this type of enzyme activity can be influenced by the nature of the fiber, the age and maturity of the plant material, the nutritional status of the animal and the types of enzyme systems provided in the supplement. As a result, it is not yet possible to consistently predict responses to many types of enzyme supplements in all feeding and management systems.
Effects of specific exogenous enzymes on in vitro ruminal microbial activities
While it is clear that exogenous enzymes can influence the solubilization of some types of dietary fiber, other enzyme-induced changes in ruminal microbial activities probably have the most significant impact on the overall production response to enzyme supplementation. A number of changes in ruminal fermentations have been associated with enzyme supplementation (Table 2). Several groups throughout the world are working to document and model these changes in ruminal fermentation. Currently, many of these changes can be demonstrated in batch cultures of ruminal bacteria by measuring the individual volatile fatty acid (VFA) production rates and by stoichiometrically estimating the amount of carbohydrates that are degraded to produce these volatile end products.
Studies of a number of enzyme preparations clearly show differences in the way individual enzymes and mixtures of enzymes alter ruminal microbial activities (Table 3). For example, use of enzyme preparations based on high levels of xylanase activity have been shown to enhance total VFAproduction and overall carbohydrate utilization by a ruminal microbial population in batch cultures provided with ground fescue hay as a substrate. This is consistent with the overall effects observed in studies that demonstrate enhanced dry matter digestion and fiber digestion.
However, enzyme preparations that contain measurable levels of fungal cellulase are more inclined to alter the relative proportions of VFA produced by a population of ruminal bacteria. This reflects rather dramatic changes in the physiological activities of the ruminal bacteria and can lead to a more efficient fermentation. These shifts in activity result in more propionate and butyrate production and less acetate production by the microbial population in the presence of the enzyme supplement. Combinations of the two enzyme systems not only increased the relative amounts of carbohydrate digested, but also beneficially decreased the acetate:propionate ratio.
Such changes in the rumen suggest that certain types of enzyme preparations cannot only have a significant impact on the digestive process, but may also have dramatic effects on the overall fermentation activity in the rumen resulting in improvements in the efficiency of nutrient utilization. The fact that some specific enzymes can improve the efficiency of ruminant production without any concomitant increase in feed intake suggests that such fermentation shifts are important in explaining some of the overall production responses (Beauchemin et al., 1995).
Effects of a specific exogenous enzyme preparation on the ability of ruminal bacteria to degrade fiber
Fibrozyme is an enzyme supplement for ruminants prepared from the fermentation extracts of Aspergillus niger and Trichoderma longibrachiatum. It has been formulated on the basis of its ability to degrade the xylans found in the hemicellulose fraction. As a result, it was designed to aid in the breakdown of the fibrous matrix in plant structural carbohydrate complexes and expose many of the bound nutrients to digestive activities in the rumen. It may be especially useful in attacking the hemicellulose which makes up 80 % of the fiber found in grains. The enzyme preparation also contains measurable amounts of cellulase and protease activity that may augment other enzymatic activities in the rumen.
As a result, its digestive activities are not limited to attack on hemicellulose. This enzyme preparation is of particular interest because many of the component proteins are produced in a naturally glycosylated form. Glycosylation is a post-translational modification of the protein molecule that is believed to be important in the processes that lead to secretion of the enzyme during production and in protecting the enzymes from destruction by the protein-degrading enzymes in the rumen (Annison, 1997; Chesson, 1993). This gives these enzymes a distinct advantage in the rumen where naturallyoccurring proteolytic enzymes work to actively destroy proteins that enter with the feed. The ability of such fungal enzyme systems to withstand ruminal degradation has been demonstrated in a number of studies (Hristov et al., 1996).
Studies have examined the effects of Fibrozyme on ruminal fermentations and have established its ability to alter microbial activities in the rumen. These effects can be rather dramatic and can be measured by removing ruminal fluid from animals receiving the enzyme supplement. Rumen fluid samples from animals receiving Fibrozyme have been consistently shown to have an enhanced ability to degrade and utilize the carbohydrates in grass hay (Table 4). These activities were often associated with an increase in the digestive activities during the initial stages of digestion and were often more pronounced in rumen fluid from animals receiving a high level of grain supplement than in rumen fluid from cattle receiving a hay based diet.
