In my viewpoint, you need to consider matrix value of Ca as well; otherwise, and as mentioned by others, Ca level in your diets incased that has adverse effect on Phytase.
Below I added some considerations regarding Phytase and its matrix value. I hope this would be applicable.
Factors affecting phytase activity
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.
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.
• Many factors influencing phytase efficacy that need to be taken under consideration
Variability in Responses
A dilemma faced by the users of exogenous enzymes is that bird responses to feed enzyme addition are variable and not entirely predictable. The factors contributing to these inconsistencies are complex, involving enzyme (enzyme source, enzyme dose, side activities present), diet (ingredient quality, diet composition, diet form, particle size), and bird (species, age, sex, individual variation) factors and their interaction. Selected dietary and animal factors of importance are briefly highlighted below.
Dietary Nutrient Density
In most cases, the use of exogenous enzymes can be beneficial only if the dietary nutrient density is marginal.
The use of microbial phytase is potentially useful and appropriate only for diets with suboptimal phosphorus levels and containing significant levels of plant-derived ingredients.
Age of Birds
theory, the potential benefits from feed enzyme additions will be greater when the digestive system is simpler, as in young birds. Young birds, especially during the first few weeks of life, have an underdeveloped digestive enzyme capacity compared with adult birds (Noy et al., 1995 ).
This belief, however, is not always true; sometimes responses may be greater during the finisher phase. The effect of bird age on recommendations of enzyme doses is a related issue, as it has not been fully elucidated (Cowieson et al,. 2006).
Inclusion Level of the Target Feedstuff
The higher the inclusion level of the target ingredient, the greater the enzyme response will be, as it will proportionately increase the contents of the substrate for enzyme action or the antinutrient(s) that is causing the problem (e.g., xylanase and wheat inclusion level) in diet formulations.
Performance Level of Control Birds
The degree of response is also governed by the existing level of performance in animals fed the unsupplemented diet. If the performance is poor, whether due to poor husbandry, marginal nutrition, or stress, then responses will be greater with enzyme addition.
Quality of the Target Ingredient
In general, the quality of an ingredient is related to the concentration of antinutrient (e.g., phytate contents, soluble NSP contents in wheat) or the content of available nutrients or energy, all of which can vary widely among batches of an ingredient.
The lower the ingredient quality, the greater will be the magnitude of improvements with added enzyme.
For example, apparently the responses of wheat AME to supplemental xylanase are determined, to a great extent, by the initial AME of the wheat sample. Large variability, ranging from 2,200 to 3,820 kcal/kg of DM, in the AME of wheat samples has been reported in the literature (Noy et al., 1995, Choct et al,. 1999). Low quality wheat samples, assaying less than 2,870 kcal/kg of AME, have been shown to respond remarkably to enzyme supplementation, whereas good quality wheat samples show practically no increment in AME.