Data suggest that use of high “biological” doses of phytase is beneficial in broilers, possibly through the destruction of phytate and thus eliminating its antinutritive effect. The dataset for turkeys are more limited and less definitive but it seems that superdosing of phytase here also has a positive effect on performance.
By Michael Bedford, AB Vista Feed Ingredients Ltd., UK
Whilst the provision of Phosphorus (P) has always been linked to the destruction of phytic acid, the benefit of a phytase has to date focused on P-release. However, some recent work suggests improved performance in response to higher inclusion rates of phytases when P is not limiting, indicating this is linked more to phytate hydrolysis than P provision. Much of this work has been conducted in broilers and piglets.
Evidence from two trials with turkeys suggests there are significant commercial benefits in feeding far greater dosages of phytase than are traditionally considered.
Response to high dosages of phytase
There are several publications spanning the last 40 years where very high levels of phytase have been employed, the earliest reported trial being that of Nelson et al. (1971) where 1 to 8 g/kg of an Aspergillus phytase containing product was fed to broilers, the units of which were not clearly defined but which approximated 165 phytase units (FTU) per gram based on the data provided. This means the highest dose employed was approximately 1320 FTU/kg feed. There was a linear improvement in body weight gain and bone ash, in addition to phytate P disappearance with each log increment in phytase dose. Since the basal diet in this study was very deficient in P, all of the responses observed were likely attributable to P-release.
Indeed P-release is the reason for most observed responses to phytases in the literature. The vast majority of studies have focused on determining the P equivalency of this enzyme invariably used in diets which were P deficient, often to the extent that the birds were still P deficient even when the highest phytase dose was employed. Relatively few studies have employed high dosages of phytase in diets which were only marginally or not limiting in P, and as a result the benefit of destruction of phytate per se is rarely tested. There are three mechanisms as to why increased phytate hydrolysis should benefit poultry, namely:
1. Greater release of P as discussed above, but in a manner which ultimately restores the calcium (Ca)/Available P (AvP) ratio closer to the optimum, or in a manner which satisfies a higher than expected P requirement. In some cases the diets into which phytases are employed have too wide a Ca/AvP ratio and as a result performance is compromised until sufficient P is released by higher levels of phytase.
2. Destruction of residual levels of phytate which may be antinutritive, even at very low concentrations.
3. Generation of inositol, which is the completely de-phosphorylated form of phytate. This compound has been shown to have vitamin like lipotrophic effects and to stimulate intake in broilers.
Higher doses of phytase will degrade more phytate (IP6), produce more P and so improve performance, but only if the bird is P deficient. It is therefore important to know how the requirements of the bird relate to the P content of the basal diet. Whereas the broiler literature tends to be relatively consistent in that it identifies the requirement for optimum growth rate of a 0-21 day-old bird at between 0.45 and 0.5% AvP, the data for turkeys is more equivocal. Some studies have found performance responses to P levels as much as 45% above that recommended by the NRC, whereas others have suggested this recommendation may indeed be in excess of requirement. As a result care must be taken in interpretation of phytase trials where P cannot be ruled out as the limiting nutrient.
If a diet is reduced in both Ca and P prior to inclusion of phytase then the subsequent performance tends to be better than in a situation when only the P is reduced, this being the case in both broilers and turkeys. Optimum performance relies on a balance between Ca and P. Phytate hydrolysis not only releases P but also Ca which was chelated by the phytate, and thus it is prudent to apply a matrix of both Ca and P to the phytase to avert an imbalance between these two elements. However, this may not be as simple as it seems since it has been shown that IP6 and IP5 have a far greater ability to chelate Ca than the lower phosphor-esters.
As a result the initial hydrolysis of phytate yields proportionately more Ca than P, whereas further hydrolysis yields yet more P but little Ca. Thus the ratio of Ca to P release may be as high as 3:1 at very low levels of phytase and decrease dramatically to less than 2:1 at very high levels. Clearly a release of a 3:1 ratio will begin to imbalance the diet, but as more and more phytase is added the error is corrected and this has been suggested as one reason for high dosages improving performance.
Nevertheless, in neither case would performance be expected to surpass that of a positive control that met requirement for Ca and P in a balanced manner. As such this effect does not relate to extra-phosphoric effects of phytase and thus does not comprise a mode of action in this regard.
If the basal diet is only marginal in P, a point will be reached whereby the dose of phytase releases more P than is needed for optimum growth rate. At this point, any further improvements in growth rate and/or efficiency with increments in phytase dose would be due to extra-phosphoric effects such as phytate destruction or inositol production. Definitive proof for such an effect needs a series of diets varying in inorganic P content where a clear optimum is defined, beyond which no further improvement is observed. Any treatment with phytase which results in performance in excess of this optima is thus likely not related to P release.
