Wheat enzyme supplementation in poultry diets: why results vary so widely

Why wheat quality itself is hard to pin down#

The paper opens by noting that wheat batches used in broiler rations differ a good deal in their feeding value, and in a number of cases this clearly holds performance back. Reviewing earlier work, Bedford points out that standard laboratory measures of grain composition have failed to consistently identify what separates a good feed wheat from a poor one. Measures such as grain weight per bushel, weight per thousand kernels and baking quality indicators including the Hagberg falling number were not shown to have a repeatable, significant link to bird performance.

Instead, the paper traces the real culprit to soluble, viscous arabinoxylans, complex carbohydrates in the wheat grain that increase the thickness of gut contents once digested. Work referenced in the review linked feeding diets rich in rye and wheat to physiological responses in the bird's digestive tract, and isolation and re-feeding studies supported arabinoxylans as the likely limiting factor. The paper reports a strong relationship between feed conversion ratio and intestinal viscosity, with an R2 of 0.94, suggesting that gains in performance scale in a straightforward way with each drop in viscosity achieved.

How grain variety, growing environment and processing complicate things#

Even once viscosity is accepted as the key mechanism, several practical factors shift how much of it ends up in the bird's gut. The paper cites work indicating that how thick the gut contents become, and so how well a wheat sample feeds, depends heavily on which variety was grown and the conditions it was grown under. Because feed mills typically receive blended wheat from mixed sources rather than single varieties, this adds an extra layer of unpredictability to any trial result.

Inclusion rate matters too. The relationship between how much wheat is included in a diet and the resulting intestinal viscosity is described as exponential, meaning that small increases in wheat inclusion near a certain threshold can produce disproportionately large rises in viscosity. A cited trial by Allen and colleagues confirmed that increasing wheat inclusion raised the ratio of metabolisable energy to gain and worsened feed conversion ratio, which in turn affected how much response enzyme supplementation could produce.

Heat and moisture applied during pelleting or expanding are also flagged as having a well-known effect of raising subsequent intestinal viscosity in the bird. The paper warns that over-processing can mask any apparent enzyme response once the thermal stability of either the enzyme itself or heat-sensitive nutrients is exceeded, a practical caution for anyone comparing pelleted and mash diets in trials.

Not all xylanase enzymes behave the same way#

One of the more striking arguments in the paper is that xylanase products are not interchangeable despite all being labelled with the same enzyme name and activity units. Commercial xylanases used in wheat-based diets come mainly from a small group of fungal source organisms, including Aspergillus, Trichoderma and Humicola, and the paper argues that differences in pH optimum, substrate preference, temperature optimum and interfering factors make direct comparison of stated international units unreliable.

A table drawn from Kluepfel and colleagues' work on three xylanases from the same organism, Streptomyces lividans, is used to show that specific activity and the Km value, a measure of how an enzyme's activity responds to substrate concentration, differ depending on which xylan substrate is tested. If enzymes from a single source organism can differ this much, the paper reasons that differences between enzymes from different organisms are likely to be larger still.

These laboratory differences carry through to bird performance. The paper notes that Humicola xylanases appear less dependent on viscosity-reduction for their benefit than Trichoderma products, and that two major xylanases from Trichoderma longibrachiatum, referred to as p1 5 and p1 9, differ substantially from each other as well. In some of the author's own data, an Aspergillus xylanase preparation actually increased intestinal viscosity while performance improved slightly, the opposite pattern to the usual Trichoderma result.

Fat type and fat-soluble vitamins add another layer#

Beyond the enzyme and the grain, the paper argues that other dietary ingredients heavily influence the scale of response seen. As digesta viscosity rises, fat digestion is impaired, and the effect is more pronounced for saturated fats than unsaturated ones, a pattern shown in the paper's data on rye-based diets with different fat sources. The explanation offered is that saturated fats depend more on emulsification, the process of breaking fat into small droplets for absorption, and that process is disrupted by viscous gut contents.

Supporting evidence comes from Pasquier and colleagues, cited as showing that raising solution viscosity from 1 to 4 cp cut the proportion of emulsified triglycerides from 80% down to a much lower figure, and sharply reduced emulsion surface area, regardless of which gum was used to create the range of viscosities tested. The paper uses this to argue that even quite low intestinal viscosities can meaningfully reduce fat digestibility, particularly in diets with saturated fat sources such as tallow.

