Agriculture Reference
In-Depth Information
adequate voluntary feed intake and growth
rates to be calculated.
was evaluated by comparing measured
and predicted data. The model was cali-
brated to predict the observed
BW
and
CFI
of each genetic strain. All strains were fed
according to the feeding phases applied in
the original experiment. All feeds con-
tained 11.5 MJ EE/kg and were assumed to
contain all other nutrients in excess, in-
cluding Lys. For the evaluation, observed
and predicted data for each strain were
compiled according to chicken strain
(broiler and pullet). The quality of fit was
tested by the procedure of Theil (1966)
in which the mean squared prediction
error (MSPE) is calculated as the sum of
squares of differences between simulated
and observed measurements divided by
the number of experimental observations.
MSPE was decomposed into error in cen-
tral tendency, error due to regression (ER)
and error due to disturbances, and expressed
as MSPE%, as suggested by Benchaar
et al
.
(1998).
Simulated and observed values were
similar across all feeding periods both for
body weight and cumulative feed intake of
broilers and pullets
(Figs 14.5
,
14.6
,
14.7
and
14.8
)
. Model accuracy, as estimated by
MSPE, was 0.01 (broilers) and 0.72 kg (pul-
lets) for BW and 0.68 (broilers) and 1.99 kg
(pullets) for CFI. Deviations between ob-
served and predicted performance values
were small, which is consistent with the
fact that model parameters were estimated
for each strain and chicken type. However,
the slope between the predicted and ob-
served BW (broilers) and CFI (pullets) was
1.04, which is higher than
1
(
P
< 0.001), in-
dicating that the model slightly underesti-
mated these parameters during the first
feeding phase, and slightly overestimated
them in older birds. In fact, more than 20%
of the observed error between predicted and
observed BW and CFI is given by the diffe-
rence between the slopes (ER error).
Because unique parameters (mainten-
ance coefficient, energy cost for protein and
lipid deposition, etc.) were applied both for
meat-type and layer-type chickens, except
for those used for bird description, the
model was able to obtain growth and intake
estimates very close to observed values.
Response to environmental conditions
When heat produced by the bird is greater
than the maximum it can lose (
THP
>
THL
max
)
to the environment the bird is hot and, there-
fore, will attempt to reduce
THP
to
THL
max
.
In this case, feed intake declines to main-
tain the heat production balance:
aFI
=
dFI
e
- (
THP
-
THL
max
)/
ME
(g/day)
The impact of
aFI
reduction on
PD
and
LD
depends on whether amino acid intake is
still sufficient to meet
pPD
(
PD
AA
) require-
ments, given that
PD
is determined by:
PD
AA
= [(
AAintake
-
AAm
) ×
k
]/
AAb
(g/day)
Lipid deposition (
LD
) is estimated as the
difference between energy intake and energy
retained for
PD
and lost as heat.
LD
= [(
aFI
×
ME
) -
THP
- (23.8 ×
PD
)]/
39.6 (g/day)
When the amount of heat loss is greater than
heat production (
THP
<
THL
min
) the bird is
cold. In this case extra heat will be neces-
sary to maintain body temperature and en-
sure
THP
=
THL
min
. The energy difference
between
THL
min
and
THP
causes mainten-
ance requirements to increase and feed in-
take will therefore increase by:
ExtraFI
= (
THL
min
-
THP
)/
ME
(g/day)
If (
ExtraFI
+
aFI
) >
cFI
(bulk constraint)
then feed intake will decline to
cFI
, and
PD
and
LD
will be adjusted accordingly, as pre-
viously discussed under constrained feed
intake.
Model Evaluation
Under adequate nutritional supply
Estimating growth and body composition
The ability of the model to estimate body
weight and cumulative feed intake (
CFI
)
