and -glucanase to barley-based poultry
diets reduces the problem of sticky
droppings (Broz and Frigg, 1986; Elwinger
and Sarterby, 1987; Rotter et al., 1989;
Brufau et al., 1991) and can also increase
the metabolizable energy content of the diet.
Apart from carbohydrases, supple-
mentary phytase has been used to improve
P digestibility in poultry diets and provide
P which is similar in biological value to
inorganic P (Qian et al., 1996a,b). The level
of phytase required varies depending upon
a number of factors, including diet
composition and bird age.
The amount of total and phytate P, Ca
and Zn and the concentration of inherent
phytase activity all affect the amount of
phytase required to release sufficient P to
meet the bird’s requirements. In an experi-
ment with day-old male broilers, Kornegay
et al. (1996) measured the efficiency of
phytase (0–1200 U) added to maize–
soybean diets with three levels of non-
phytate P of 2.0, 2.7 or 3.4 g kg^1.
Increasing levels of phytase linearly
improved body weight gain, feed intake,
toe ash % and apparent retention (% of
intake) of total Ca + P. Phytase linearly
decreased P excretion. The size of the
response to phytase was inversely related
to the level of non-phytate P in the diet. In
contrast to phytase, increasing supple-
mentation of non-phytate P increased P
excretion and reduced P retention. In this
experiment, 939 U kg^1 of microbial phytase
activity were required to replace 1 g of
defluorinated phosphate for broilers given
maize–soybean diets.
Schoner et al. (1993a) showed that the
amount of phytase required increases as
dietary Ca levels increase. There were three
levels of Ca tested in the Schoner et al.
(1993a) experiment (6, 7.5 and 9 g kg^1 ).
All diets were based on a maize–soybean
combination that was P deficient (3.5 g
kg^1 total P). In the diet with 6 g kg^1 Ca,
1 g of inorganic P was equivalent to 570 U
phytase. Increasing Ca from 6 to 9 g kg^1
required more phytase to be added to meet
the bird’s P requirements.
Phytase supplementation not only
affects/or is affected by P and Ca level, but
also Zn. Thiel et al. (1993) found Zn reten-
tion in broilers increased by an amount
equivalent to 15 mg kg^1 of supplementary
Zn with the addition of 700 U kg^1 of
phytase. This response occurred in a diet
which contained 30 mg kg^1 Zn. As addi-
tional Zn (4, 9 and 15 mg kg^1 ) was added,
the effect of phytase on Zn retention
diminished. The Zn and Ca experiments
described show how important it is to
know the mineral balance of a diet before
recommending an optimum concentration
of exogenous phytase.
Schoner et al. (1993a) also showed that
the inorganic P equivalence of phytase is
dependent upon broiler age (30–70 days of
age). Using liveweight as the criterion,
broilers at 44 days of age required 570 U of
phytase to release 1 g of P. By 70 days of
age, the amount of phytase required to
release 1 g of P had increased to 850 U.
This result suggests that phytase efficiency
declines with increasing age.
Increasing the P digestibility of a diet
can significantly reduce the amount of
inorganic P required in the diet and reduce
P pollution. In those countries where there
are limits on P addition to the soil, Schoner
et al. (1991) estimated that a 14% increase
in P retention would allow a 50% increase
in the stocking rate of broilers from 350 to
525 birds per dung unit (1 dung unit = 50
kg of P 2 O 5 ). This means that more birds can
be grown on a given area without exceed-
ing application levels of waste P to land set
by legislation.LayersConsiderably fewer experiments have been
conducted with laying hens than broilers
and the measured effect of enzymes on
digestibility is smaller. Enzyme supple-
mentation of barley-rich layer diets had no
significant effect on layer production or
feed:egg (kg kg^1 ) efficiency in a number of
experiments (Al Bustany and Elwinger,
1988; Nasi, 1988 (experiment 1); Aimonen
and Nasi, 1991; Gruzauskas et al., 1991 (two
experiments); Jeroch, 1991 (experiments 1
and 2); Benabdeljelil and Arbaoui, 1994Feed Enzymes 411