Response of Laying Hens to Amino Acid Intake - Nutritional Modelling for Pigs and Poultry - page 265

of the many advantages of this technique is
that it is possible to verify that the amino
acid under test is first limiting in the dilu-
tion series, unlike the situation with the
graded supplementation technique (Gous
and Morris, 1985). In all cases the control
treatment confirmed that the amino acid
under test was indeed first limiting in the
dilution series used.
Fitting the Reading model to response
data such as those measured here has many
advantages over other curve-fitting proced-
ures used to interpret response data (Morris,
1983, 1999) particularly because the coeffi-
cients generated have biological meaning,
and the optimum economic intake of the
limiting amino acid may be determined by
considering the marginal cost of amino acid
and the marginal revenue for eggs. This
model has thus been used widely to describe
responses of laying hens to limiting amino
acid intake.
The intakes of Lys, Met+Cys and Thr re-
quired per gramme of egg output ( a coeffi-
cient) were calculated to be 8.58, 7.96 and
6.73, respectively. These values are very
close to those reported by Fisher and Morris
(1970) for Met ( 4 mg/g), McDonald and
Morris (1985) for Lys ( 9 mg/g) and Met (4.7
mg/g), Pilbrow and Morris (1974) for Lys
(9.5 mg/g), Gous et al . (1987) for Lys (11.03
mg/g) and Met (3.02 mg/g) and Huyghebaert
and Butler (1991) for Thr (8.7 mg/g) despite
these authors having worked with different
strains. That the coefficients have biological
meaning is apparent when comparing the
intake required with the amount of each
amino acid in the egg. The efficiency of util-
ization of the digestible amino acids for egg
production calculated in this study was
0.97, 0.87 and 0.85 for Lys, Met+Cys and
Thr, respectively.
The Reading model is not the ideal
means of determining the maintenance re-
quirement for amino acids. It has been ar-
gued that the maintenance requirement
should be based on body protein and not
body weight (Nonis and Gous, 2008), and
that this should be measured independently
of egg production (Burnham and Gous, 1992).
Nevertheless, the b coefficients obtained here
for Met+Cys and for Thr are close to those

(1970) obtained a value of 25 mg/kg for Met;
McDonald and Morris (1985) obtained a
value of 31 mg/kg for Met and Huyghebaert
and Butler (1991) obtained a value of 43.5
mg/kg for Thr). The Lys required for main-
tenance obtained in this study (36.6 mg/kg)
was considerably lower than that reported
by Pilbrow and Morris ( 90 mg/kg), but the
same as that reported by Gous et al . (1987)
(37.0 mg/kg). The coefficients measured by
Bonato et al . (2011) using a direct method
of measurement differed from those re-
ported here for Met+Cys ( 19 mg/kg) and for
Thr ( 17 mg/kg). Clearly, more research needs
to be conducted to resolve the issue of main-
tenance requirements for laying hens.
The pattern of food intake measured,
namely, a marginal increase and then a se-
vere decrease as the feed amino acid con-
tent was reduced has been reported in all of
the response trials in which the dilution
technique has been used (Fisher and Mor-
ris, 1970; Pilbrow and Morris, 1974; Gous et
al ., 1987; Huyghebaert and Butler, 1991).
The initial increase in intake is an attempt
by the birds to consume sufficient of the
limiting amino acid to enable them to con-
tinue to lay at their potential, but as the de-
ficiency becomes more severe, an increase
in intake is no longer possible presumably
because the bird would become too hot, or
the bulkiness of the food would prevent the
bird from consuming what was required.
The reason for the reduction in intake has
not been established, but it is presumably
linked to the ovulatory control mechanism
in some way, such that yolk synthesis no
longer takes place continuously, ovulation
becomes more sporadic, and the require-
ment for amino acids for egg production de-
clines.
As a consequence of the reduced intake
of the limiting amino acid both rate of laying
and egg weight decline. The rates of decline
are, however, not the same, as has been dem-
onstrated by Morris and Gous (1988) with
laying hens and by Bowmaker and Gous
(1991) with broiler breeders. A graph similar
to that produced in the preceding references
indicates that the relative change in rate of
lay is greater than that in egg weight ( Fig. 19.1 )
and that the reductions in both rate of lay and