Do phytases always work in poultry nutrition?

Some producers complain that their meat or egg type chickens suffer from problems in bone or egg shell quality when they include phytase in the diet as a partial substitute for the inorganic phosphorus sources (MCP; DCP; bone meal) according to the matrix value provided by the manufacturing company. Under field conditions, phytase activity is controlled by several parameters. This article will spot the light on utilization of phytase enzyme in poultry nutrition and the factors influencing its activity.

Phytate (phytic acid) compound is formed in the plant via phosphorylization of myoinositol to form myoinositol hexaphosphate (IP6). Plant based diets contain approximately 1% phytic acid and 0.28% phytate bound phosphorus. Phytic acid has several adverse effects on poultry nutrition (binding phosphorus and reducing its availability; forming insoluble complex with some cations such as calcium, zinc, iron and magnesium; lowering protein/amino acid digestibility due to forming insoluble complex with protein, increased endogenous losses, and inhibition of proteases; reducing energy digestibility due to increased endogenous losses, lowered fat digestion, and to a lesser extent inhibition of amylase enzyme).

Phytase enzyme (myo-inositol hexaphosphate phosphohydrolase) has become a common tool utilized by poultry nutritionists to lower feed costs, reduce the negative impacts of phytic acid on the nutrient digestibility and environment. Fytase units (FTU) is used for measuring phytase enzyme activity; one FTU unit is defined as the amount of enzyme that required to liberate 1 µMol of inorganic phosphorus per minute from 5.1 mMol/L sodium phytate at 37º C and pH 5.5.

Phytase starts release of phosphorus atoms from phytic acid by hydrolysis of fully phosphorylated phytate compound (IP6) to less phosphorylated compounds (myoinositol penta phosphate (IP5), myoinositol tetra phosphate (IP4) and myoinositol tri phosphate (IP3). Under in vivo conditions, the hydrolysis does not continue to yield myoinositol di phosphate (IP2), myoinositol mono phosphate (IP1) and myoinositol. The capacity of IP6 to bind protein and minerals in the digestive tract is much higher than that of IP5, and that of IP5 is much higher than that of IP4 and IP3. For instance, the binding capacity of IP3 to calcium is only 11% of that of IP6. Therefore, it is important that most of IP6 and IP5 been hydrolyzed in the upper digestive tract as complete as possible to reduce calcium binding in the intestine. On average, phytases degrades 35% to 50% of feed phytic acid.

Phytases can be classified according to the initial site of phytate hydrolysis in myoinositol hexaphosphate ring into 3-phytase or 6- phytase. The 3-phytases liberate P moiety at position C(IP3), while the 6-phytases liberate P moiety at position C(IP6). Phytases can also be classified according to their origin into fungal and bacterial phytases. Fungal phytases are either derived from Aspergillus niger (3-phytase) or from Peniophora lycii (6 phytase). Bacterial phytases are derived from E.coli, Citrobacter braakii or Buttiauxella spp, and all considered 6-phytases.

Examples of some commercial phytase products

Factors affecting phytase activity

1) pH

pH of 2.5 to 5.5 is the optimum range at which maximum benefits are obtained from phytase. Research studies revealed that bacterial phytase is more effective than fungal phytase at the above mentioned pH range.

2) Resistance to endogenous protease

Phytase enzyme is a proteinaceous compound, so, it can be susceptible to hydrolysis by pepsin enzyme in the GIT. Bacterial phytases revealed higher resistant against hydrolysis than fungal phytases.

3) Resistance to temperature

Due to its protein nature, phytase enzyme can be inactivated if exposed to a high temperature (above 70-75 ° C) as that occurs during feed conditioning and pelletizing. If the temperature exceeds 80 ° C as that occur during feed pelletizing, phytase enzyme should be protected either by coating or by providing it in an intrinsically thermostable form. Unprotected phytases can be utilized in pelletized feed in liquid form via spraying on the feed post pelleting.

4) Dietary Factors

a) Mineral content

Although doubted in some literature, high level of dietary calcium has been reported to adversely affect phytase efficacy either via complexing with phytic acid in the small intestine and forming an insoluble complex or via binding to the active site of phytase enzyme. Similarly, higher levels of iron or zinc make insoluble complex with phytate and subsequently decrease the phosphorus releasing efficacy of phytase. Recently, high dietary sodium has been revealed to decrease phytase activity. Inspection of the mineral content of farm drinking water should be conducted at least once per year as most water of desert areas contains very high level of some minerals that could hinder phytase activity. Phytases that efficiently hydrolyze phytate in the proximal gut (crop and proventriculus) will counteract the adverse effect of high dietary minerals.

b) Dietary phytate level

Although phytase activity (phytate hydrolysis) is increased in diets containing low level of phytic acid, the absolute amount of liberated phosphorus is high in diets containing high level of phytic acid compared to those with low phytic acid. The degree of phytate hydrolysis and phosphorus release was at its highest level in rice polish followed by rice bran, wheat bran, sunflower meal, wheat middling, canola meal, soybean meal, corn gluten meal, full fat soy, wheat, corn, barley, sorghum, and corn DDGS respectively.

c) Feed acidification

As the acidic pH is the optimum pH for phytase activity, adding organic acids in the feed such as citric acid has been reported to increase phytase activity.

Phytase matrix value

Although each phytase product has its own matrix value that published by the manufacturing company, these matrices are not derived in the same way. The matrix involves values for Av. phosphorus, calcium, protein/amino acids, energy, and sodium. Research results and field experiences revealed that there should not be a fixed matrix value for phytase in all feed formulations. Poultry nutritionists can have their own matrix values that can be more reserved and have more safety margin than the values published by the manufacturing companies.

Conclusion

To get maximum benefits from phytase inclusion, phytase enzyme should be characterized by having a maximum activity at a pH range of 2.5 to 5.5, hydrolyzing most phytate esters (IP6 and IP5) to at least IP4 in the upper part of the digestive tract as quickly as possible, resistant to endogenous proteases and heat stable. Properly acting phytase will reduce feed costs, improve efficiency of phytate phosphorus and other nutrient releases, and protect the environment from phosphorus pollution.

References

Dersjant-Li, Y., Awati, A., Schulze, H. and Partridge, G. 2015. Phytase in non-ruminant animal nutrition: a critical review on phytase activities in the gastrointestinal tract and influencing factors. J Sci Food Agric. 95: 878–896.
Selle, P.H. and Ravindran, V. 2007. Microbial phytase in poultry nutrition. Anim. Feed Sci. Technol. 135: 1-41.
Web search in phytase producing companies
Field experience