relatively expensive. Plants store P in the
form of phytic acid which cannot be
broken down by monogastric animals
unless there is inherent phytase activity in
the feed. The enzyme phytase breaks down
phytic acid releasing six phosphate
molecules, and has been available commer-
cially since the mid 1990s. Addition of
phytase to the diet allows substantially less
inorganic P to be included in the diet and,
as a result, reduces the amount of P
excreted. Lower P waste is advantageous
especially in intensively farmed environ-
ments where it is viewed as a pollutant.
Phytase, according to Jongbloed
(1997), is used in >80% of Dutch pig feeds
and has been partly responsible for a
reduction of 60% in the excretion of P by
pigs in The Netherlands between 1973 and
- Phytase is also widely used in other
parts of the world. Increasing environmen-
tal pressure for the inclusion of phytase in
all animal diets is likely to continue.
Unfortunately, addition of phytase,
like other enzyme products, cannot be
relied upon to release a fixed amount of P
from every diet. Different ingredients have
varying concentrations of phytate P and an
inherent level of plant phytase. This means
that the availability of phytate P will be
higher for ingredients with inherent
phytase than would be expected from the
concentration of phytate P.
As well as releasing bound P, phytase
may have a role in improving the
metabolizable energy content of protein
meals. Rojas and Scott (1969) showed that
incubation (in vitro) of both cottonseed
meal and soybean meal with a phytase
improved the metabolizable energy content
of both meals. Rojas and Scott (1969) also
demonstrated that addition of phytase
decreased the amount of gossypol in
cottonseed meal as well as reducing total
phytate. The authors suggest part of the
improvement in metabolizable energy
digestibility is due to the breakdown of
bonds between phytate and other com-
pounds such as protein. Therefore, any
breakdown in phytate–protein complexes
is likely to increase protein digestibility.
Because enzyme products can be rela-
tively expensive, alternative sources of
phytase from plants have been sought. The
intrinsic phytase activity in plant materials
is higher in wheat, wheat bran, barley and
triticale but lower in maize and legume
seeds which have been heat treated
(Eeckhout et al., 1994; Schroder et al., 1996).
Of 285 samples from 51 feedstuffs tested by
Eeckhout et al. (1994), only 13 were found
to have >100 U kg^1 of intrinsic phytase
activity. All other samples had zero or very
small amounts of phytase (Table 19.3).
Han et al. (1998) tested the value of
wheat middlings as a practical phytase
source for pigs. Their experiment showed
gain, feed intake, feed conversion ratio
(FCR) and metatarsal bone strength were
equal for a maize–soybean diet with 150 g
kg^1 wheat middlings, maize–soybean plus
1200 U phytase or maize–soybean plus 2 g
kg^1 inorganic P.
To reduce the diet cost, Han et al.
(1998) reduced the level of phytase to 300 U
kg^1 and added citric acid (15 g kg^1 ) to
maize–soybean diets containing either 100
or 150 g kg^1 wheat middlings. The
addition of both phytase and citric acid
improved gain and FCR compared with the
diet without citric acid. The authors
suggested that the additional citric acid had408 D.I. Officer
Table 19.3.Feed ingredients with >100 units kg^1 of phytase activity.
Cereals Wheat by-products
Barley 582 Fine bran meal 460
Rye 5130 Fine bran pellets 2573
Triticale 1688 Middlings 4381
Wheat 1193 Feed flour 3350
Bran 2957
Source: Eeckhout et al.(1994).