Xylanase inhibitors in UK wheat varieties: Survey of incidence and significance to pig and poultry feed



This review was undertaken to investigate the impact of xylanase inhibitors (XIs) in wheat on UK industries such as animal feed and enzyme manufacturers, poultry producers and wheat growers, and indicate how these sectors could benefit from further research and development in this area. 

Reduction of viscosity in wheat-based animal feed
Wheat is a major ingredient in animal feed (typically up to 65 %) but non-starch polysaccharides (NSPs), and mainly the arabinoxylans (AX) in feed wheat confer high viscosity in the GI tract. For the past two decades, wheat-based feed for monogastric animals (pig and poultry) has been supplemented with enzyme cocktails (exogenous enzymes derived from fungal or bacterial sources) to reduce viscosity in digesta.  AX hydrolysis by enzymes such as xylanases reduces viscosity and results in improved digestion, improved Feed Conversion Efficiency (FCE) and healthier flocks, particularly with young, growing birds such as broilers.

Presence of XIs in wheat
In recent years, several xylanase inhibitor (XI) proteins have been found in wheat and other cereals, and have been described in the scientific literature.  Most studies have focussed on protein characterisation, their mode of action, substrate specificity, occurrence and distribution within the plant.  Only few reports illustrate how XIs present in wheat may interfere with current industrial processes relying on xylanase addition for product improvement.

Variation in XIs between wheat varieties
XIs have been found in all wheat varieties tested so far. Most varieties tested were bread wheat varieties, although some feed wheat was also included.  In addition, most work reported has been on white flour, whereas for feed purposes, characterisation should be carried out at the whole-grain level. 

The levels of XIs in European wheat reported in the literature vary between wheat sources, and may be partly environment dependent. The impact of XIs may be greater on feed wheat varieties vs. bread wheat varieties: most feed wheats contain the 1BL/1RS translocation and are highly viscous, thus needing more exogenous xylanase addition to reduce viscosity. 

The continental work showed that the varieties Rialto and Cadenza, both 1BL/1RS wheats, contained high levels of XIs.  In addition, all the non-1BL/1RS feed wheat varieties tested also tested high for XI levels.  To specifically quantify how XIs affect wheat quality and exogenous enzyme performance in the UK, the correlation between wheat AME and XI levels needs to be investigated using UK-grown wheat varieties.

Interaction between XIs and commercial enzymes
Both poultry and non-poultry producers have been relying on addition of xylanase-containing enzyme cocktails to reduce feed viscosity in monogastric animals.  However, enzyme efficacy is variable.  There are several possible reasons for this, but the XIs present in wheat, which makes up the bulk of feed, are likely to contribute to this variability as it has been shown that the commercial enzymes are inhibited by XIs. 

Survival of XI during the feed manufacturing process
XI survival during feed processing has not been tested, and such knowledge is lacking regarding how much actual inhibition is still present in processed feed.  All previous research has focussed on wheat and/or wheat flour and has used purified proteins and inhibitors. 

The actual amount of XI present in the end product after processing is likely to differ significantly from the starting material, and other components present in processed feed may also impact on xylanase and XI interactions.  Therefore, an interesting approach for the feed industry would be to study the effect of processing at different temperatures, water contents, pressures and recipe on the survival of the inhibitors, and the nutritional value of the feed in relation to actual XI activity.

Quantification of XI impact on xylanase performance in feeds
Currently, feed trials researching the benefits of xylanase addition do not take the presence of XIs into account, nor the fact that different feed wheat varieties may contain varying XI levels.  Therefore, to address optimal quantification of xylanase performance in wheat-based animal feed, more research needs to be conducted. Such research will need to be carefully designed to ensure that results are not artefactual and that they reflect the action of the native (endogenous) XIs. 

Screening requirements for XIs in UK wheats
Even though few of the varieties tested so far are currently grown in the UK, and few are representative of feed types, it is to be expected that similar levels of XIs are present in UK lines. However their impact has not been quantified. 
Initial screening of XI levels in UK-grown feed wheat appears the simplest and most direct way to establish variation.  This would also help direct future research, such as more extensive investigations into potential new low-XI wheat breeding lines, wheat nutritional quality and animal feed performance.

Development of new XI assays
All assays reported in the literature currently rely on spectrophotometric assays, and these are not representative of the environment in which the enzymes would normally work.  A more realistic way of measuring XI impact on feed quality would be through quantifying their impact on viscosity, with and without exogenous xylanases. Enzyme companies currently use similar methods to quantify the impact of exogenous xylanases on feed viscosity. 

In addition, assays described thus far have not accounted for possible xylanase contamination from fungal residues potentially present on grains, thereby possibly underestimating xylanase inhibition. It is not clear to what extent previous studies have been influenced by endogenous levels of xylanases in the grain or contamination with enzymes of fungi colonising grain after harvest. This impact should be quantified.

A number of areas for further work and development were identified:
1. Assessment of varietal variation in XIs in UK wheat by rapid, informative lab-based screening;
2. Development of XI level assays adapted to the animal feed industry;
3. Evaluation of fungal xylanase contamination in stored feed wheat;
4. Studies on XI survival during feed processing;
5. Quantification of xylanase performance in wheat with and without XIs.

Cereals & Oilseeds
Project code:
01 May 2005 - 31 August 2005
AHDB Cereals & Oilseeds.
AHDB sector cost:
£18,018 from HGCA (Project No. 3181).
Project leader:
T. Verhoeven, F. Short & R. Weightman, ADAS Consulting Ltd Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ . A. Cowieson & M. F. Isaksen, Danisco Animal Nutrition Ltd, PO Box 3, Boothby Graffoe, Lincoln, Lincolnshire LN5 0XB