Use of enzymes in ruminant diets

Enzymes have been used extensively in the diets of poultry and pigs, and as silage additives, but, until recently, not much in the diets of ruminants (Beauchemin et al., 1995; Beauchemin et al., 1999a; Yang et al., 1999), due to the view that feed enzymes would be rapidly degraded in the rumen before they could have any effect (Beauchemin et al., 1995; Beauchemin et al., 1999a; Kung et al., 2000; Bowman et al., 2002a) and due to the high cost of enzymes and inconsistency of responses (Yang et al., 1999).

High costs of livestock production, the potential economic returns from effective enzyme supplementation, and the availability of new enzyme preparations have renewed interest in the potential use of feed enzymes for ruminants (Yang et al., 1999; Beauchemin et al., 2000; Yang et al., 2000).

A growing body of evidence shows that spraying enzymes directly onto feeds, just before feeding, can improve animal performance (Kung et al., 2000) by increasing dry matter intake, particulate passage rate and digestibility of dry matter and fibre (Lewis et al., 1999).

Fibrolytic enzymes applied, in the right amount, to the forage portion of rations prior to feeding improves lactational performance of early and mid-lactation cows as well as growth in steers (Lewis et al., 1999).

Results have been inconsistent and not all responses have been positive (Yang et al., 1999; Kung et al., 2000; Yang et al., 2000; Bowman et al., 2002a).

Responses to enzyme supplementation differ for dry and fresh forages and silage, and when the enzyme is infused into the rumen, applied to the whole diet or a component of the diet (Yang et al., 2000).

This paper looks at the research that has been done on the use of exogenous enzymes in ruminant diets.

Classification of enzymes and enzyme preparations

There are hundreds of enzyme products marketed for livestock. These are all derived mostly from four bacterial species (Bacillus subtilis, Lactobacillus acidophilus, L. plantarum, and Streptococcus faecium) and three fungal species (Aspergillus oryzae, Trichoderma reesei and Saccharomyces cerevisiae) (McAllister et al., 2001).

Complete digestion of complex feeds requires hundreds of enzymes (McAllister et al., 2001). Enzyme preparations for ruminants are mainly marketed based on their capacity to degrade plant cell walls (cellulose and xylan) and are referred to as cellulases and xylanases (Morgavi et al., 2000a; McAllister et al., 2001).

These preparations contain many enzymes, including secondary enzymes such as amylases, proteases or pectinases. The degradation of cellulose and hemicellulose alone requires several enzymes. Differing proportions and activities of these enzymes has an impact on the efficacy of cell wall degradation by the product (McAllister et al., 2001). A blend of cellulase and xylanase is more effective than cellulase alone (Schingoethe et al., 1999).

The type and activity of enzymes produced can vary a lot, even within a single microbial species. Enzyme products for ruminants are usually standardised by blending crude enzyme extracts to obtain specified levels of one or two enzyme activities (McAllister et al., 2001). The effectiveness, rate of passage and stability of exogenous enzymes differs among enzyme preparations and types. Because of the difference in commercial preparations, the enzyme activity needs to be described when a study is done. Commercial suppliers need to provide information such as activity rates and enzyme types (ZoBell et al., 2000).

Effects of feed enzymes in forage diets Direct application of fibrolytic enzyme preparations to forages has been shown to improve fibre digestibility (Lewis et al., 1996; Beauchemin et al., 1999a). Adding fibrolytic enzymes to grass hay before feeding beef steers was found to increase dry matter intake (DMI), rate of passage, and digestibility of dry matter (DM), neutral detergent fibre (NDF) and acid detergent fibre (ADF) (Lewis et al., 1996).

Feng et al. (1996) and Beauchemin et al. (1999b) found a reduced ruminal retention time of particles with enzyme supplementation indicating a faster rate of particle size reduction in the rumen which contributes to the greater intake (Feng et al., 1996).

