Plants and plant products are a major component of animal feed. However, many plants have the capacity to synthesise unique chemicals that serve as defence mechanisms against insects. These compounds are mainly indigestible by animals and are referred to as anti-nutritional factors.

Dr Nasser Odetallah

These naturally occurring compounds – including protease and trypsin inhibitors, goitrogens, alkaloids, oxalates, phytate, phyto-haemagglutins (lectins), urease, lipoxygenases, cyanogenic glucosides, anti-vitamin factors and microbial metabolites – may impair digestibility and metabolism, reducing growth in many species. Legume seeds, such as soybeans and others are excellent protein sources, but their dietary inclusion is limited because they all possess one or more of the aforementioned anti-nutritional factors.

Raw soybeans contain other compounds which can cause severe absorption of vitamin B12 and other amino acids and reduce the activity of vitamin A. Even after the cooking/extraction process of soybeans into soybean meal (SBM), such meals will still contain other anti-nutritional complex carbohydrates, including the beta-mannans and the anti-nutritional oligosaccharides, stachyose and raffinose.

Furthermore, in order to formulate to the high protein requirements of most young animals, farmers tend to use a number of byproduct meals that contain high amounts of proteins such as meat and bone meal, fish meal, blood meal and the less digestible feather meal. Nevertheless, SBM remains to be the major protein source for monogastric diets. However, the presence of beta-mannans in soybean meal, and especially in the 44 per cent SBM, as well as the other anti-nutritional oligosaccharides will provide a favourable environment to the gut micro flora and along with the high-protein content of the diet might result in a disturbance to the natural colonisation of gut microflora.

Poultry, in general, lack the innate enzymes capability, e.g. mannanase and glucanase, to degrade these anti-nutritional factors and have less than optimal levels of other enzymes such as amylases and proteases resulting in less than optimum utilisation of the diet and a lower overall performance by the animal. Supplementation of the diet with an enzyme or a mixture of enzymes will result in improving dietary response via degradation of specific substrates, resulting in an improvement of the general digestibility of corn-SBM diets.

Beta-Mannans

Beta-mannans are a group of closely related heat-resistant compounds that survive the drying/toasting phase of soy bean processing. beta-mannans comprise about 1.3 per cent of the 48 per cent-CP SBM product and 1.5 to 1.7 per cent of the 44 per cent-CP SBM product, with an estimated beta-galactomannan content of 1.83 per cent and 2.22 per cent, respectively. Mannans, mainly associated with the hull and fibre fraction of SBM, are intensely anti-nutritional due to their highly viscous properties, likely decreasing the efficiency of the monogastric' carbohydrate utilization by partially blocking key sites on the intestinal surface and resulting in a lower performance.

Beta-mannans' effects can be reduced by the use of dietary mannanase enzymes (beta-D-mannanase), which degrade some of these complex polysaccharides into smaller units, some of which can be utilised for energy production.

Anti-Nutritional Oligosaccharides

Approximately 40 per cent of SBM is made up of crude fibre and various polysaccharides and oligosaccharides. Stachyoses and raffinoses are non-starch polysaccharides (NSP) that has anti-nutritional properties and are present in soya beans. Stachyose [Gal alpha(1-6)-Gal alpha -(1-6)-Glu alpha-1 beta-2Fru] and raffinose [Gal alpha-(1-6)- Glu alpha-1 beta-2Fru] represent a significant amount of potential dietary energy present in SBM. However, the alpha-galactoside family of oligosaccharides has been implicated in reducing SBM true metabolisable energy (TMEn), fibre digestion and transit time in chickens. It has been well documented that that the alpha-galactoside oligosaccharides are also a major cause of hydrogen gas production from SBM in poultry.

Endogenous enzymes in poultry are specific for alpha-linked carbohydrates similar to that in starch, but they are not active against SBM oligosaccharides because the alpha-galactosides cannot be hydrolysed due to the lack of alpha-1,6 galactosidase activity in the intestinal mucosa. Consequently, undigested oligosaccharides pass to the distal end of the intestine where they are fermented by gut microflora to volatile fatty acids (VFA) and gases, thus yielding less energy and creating digestive disturbances. Raffinose and stachyose significantly reduced TMEn values of adult Leghorn roosters when added to the feed at levels equal to that found normally in SBM. Dry matter digestibility also appears to be negatively affected, thus decreasing significantly as stachyose levels increase.

