Review articlePhytate-degrading enzymes in pig nutrition
Introduction
The acceptance of exogenous feed enzymes by pig and poultry producers over the last two decades has been a remarkable development. Originally, the main focus was on the inclusion of non-starch polysaccharide (NSP) degrading enzymes, with predominantly β-glucanase or xylanase activities, in barley- and wheat-based broiler diets. Initially, the commercial introduction of phytate-degrading enzymes in 1991 attracted little interest outside of The Netherlands. However, after a considerable lag phase, the usage of microbial phytases gathered momentum such that their inclusion in pig and poultry diets presently exceeds that of the NSP enzymes. To some extent this has been predictable as the substrate, phytate, is invariably present in practical pig and poultry diets. Moreover, phytase effectively represents a viable, alternative source of phosphorus (P) and reduces P excretion, which is ecologically beneficial and global P reserves are not renewable. Environmental P pollution is a major concern, which is being increasingly transposed into legislation designed to curb losses of P in effluent from pig and poultry units. In relevant countries, prohibition of meat-and-bone meal inclusion in monogastric diets has also provided real impetus for phytase acceptance. Furthermore, the so-called ‘extra-phosphoric’ effects of phytase may enhance macro- and trace mineral availabilities and the utilisation of protein/amino acids and even energy. That these issues remain controversial does not reflect well on the scientific research effort and is somewhat inconsistent with the increasing acceptance of phytase in practice. Thus, the purpose of this paper is to review phytate-degrading enzymes in pig nutrition to identify areas for instructive research that may resolve these outstanding issues.
Section snippets
Background
Phytate, derived from plant-sourced feed ingredients, is invariably present in practical pig and poultry diets. Phytate-degrading feed enzymes have been included in monogastric diets for more than fifteen years and this is an increasingly common practice. Notionally, microbial phytase has the capacity to hydrolyse dietary phytate, the mixed salt of phytic acid (myo-inositol hexaphosphate; IP6) to liberate six P moieties and inositol in the porcine gastrointestinal tract. In practice, however,
The enzyme: phytase
Phytase (myo-inositol hexaphosphate phosphohydrolase) is the requisite enzyme to hydrolyse the phytate molecule and release phytate-P. This enzyme was first detected in rice bran a century ago (Suzuki et al., 1907) and is widely present in nature. However, in respect of swine nutrition, there are four possible sources of phytate-degrading enzyme activity. These include (i) endogenous enzymes generated by the small intestinal mucosa, (ii) microfloral enzyme activity that is mainly present in the
The substrate: phytate
It has been estimated by Lott et al. (2000) that the total global harvest of barley, maize, sorghum, wheat, cottonseed, rapeseed and soyabean, which are key feed ingredients for pigs, contains approximately 16 million tonnes of phytate. Phytate is predominantly present as the myo-inositol hexaphosphate ester (IP6) (Kasim and Edwards, 1998) coupled with magnesium (Mg) and potassium (K) as a mineral–phytate complex. Indeed, Lott et al. (2000) proposed that the model substrate is one in which IP6
Phytate hydrolysis by phytase in pigs
The extent of phytate hydrolysis and liberation of phytate-bound P generated by microbial phytase in the gastrointestinal tract of pigs, proximal to the large intestine, is critical. The extent of phytate hydrolysis, coupled with dietary phytate concentrations, determines the actual P equivalency of phytase and governs the extent to which dietary P levels may be reduced and P excretion abated. However, as noted previously, the role of alternative sources of phytase activity cannot be
Impact of microbial phytase on P digestibility and P equivalency estimates
The capacity of microbial phytase to improve total tract digestibility of P is established. For example, Dungelhoef et al. (1994) reported that 750 FTU kg− 1 phytase increased P availability in maize by a three-fold factor (0.56 versus 0.18). However, P availability responses to phytase in wheat and triticale were modest due to the presence of intact plant phytase activity in non-pelleted diets. Bruce and Sundstol (1995) investigated the effects of supplementing oats-based diets with phytase
Growth performance responses generated by phytase
Predictably, phytase feed enzymes have the capacity to enhance growth performance of pigs offered P-deficient diets. A more controversial area is whether or not phytase enhances growth performance of pigs offered P-adequate diets and, if so, the genesis of these responses should be elucidated. Probably, Beers and Jongbloed (1992) reported the first example of phytase enhancing performance in pigs offered P-adequate diets. They reported that 1450 FTU kg− 1 phytase increased growth rate by 12.8%,
The impact of phytate and phytase on utilisation of protein and amino acids
That microbial phytase has the capacity to increase ileal digestibility of amino acids and enhance protein availability in pigs remains a contentious issue. One contributing factor is that the underlying mechanisms whereby phytate depresses protein/amino utilisation have not been clearly identified. Consequently, it is relevant to consider these mechanisms, which have been discussed previously by the authors (Ravindran et al., 1995, Ravindran et al., 2001, Selle et al., 2000, Selle et al., 2006
The impact of phytate and phytase on utilisation of energy
On the basis of apparent metabolisable energy (AME) assessments, microbial phytase has been shown to quite consistently enhance energy utilisation in poultry (Selle and Ravindran, 2007). However, a convincing rationale has yet to be developed to explain the energy effect of phytase in broilers although it is possible that phytase collectively increases utilisation of energy derived from protein, lipid and starch (Baker, 1998). Any enhancement of protein digestibility would have a positive
Factors influencing the efficacy of phytase
Almost certainly, numerous factors interactively influence the efficacy of microbial phytases. Clearly, these may include dietary substrate levels, the inclusion rate of phytase and the particular phytase used. In pigs, an inclusion rate of 500 FTU kg− 1 is commonly recommended but there are indications that this may be conservative and higher rates are justified. For example, in a comparative study, Zhang et al. (2000) reported that growth performance responses in weaner pigs to two exogenous
Summary
In essence, apart from the obvious ecological considerations, one interpretation of the benefit of microbial phytase to pig production is that they provide nutritionists with the opportunity to reduce dietary specifications without compromising performance; thereby, decreasing feeding costs and increasing the viability of the industry. However, it will be a real challenge to fundamental and applied nutritional research to define the most effective reductions in specifications that can be
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