Unfortunately there are no such turkey trials in the literature. Nevertheless we have recently conducted two trials with turkeys, to eight and 12 weeks of age, respectively, where a P adequate control was compared with a P reduced diet supplemented with up to 2500 FTU/kg feed of an evolved E. coli phytase. In both trials the negative control clearly reduced performance due to removal of P but this was quickly restored with only 500 FTU/kg feed of the enzyme, subsequent increments resulting in significantly improved gain and, extraordinarily, feed conversion ratio (FCR) compared with the positive control.
In general, P reduced diets result in reduced intake and gain but the effect on FCR is muted. Phytase addition to such diets restores gain and intake but does not influence FCR. As such this marks these trials out as being of interest since the final weights of the 1000 - 2500 FTU/kg feed treated birds were almost 5% greater than the positive control whilst the FCR was significantly lower than the positive control despite the greater body weights (Figure 1
This suggests that the benefit observed with the highest dose of enzyme may not have been due to P release. Definitive evidence that non-P related gains can be had in commercial diets through use of high dosages of phytase has recently been shown in four independent broiler trials where both a positive control and a positive control with supplemental inorganic P were significantly outperformed by a positive control with added phytase (500 FTU/kg), or by a negative control which was only marginally reduced in P but supplemented with significant levels of phytase (1000 – 1500 FTU/kg). Given that there was no response in the broiler trial to addition of extra P, correction of the Ca:P ratio or a deficiency of P is not the likely mechanism.
Antinutritive effect of phytate
Phytate is known to be antinutritive for several reasons; it not only chelates minerals, reduces protein digestibility and promotes endogenous losses, but also seems to influence immune parameters as well. Much of this may be as a result of its highly charged nature under physiological conditions but recent work suggests it may also be due to its ability to order water in such a way as to limit protein solubility.
Protein digestion relies first on its dissolution, the restriction of which has a myriad of consequences for host nutrition and subsequent microbial populations. As a result the presence of phytate in the diet degrades animal performance and as a result its destruction will allow for performance improvements which are not achievable through addition of extra P. Its destruction may also yield significant quantities of inositol, which has been shown to enhance growth rate and efficiency in broilers in the diet.
What constitutes a high dose?
The scale of the beneficial response to phytase addition relates not only to the dosage employed, but also to the type of phytase and the scale of reduction in dietary P. The type of phytase influences the response. The extent of P reduction is also important because intake and P are closely linked. Addition of phytase to P-reduced diets will restore intake provided enough phytate P is available to make up the deficiency. Continued logarithmic increments in dose beyond this point continue to stimulate intake further in a linear fashion which will contribute to an improvement in both gain and FCR. Consequently the greater the P reduction in the phytase diet, the greater the phytase dose needed to elicit an extra-phosphoric effect.
When considering high doses of phytases, it is essential to understand the definition of “high dose”. Dose invariably relates to units of enzyme, which is determined in an in vitro assay. There are several methods for measurement of phytase activity, the most commonly used method being that of AOAC International (Association of Analytical Communities, the scientific association dedicated to analytical excellence). The pH of this assay and almost all others used commercially is set at 5.5 which is quite disparate from the pH encountered in the gastric region where phytases are known to act. Since the four major classes of phytase on the market (Peniophora, Aspergillus, E. coli and evolved E. coli) vary quite markedly in their pH profiles, there is little relationship between the number of units determined in the standard assay at pH 5.5 with that at pH 3, for example, which approximates average gizzard pH.
If each of the four classes of enzyme were dosed at 100 units at pH 5.5, then there would be 20-30, 50-60, 100 and 150 units, respectively, at pH 3. Research has shown that if all phytases were assayed and dosed using units determined at pH 3, then there is a much better relationship between units of activity measured and biological efficacy. Thus, it is clear that when high doses of phytase are discussed, reference should always be made to a unit which translates to biological efficacy. 500 FTU of an evolved E. coli phytase delivers the same number of units at pH 3 as ~4000 FTU of a Peniophora phytase. As a result, it is essential to consider the source of the phytase as well as the units employed if a meaningful interpretation of the literature is the desired outcome.
In essence, conversion to pH 3 units is most helpful, and in the context of this paper, high dosages of phytase refer to use of 1000 FTU/kg feed at pH 3 or greater (~4000, 2000, 1000 and 650 FTU/kg feed at pH 5.5 for Peniophora, Aspergillus, E. coli and evolved E. coli, respectively).
Data are beginning to emerge which suggest that use of high “biological” doses of phytase are beneficial with regards to performance of broilers, possibly through the destruction of phytate rather than the provision of P per se. The dataset for turkeys are more limited and less definitive but it seems that the same may well be true. Benefits of superdosing phytases are dependent upon the diet structure and the dosage of phytase used (the dosage needed to achieve such superdosing benefits varying with phytase source) which means that care must be taken when interpreting the literature or trying to implement such strategies in turkey production.
This is an edited version of the paper that was published in the proceedings of the Turkey Science and Production Conference 2012. References are available on request.