Fiber-degrading enzyme substrate interactions critical to efficacy in poultry diets
Chicken performance remains the only available measure when judging fiber-degrading enzymes
Despite widespread uptake across the poultry industry, considerable debate continues around how to achieve best results with fiber-degrading feed enzymes.
Much of that debate relates to the complexity and variety of potential non-starch polysaccharide (NSP) substrates such enzymes can target, a situation which has led to a growing body of literature advocating the use of products with multiple NSP enzyme activities.
The problem faced by end-users is that much of the research used to promote these multi-enzyme products is incomplete, and contains conclusions - potentially with huge commercial implications - based on the results of experiments not adequately designed to support such interpretation. In many cases, it is simply not possible to link performance benefits to the additional claimed activities as it is only the complete product which is evaluated.
Further, in the large number of studies carried out to date, it is not usually the enzyme with the greatest number of activities that produces the best performance. As a result, until a definitive study is published to show that all components within a multi-enzyme product are necessary for optimum benefit, there is no evidence to support their use over an optimized single-component NSP enzyme.
The potential for confusion is further compounded by the considerable misunderstandings that surround the relative importance of the various NSPs present in monogastric diets, as well as their differing properties.
The most significant for poultry producers are the arabinoxylans, often referred to as xylans, and β-glucans, including cellulose and mixed-linked β-glucans, whilst others such as mannans, arabinans, galactans and xyloglucans are of considerably less importance.
Subsequent interactions within the gastric environment, both between the various NSPs and with any applied NSP enzymes, are also complex.
The problem is that despite the negative effects associated with NSPs in poultry diets - increased digesta viscosity, wet litter, reduced nutrient utilization - being widely recognized, the mechanisms by which NSPs cause these negative effects, and how NSP enzymes might overcome them, are much less well known.
Modes of action
At the most basic level, NSP enzymes are thought to act both by eliminating the nutrient encapsulating effect of the plant cell wall, and by breaking down the long-chain, soluble NSPs (particularly arabinoxylans and mixed-linked β-glucans) which can increase digesta viscosity. Each effectively improves access to feed materials for the bird's digestive enzymes, the former by exposing nutrients such as starch and protein stored within plant cells, the latter by enabling free movement of digestive enzymes, and subsequent end-products of digestion, within the gut.
An additional mechanism comes from potentially beneficial end-products of cell wall degradation by NSP enzymes. In addition to providing additional energy through direct absorption, following conversion by intestinal microbes into volatile fatty acids, it has been found that specific short-chain oligosaccharides produced by certain xylanases can have a positive effect on bird performance by promoting the populations of beneficial microbes over those that are pathogenic.
However, few, if any, commercial NSP enzyme products are purified enzyme proteins. The vast majority contain other material, such as a carrier or byproducts of manufacture, which often contributes additional side enzyme activities - as many as 43 in one commercial product tested.
When considering the potential role of multi-enzyme dosing, therefore, it is important not to attribute performance benefits to specific enzyme activities unless a clear mode of action is identified. Such side activities may be at very low levels, or insufficiently thermostable to survive feed processing, and any effect they have might be beneficial, superfluous or even detrimental.
Until the definitive studies needed to determine such detail have been carried out, NSP enzyme product efficacy can only truly be judged by the impact on performance within the bird.
It appears from the research carried out so far that enzyme characteristics and dose are more important than the number of claimed activities present.
If we consider one of the key mechanisms by which NSP enzymes improve nutrient digestibility, namely the opening up of plant cell walls to improve access to key nutrients such as starch and protein stored within, the possible reasons for these results become clearer.
The basic structure of a cereal grain is shown in Figure 1, with two "barriers" for endogenous digestive enzymes to overcome - the thick aleurone layer covering the grain and the thinner cell walls of the endosperm. The breakdown of these insoluble NSPs is the primary mechanism by which NSP enzymes can improve nutrient digestibility in corn-based rations, where high digesta viscosity is not an issue.
If an essentially "single enzyme" xylanase (plus any minor side activities which may be present) is able to so effectively disrupt these cell wall structures in cereal grains, can any significant extra access to nutrients be provided by addition of a mannanase, arabinanase or galactanase to target the other very minor polysaccharides present? And just as importantly, is further cell wall breakdown actually desirable?
It has been shown that even relatively low levels of the monosaccharide xylose (2.5 percent), for example, can have a negative effect on bird performance, suggesting that complete degradation of some NSPs to their component mono-saccharides is not actually beneficial. It also appears to be oligosaccharides from partial hydrolysis of NSPs by xylanase that are implicated in optimizing intestinal health.
If, as current thinking suggests, the fall in NSP enzyme performance sometimes seen when optimum dose levels are exceeded is due to breakdown of these beneficial oligosaccharides into less beneficial mono-saccharides, the same may be true for the effect of additional enzyme activities included in multi-enzyme products.
Clarification will only come from standardization in the way in which enzyme products and their activities are described in the scientific literature, and in how the effects of additional activities are measured and reported. Until that time, it is performance within the bird and subsequent return on investment that remain the only true measures by which to judge product choice.