Feed enzymes support the challenge of growing food demand
The growing human population will create an increasing demand for food, including meat and other animal protein products. It is expected that poultry demand will grow fastest, followed by pigs. Aquaculture will increase as well, but from a small base. Ruminant growth will be less strong than monogastrics, but dairy expansion is predicted in China. In order to support the rising demand for feed, animals may consume different diets in the future compared with those offered today. A trend is developing in which coproducts and byproducts are incorporated into monogastric feeds at levels not considered prior to the year 2000.
Although the inclusion of these alternative products will depend on the price of grains and soy, the move towards more consistent use of poorer feed ingredients is likely to grow over the long term. Feed ingredients also bring with them anti- nutrient factors (ANFs) which reduce the efficiency of absorption of the nutrients and therefore, raise the cost of producing the protein. Both efficient animal production and environmental management face significant challenges unless these poorer ingredients are made more nutritious, thus achieving a consistently efficient protein production, improved health and lower environmental footprint. The use of enzymes will be integral to meeting this challenge by providing a means of reducing the effects of the ANFs, breaking down feed components that the animal cannot and releasing more nutrients.
Enzymatic activity is substrate specific. Therefore, the benefit of an individual enzyme may be calculated independently, whether or not it is used in combination with other enzymes and additives. Combinations may exhibit additional improvements beyond the measured release of energy, amino acids and minerals, such as better-balanced gut microbiota. Consequently, the major enzymes today, phytase and xylanase, individually or in addition to the increasingly important protease, will complement each other due to their actions on different substrates in the animal's gastrointestinal tract. While protease is a more recent addition to the feed enzyme portfolio as a mono-component product, phytase and xylanase have widespread use, particularly in poultry and swine feeds. Even when alternative feed ingredients are not used, these enzymes are necessary to act upon specific substrates, as ANFs are present in all raw materials.
Phytate serves as a phosphorus (P) reservoir during seed germination and acts as a protectant against oxidative stress during the life of the seed. It is present as a mineral-phytate complex and the majority of P in feedstuffs of plant origin is present as phytate-P. The level of total phytate-bound P may be as high as 80 percent, as seen in rice bran. Exact levels in typical feed vary considerably within and between feed ingredients. One issue arising from the presence of phytate-P is that the undigested P will be excreted and creates an environmental hazard. Alternative sources of P include minerals (such as dicalcium phosphate) and meat and bone meal, in which the P is highly digestible and thus may balance the diet for the animal, but do little for the environment. Releasing the P from phytate reduces the environmental load and also reduces the cost of the feed, as other sources of P are required at lower levels.
Additionally, phytate chelates other minerals, such as Zn, Cu, Ca and Fe, reducing the availability of these minerals. Also, phytate has the capacity to bind protein, which in turn may depress amino acid digestibility. In poultry, particularly, phytate depresses energy utilisation as well.
The assumption for microbial phytase is that optimum activity occurs at a low pH and, therefore, phytase is active in the gizzard and proventriculus of broilers, with the latter, particularly, having a pH of around 2. In pigs, the main site of activity is the stomach. The newest generation of phytases most probably will complete their activity in the acid environment of the stomach.
The advent of bacterial phytases raised the level of bioefficacy in animal feed. The latest generation phytases offer further improvement as indicated by their higher matrix values, which are highly dependent on the ingredients and test conditions. At the same time, further benefits may be ascribed to phytase, as more is understood of its mode of action. Ongoing research continues to reveal further value of phytase to the producer.
Non-starch polysaccharides (NSPs) belong to a group of carbohydrates referred to as dietary fiber. NSPs are poorly digested in the small intestine, but are completely or partially digested by microbes leading to short chain fatty acids (SCFAs) that may be absorbed from the small or large intestine as a result of fermentation. NSPs are divided into cell wall components and non-cell wall components and include cellulose, hemicellulose, pectins and hydrocolloids. Xylan is the major component of hemicellulose and is the second most abundant polysaccharide in nature after cellulose. Hemicelluloses are storage polymers in seeds and structural components of woody plants.
Cereal grains such as wheat and barley, due to the nature of their soluble NSP levels, raise the viscosity in the intestine, which slows down feed intake, has an unwanted effect on bacterial proliferation and entraps nutrients. The main target for xylanase in corn is the destruction of the endosperm wall, thereby releasing trapped proteins and starch. Higher levels of insoluble fiber, found in wheat byproducts and coproducts from the bioethanol industry (corn- or wheat- based Dried Distillers Grain Solubles; DDGS), would speed the passage of nutrients through the gut, reducing the potential for absorption. Xylanase militates against this effect and should permit the greater use of raw materials with lower nutritional value, thereby increasing the flexibility of feed formulation and reducing feed cost. There should also be a reduction of faecal mass.
