All phytases have a matrix recommended by the supplier but the method used to generate it can vary.  Why does the method matter?  In the end the bird or pig still grows, right?  There are two methods for phytase matrix development:  direct and indirect.   The matrix method influences the accuracy of the recommended matrix values and “indirect” methods overestimate the values because of assumptions made. 

The direct method

The direct method uses digestibility to measure phytase contribution.  For example, let’s look at phosphorus (P).  The direct method measures and compares P digestibility of the animal fed a low phosphorus diet without phytase (the negative control) with those fed that same low P diet including graded levels of phytase.  The difference between these treatments becomes the matrix for each level of phytase (Figure 1). In this way, direct determination of phosphorus, calcium, other minerals, energy, and amino acids digestibility can be measured when phytase is added.  

Deriving matrix values through the direct method is time consuming and costly as multiple studies need to be run to derive and validate the matrix recommendations specific to age and physiological status.

measuring change in P digestibility

The indirect method

The indirect method has two options:  available equivalence and available calculated.  Available equivalence also known as the reference index method uses a reference curve of a performance parameter, for example bone ash or body weight gain, as a function of graded levels of inorganic phosphorus added to a low P diet. The performance of different levels of phytase is then plotted against this reference curve to derive the amount of available phosphorus released by phytase. Figure 2 shows that a phytase at 500 FTU/kg feed, compared to the reference curve, is equivalent to adding 0.148% available P.  This method uses data from multiple phytase inclusion levels and curve fits these data to provide matrix values for a wide range of phytase levels. However, curve fitting assumptions may be misleading when going beyond the range covered by the reference curve. Furthermore, responses are dependent on the animal’s age and physiological status.

Tibia Ash and the monocalcium phosphorus standard curve

The available P calculated method determines nutrient release at one inclusion of phytase, typically 500 FTU/kg feed, vs. that of inorganic P with all other phytase release values extrapolated from this.  The calculation method may also use published curves as a basis of extrapolation, often derived from earlier fungal phytases which are biochemically very different.  Figure 3 shows the available equivalence for 500 FTU/kg feed as 0.148% avP (taken from figure 2) with higher release values calculated using improvement coefficients taken from Rosen, 2001.  This method is relatively low cost, as it relies on only minimal animal derived data as well as improvement values for other phytases that may not share the same response and specificity as the test phytase.

500 unit/kg feed response measured with other levels extrapolated.

Indirect methodologies benchmark phytase response as if it were parallel with that of the inorganic P source.  But phytase response shifts according to the level and amount of substrate present and how active that phytase is on lower esters of phytate.  This leads to the overestimation on nutrient release depending on where the user is on the reference curve, and possibly a higher release than substrate availability. This is particularly true if performance parameters, such as bodyweight gain and FCR are used in P release calculations as these are influenced by the extra-phosphoric effects of phytase.  Additionally, this method can be confusing as not all types of inorganic P have the same availability and the benchmark source set at 100% bioavailability has shifted through the years as the original reference is no longer available.  Current available P values for inorganic sources have been ‘re-benchmarked’ relative to a new source.  All of this lends increased variance to the release numbers derived under this methodology.

The most accurate

In the end, the direct method allows for a more accurate picture of what the animal is getting from each level of phytase inclusion, generating more reliable matrix values.  So be direct and ask your supplier where their phytase matrix values came from.