Extra-phosphoric effects of phytase
The effects of phytase beyond phosphorus release are termed ‘extra-phosphoric’ effects of phytase. The primary effect of phytase is the improvement of phosphorus availability through the release of phosphorus from phytate. However, phytate also forms stable complexes with proteins and minerals like calcium, zinc, and iron in the digestive tract and prevents nutrient absorption (Woyengo and Nyachoti, 2013). Thus, the extra-phosphoric effects of phytase are related to the improvement of digestibility of energy, amino acids, and minerals through the dissociation of such complexes (Selle and Ravindran, 2008).
The extra-phosphoric effects of phytase provide economic advantages in diet formulation and enhance the value of dietary phytase. The matrix values for calcium release in a digestible calcium basis seems to be similar to the digestible phosphorus release. However, the assignment of matrix values for other minerals and amino acids should be adopted with caution (Cowieson et al., 2017), as the effects are more variable and have not been fully elucidated (Adeola and Cowieson, 2011). Particularly in the case of amino acids, there is evidence to support the use of amino acids matrix values, but because the effects are not as obvious or consistent it is appropriate to use a more conservative approach (Cowieson et al., 2017).
The use of phytase above conventional levels (500 to 1,000 FTU/kg) seems to have the potential to improve growth performance beyond what is expected with adequate phosphorus levels (Zeng et al., 2014). The exact mode of action of high phytase levels remains unknown, but it is believed to be related to extra-phosphoric effects due to greater degradation of phytate (Adeola and Cowieson, 2011; Cowieson et al., 2011). The greater degradation of phytate removes most of the antinutritional effects of phytate, further improving digestibility of energy, amino acids, and minerals (Selle and Ravindran, 2008). Moreover, the complete degradation of phytate releases myo-inositol, a vitamin-like compound with many metabolic functions (Laird et al., 2018; Moran et al., 2018).
The use of high levels of phytase appears to have potential for a greater effect on nursery pig performance (Zeng et al., 2014; Gourley et al., 2018; Laird et al., 2018), with less evidence for effect on grow-finish pig performance (Holloway et al., 2016; Miller et al., 2016; She et al., 2018). Moreover, the effects of high phytase levels appear to be greater if the levels of phosphorus, calcium, and other minerals are marginal in the diet (Zeng et al., 2014; Miller et al., 2016; Laird et al., 2018). Also, it has been suggested that the effects of high phytase levels follow a curve of diminishing returns, with most of the beneficial effects generated within the dose of phytase necessary to destroy 30 to 40% of the dietary phytate and proportionately lower effects thereafter (Cowieson et al., 2017).