Xylanases tend to have an optimum pH activity that is close to neutral. Evidence suggests that in broilers the crop, normally alkaline, mostly is bypassed in ad libitum feeding regimens and the gizzard and the proventriculus have an acid environment. The small intestine pH varies from mildly acid to mildly alkaline, which generally results in pH levels too high for some xylanases. The moisture content of the digesta in the anterior gut is low too, which is not conducive to enzymatic activity, which requires a reasonable level of moisture. Given this information, the non-starch polysaccharide (NSP) enzyme activity in the anterior gut may be low. In poultry, the activity of xylanase may occur mainly in the small intestine, although some activity in the crop is possible depending on the feeding regimen. For pigs, the stomach has the potential as an important site of activity, with a pH of between 3 and 5. Therefore, some or all of the activity of xylanase and phytase could be in the same segment in swine.
Vegetable protein meals introduce another form of ANF. The main source of protein in animal feed on a global scale is soybean meal, which represents 55 percent of the global production of oil seeds. However, other sources are used also, such as canola meal, DDGS, soybean hulls and peas. Soybean meal is a popular source of protein for livestock and aquaculture because it has a high concentration of protein (up to 49%), which is highly digestible and well balanced for non-ruminants. However, quality varies from region to region. Soybeans contain ANFs, which are known to depress growth performance in swine. These include trypsin inhibitors, phytate, oligosaccharides, antigenic factors (eg glycinin) and lectins; the latter interfere with absorption of nutrients. Other vegetable protein sources also have a combination of both valuable available amino acids and undesirable ANFs.
Overall, protease has the potential to show multiple benefits. It reduces the effects of the ANFs described above. Consequently, it lowers the risk associated with poorly balanced feed formulation and variation in the nutritional quality of feed ingredients. Protease also allows for the use of poorer quality raw materials at higher inclusion rates. Finally, it allows for a lower protein level of the diet and feed cost.
However, one enzyme may have multiple nutrient benefits beyond its primary action. Protease, for example, breaks down the kafirin protein complex protecting the starch granule in sorghum, which has the additional benefit of releasing energy indirectly for use by the host animal. Therefore, although there will be important improved amino acid digestibility (and associated energy) values for protease with sorghum, the full energy value will prove to be significantly higher due to the indirect effects.
It may be expected for protease to be more active in the small intestine, given the typical pH profile of commercial products. There is potential for overlap in its activity with phytase and xylanase in the proximal duodenum, but the protease activity could continue even after the phytase and xylanase have become inactive. Protease and amylase activity could overlap as well, which may be advantageous for certain feed ingredients.
Why supplement with multiple enzymes
All raw materials contain a mixture of ANFs coming from fiber, phytate and factors sensitive to protease action. Much of the P in corn is bound to phytate, but can be released with the addition of phytase. Corn also has levels of insoluble NSP that may be broken down by xylanase. A co-product from the bioethanol industry (corn- or wheat- based DDGS) has high levels of NSP that show enhanced value from the presence of xylanase. Soy nutritional value benefits from phytase (phytate breakdown), fiber degradation from xylanase and the reduction of ANFs from protease supplementation.
Consequently, the mode of action of each enzyme may be additive. Supplementing with multiple, existing enzymes brings more nutrient release and greater reduction of the ANF effects of the diet than might be achieved with a single enzyme. Nortey et al. (2007), showed phosphorus digestibility was improved by adding either phytase or xylanase, but was greatest when the two enzymes were combined in a wheat-based diet (Figure 1). This work also showed phytase and xylanase improved ileal energy and lysine digestibility. This illustrates how multiple enzymes may show levels of improvements on single nutrients beyond their primary action.
Figure 1. Effect of xylanase and phytase individually and in combination on phosphorus digestibility in pigs (Nortey et al., 2007)
Given both the ever-increasing demand for food allied to a long-term increasing scarcity in resources, better utilization of all available feed ingredients will be critical in order for animal protein production to satisfy the growing global demand. Use of feed enzymes has emerged as offering an important contribution towards a potential solution for sustainable animal production. Enzymes help not only to increase the availability of costly nutrients, but also improve animal performance by way of reducing the damaging effects of ANFs and, therefore, contribute to lower animal production costs. Finally, enzymes reduce the potential excretion of minerals, nitrogen and carbon, which may be higher in ingredients of poor quality. Generally, the effects of enzymes are more profound when used in combination.
New and improved enzymes will be developed. Today and in the future, animal production in all its forms will benefit from the use of ever-evolving enzyme technology and application.
Article made possible through the contribution of Dr. Howard Simmins, InSci Associates Ltd, and Dr. Ajay Bhoyar, Novus International, Inc.