Agriculture Reference
In-Depth Information
protein mass. This choice for either expres-
sion has an important (theoretical) meaning
because it implies that either time (or age)
or state (current protein mass) is considered
the driving force for protein deposition.
This also has practical consequences in the
way compensatory growth is predicted.
When protein deposition is described as a
function of time, the pig may lose (part of)
its growth potential because of ageing. This
is not the case when protein deposition is
described as a function of current protein
mass. The study of Lister and McCance
(1967) is inconclusive because refeeding the
pigs after
1
year of severe feed restriction
resulted in growth rates similar to that of
the control group. However, the restricted
pigs stopped growing at a lower body weight,
suggesting that both time and current state
play a role in protein deposition. In InraPorc,
we chose to describe the potential protein
deposition as a function of the current pro-
tein mass. Initial simulations indicated that
this allowed for a better prediction of com-
pensatory growth after a period of feed re-
striction using the data of Bikker
et al
.
(1994, 1996).
Different approaches have been taken
for the empirical modelling of protein de-
position or body weight gain (Black
et al
.,
1986; Emmans and Kyriazakis, 1997; Schulin-
Zeuthen
et al
., 2008). We felt most comfort-
able with a function that would account for
the concept of maturity (i.e. protein depos-
ition should tend to zero) but without attrib-
uting a specific biological meaning to the
Gompertz function or to its parameters.
When written as a differential equation, the
Gompertz function is often parameterized
to include the initial protein mass, the pro-
tein mass at maturity and a shape parameter.
The mature protein mass of pigs is around
30 kg, but the protein mass at slaughter is
much lower. When data up to a normal
slaughter weight are fitted to a function like
the Gompertz, it is not uncommon to obtain
estimates of the mature body weight that are
biologically unrealistic. This is a problem of
fitting partial data to a function that de-
scribes growth throughout life. Rather than
restraining mature protein mass within bio-
logically reasonable limits, we preferred to
parameterize the Gompertz function with
parameters that have a practical meaning
and that best describe the protein depos-
ition during the productive life of the ani-
mal. The Gompertz function for protein de-
position was parameterized in InraPorc by
the initial protein mass, the mean protein
deposition during the productive life and a
precocity parameter. By default, the initial
protein mass is calculated from the initial
body weight. Because we used a fixed rela-
tionship to estimate body weight from pro-
tein and lipid mass, the user will have the
possibility of adjusting (within reasonable
limits) the initial protein mass for a given
initial body weight. The mean protein de-
position determines the difference between
the initial and final protein mass at the end
of a simulation. For a given feed intake,
changing the mean protein deposition will
therefore change the body weight gain and
body composition. The precocity parameter
represents the shape of the Gompertz func-
tion. For a given initial protein mass and
mean protein deposition, the protein mass
at slaughter will be known, but not the tra-
jectory to get there. A high value for the pre-
cocity parameter results in an early matur-
ing animal, while a low value results in a
late maturing animal.
Maintenance Energy Requirement
As explained before, maintenance is a fun-
damental concept in biology but difficult to
measure in growing animals. We feel that
the fasting heat production is the best meas-
urable indicator for the maintenance energy
requirement in growing animals. There is
strong evidence that the fasting heat pro-
duction in growing pigs varies with body
weight raised to the power 0.60 (Noblet
et al
., 1999) and not to the frequently used
scalar of 0.75. The latter is derived from
comparing maintenance in different mature,
non-producing species (e.g. from mice to
elephants) and it is not surprising that other
values are found for growing animals of dif-
ferent body weights within a species. The
choice of an appropriate scalar has important
consequences for the change in maintenance
