Plant phytase

It has been known for more than 50 years that plant phytase has the ability to hydrolyse phytate (McCance and Widdowson, 1944; Hill and Tyler, 1954) and its effectiveness for improving P digesitibility in pigs and poultry has been clearly shown (Nelson, 1967; Newton et al., 1983; Bagheri and Gueguen, 1985). Phytase activity has been reported in a wide range of seeds, such as rice, wheat, barley, maize, rye, soybean and oil seeds (Reddy etal., 1982; Gibson and Ullah, 1990; Eeckhout and De Paepe, 1994); however, phytase activity of seeds varies greatly among species of plants (Table 10.1). With the exception of wheat, rye and triticale, most dormant seeds contain very low phytase activity. Phytase activity in maize and soybean meal is so low that it is not of practical importance. Barley may or may not contain nutritionally significant levels of phytase activity (Pointillart, 1993). The majority of the phytase activity in wheat, rye and triticale is in the bran. Diets formulated using ingredients having high phytase activity, such as wheat bran, wheat, triticale, rye bran and wheat middlings, promote greater absorption of phytate P (Pointillart, 1991,

1993). Eeckhout and De Paepe (1991) reported that microbial phytase was 74% more efficient in vivo (fed to pigs) than phytase in wheat middlings when added at an equal in vitro activity level (500 U kg-1). They showed that genetically modified microbial phytase was active over a wider pH range than wheat middlings phytase. They suggest that microbial phytase may be more active at pH levels present in the stomach than wheat phytase.

Although some feed ingredients contain native phytase activity, steam-pelleting used in the manufacture of many commercial pig and poultry feeds results in significant losses of this intrinsic phytase activity. Because of variation of phytase activity among and within plant species, damaging effects of pelleting during feed manufacturing and the lack of availability of feed ingredients of high phytase activity, the presence of residual phytase activity often may not be considered in diet formulation when feeds are pelleted.

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