Agriculture Reference
In-Depth Information
thermolysis capacity multiplied by the simu-
lated metabolic weight (Eqn 9.13). In a dynamic
model, it is represented as an accumulation
variable for the duration of the simulation:
Maximum feed intake
The thermostat approach is limited because
the quantity of feed ingested by the animal
cannot be infinite. The quantity of feed con-
sumed (grammes) is limited by a maximum
consumption (grammes) defined as the refer-
ence consumption (grammes) multiplied by
a physical capacity (Eqn 9.17), an adjustable
value, set to
2
in INAVI, which means that in
1
h (time step of the model), the animal can-
not ingest more than twice the reference feed
intake. This value of
2
corresponds to a prac-
tical situation in periods of starvation. It might
be adjusted for more restrictive systems.
Thermostat = ∫(
HP
- Thermolysis
capacity ×
BW
0.75
) (9.13)
A positive or negative balance of the thermo-
stat induces, respectively, a decrease or an
increase in
MEI
at the next time step of the
simulation. The correction (Eqn 9.14) is de-
termined by a modifiable factor (Adaptation
level) that translates the calories of heat pro-
duced into
MEI
calories. Its value is set to
1.5 kcal
EMI
/kcal
HP
but can be modified by
the user. Corrected
MEI
is then calculated as
the difference between
MEI
ref
and
MEI
cor-
Maximum consumption = Physical
capacity × Reference consumption
(9.17)
The simulated
MEI
is thus defined in the
following way:
MEI
correction = Adaptation level
× Thermostat (9.14)
Corrected
MEI
=
MEI
ref
-
MEI
correction
MEI
= Corrected
MEI
, if Corrected
MEI
≤ Maximum consumption
MEI
= Maximum consumption,
if Corrected
MEI
> Maximum
consumption
(9.15)
Mobilization of the reserves
(9.18)
In some cases, the animal has no more access
to the feed (e.g. at night). The thermal bal-
ance becomes negative and body reserves be-
come the only source of energy for mainten-
ance requirements. In that case,
MEI
is equal
to 0 and
MEdc
becomes negative, representing
the energy deficit related to the maintenance.
In this situation, NED does not represent
the deposited energy anymore, but rather the
energy needed to cover the maintenance
requirements. The efficiency of deposition
(
Ed
) becomes meaningless and thus
NED
is
defined according to Eqn 9.16:
The architecture shown here defines the base
of INAVI functioning. It is a simplified en-
ergy model with two versions (submodels).
The first one establishes the reference en-
ergy balance (heat production, thermolysis
capacity), which is calibrated with the refer-
ence data (growth, feed intake) provided by
the user. The second is the simulation sub-
model, which takes into account nutritional
and environmental factors and their conse-
quences for the energy balance. To maintain
this balance, feed intake is regulated through
NED
=
MEdc
, if
MEdc
< 0
NED
=
MEdc
×
Ed
, if
MEdc
≥ 0
(9.16)
Taking into Account Nutritional
and Environmental Factors and
their Effects on Energy Balance
In such conditions, weight gain actually rep-
resents weight loss related to the use of the
body reserves. The energetic value of depos-
ition (
Ved
) is no longer bound to the lipid:pro-
tein ratio of growth, but takes the value of
7.4 kcal/g of weight loss per calorie of
NED
(i.e. raw energy of
1
g of lipid (9.3) multiplied
by efficiency of use (0.8)) since body lipids are
considered as the only source of energy.
General approach
In the conceptual approach of INAVI, simu-
lation conditions (i.e. environmental and
nutritional factors) influence energy flows