Form of forage to which the enzymes are added

When enzymes are added directly to grass there is a greater DM and NDF disappearance, compared to the control, when the enzymes are applied to wilted or dried grass but not if they are added to fresh or rehydrated grass (Feng et al., 1992; Feng et al., 1996). This may be because when the fresh grass was dried the enzyme activity might have been reduced (Feng et al., 1996).

When to add the enzymes

Maximum benefit can be achieved from the enzymes when they are direct-fed (added directly before feeding) rather than applied at harvest (Feng et al., 1996).

Effect of enzymes in concentrate diets

Enzyme mixtures have also been shown to be beneficial in high concentrate diets for dairy and feedlot cattle, probably by helping to overcome the depression in fibre digestion caused by low ruminal pH (Lewis et al., 1996; Beauchemin et al., 1999a; Morgavi et al., 2000a).

Effect of enzymes on milk production in dairy cows

Cows on enzyme treated diets have been reported to produce 5 to 25% more milk (Yang et al., 1999).

Lewis et al (1999) found that cows in midlactation consuming forages treated with enzymes (cellulases and xylanases) produced more milk (27.2 vs. 25.9 kg/d) of the same composition, and digested more DM per day than cows on the control forage. They also gained weight instead of losing it indicating that there was more energy available over and above that required for the extra milk production.

In a study by Schingoethe et al. (1999) milk production increased two to four weeks after the cows started consuming the treated forages and the response was maintained throughout the rest of the experimental period.

Kung et al. (1997) found no effect on milk production, milk composition or DMI when the cows received a maize-based topdressing containing 10 g/d of a supplement containing a live yeast culture (Saccharomyces cerevisiae) and enzymes. However in another experiment they fed high producing cows in early lactation the supplement at 0, 10 or 20 g/d. These cows produced 36.4, 39.3 and 38.0 kg of milk (3.5% FCM) per day respectively.

Inconsistencies in results

Many of these studies have shown increased milk production with enzyme supplementation while others have shown no effect (Yang et al., 2000). Results with beef cattle have also varied (ZoBell et al., 2000).

These inconsistencies could be due to factors such as diet composition (differing substrates), type of enzyme preparation, complement of enzyme activities, amount of enzyme applied and to what fraction of the diet, enzyme stability, method of application and animal differences as well as whether the cows are in a negative energy balance and able to respond to increased available energy (Yang et al., 1999; Yang et al., 2000; Morgavi et al., 2000b; Knowlton et al., 2002; Bowman et al., 2002a).

Enzyme level

Response to level of enzyme supplementation is not linear, thus the optimum concentration needs to be investigated (Beauchemin et al., 2000).

Optimum enzyme levels depend on the type of forage (Beauchemin et al., 1995).

Digestibility increases more with the addition of a certain level of enzyme mixture (probably about 2 g per kg feed) than with lower or higher levels (Lewis et al., 1999; Yang et al., 1999; Beauchemin et al., 2000).

Over-treatment with enzymes could result in less chewing of forages and less saliva production and thus lower rumen pH and fibre digestion which could result in lower milk production. Excessive binding of enzymes to the substrate might hamper attachment of rumen microorganisms to the fibre. High concentrations of enzyme treatment could release antinutritional factors (such as phenolic compounds) reducing microbial digestion (Kung et al., 2000).

Factors such as substrate specificity, moisture level of the feed, time required for enzymes to interact with the feed, pH, and temperature of the feed could affect binding of the enzyme with the substrate and are therefore important (Beauchemin et al., 1995).

Method of enzyme application

In order to maximise the benefits of using fibrolytic enzymes in dairy cow diets, the method of enzyme delivery needs to be considered (Yang et al., 2000). As forages are consumed by ruminants in a variety of ways, evaluating a variety of methods of delivering exogenous enzymes is necessary (Lewis et al., 1996).

