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precisely modify the EAA composition of the
experimental diets. However, the lower util-
ization efficiency of crystalline AAs com-
pared to protein-bound AAs suggested by
many authors (Dabrowski and Guderley, 2002)
may influence the estimation of the EAA pat-
tern. In this study, all the essential amino acids
were supplemented in each experimental
diet to ensure a balanced mixture and instead
of feeding the animals once daily, the animals
were fed several times a day ad libitum .
The observed changes in N deposition
when dietary amino acids were reduced
from the BD were used to calculate the opti-
mum balance between the EAAs in the N
balance trial. In both methods, the dilution
of valine promoted the greatest reduction in
N deposition in period I ( 6 to 21 days), fol-
lowed by leucine in periods II and III ( 22 to
37 days and 38 to 53 days). This indicates
that valine and leucine were the first limit-
ing AAs in the BD given in periods I, II and
III, respectively. This would lead to the fea-
ther abnormalities observed in the valine
and leucine treatments, similar to those ob-
served by Gruber et  al . (2000). As dietary
valine and leucine levels decreased, a simi-
lar progressive feather abnormality became
apparent and gave the feathers a ragged ap-
pearance (Robel, 1977), and was respon-
sible for a decrease in body weight and feed
conversion (Farran and Thomas, 1992).
As protein is essential for both growth
and maintenance, nitrogen deposition is af-
fected by the level of protein intake and by
the quality of the dietary protein (Wang and
Fuller, 1989). As the individual feed supply
was controlled and kept constant during the
experimental period, N intake was similar
between dietary treatments. The decrease in
protein efficiency ratio observed during the
experimental period may be explained
largely by the fact that as body weight gain
increased with age, protein requirements
for maintenance also increased (Brody,
1945) while feed or protein intake did not
increase proportionately (Scott et al ., 1969),
hence the quantity of protein available for
growth (weight gain) decreased. However,
the protein efficiency ratio is not a good in-
dicator of protein quality because this
method does not consider the quantity of
protein used for maintenance, and values of
protein efficiency ratio vary with levels of
protein intake. Additionally, body weight
gain does not necessarily correspond to
body protein gain. However, protein quality
( b ) calculated using the Goettingen approach
is a parameter that plots the slope of the ex-
ponential function. Thus, the dietary protein
quality ( b ) is independent of NI but is lin-
early dependent on the concentration of the
limiting AA in the feed protein ( c ). The ob-
served protein quality ( b ) declined following
dilution of the crystalline AA under study.
Due to EAA dilution protein quality was sig-
nificantly affected. According to Samadi and
Liebert (2008) the daily AA requirement for
equal daily protein deposition is dependent
only on the efficiency of utilization of the in-
dividual dietary AA under study, and this
was established by the model parameter ( bc -1 ).
Consequently, it is possible to compare the
model parameters ( bc -1 ) of individual AAs
directly. Using this procedure for evaluating
the optimal AA ratio, comparisons are only
allowed within equal age periods because
varying NR max T depending on age affected
the established value of bc - 1 . Thus, the opti-
mal lysine to EAA ratio was derived by divid-
ing the efficiency of utilization of lysine by the
efficiency of utilization of the EAA.
The dilution method is generally ac-
cepted as an efficient and rapid tool to esti-
mate the ideal EAA profile (Baker, 2003).
This method was initially outlined by Wang
and Fuller (1989) in pigs and is based on the
concept that each EAA is equally limiting
for protein accretion. In broiler chickens,
estimation of the ideal dietary EAA profile
has already been applied using the dilution
method (Gruber et al ., 2000; Roth et al ., 2001)
but only from 7 to 28 days post-hatching.
Roth et al . (2001) estimated the EAA profile
for broiler chicks using the dilution method,
and values obtained ( Table 21.4 ) are very
similar to those estimated in the present
study using the broken line model and the
Goettingen approach. The data are also con-
sistent with the recommendations of Brazil-
ian tables (Rostagno et al ., 2011) for periods
I and II and the Illinois ideal protein pattern
(Baker and Han, 1994; Baker et al ., 2002) in
period I.
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