Substrates for enzymes are present in differing amounts, based upon feed formulation (i.e. corn, wheat, rye, barley). Each feed grain contains varying amounts of non-starch polysaccharides (NSPs), which are considered anti-nutritional factors because they are not digested by poultry. NSPs bind nutrients and increase the viscosity of digesta material, which leads to negative effects on performance (i.e. lower body weight gain). Arabinoxylan (xylan) is the most prevalent NSP in feed grains and can have detrimental effects on poultry performance.
The plant cell wall is the outermost layer of a plant cell. These cell walls generally consist of cellulose and hemicellulose. Cellulose is a linear chain of glucose sugar units linked together, giving it a rigid structure that is very difficult to break down. Hemicellulose consists of different types of sugars (i.e. xylan). It is present alongside cellulose in almost all plant cell walls, but is more flexible than cellulose. In general, we can think of plant cell walls as having long ‘cables’ providing shape and rigidity (cellulose) and a ‘net’ (hemicellulose) woven in and around the ‘cables’. The ‘net’ can keep starch and other necessary nutrients trapped.
Xylan is a long chain polymer of xylose sugar units. Four sugar units linked together are shown below. In nature, xylan exists as a very long chain polymer with hundreds of sugars linked together.
This long chain can include arabinose, as branched units (green, image below). The asterisks (*) indicate other potential sites for arabinose linkage, which can modify properties of the xylan. Highly branched xylan has a lower capacity to bind cellulose, making it more soluble. Soluble (sometimes referred to as water-soluble) xylan is very viscous (making a thick solution), and actually increases the viscosity in the chicken gut. Overall, this decreases the passage rate, feed intake, and nutrient digestibility. Thus, the anti-nutritional property of soluble xylan is related to digesta viscosity.
Less branched xylan has a higher capacity to bind cellulose, making it more insoluble. This tight binding limits the ability of the plant cell wall to be broken down and release nutrients. Thus the anti-nutritive character of insoluble xylan limits nutrient release.
Overcoming Anti-nutritional Properties of Xylans
The exact xylan content of each feed grain determines the type and severity of anti-nutritional properties. The type and amount of xylan contained in the primary grains used in poultry production is shown in the table below (as a percentage of total NSP). For example, wheat has a high percentage of soluble xylan, which leads to viscosity related problems. Wheat also has a high percentage of insoluble xylan, which limits the availability of key nutrients (i.e. starch). Fortunately, xylanase enzymes can be used to break down each form of xylan and reduce these negative effects.
Table: Soluble and Insoluble content of xylan in common feed ingredients as % total NSP1
|Ingredient||Soluble Xylan||Insoluble Xylan|
Selecting the Right Xylanase Enzyme
Care should be exercised when selecting a xylanase feed additive, as not all xylanase enzymes are the same. A xylanase’s ability to breakdown soluble or insoluble xylan comes from its inherent protein structure. This protein structure can be one of two categories in xylanases: GH10 or GH11, each have different specificities for either soluble or insoluble xylans. Some xylanases only breakdown soluble xylan and do not preferentially breakdown insoluble xylan. The opposite is true as well (i.e. insoluble xylan breakdown preferred over soluble xylan). The best xylanase to apply as a feed additive, across a variety of feed formulations, must have dual-functionality, with the inherent capability to breakdown both soluble and insoluble xylan. A better understanding of the specific xylan content of your feed formulation leads to a more informed decision regarding xylanase enzyme selection, and ultimately will provide improved animal nutrition and performance.
References or further reading
1Poultry Feedstuffs: Supply, Composition, and Nutritive Value edited by J. M. McNab, K. N. Boorman. Vol 26
Saeed, F., et. al. Arabinoxylans and Arabinogalactans: A Comprehensive Treatise. 2011. Critical Reviews in Food Science and Nutrition, 51:5, 467-476.
Choct, M. Feed Non-Starch Polysaccharides: Chemical Structures and Nutritional Significance. 1997. Feed Milling International.
Izydorczyk, M.S., et. al. Effect of Molecular Size on Physical Properties of Wheat Arabinoxylan. 1992. J. Agric. Food Chem. 40, 561-568.