In vitro digestion is improved if the enzyme is sprayed onto the diet 24 hours before incubation but not if the enzyme is added directly into the digestion vessel (Morgavi et al., 2000b). Enzyme effectiveness is enhanced by applying the enzyme to the feed rather than direct infusion into the rumen, and to dry rather than moist feeds, but there appears to be no difference in effect when enzymes are applied to concentrate or dry forage (Rode et al., 1999).

Digestibility and milk production are increased when the enzymes are added to the concentrate or forage but not if to the whole TMR (Yang et al., 2000).

The higher digestibility of forages that have had enzymes applied before feeding than when enzymes are ruminally infused, suggests that it is more effective to apply enzymes directly to the forage than to infuse them ruminally (Lewis et al., 1996; Yang et al., 1999).

This may be because adding the enzyme to the feed enhances the binding of the enzyme to the feed substrate (Yang et al., 1999; Yang et al., 2000), or there could be a conformational change (Kung et al., 2000), increasing the resistance of the enzymes to proteolysis and lengthening their residence time in the rumen (Yang et al., 1999, Kung et al., 2000; Yang et al., 2000). When ruminally infused the enzymes could pass from the rumen before sufficient contact with the forage has taken place (Lewis et al., 1996) or the enzymes could be degraded by ruminal bacterial proteases (Lewis et al., 1996; Kung et al., 2000).

Proportion of the diet to which the enzymes are applied

The proportion of the diet to which the enzyme is applied must be considered to ensure maximal beneficial response (Bowman et al., 2002a).

Digestion is increased when the enzyme is added to the concentrate portion of the diet but not if only to the supplement or premix. The reason for the ineffectiveness of the enzyme applied to the supplement could be that the high temperatures of the pelleting process could reduce the activity of the enzyme.

An alternative for future studies could be to add the enzyme after the pelleting process (Bowman et al., 2002a).

Better digestion when the enzyme is applied to a larger proportion of the diet could be due to better dispersion of the enzyme in the rumen. Applying enzymes to feed could create a slow release mechanism by which the enzyme is released into the rumen fluid as the feed is digested. Premix has a low residence time in the rumen reducing the time for the enzyme to be released and have its effect (Bowman et al., 2002a).

When should animals receive enzyme supplementation?

Since exogenous enzymes increase the digestibility of the diet (Rode et al., 1999), the beneficial effects of feed enzymes on animal performance could be the greatest for ruminants in a negative energy balance, such as early lactation cows (Rode et al., 1999; Beauchemin et al., 2000).

Rode et al. (1999) found that supplementing the diets of early lactation dairy cows with a fibrolytic enzyme mixture can enhance milk yield and nutrient digestibility. The magnitude of response was much higher in this study, using early lactation cows, than in previous studies with cows in mid-lactation (Rode et al., 1999).

This is in agreement with the study by Schingoethe et al. (1999) where they found that cows that started receiving the enzyme treated forage during the first 100 days postpartum produced 9 to 15% more milk than cows fed the control diet. Production was not increased when cows were in midlactation when the experiment was started (Schingoethe et al., 1999).

When to start supplementing enzymes to dairy cows

The best time to start feeding enzymetreated forages would most likely be soon after parturition. There is no added advantage to starting prepartum, and starting a few weeks after calving would only delay the increased production caused by enzyme treatment (Zheng et al., 2000).

In the study by Rode et al. (1999) the higher milk yield occurred within the first week of lactation (when the enzyme supplemented diet was supplied immediately after calving), although in the study by Schingoethe et al. (1999) the response only occurred in the second to fourth week.

From the above it can be concluded that dairy producers can benefit most if they start feeding enzyme treated forages early in lactation (Schingoethe et al., 1999).

When is it optimal to feed enzymes to feedlot steers

Feedlot calves are the most receptive to enzyme treatment in the beginning of the feeding period (Dvorak, 2000).

Effect of enzymes on pollution

Using enzymes to improve digestion would also reduce faecal material and hence pollution (Kung et al., 2000) as well as supplementing enzyme activity in the faeces, speeding up the decomposition of waste (McAllister et al., 2001). Knowlton et al. (2002) found a decrease in excretion of faeces and N only in later lactation cows.

