INAVI: A Practical Tool to Study the Influence of Nutritional and Environmental Factors on Broiler Performance - Nutritional Modelling for Pigs and Poultry - page 108

General diagram of energy utilization every genotype, MEm can be considered as

a function of body proteins (Whittemore,
1976; Emmans and Fisher, 1986). Some au-
thors even go further by introducing a factor
dependent on body weight gain (Black et al .,
1986). However, none of these methods can
actually represent a real situation. There-
fore, in INAVI, the problem related to the
determination of MEm led to a simplifica-
tion of the system by considering MEm as a
function of metabolic weight, which can be
adjusted by the user. MEm is thus defined as
the product of metabolic weight by an index
of maintenance ( IM ), an adjustable param-
eter related to the broiler 'growing shape'
(see 'Step 2: calibration of the model'):
MEm = IM × BW 0.75

INAVI has to remain as simple as possible to
remain user-friendly. The representation
adopted (Fig. 9.1 ) by INAVI is based on the
use of the metabolizable energy (ME). Feed
intake is the main input of the model and is
used to calculate metabolizable energy in-
take ( MEI , kcal) (defined as the product of
feed intake (grammes) by ME dietary content
(kcal/kg)). A part of MEI is used for mainten-
ance requirements ( MEm , kcal) and physical
activity ( EPA , kcal). The remaining energy
is then defined as the metabolizable energy
available for growth ( MEdc , kcal):
MEdc = MEI − MEm − EPA (9.1)

(9.5)

A fraction of MEdc is deposited in tissues,
the net deposited energy ( NED , kcal), while
the remaining part corresponds to the pro-
duction of heat associated with these de-
positions. NED is estimated with MEdc and
a deposition efficiency coefficient ( Ed ):
NED = MEdc × Ed

Energy deposition efficiency ( Ed )

Ed controls the efficiency of use of MEdc .
A single parameter does not allow differenti-
ation of the specific efficiency of protein or
lipid deposition. Nevertheless, the coefficients
of efficiency of deposition of lipid and protein
can vary from 0.6 to 0.8 and from 0.4 to 0.6,
respectively (De Groote, 1974; Emmans and
Fisher, 1986). The use of a single parameter
has the advantage of simplifying the system
and reducing the number of model param-
eters, but it can be less flexible. Since the ob-
jective of the first version of INAVI was not
the prediction of body composition, the use
of two coefficients was not required.
An Ed value of 0.6 is frequently suggested
in the literature as an average (De Groote,
1974). Beyond an oversimplification, Ed rep-
resents a global metabolic parameter describ-
ing the global state of syntheses. We consider
that the regulations of these syntheses are
probably less sensitive to the feed character-
istics, environmental conditions or age than
MEm , especially in the young growing animal.
Therefore, in INAVI, the value of Ed is considered
to be constant (0.6).

(9.2)

NED is the energy of lipids and proteins de-
posited, and represents a fraction of the
weight gain. The transformation of NED into
weight gain (Gain, grammes) is estimated by
the energetic value of weight gain; that is, the
number of calories corresponding to 1 g of
weight gain ( Ved , kcal/g):
Gain = NED / Ved (9.3)

Total body weight (BW) is then calculated
from weight gain:
Total body weight = Initial body weight

+ ∫ Gain (9.4)

Estimation of MEm , Ed and Ved

Energy for maintenance ( MEm )

Maintenance is widely dependent on geno-
type, feed composition, physical activity and
the environment (Van Milgen et al ., 1998).
It is likely that considering MEm as a single
function of metabolic weight in broilers of
very different growing speeds can be a mistake.
To better take into account the potential of

From NED to weight gain in INAVI, the
energetic value of weight gain ( Ved )

With a variable value of MEm and a nearly
constant Ed , the description of growth is