Finding correct inclusion levels of phytase in broiler diets

A ratio of Calcium (Ca) to Available Phosphorus (AvlP) must be attained in broiler diets if bone development and performance are to be normal. The phytase enzyme is added to most diets as a standard inclusion, and its addition changes the consideration of how we should add Ca and AvlP.

Nutritionists can take their own precautions when formulating diets to ensure that they use phytase correctly and that the Ca and AvlP inclusions are within acceptable levels. This is particularly true when higher than normal doses of phytase are used.

Far from complete
Our understanding of the Ca and AvlP requirements of broilers is far from complete. Researcher Angel tabulated the scientific findings with regards to Ca and AvlP recommendations in broiler diets from 1992 to the present (Table 1),and comments that “there is poor consensus on what the needs are for Ca and P in the modern broiler”.

There has been a tendency not to look beyond the concentrations used in studies or indeed, when comparing studies. Factors such as the actual intake of the nutrient and other related nutrients, dietary sources of macro-minerals, the performance of the birds and the concentrations of other key nutrients such as vitamin D and micro-minerals are mostly ignored. The fact that birds use both the Ca and the P in the diet differentially, dependant on the level of the nutrient in the diet adds a further complication. Candidly, the production of specification for broiler diets for “Starter” and “Finisher” phase is less than adequate for commercial nutritionists who feed four or five phases of feed.

Effect on metabolism
The primary breeder recommendations are at odds with the scientific data, although they appear to be in agreement with each other (Table 2). They exceed the absolute levels determined scientifically but the Ca:AvlP ratios fall into the range that we would expect. Thus the chances are they would not cause any major problems, but they would be very expensive.

The use of phytase in broiler diets should probably be considered to be the norm. Angel comments that much of the recent work on Ca and P has focused on the use of phytase in the diet, and that we can rarely surmise what the actual requirements for either Ca or P are from these publications. The nutritional physiology of Ca and P are inextricably linked and phytase efficacy is a complex function of dietary Ca, total P and phytate-P concentrations. Any future studies conducted on the Ca and P requirements of broilers should include phytase in their design because the enzyme has a material effect on their metabolism.

Phytate and Phytase
Some understanding of the relationship between Ca, P and phytase needs to be acquired before making any nutritional decisions. Much of the phosphorus contained in plant material occurs in the phytate form (myo-inositol hexaphosphate; IP6). The average poultry diet contains about 10 g/kg of IP6, which has a phosphorus content of 282 g/kg. Exogenous phytase degrades about 35% of dietary phytate at standard inclusion rates. The addition of the enzyme to a diet improves phosphorus digestibility by at least 1g/kg. This leads to a saving of about 5 to 6 kg of inorganic phosphorus.

The mode of action of phytase and its interaction with other dietary components was reviewed in 2009. Under conditions of low pH, the presence of H-ions results in most of the negatively charged phosphate groups in phytate becoming protonated. Once protonated and uncharged, the phosphate groups will no longer attract or chelate with positively charged ions such as Ca or other feed components such as amino acids. In this form, the phytate molecule is susceptible to phytase attack and breakdown.

As the pH increases in the small intestine, the phosphate groups become negatively charged and chelate with metal ions. Should the pH increase sufficiently, the metal- phytase complex becomes inert to phytase digestion. A typical broiler diet contains around 8-10 g/kg of Ca, of which about one third is bound in the ileum into Ca-phytate complex. Thus the removal of phytase leads to a sparing effect of about 1 g/kg of Ca.

Phytase efficacy
Phytase efficacy is variable and a number of factors play a role in this regard:

·         The manner in which phytase is applied to and distributed within the feed, are the most important factors impacting on phytase efficiency. Simple blends of enzyme and carrier can’t be compared to products where the enzyme is encapsulated into the carrier. Mixtures of fine powders lead to poor distribution in the feed and may be less heat tolerant during the pelleting process.

·         Phytase requires a substrate upon which to act. The ingredients used in the make-up of the diet contain variable amounts of phytate. Not only do different ingredients contain differing amounts of IP6, but there is variability in the levels measured for each ingredient. Clearly, the amount of substrate in a diet will depend on both the formulation and ingredients used.

·         Higher molar levels of Ca increase the formation of insoluble complexes which has a negative impact on phytase efficacy.

·         Phytase exhibits a typical, non-linear dose response and can’t be used as a standard ingredient and simply formulated into a diet.

Practical considerations
Nutritionists are required to decide on the nutrient contribution they should use for phytase and then what Ca and AvlP levels are appropriate. Predicting phytase response is not straight forward. The various suppliers of commercial phytase claim AvlP contribution in the range 1 to 1.3 g/kg, although making a single, firm recommendation as they all do is perhaps unrealistic. When higher levels of phytase are used caution is required.

Increasing phytase levels is most likely to work if the value being ascribed to a single dose is conservative. Assuming that a single dose will yield 1 g/kg of AvlP, it would be realistic to assume that a double dose may yield 1.3 to 1.4 g/kg of AvlP. If it is assumed that a single dose yields 1.3 g/kg, then it would perhaps be unwise to increase the AvlP yield too much beyond 1.4 or 1.5 g/kg. Even a conservative approach will be cost effective. Phytase impacts on the anti-nutritional properties of IP6. Its removal will have a positive outcome on bird performance. Higher doses of phytase may be justified on these grounds alone.

Cowieson and Bedford believe that phytase enzymes perform most efficiently when the Ca:AvlP ratio is less than 2.2:1. Although the Ca levels determined using ratios of this type are low and would alarm some nutritionists, practical experience would indicate that this is a reasonable assumption. Indeed, researcher Narcy has shown that Ca levels of 6 g/kg combined with 3.9 g/kg of AvlP resulted in the best daily gain in starter diets. There is enough scientific evidence to show that Ca levels of 7 g/kg or more are unnecessary.

Conditions count
No decisions about phytase, Ca and AvlP levels should be made in isolation. The type of enzyme, the ingredients being used, bird performance levels and the specification being used all need to be considered. For example, if using the breeder recommendations (Table 2) as opposed to more moderate figures based on scientific research, a less conservative approach may be appropriate when considering phytase contributions. In cases where environmental conditions limit broiler performance, lower levels of Ca and AvlP levels may be considered.

A few simple mistakes in the formulation process can lead to potential problems (Table 3). Using the Ross 308 recommendation as a starting point I have demonstrated how a few small errors and or omissions may impact on the Ca:AvlP ratio. These simplistic examples may not be problematic individually, but often they occur together. The worst case scenario is also shown. It is easy to see how problematic the over inclusion of Ca can be. It may be judicious to use lower Ca specifications to avoid potential problems. Over estimating the P contribution of phytase could also have serious implications.