It is interesting to note that the enhanced digestive abilities of these populations is not always reflected in in vitro dry matter digestion (IVDMD), but is often readily observed when examining the overall carbohydrate utilization (mM carbohydrates degraded) by the microbial population during a 12 hr incubation period (Table 4). Such studies have established a role for Fibrozyme and similar enzyme preparations in strategies for enhancing overall microbial activities in the rumen. The enzyme preparations appear to have the ability to change the way microbial populations in the rumen degrade and process fibrous substrates. Optimal applications of exogenous enzymes will require special consideration of diets and management practices, but it is clear that these types of preparations will have a place in modern ruminant feeding programs.
Effects of Fibrozyme on milk production in field trials
While laboratory studies have demonstrated the potential effects of Fibrozyme on digestive processes in the rumen, several recent field trials have demonstrated the impact of this exogenous enzyme preparation on the performance of dairy cattle in practical management systems (Table 5). Improved milk production in cattle receiving total mixed rations (TMR) or pelleted diets supplemented with 15 g of Fibrozyme per head per day range from no increase in milk production to an increase of 6.2 lbs of milk per cow each day in high-producing dairy cows.
Such changes in milk production are often associated with increases in voluntary intake, but may also be the result of increases in the efficiency of the ruminal fermentation process as reflected in altered ruminal fermentation patterns. It is clear from the field data that responses to exogenous enzyme supplementation can be quite variable. This may be partially due to the inability of specific enzyme preparations to enhance the digestion of certain complex structural carbohydrates. Factors such as the maturity of the forage or the amount of lignification can influence the composition and structure of plant polysaccharide and can inhibit the activities of some enzyme preparations. Such factors could be expected to limit the ability of the enzyme preparation to stimulate animal performance. It will be important to identify plant, animal and microbial factors that influence exogenous enzyme activities in the rumen before predictable reponses to enzymes can be obtained.
Defining appropriate dose rates for enzyme preparations
The level of enzyme supplementation is an important consideration when defining strategies for using specific enzyme preparations. Both in vitro digestion studies and production studies suggest that ruminal responses to different levels of enzyme supplementation are not linear and that optimal activities of specific enzyme preparations occur over narrow ranges of inclusion rates.
Inclusion of enzyme above the optimal level can depress microbial actives in in vitro systems (Figure 1). This type of depressed activity appears to be reflected in performance responses and has been demonstrated in studies with beef cattle (Figure 2). This characteristic of enzyme supplements can account for the lack of response to high levels of enzyme supplementation in dairy cattle (Sanchez et al., 1996). The fact that it is possible to both over-feed and under-feed enzymes makes their application complex.
The reason for depressed responses with increased levels of enzyme supplementation is not clearly understood, butmay reflect the abilities of certain enzymes to down-regulate or depress the normal enzyme systems produced by the ruminal microbial population. Such depressions emphasize the need for dependable systems for evaluating enzyme activities and their effects on microbial activities in the rumen.
Basic mechanisms which explain the effects of fibrolytic enzymes in the rumen
Exogenous enzyme preparations are often used at low concentrations. At these concentrations the enzymes would not be expected to contribute to extensive amounts of fiber digestion in the rumen even if conditions were optimal for their activities.
For example, providing an animal with 1500 IU of xylanase, could only result in a 1 % increase in the soluble fibrolytic enzymes are related to their ability to enhance the initial degradation of plant structural carbohydrates and complement normal enzymatic activities associated with ruminal microorganisms. It is likely that exogenous enzymes act in the rumen shortly after feeding and during a period just prior to bacterial colonization and prior to the time when naturallyoccurring microbial enzymes develop and initiate fiber digestion.
As a result, the exogenous enzymes can be expected to complement the endogenous enzyme systems associated with the fibrolytic activities of the microorganisms in the rumen and allow for greater digestion of fibrous substrates during the critical preliminary stages of digestion. The overall effects of enzyme supplementation may be a result of their ability to expose slowly-degraded substrates to microbial attack.
