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enzymes, and possible ways of optimising the effects of combining these enzymes.


Causes for sub-optimal performance of combined phytase and NSP-degrading enzymes


1. Phytase sources/types Phytases are produced to a greater extent by microorganisms and plants, and to a lesser extent by animals. And among microorganisms, the major phytase producers are fungi, yeast and bacteria. Plant phytases are not as effective as microbial phytases, this is due to: - Their narrower optimum pH range - Greater resistance of microbial phytases to protease enzymes in the gastro-intestinal tract due to a higher degree of glycosylation


2. Endogenous Phytase Activity (EPA) Viscous grains such as wheat, rye and barley, and their milling by- products have higher endogenous phytase activity than corn, sorghum and oilseed meals. Digestibility of P is high in diets containing high EPA ingredients, and the impact of phytase on P availability and utilization in these diets, based on these feed ingredients is lower.


3. Dietary NSP composition As an example, the efficacy of NSP-degrading enzymes in wheat, rye or triticale-based diets can be higher if the NSP-degrading enzymes have higher xylanase activity, because the major NSP in these feedstuffs are arabinoxylans. On the other hand, corn has lower


NSP content than wheat or barley, and thus, the lack of synergistic interactions between phytase and xylanase, observed in corn broiler studies (Choct 1997).


4. Dietary digestible phosphorus concentration Studies show that the catalytic activity of phytase is inhibited by its end product, inorganic P (Greiner et al. 1993), indicating that an increase in dietary concentration of non-phytate P can result in decreased hydrolysis of phytic acid by phytase. In addition, the efficiency of absorption of P from gut is negatively correlated to dietary digestible P concentration (Saddoris et al. 2010), which means excess dietary supply of non-phytate P can result in reduced utilization of P released from phytic acid by phytase.


5. Dietary concentration of multivalent cations Phytic acid can react with multivalent cations at intestinal pH to form insoluble phytic acid-mineral complexes that cannot be hydrolysed by phytase. Thus, an increase in dietary concentration of multivalent cations can result in reduced efficacy of phytase on phytic acid hydrolysis and hence synergism between phytase and NSP-degrading enzymes. Of the multivalent cations that are present in poultry diets, Zn2+


by Fe2+


is the most potent inhibitor of phytic acid hydrolysis followed , Mn2+


, Fe3+ , Ca2+ and Mg2+ in decreasing order of potency


(Woyengo and Nyachoti, 2011). The addition of vitamin D to the diets, which increases the Ca


absorption (Bronner 1987), or organic acids such as citric acid, which increase the solubility of Ca-phytic acid complexes (Boling et al. 2000), may alleviate the negative effects of Ca on phytic acid hydrolysis by phytase, and hence result in improved performance and bone mineralization (Woyengo and Nyachoti, 2011).


Concluding remarks - The full potential or synergism between phytase and NSP- degrading enzymes in poultry diets depends on several factors; this is the source of variations observed in diets. - Plant phytases are not as effective as microbial phytases. - The synergistic effects are more pronounced in diets that use NSP-degrading enzymes with enzyme activities that target most of the major NSPs in the basal diet. - Phytase and NSP-degrading enzymes may not interact synergistically or additively in poultry diets if the basal diet has high endogenous phytase activity. There is, however, a need to establish the endogenous phytase activity level beyond which phytase and NSP-degrading enzymes are unlikely to act synergistically or additively in basal diets for poultry. - Excess dietary supply of non-phytate P can result in reduced utilization of P released from phytic acid by phytase. - The addition of vitamin D or organic acids such as citric acid, which increase the solubility of Ca-phytic acid complexes, may alleviate the negative effects of Ca on phytic acid hydrolysis by phytase.


PAGE 18 MAY/JUNE 2018 FEED COMPOUNDER


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