Agriculture Reference
In-Depth Information
a personal preference for the power func-
tion because it enables comparison with the
maintenance energy requirement, which is
expressed relative to
BW
0.60
. When the par-
ameter '
b
' for the power function is less
than 0.60, the difference between feed in-
take and maintenance declines with in-
creasing body weight.
The functions we proposed initially
have the limitation that there is no explicit
control of feed intake relative to the energy
expenditure, so the animal would become
excessively lean or fat if the simulation were
to be carried out for a long period of time.
This is why we included a new function in
InraPorc where feed intake is expressed as a
Gamma function of the maintenance energy
expenditure (i.e.
Y
= (
a
× (
b
×
BW
× exp
(-
b
×
BW
)) + 1) ×
c
×
BW
0.60
). The parameters
'
a
' and '
b
' are user inputs, while '
c
' is a con-
stant, which depends on whether intake is
expressed on a quantity basis or on an en-
ergy basis (e.g. it is 0.75 MJ/(kg BW
0.60
)/day
when intake is expressed on a net energy
basis). With increasing body weight, feed
or energy intake will approach the main-
tenance energy requirement so that the ani-
mal stops growing and attains maturity.
The Gamma function contrasts with the
other three functions because it allows for
a reduction in feed intake when body
weight increases. Compared to the other
functions, it also provided the most accur-
ate fit to observed data on feed intake
(Vautier
et al
., 2011a,b). Because protein
deposition is described by a Gompertz
function (which also becomes zero when
the animal attains the mature protein
mass), lipid deposition will be controlled.
This approach contrasts with that of Fer-
guson
et al
. (1994), who explicitly mod-
elled genetically determined protein and
lipid depositions using Gompertz func-
tions to predict feed intake. These funda-
mentally different approaches can be sum-
marized as 'do animals grow because they
eat or do they eat because they want to
grow?' Even though we feel that the second
approach may be biologically more appro-
priate, we opted for the first approach be-
cause feed intake is easier to measure and
control than lipid deposition.
The Gamma function also predicts that
feed intake equals the maintenance energy
requirement at zero body weight (i.e. around
conception), which is, of course, debatable.
Alternatively, an exponentially declining
function of maintenance could be used
(
Y
= ((
a
× exp(-
b
×
BW
) + 1) ×
c
×
BW
0.60
)) so
that energy intake would be highest at con-
ception. Although we tested this function,
we did not include it in the InraPorc soft-
ware tool because its fit to experimental
data was not as good as that of the Gamma
function.
In the InraPorc software tool, it is left to
the user to decide whether voluntary feed
intake is determined by quantity (dry mat-
ter) or energy (DE, ME or NE). This has of
course an impact on how the animal re-
sponds to diets with different energy dens-
ities. If the user decides to define voluntary
feed intake on a dry matter basis, feeding a
high energy diet (e.g. with a high fat con-
tent) results in a greater energy supply and
growth rate than feeding a low energy diet.
However, if energy intake is assumed to be
regulated on an NE basis, there will be no
difference in growth rate between both
diets. As discussed before concerning en-
ergy homeostasis in mature animals, it is
likely that the regulation of feed intake is
described best on an NE basis. However,
there may be situations where the physical
feed intake capacity of the animal is limited
(especially in younger pigs fed high fibre
diets), and expressing intake on a DE or dry
matter basis (or on a bulk basis) may then be
more appropriate. We realize that leaving
the choice to the user has an important im-
pact on the model response.
Potential Protein Deposition
A Gompertz function was used to describe
the potential protein mass. The Gompertz
function is a sigmoidal function so that pigs
will attain a constant protein mass at matur-
ity. The first derivative of the Gompertz func-
tion describes the potential protein depos-
ition. This first derivative can be expressed
relative to time or as a function of current
