Agriculture Reference
In-Depth Information
is reduced (in relation to the lower protein
synthesis). Yet, the adaptation capacities of
the animal are quickly exceeded, explaining
the decrease in
Ed
in real EAA deficiency
conditions.
An increase in animal density is associ-
ated with a decline in the performances
(Feddes
et al
., 2002). The influence of
density on
Tp
was taken into account by
adjusting the model results according to
the data of Feddes
et al
. (2002). In INAVI,
we therefore considered there to be an in-
crease in temperature of 0.18°C per sup-
plementary kg/m² (
Fig. 9.5a
). However,
when simulation stocking rate was below
20 kg/m² or lower than the reference, the
influence of animal density was considered
to be negligible.
From
4
to
8
weeks of age, the growth of
the broilers exposed to a temperature of
35°C is a parabolic function of relative
humidity, with an optimum between 60%
and 65% (Yahav
et al
., 1995). Indeed,
under high temperatures (greater than
28°C), the main method of thermolysis is
hyperventilation (panting), thus heat losses
are mainly latent. A relative humidity that
is too high reduces the efficiency of therm-
olysis and the animal quickly becomes
hyperthermic, with a decrease in feed in-
take and growth. In contrast, when relative
humidity is lower than the optimum,
thermolysis is easier but water losses due
to hyperventilation cannot be totally com-
pensated for and quickly lead to a respira-
tory alkalosis, also affecting growth (Teeter
et al
., 1985). Based on the data of Yahav
(2000), we therefore considered an influ-
ence of +0.095°C per % of relative humid-
ity higher or lower than the optimum as
shown in
Fig. 9.5b
.
Air speed (m/s) above the animals re-
duces the sensation of heat by convection,
which increases the feeling of coolness.
The effect of air speed (from 0.5-
3
m/s) on
growth was measured by Yahav
et al
. (2001)
at high temperatures (35°C). The adjust-
ment of the INAVI results based on the re-
sults of Yahav
et al
. (2001) led to a decrease
in
Tp
of 3.3°C between 0 and 2.5 m/s, while
for an air speed over
3
m/s,
Tp
was only
reduced by 2.2°C (
Fig. 9.5c)
. This relation-
ship is only valid for
T
Tindoor
values below
26°C. Above 26°C, we considered there to
be a linear decrease (empirical estimate) of
5°C for an increase in air speed of
1
m/s as
shown in
Fig. 9.5c
.
Diet particle size
Results from experiments by Quentin
et al
.
(2004) were used to represent the influence
of levels of fine particles on feed intake
(through a variable called 'prehensibility'
directly modulating feed intake both in ref-
erence and simulation submodels) and
PAL
.
The performances of broilers (
15-
35 days of
age) were compared by feeding them with
pellets (0% fine particles) or meal (100%
fine particles). In broilers fed with meal,
feed intake decreased by 18% and
PAL
was
estimated to be 140% of that on the pelleted
diet. Between these values, the evolution of
PAL
and prehensibility were both con-
sidered to be linear as shown in
Fig. 9.4c
,
even if the fine level could also fit a linear-
plateau model, that is, if the proportion of
fine particles is lower than 30%, they have
no effect on
PAL
(Quentin
et al
., 2004).
Environmental factors: the notion
of perceived temperature
Estimation of perceived temperature
The environment strongly influences the
heat loss of the chicken, specifically the
combination of several parameters such as
indoor temperature (
T
Tindoor
); air relative hu-
midity (%) and air speed (m/s) above the ani-
mals (Yahav, 2000). Animal density (birds/
m²) also modulates the actual perception of
heat. The combination of these factors de-
termines a 'perceived temperature' (noted
Tp
), actually felt by the animal, which influ-
ences the balance between thermogenesis
and thermolysis. In INAVI,
Tp
is calculated
as the sum of
T
Tindoor
with additive effects of
these factors (
E
density
,
E
humidity
and
E
airspeed
):
Tp
=
T
Tindoor
+
E
density
+
E
humidity
+
E
airspeed
(9.20)
