Agriculture Reference
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in terms of body protein content or body
weight (Bonato
et al
., 2011). A parallelism
test (Kaps and Lamberson, 2004) was previ-
ously carried out to compare the slopes of
the response to increasing amino acid in-
take on N balance for these genotypes. The
regression coefficients obtained for Lys and
Val did not differ (
P
> 0.05) between the two
genotypes, and hence, only one equation is
needed to describe bird response, inde-
pendently of genotype (Bonato
et al
., 2011).
The responses of Bovans White and Cobb
roosters were again compared when deter-
mining the maintenance requirements for
Lys and Val.
collection period) individual body weight
was recorded. Birds were fed the experimen-
tal diets directly into crop by gavage every
24
h for the next
72
h. During this period, a
N-free diet was supplied
ad libitum
in the
feeder and its intake was measured during
the
72-
h period. Excreta collection started
24
h after the first feeding and ended
24
h
after the last feeding. Excreta were weighed
per individual bird at the end of the collec-
tion period, and then processed.
Experimental diets
The experimental diets were formulated ac-
cording to the dilution technique (Fisher
and Morris, 1970). Two basal diets were for-
mulated in each experiment. One was free
of nitrogen (N-free diet) but contained ad-
equate metabolizable energy (ME), mineral
and vitamin levels as recommended by Ros-
tagno
et al
. (2005) and Rostagno
et al
. (2011).
The other basal diet (concentrated diet) was
formulated to contain the same ME, mineral
and vitamin levels as the N-free diet, but
also included protein, with the test amino
acid as the first-limiting amino acid, being
0.2 (Met+Cys, Thr and Lys trials) and 0.4 (Val
trial) of the 'ideal' requirement. The N-free
and the concentrated basal diets were mixed
in different proportions in order to obtain the
intermediate amino acid dietary levels.
The composition of the six basal diets
used to determine the maintenance require-
ment for amino acids is presented in
Table 15.1.
Daily feed allowance consisted of a mixture
of N-free and N-containing diets, and changed
according to the diets, as shown in
Table 15.2.
The last amino acid level in each trial, as
shown in
Table 15.1
,
was used to verify if
indeed the test amino acid was the limiting
amino acid in the diet, and not to determine
maintenance requirements.
Birds and experimental design
In each experiment,
42
birds of each strain
were used, except for the Lys trial (
36
birds). Birds were selected in order to ob-
tain the same average body weight at the
beginning of the experiments. Bovans White
roosters weighed, on average, 2.02 ± 0.2 kg
in the Met+Cys trials, 2.11 ± 0.2 kg in the
Thr trials and 2.05 ± 0.2 kg in the Lys trial;
and Cobb roosters weighed, on average,
5.54 ± 0.6 kg in the Met+Cys trials, 5.78 ±
0.7 kg in the Thr trials and 4.10 ± 0.5 kg in
the Val trial.
Roosters were individually housed in
metabolism cages (0.4 × 0.5 × 0.6 m) each
equipped with a nipple drinker and a trough
feeder. In each experiment, birds were fed
six diets (treatments) containing graded levels
of the test amino acid, with six birds (repli-
cates) per treatment, except for the Lys trial,
which included five treatments, but also six
replicates each. Birds were randomly dis-
tributed among treatments.
Experimental procedures
Feed was withdrawn two days before the
experiments started to allow the emptying
of the digestive tract. During this period,
60 ml of water with sucrose (
1:
1) was fed
once daily directly into the crop. In the
morning of the third day (beginning of
3-
day
Laboratory analysis
The excreta produced daily were individu-
ally weighed and stored in a freezer (-20°C)
until the end of the trial. Samples were