Phytase supplementation

Some of the phosphorous (P) in the diet is provided in the form of phytate. In order to be used by the animal, the P has to be released from phytate by the enzyme phytase, which is produced by the rumen ecosystem. Rumen phytase activity could be saturated by the high levels of phytate phosphorous in diets rich in grains (Bravo et al., 2002). This justifies investigating phytase supplementation in ruminant diets.

Bravo et al. (2002) investigated the effects of fungal phytase addition on P solubility (availability) in lactating goats and dry cows. They found that fungal phytase supplementation increased solubility of rumen P when applied to a concentrate diet, but not when applied to a forage diet.

This could be because phytase supplementation could have improved starch degradation, as phytate P binds with starch. Fungal phytase activity is highly dependent on pH, so the lower pH caused by the concentrate diets probably led to increased phytase activity, while the rumen pH induced by the forage was probably less optimal for phytase.

Potential mode of action

The mode of action of exogenous enzymes is very complex and needs to be understood before enzyme supplementation can be used to improve the efficiency of feed utilisation in ruminants and reduce waste production (McAllister et al., 2001). Several potential modes of action as to how exogenous fibrolytic enzymes improve feed utilisation in ruminants have been proposed.

Exogenous enzymes alter feed utilisation in ruminants through their effects on the feed before ingestion, and enhancing digestion in the rumen and/or postruminal digestive tract (Morgavi et al., 2000a; McAllister et al., 2001). These effects are intertwined; the enzyme mediated alterations to the feed before ingestion likely affect the ruminal and post-ruminal digestion of the nutrients (McAllister et al., 2001).

The exogenous enzyme mixture could have a pre-ingestive effect by starting to attack the plant fibre and removing structural barriers that limit microbial digestion or releasing soluble carbohydrates. This could make ingestive mastication easier, reducing the time spent eating per unit fibre (Beauchemin et al., 2000; McAllister et al., 2001; Bowman et al., 2002a).

Pre-ingestive enzyme/feed interaction is necessary for there to be any significant beneficial effects on ruminal digestion.

Longer incubation before feeding results in greater changes to the fibre structure that can be exploited faster. The necessary pre-ingestive feed/enzyme interaction would not take place if the enzyme was applied to the feed as a solid preparation. These findings have practical implications as to whether the enzyme is applied as a solid or a liquid and how long the enzyme is left on the feed before feeding (Nsereko et al., 2000).

Enzymes enhance the hydrolytic capacity of the rumen due to added enzyme activities (acting directly on the feed) and synergy with rumen microbes by stimulating numbers of rumen bacteria (Kung et al., 2000; Morgavi et al., 2000a; Nsereko et al., 2000; McAllister et al., 2001). They may remain active in the lower digestive tract, contributing to the post-ruminal digestion of fibre, or they could indirectly improve nutrient absorption in the lower tract by reducing viscosity of intestinal digesta (McAllister et al., 2001).

Exogenous fibrolytic enzymes may accelerate the rate of digestion by enhancing attachment to, and colonisation of, plant cell wall components of feeds, and improving access to the cell wall matrix by ruminal microorganisms (Nsereko et al., 2000).

Enzymes could also change the site of nutrient digestion or improve the palatability and change the pattern of feed consumption (Kung et al., 2000).

Enzyme stability

Enzyme mixtures remain effective for at least several months after being applied to the feed (Yang et al., 1999) and contain sufficient activity to be effective at very low temperatures (Schingoethe et al., 1999).

Until recently it was assumed that unprotected enzyme feed additives would be inactivated in the rumen since dietary proteins are usually rapidly degraded in the rumen (Morgavi et al., 2001).