Conclusions
Recent studies with specific enzyme supplements have shown that some of these preparations can enhance the solubilization of the fibrous components of ruminant diets. These activities can improve intake and stimulate animal performance. However, some enzyme supplementation strategies can also beneficially alter microbial activities in the rumen and change ruminal fermentation patterns. Some of these changes may have dramatic effects on production efficiency and may account for a large proportion of the beneficial activities associated with the use of low levels of exogenous enzyme in ruminant feeds.
References
Annison,G. 1997. The use of enzymes in ruminant diets. In: Biotechnology in the Feed Industry, Proceedings of the 13th Annual Symposium. (T. P. Lyons and K. A. Jacques, eds.). Nottingham University Press, Nottingham, Leics., UK. p.115.
Annison, G. and M. Choct. 1993. Enzymes in poultry diets. In: Enzymes in Animal Nutrition: Proceedings of the 1st Symposium. Institut fur Nutzienwissenschaften, Zurich. (C.Wenk and M. Boessinger, eds.). p. 61.
Beauchemin, K. A., L. M. Rode and B. J. H. Sewlt. 1995. Fibrolytic enzymes increase fiber digestibility and growth rate of steers fed dry forages. Can. J. Anim. Sci. 75:641.
Chesson, A. 1993. Feed enzymes. Anim. Feed Sci. and Technol. 45:65. Dawson, K. A. 1993. Probiotics and enzymes in ruminant nutrition. In: Enzymes in Animal Nutrition: Proceedings of the 1st Symposium. Institut fur Nutzienwissenschaften, Zurich. (C.Wenk and M. Boessinger, eds.). p. 89.
Feng, P., C. W. Hunt, G. T. Pritchard and W. E. Julien. 1996. Effect of enzyme preparations on in situ and in vitro degradation and in vitro digestive characteristics of mature cool-season grass forage in beef steers. J. Anim. Sci. 74:1349-1357.
Histrov, A., T. A. McAllister and K. -J. Chen. 1996. Exogenous enzymes for ruminants: Mode of action and potential applications. Proc. of the 17 Western Nutrition Conference, Edmonton, Alberta.
Howes, D., J. M Tricarico, K. A. Dawson and P. Karnezos. 1998. Fibrozyme: The first protected enzyme for ruminants: Improving fiber digestion and animal performance. In: Biotechnology in the Feed Industry, Proceedings of the 14th Annual Symposium (T.P. Lyons and K.A. Jacques, eds.) Nottingham University Press, Nottingham, UK. p.393.
Johnson, R., P.Williams and R. Campbell. 1993. Enzymes in pig production. In: Enzymes in Animal Nutrition: Proceedings of the 1st Symposium. Institut fur Nutzienwissenschaften, Zurich. (C.Wenk and M. Boessinger, eds.). p.49.
Lewis, G. E., C .W. Hunt, W. K. Sanchez, R. Treacher, G. T. Pritchard and P. Feng. 1996. Effect of direct-fed fibrolytic enzymes on the digestive characteristics of a forage-based diet fed to beef steers. J. Anim. Sci. 74:3020-3028.
Tricarico, J. M., K. A. Dawson and K. E. Newman. 1998. Effects of an exogenous microbial enzyme preparation (Fibrozyme) on ruminal digestion of fescue hay. J. Anim. Sci. 76 (Suppl. 1):289.
Sanchez,W. K., C.W. Hunt, M. A. Guy, G. T. Pritchard, B. Swanson, T. Warner and R. J. Treacher. 1996. Effect of fibrolytic enzymes on lactational performance of dairy cattle. Proceedings of the American Dairy Science Association. Corvallis Oregon.
Van Soest, P. J. 1994. Nutritional Ecology of the Ruminant (Second Edition). Cornell University Press, Ithaca. p. 119.
Authors: KARL A. DAWSON and JUAN M. TRICARICO
Department of Animal Sciences, University of Kentucky, Lexington, Kentucky, USA
Author: KARL A. DAWSON and JUAN M. TRICARICO (Courtesy of Alltech Inc.)
Who saw this article? New!
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