However, stachyose and raffinose can be hydrolysed to glucose, galactose, or corresponding disaccharides in the proximal small intestine via supplementing the feed with an enzyme mixture containing the proper oligosaccharidase.

Beta-Mannanase

Mannanase enzymes are endo-hydrolyases that are typically fermentation products of Bacillus spp. This enzyme typically degrades beta-mannans present in feed. Beta-mannanases cleave randomly within the 1,4-beta-D-mannan main chain of galactomannan, galactugluco-mannan and mannan. Since beta-endo-mannosidases improve the digestibility of all types of mannan, and because a major source of mannan in soya is the hull, the effect of beta-mannosidase seems to be more pronounced when supplemented to diets containing SBM-44, which contains hulls, as opposed to SBM-48. Use of mannanases in feed has increased over the last two decades, constituting almost 10 per cent of the overall enzymes used in corn-SBM poultry diets globally.

Alpha-Galactosidases / Endo-xylanases

The increased use of different feed ingredients requires the use of exogenous pentosanases such as galactosidases, xylanases and glucanases. The use of such enzymes, either individually or in a blend, results in degrading the complex NSP and releasing other nutrients that might be bound or otherwise trapped within the complex net-like NSP. The use of these enzymes will result in liberating energy and limited amounts of proteins resulting in improving growth performance parameters in all types of poultry.

Proteases

Much of the early work in the 1950s and 1960s involving proteases supplementation to cereal-based diets did not result in improvements in bird performance. However, recently, supplementation of poultry diets with enzyme mixtures, including proteases and amylases, have produced significant improvements in growth performance. More recently, researchers were able to estimate the value such enzymes play in sparing protein sources both in vivo and in vitro.

The use of enzyme mixtures, such as phytases with protease, xylanases and amylases has been on the rise. While the synergistic effect of these enzymes, if present, is not completely understood, their effects are complementary rather than substitutive as previously thought.

Few years ago, many researchers reported that supplementing corn-SBM broiler starter diets with enzyme preparations containing a mixture of xylanase, protease and amylase resulted in improvements in bodyweight but the effects were seldom significant on feed conversion ratio. However, higher doses of enzyme mixtures were recommended in order to produce a significant effect on apparent ileal nitrogen digestibility or faecal nitrogen digestibility. Short et al., (2002) also reported an improvement in average egg production upon supplementing the diets of laying hens with an enzyme mixture that contained both xylanase and protease.

Prior to work carried out in 2002, there was no reporting of benefits of individual proteases in corn-soybean poultry diets. In addition, Novus found a reduction of the viscosity of the contents of the small intestines of birds fed diets supplemented with the protease enzyme. Over the past few years, and with the increased prices of SBM and corn, the need to use proteases in corn-soy diets became indispensable to help save on feed costs by reducing the amount of proteins in the diet. Cibenza DP100 (Novus International, Inc), a broad-spectrum ser-protease, for example, allows the producer to use diets formulated with proteins and amino acids that are on average five to 10 per cent lower than the recommended industry standards and still obtain the same performance sought by the producer and thus significantly reducing the cost of the diet.

Conclusion

Over the last decade, interest in the use of commercial enzyme preparations in corn-SBM diets has increased dramatically. Still, other than phytase, there are only a handful of commercial and experimental enzyme preparations available for use in corn-soybean diets. Such preparations normally contain one or more of the following enzyme activities: beta-mannanase, alpha-galactosidase, alpha-amylase, xylanase and beta-glucanase, and most recently, proteases.

Most of the research reported so far indicates that supplementing corn-SBM diets with these enzymes results in increased body weight and improvement in feed efficiency accompanied in some instances with a reduction of gut viscosity and a shift to a more favourable gut environment for the growth of beneficial bacteria.

Less is known about the change in nutrient digestibility and utilisation of corn-soy diets upon the use of these commercial enzymes, which requires further investigation.

December 2010