These patterns can be in part explained by a significant negative


correlation between the digestibility of amino acids in the control diet and the level of response to phytase addition (Figure 2). Clearly, phytase has less of an effect on amino acids where the inherent amino acid digestibility is higher, such as methionine, as opposed to one where it is low like cysteine. The well-known involvement of phytase in reducing endogenous protein loss is also reflected in these trends, for example by the high response on threonine digestibility.


Figure 2. Effect of diet digestibility on response to phytase in broilers


30


This pattern is logical given the preference phytases have for


the highly phosphorylated IP6 and IP5 isomers of phytate. Initial phosphorus release comes disproportionally from IP6 and IP5 forms of phytic acid. As inclusion increases, phosphorus release is increasingly from lower esters such as IP2 or IP3. Such lower esters are substantially less anti-nutritional and so whilst more phosphorus may be released, additional amino acids may not.


Dietary phytate The nutritional make-up of the diet itself is an important factor in determining the effect phytase will have. As the level of phytate in the control diet increases, so the ileal amino acid digestibility decreases (Figure 4).


20 10


Figure 4. Effect of dietary phytate level on amino acid digestibility


Control amino acid digestibility vs. Phytate-P % 0 0.80 -10 0.5 0.6 0.7 0.8 0.9 Coefficient of apparent ileal amino acid digestibility in the control groups (%) 0.75


As diet digestibility increases so the effect of phytase on amino acids reduces.


Ensuring these distinct amino acid release patterns are reflected 0.65


in matrix values of phytase products when formulating feed is important for precision production. Phytase matrix values which don’t follow these trends should be approached cautiously.


Dose response At 500 FYT (units of phytase) per kg feed the amino acid digestibility response was ~2% and rose to over 4% when the dose was doubled. However, when the dose was increased beyond ~1000FYT per kg little additional improvement in amino acid digestibility was seen (Figure 3).


Figure 3. Phytase dose response on ileal amino acid digestibility in broilers


30


Control amino acid digestibility (%) low digestibility mid digestibility high digestibility


20 0.0 0.5 Phytate-P %


As the level of phytate in the diet increases so ileal amino acid digestibility decreases.


This negative effect of phytic acid on amino acid digestibility is


the result of an increase in endogenous protein loss from the intestine to maintain gut integrity, rather than a direct impact on dietary protein digestibility. Not surprisingly therefore, the efficiency of phytase to destroy dietary phytate has a significant bearing on amino acid digestibility. In the review, when the amount of phytate destroyed was less than


10% the improvement in digestibility was 1.5-3.0%. However, when the amount increased to 50%, amino acid coefficients increased up to 10%. The practical importance of this relationship, is how scientifically inappropriate it is for different phytases to have substantially different amino acid matrix values, when equivalent phosphorus release (phytate degradation) values are presented.


10 0 -10 500 1000 Phytase dose (FYT/kg)


The digestibility of amino acids increased up to ~1000FYT and then plateaued.


1500 2000


PIG EVALUATION In a similar review on the effect of phytase in pigs, a meta-analysis of the data from 27 peer-reviewed papers was carried out. In these trials phytase was in most cases fed at levels between 500 and 1200 FYT/kg feed with a mean of 850 FYT /kg feed. This average dose was associated with a digestible phosphorus release of ~0.07% and a mean ileal amino acid digestibility improvement of 2.8%. This shows


FEED COMPOUNDER MAY/JUNE 2017 PAGE 35 1.0 1.5 1.0


0.70


Effect of phytase on apparent ileal amino acid digestibility (change relative to the control, %)


Effect of phytase on apparent ileal amino acid digestibility (change relative to the control, %)


Control amino acid digestibility


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