Phytases have traditionally been employed to reduce inorganic phosphate usage in monogastric rations, saving money and reducing phosphorus (P) pollution. However, two recent turkey trials show improved performance in response to higher phytase inclusions when phosphorus is not limiting, indicating this is linked more to phytate hydrolysis than phosphorus provision and suggesting commercial benefits from higher phytase doses.

The majority of phytase studies have focused on determining phosphorus released from phosphorus-deficient diets, often with the animals still phosphorus deficient even with the highest enzyme dose.

Few studies have employed high phytase dosages in diets marginally or not limiting in phosphorus, and thus the benefit of phytate destruction per se is rarely tested. There are three mechanisms by which increased phytate hydrolysis could benefit poultry, namely:

  1. Greater release of phosphorus, restoring the optimal calcium (Ca):available phosphorus ratio, or satisfying a higher than expected phosphorus requirement.
  2. Destruction of phytate, which may be anti-nutritive, even at very low concentrations.
  3. Generation of inositol, shown to have lipotrophic effects and stimulate intake in broilers, through complete phytate de-phosphorylation.

Response to high phytase doses - phosphorus release  

Higher phytase doses will degrade more phytate, produce more phosphorus and so improve performance, but only if the bird is phosphorus deficient. It is therefore important to know how the bird requirements relate to the diet phosphorus content, and turkey data is somewhat equivocal.

Optimum performance also relies on a balance between calcium and phosphorus. Phytate hydrolysis releases phosphorus and also calcium chelated by the phytate, and thus it is prudent to apply a matrix for both calcium and phosphorus to the phytase to avert an imbalance.

This may not be as simple as it seems since the initial hydrolysis of phytate yields proportionately more calcium than phosphorus, whereas further hydrolysis yields more phosphorus but little calcium. Thus the ratio of Ca:P released may be as high as 3:1 at low levels of phytase and less than 2:1 at high levels. A release of a 3:1 ratio will begin to imbalance the diet, but as more phytase is added this is corrected, one reason why high dosages may improve performance. Nevertheless, neither would be expected to perform better than a diet that met calcium and phosphorus requirement in a balanced manner. As such, this effect does not relate to extra-phosphoric effects of phytase.

Response to high phytase doses - phytate destruction  

If the diet is just marginal in phosphorus, a point will be reached where phytase releases more phosphorus than is needed for optimum growth. At this point, further improvements in performance with increasing phytase would be due to extra-phosphoric effects. In two recent turkey trials, a phosphorus adequate control was compared with a phosphorus-reduced diet supplemented with up to 2500 FTU/kg of an evolved E. coli phytase.

In both trials, phosphorus removal reduced performance, which was restored with 500 FTU/kg phytase. Subsequent enzyme increments resulting in significantly improved gain and, extraordinarily, feed conversion ratio compared with the positive control.

Phytase addition to low-P diets normally stimulates gain and intake but does not influence feed conversion ratio. What marks these trials out as being of interest is that the final weights of the 1000 - 2500 FTU/kg treated birds were almost 5 percent greater than the positive control whilst despite this the feed conversion ratio was significantly better.


This suggests that the benefit observed with the higher enzyme doses may not have been due to phosphorus release. Definitive evidence of non-phosphorus related improvements has recently been shown in four broiler trials where both a control and a control with supplemental inorganic phosphorus were significantly outperformed when phytase (500 FTU/kg) was added to the former, and by a negative control with marginally reduced phosphorus supplemented with higher phytase levels (1000–1500 FTU/kg). Given that there was no response in these trials to added phosphorus, correction of the Ca:P ratio or phosphorus deficiency is not the likely mechanism.

Phytate is known to be anti-nutritive. It chelates minerals, reduces protein digestibility and promotes endogenous losses, and also influences immune parameters. As a result, the presence of phytate in the diet degrades animal performance and its destruction will allow performance improvements not achievable through additional phosphorus. Its destruction may also yield inositol, which has been shown to enhance growth rate and efficiency in broilers.

Empirical expectations  

The response to phytases in poultry, whether performance or bone ash, is log-linear. Many studies employing multiple but linearly spaced dosages of phytase often incorrectly state that an asymptote has been reached at a given dosage; logarithmic dose increments would be required to definitively prove this point. 

Where wider phytase ranges have been employed, performance improvements beyond initial expectations have been observed with high dosages. The scale of response noted relates to the dosage employed, the type of phytase and the dietary phosphorus reduction.

What constitutes a high dose?  

Dose invariably relates to enzyme units, which is determined in in vitro assays. The pH of commonly used assays is usually set at 5.5, which is disparate from the acidic pH in the gastric region where phytases act. 

Since phytases vary quite markedly in their pH profiles, there is little relationship between the units determined in the standard pH 5.5 assay with activity in the animal. Thus, when high doses of phytase are discussed, reference should always be made to biological efficacy. 

Recent trials suggest that use of high “biological” superdoses of phytase are beneficial to poultry performance, probably through phytate destruction rather than phosphorus release per se. Benefits of superdosing phytases depend on diet structure and phytase dosage and source, meaning that care must be taken when trying to implement such strategies in turkey production.