Agriculture Reference
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and hence, the result is closer to the ob-
served value.
NR
max
T
is dependent on the genotype
and changes with age, as shown by our
results. This value decreased from period I
(
14
to 28 days) to period III (
96
to 112 days)
and would tend to reach zero when growth
of protein ceases in adult birds and only fat
is deposited thereafter (Samadi and Liebert,
2006b, 2007a; Marcato
et al
., 2010).
Traditionally, the potential for growth
has been described by the Gompertz growth
model, based on the body or body protein
weight (
BP
) vs time (
t
). By dividing the first
derivative of this equation by the metabolic
weight of the bird (
dBP
/
dt
÷
BW
k
0.67
) it is
possible to determine the nitrogen depos-
ition rate (
BP
÷ 6.25) in mg/BW
k
0.67
/day.
Considering the results of Martin
et al
.
(1994) obtained on the same basis as in this
study (mg/BW
k
0.67
/day), the
ND
max
T
values
were 860 mg/BW
k
0.67
/day, 865 mg/BW
k
0.67
/day
and 791 mg/BW
k
0.67
/day for the Hisex, Ross
Brown and Amber-Link strains, respect-
ively. The differences between their values
and those of the present study are related to
methodological aspects. Martin
et al
. (1994)
used the comparative slaughter technique
to determine protein deposition, and the
dietary nitrogen intake levels were in agree-
ment with practical conditions, in contrast
with the conditions applied in this study.
Other differences between the above-
mentioned approaches and the approach ap-
plied in this study are that values for
ND
max
T
were determined using non-destructive
methods and the birds were subjected to high
levels of nitrogen intake to characterize the ni-
trogen use limit of this modern strain, which
provided knowledge that can be used to
understand and explore their full deposition
potential, thereby limiting dietary amino acid
levels to minimize nitrogen excretion and
avoid the use of excessive levels of nutrients.
The theoretical maximum rate of pro-
tein deposition (
PD
max
T
) obtained from
ND
max
T
characterizes the genetic potential of the
strain and it is not possible to attain this by
improving the diet; therefore, different at-
tainable threshold values can be expressed
as a percentage of
PD
max
T
(Samadi and
Liebert, 2006a).
The three amino acids under study pro-
duced different rates of deposition of nitrogen
(Fig. 20.3
,
20.4
and
20.5
). This is due to the
fact that each amino acid has a different
composition in the protein being formed.
However, as this method does not use the
comparative slaughter technique, it is not
possible to determine these concentrations.
However, their importance may be observed
in nitrogen deposition.
To estimate dietary amino acid re-
quirements it is necessary to know the effi-
ciency of utilization of each amino acid. In
the approach taken in this study the effi-
ciency is obtained from the relationship
between
b
(protein quality of the feed) and
c
(concentration of the limiting amino acid
in dietary protein). To compare the effi-
ciencies of amino acid utilization found in
this study with those in the literature,
a calculation must be performed. Consider-
ing a protein deposition rate of 60% for
each growth phase, based on the maximum
potential for protein deposition and the
Lys, Met and Thr contents of the body plus
the feathers, according to Emmans (1989)
and considering the estimated amino acid
intake (
Tables 20.5
,
20.6
and
20.7)
, the cal-
culated efficiencies for body protein de-
position of the individual amino acids in
the three growth phases are 86%, 73% and
75% for Lys, 75%, 73% and 65% for Met
and 94%, 86% and 78% for Thr. The val-
ues obtained in this study are within the
range of those reported in the literature
(Hurwitz and Bornstein, 1973; Martin
et al
., 1994; Edwards
et al
., 1997, 1999;
Edwards and Baker, 1999; Fatufe
et al
.,
2004). The great variation among the re-
ported values is due to the use of different
approaches and, moreover, these results
are mostly for broilers because no studies
using pullets were found in the literature.
However, the efficiency of amino acid util-
ization is an important dietary parameter
that must be thoroughly studied because it
represents how the birds use an amino acid
for growth.
All of the information regarding the
Dekalb White pullets that was obtained in
this study using the Goettingen approach
was applied in the
LAAI
equations to provide