However, not all enzymes are subject to extensive degradation by microbial proteases in the rumen (Yang et al., 1999). Different enzyme additives have been shown to be more stable in the rumen depending on the origin and type of activity (Hristov et al., 1998; Morgavi et al., 2001). Applying enzymes to dry feed in a liquid form increases adsorption of enzymes to the feed, which could increase the resistance of the enzymes to proteolysis and prolong their viability in the rumen (Beauchemin et al., 1999a).

Many fungal and bacterial cellulases and xylanases are glycosylated which may partially protect them from degradation by proteases (Kung et al., 2000). Significant portions of exogenous enzymes escape ruminal digestion and remain active in the small intestine (Hristov et al., 1998; Yang et al., 1999; Morgavi et al., 2001).

Susceptibility of the enzymes to proteases is influenced by secondary and tertiary protein conformation. Differences could also be attributed to carriers and stabilisers, thus manufacturing practices influence stability. The presence of substrate also increases enzyme resistance to proteolytic inactivation, which is why adding feed enzyme additives to the feed contributes to their stability (Morgavi et al., 2001).

Due to variation amongst cows, some cows could have a more favourable ruminal environment for the expression of exogenous enzyme activity, and could therefore benefit more from the additions of enzyme additives to the diet (Morgavi et al., 2001).

Practical implications

Feeding enzymes allows for more economical milk production by making feed utilisation more efficient, or increasing the amount of forage in the ration or replacing it with lower quality forages while sustaining previous milk production levels (Lewis et al., 1999).

There is no risk of residues in tissues or milk (Beauchemin et al., 1999a).

In order to apply this technology successfully, a more detailed knowledge of exogenous enzyme interactions with the feed, the host and the rumen microorganisms is necessary (Morgavi et al., 2000a). Variability of results must be reduced for enzyme supplementation of ruminant diets to gain acceptance in the cattle industry (Bowman et al., 2002a).

The enzymes must match the feed (Yang et al., 1999; McAllister et al., 2001). In order to engineer enzyme preparations to overcome the constraints in feed digestion, the factors that limit the rate and extent of digestion need to be known.

For example, in maize the protein matrix surrounding the starch granules dictates the rate and extent of starch digestion in the grain. Thus exogenous enzymes designed to improve the utilisation of maize should contain proteases, rather than amylases, so that the protein matrix can be digested, exposing the starch granules to digestion by endogenous ruminal or host enzymes (McAllister et al., 2001).

Putting the experimental results into practice is not easy. Comparison between experiments is very difficult because many enzyme products are poorly defined. Applying an enzyme preparation at a certain concentration gives little information with regards to potential effects on animal performance if a different product is applied or even if this one is applied at a different concentration or in a different way. Thus it is important to understand the mode of action to obtain consistent positive responses (McAllister et al., 2001).

More studies need to be done on the mode of action (Beauchemin et al., 1999b ; Bowman et al., 2002a). The best method of application and specific, optimal enzyme concentrations also need to be determined (Kung et al., 2000; Bowman et al., 2002a) so that specific enzyme supplements can be formulated and used with a high success rate (Rode et al., 1999; Beauchemin et al., 2000). The amount applied, and hence the cost, can be minimised by including the right enzymes in the preparation (McAllister et al., 2001).

Conclusion

Future use of enzymes as direct-fed additives depends on cost and convenience of application (Feng et al., 1996). To ensure ‘on-farm’ efficacy of enzyme products factors such as product formulation, method of application, animal effects and the amount of enzyme applied need to be considered (Bowman et al., 2002b). A lot more research needs to be done before commercial exogenous enzymes should be made available to commercial cattle producers. Research needs to focus on the substrates, management practices which could also affect results, as well as time, rate and method of enzyme application (ZoBell et al., 2000). Fine tuning needs to be done before enzyme supplementation can be a consistently cost-effective option for ruminant diets.

References

The references are available from the author or AFMA.

Author: Evelyn Malleson
University of Pretoria

The use of enzymes in ruminant diets - Image 1

The previous article is a special collaboration from AFMA South Africa
(Animal Feed Manufacturers Association) and their magazine AFMA Matrix.
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