The phosphorus (P) matrix value of a phytase gives an indication of how much P will be released from phytate-P.
Exogenously added phytase in feed is used to liberate the phosphorus (P) which is bound to the phytate present in raw materials. This lowers the feed cost by reducing the amount of inorganic P that needs to be added.
Increasing the level of phytase will lead to increased degradation of phytate, and thereby more P release (a higher P matrix value). This allows further reduction of inorganic P sources, lowering the cost of the feed even further.
Recent phytases coming onto the market claim extremely high P matrix values. The question arises if there is enough phytate-P in the feed to support these high matrix values. If not, these high P matrix values will only be theoretical, presenting a risk that the formulated feed will be deficient in P.
Sources of phytate-P in feed
Although cereals have the highest inclusion levels in feed, the inclusion level of the protein source has the biggest influence on the total phytate-P level in the feed as proteins contain higher levels of phytate-P (Table 1).
Cereal by-products are also high in phytate-P, however their inclusion level in feed is mostly limited. In the group of protein sources, sunflower seed meal and rapeseed meal have a higher level of phytate-P compared to soybean meal, while DDGS has a low phytate-P content. Animal-derived feedstuffs (processed animal proteins (PAP), meat and bone meal, as well as whey protein and plasma protein) do not contain any phytate-P. The range of phytate-P levels which can be obtained during feed formulation in different feeds with or without animal-derived feedstuffs is shown in Table 2.
Some phytate-P in feed cannot be degraded
Current phytases, including OptiPhos® Plus, can degrade phytate completely, removing all six P groups on the inositol molecule once the phytase makes contact with the phytate molecule. However, part of the phytate-P is trapped within the fibrous structure of feed materials, in particular the cereal by-products, and is thereby physically not reachable by the phytase.
Several research trials run by Huvepharma in recent years have shown that a maximum of 80% of the phytate-P can be ileal-digested in pigs and poultry with high inclusion levels of OptiPhos Plus. Some nutritionists and researchers will only consider a maximum feed phytate degradability of 70%, especially when a high level of cereal by-products is used.
Based on these estimates, the maximum P (available P) which can be released from total phytate-P in feed with or without animal products (as shown in Table 2) is shown in Table 3. As the table illustrates, in commercial feed formulation there is little reason why a phytase should be added at a level where it claims P matrix values about 2.0 g/kg feed. With OptiPhos Plus, the P value at 2,000 FTU/kg is 2.0 g/kg for poultry, while for pigs it is 2.1 g/kg. Therefore, it can be concluded that in high performing commercial feeds, an inclusion level greater than 2,000 FTU/kg is not justified.
Feed with high levels of phytate-P (2.8 - 3.5 g/kg), which might justify higher inclusion levels of a phytase, are typically composed of ingredients with a lower nutritional value which will compromise the performance of the animal and thereby decrease the commercial value of the feed. In theory, a feed with high levels of phytate-P can be formulated (and this is often done in research studies) which would justify the use of a phytase at a similarly high level to yield P matrix values about 2.0 g/kg. However, animal performance on this feed is almost always below industry standards.
Estimating the phytate-P levels in feed: based on feedstuff database, NIR or wet chemistry?
The question also arises as to the best method for estimating the phytate-P level in the feed. There are two ways to do this:
- calculating a value based on the declared phytate-P found in feedstuff databases
- analysis
Analysis can be through NIR (near-infrared spectroscopy) or by wet chemistry. It should be noted that the accuracy of analysis using NIR at the low levels associated with phytate-P is quite poor, and errors of 10-20% are common. Similarly, the determination of the phytate-P levels by wet chemistry also produces an analytical error of up to 20%. Even though it can be argued that calculating phytate-P in feed based on the feedstuff database and the feed composition does not always reflect the local situation, it will still give the best estimate.
Another option can be to use the declared phytate-P levels from two different feedstuff databases, as this can achieve a more accurate estimate of the real phytate-P levels in feed. For instance, as can be seen in Table 1, the declared phytate-P levels provided by CVB are higher compared to those from INRA, so final feeds might have higher (CVB) or lower (INRA) phytate-P levels. Using both values can give a 'minimum - maximum' range.
In summary, claiming matrix values higher than 2 g P/kg feed makes no sense in commercially viable feeds as only 70-80% of the phytate-P can be destroyed. The theoretical calculation of the feed phytate-P levels from feedstuff databases is still the best way to estimate the real phytate-P levels in feed and is probably more accurate than estimation by NIR or wet chemistry.
