Agriculture Reference
In-Depth Information
within the animal, thus inducing an adapta-
tion of feed intake. One or several factors
can influence variables such as
Ed
,
Ved
, etc.
Therefore, in order to study the respective
effects of these factors on a variable, we
defined specific response laws; that is, the
relationship between an influencing factor
(e.g. ME content) and the variation of this
variable, represented by a dimensionless
weighting coefficient (variation factor,
VF
).
Except for two factors (limiting amino acid
level and fine particle level),
VF
values are
always expressed as a function of the diffe-
rence between simulation and reference
values of the factors. Furthermore, this ap-
proach allows users to modify existing laws
or implement new ones in the model.
The variable in the simulation is then
calculated as the variable value in the refer-
ence conditions multiplied by the product
of all
VF
values as shown in Eqn 9.19:
Dietary crude protein content
In the model, dietary crude protein (CP) con-
tent was segregated from essential amino
acids (EAA) requirements
sensu stricto
. In
practice, feed formulation is generally car-
ried out with minimum levels of EAA and a
minimum level of protein. However, CP
content (%) was taken into account in the
function of ME content by calculating an
ME:CP ratio.
The increase in CP content (i.e. decrease
in the ME:CP ratio) of the feed is associated
with a decrease in fattening and an increase
in protein deposition, thus decreasing the
value of
Ved
. To calculate these variations,
the body composition of broilers fed with
regimes of protein content varying from
16% to 36% were used to measure the vari-
ations of
Ved
from 0 to 49 days of age (Jackson
et al
., 1982). A linear relationship between
ME:CP and
Ved
was found, and we there-
fore considered that an increase of
100
units
of ME:CP resulted in a 20% increase in
Ved
(Fig. 9.4b
)
.
The effect of the ME:CP ratio on
Ed
was estimated by adjusting
Ed
in order to fit
simulated performances to measured per-
formances from Sklan and Plavnik (2002).
The increase in CP content from 18% to
24% (ME:CP values of
178
and
133,
respect-
ively) induced a decrease of 7% in feed con-
sumption and an increase in growth of 5.5%.
The authors also measured a 6% decrease in
feed conversion. After adjustment, the effect
of ME:CP was estimated to be linear, with an
improvement in deposition efficiency (
Ed
) of
10% for an increase of
100
units in the ME:CP
by a higher proportion of lipids deposited in
total weight gain. The articulation of
Ed
and
Ved
in connection with the ME:CP ratio em-
phasizes the importance of examining the
intermediary metabolism and consequently
the gain content according to the supplied
nutrients and the tested bird lines.
Variable = Variable ×
Π
VF
(9.19)
Feed characteristics
Dietary ME content
Several authors have reported an increase
in carcass fattening with the increase in
dietary ME content (Jackson
et al
., 1982;
Leeson
et al
., 1996). In our approach, this
fattening is connected with an increase in
Ved
of 13% for an increase in ME content
of 1000 kcal/kg (in comparison with the
reference ME content) as shown in
Fig. 9.4a
.
However, when simulating an increase in
ME content from 2900 to 3400 kcal/kg,
BW
was decreased by 7%, which is not realis-
tic. The adjustment of
Ed
was thus neces-
sary to remove this paradoxical effect from
INAVI. The regulation of
Ed
according to
ME content was based on the data of Jack-
son
et al
. (1982), with an increase or a de-
crease of
Ed
of 7% and 6% for an increase
or a decrease in ME content of 1000 kcal/kg,
respectively
(Fig. 9.4a)
. This balance between
Ed
and
Ved
might be connected to the ME
origin, which could be composed of more
digestible nutrients.
Dietary amino acid content
The EAA content of the feed is not taken into
account in absolute values but by considering
the level of the first limiting EAA. In INAVI,