Because fat digestion affects absorption of fat-soluble vitamins, the review reports that liver vitamin A and E content was influenced by both fat type and xylanase addition. In one trial, birds on a rye-plus-tallow diet without enzyme treatment developed rickets severe enough that the diet was removed from the trial by four weeks of age, attributed to poor vitamin D absorption linked to poor fat digestibility. The implication is that in diets marginal in fat-soluble vitamins, enzyme addition can produce a larger apparent benefit than digestible energy and protein improvements alone would predict.

Microbial challenge and immune status shift the response#

The review links much of the variation in enzyme response to differences in gut microbial populations between trials. Enzyme addition to wheat-based diets is reported to reduce ileal microbial populations while increasing caecal fermentation, likely because enzymes free up small fermentable carbohydrate fragments for the hindgut. The paper observes that in floor pen trials the bulk of the enzyme response occurs during the grower and finisher stage, from 22 to 42 days of age, while in cage trials, where litter-borne microbial challenge is lower during the finisher phase, the response shows up earliest in the starter period instead.

Further circumstantial support comes from reports that antibiotics can offset the negative effects of viscous grains in rye-based diets, and from a cited demonstration that a combined antibiotic treatment of monensin and avoparcin acted together with a xylanase to improve outcomes in wheat-based diets. The paper also draws on evidence that fat digestion was impaired to a greater extent in conventional rats than in germ-free rats, consistent with microbes contributing to bile salt breakdown and fat saturation, both of which work against good emulsification.

A further example concerns disease challenge: broiler performance after a coccidial challenge was worse on wheat-based diets than maize-based ones, but xylanase addition to the wheat diet partly offset the growth depression caused by the challenge. The paper suggests this points to enzyme supplementation interacting with immune status, possibly through improved absorption of fat-soluble vitamins that support immune function, though this remains an interpretation rather than a directly tested mechanism.

Bird breed, age and individual variation in wheat enzyme supplementation trials#

The paper also flags bird-level factors that are easy to overlook when comparing trial results. Some broiler breeds reportedly show higher intestinal viscosity than others when fed the same diet, and therefore respond to xylanase to a lesser extent, meaning breed needs to be accounted for when interpreting the literature on wheat enzyme supplementation. Age matters too: intestinal viscosity is reported to decline as birds get older, work attributed to Petersen and colleagues, though the paper cautions that a given viscosity may do more damage in an older bird with a bacterially rich gut than in a young, relatively sterile one.

Perhaps the most striking finding cited is from Hughes and Choct, who reported that individual bird variation on the same low-metabolisable-energy wheat diet could be very large. Birds grouped as Low, Average and High apparent metabolisable energy showed values across the three groups of 9.6, then 12.2, then 14.7 MJ per kg of dry matter, with feed conversion ratios of 1.77, 1.96 and 2.07 in turn. Notably, this variation was not explained by intestinal viscosity but correlated instead with ileal starch digestibility, suggesting that viscosity reduction interacts with other factors such as gut transit time and microbial activity that differ from bird to bird even within a single flock on an identical diet.

What this means for interpreting enzyme trials#

Bedford's conclusion is that intestinal viscosity remains the principal factor governing the size of the response to xylanase in wheat-based diets, but that a long list of other variables, grain variety and growing conditions, enzyme source organism and isoenzyme, fat type, microbial challenge, bird breed, age and individual variation, all modify how that viscosity effect plays out in practice. The paper argues that enzymes which do not effectively reduce viscosity, mainly those from Humicola and some Aspergillus sources, are likely to work through different and less well understood mechanisms.

For readers comparing this review against other enzyme research indexed alongside it, the study on a composite enzyme supplement fed to young calves and the work on enzyme digestibility changes after sorghum germination both illustrate how enzyme effects depend heavily on the specific feedstuff and animal involved, a theme that runs through Bedford's argument as well. The practical message from this review is caution rather than simple extrapolation: a positive or negative result in any single trial reflects a combination of grain, enzyme product, diet composition and the bird's own microbial and physiological state at the time, not necessarily the enzyme's inherent worth.

We think the paper's strongest contribution is less any single number and more its insistence that enzyme literature cannot be compared across studies without accounting for these interacting factors, a caution still relevant to anyone reading feed trial results uncritically. The full proceedings sit within the RAAN Conference Proceedings collection, part of the wider Recent Advances in Animal Nutrition community held in this index's research library.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Integrity Systems (NLIS and LPA): which runs NLIS, LPA and the National Vendor Declaration
• Feedipedia animal feed database: an open-access database of animal feeds
• Poultry Hub Australia: poultry science information

Questions#

— The Livestock Library team