Agriculture Reference
In-Depth Information
32
30
28
26
24
22
20
18
16
4
weeks
5
weeks
6
weeks
Fig. 9.2.
Proportion of fast-growing broilers (
12
pens,
25
chickens) observed standing at
4,
5
and
6
weeks of
age. Values were obtained by scan-sampling from five observations during four periods (
8:
00,
12:
00,
16:
00
and
20:
00) of day time. (From Quentin, 2004.)
to Wenk and Van Es (1980),
PA
represents
7% to 15% of
MEI
in fast-growing broilers
(from
20
to 40 days of age) in normal meas-
urement conditions in respiratory cham-
bers. We therefore fixed
AU
at a value of
1.5 kcal/%
PAL
/g
BW
so that
EPA
repre-
sented 10% of
MEI
. However, this parameter
is considered as a constant, independent of
BW
,
PAL
and the age of the animal. The cur-
rent determination of AU lacks precision but
this value can be modified by the user (accord-
ing to new research or calculations).
Are the limits on feed intake
physical or physiological?
A decrease in dietary energy content as well
as changes in the physical characteristics of
the diet (e.g. pellets vs meal) tend to in-
crease feed intake in relatively large propor-
tions, before the physical limit of the animal
is reached. This seems to indicate that broil-
ers are almost always physically capable of
ingesting more feed. On the other hand, the
range of physiological adaptation may be
more limiting. Body temperature is never
constant, but since broilers are homoeo-
thermic animals, body temperature variations
cannot exceed
2-
3°C without jeopardizing
the survival of the animal (De Basilio
et al
.,
2001). Fast-growing broilers produce more
heat than laying hens or slow-growing broil-
ers, and are also more sensitive to the tem-
perature of their environment. Therefore, a
major problem for these animals is to dissipate
the heat produced by feed intake and their
metabolism (Yalcin
et al
., 1997).
Feed Intake Regulation: The Notion
of a Thermostat
The main hypothesis regarding feed intake in
the current model is based on the adaptation of
the animal to its requirements under optimal
production conditions. The calculation of en-
ergy requirements from body composition and
the description of the energy balance of the
animal is the classical way to predict feed intake
in broilers (Emmans and Fisher, 1986). Even if
this approach is successful in 'optimal' condi-
tions, predictions are not always fully accurate in
'non-optimal' ones. In practice, there is always
at least one constraint. The thermolysis cap-
acity of the animal seems to be a factor regulat-
ing feed intake, representing a major limit on
its metabolic capacities for adaptation.
Modelling feed intake regulation
In order to regulate energy intake, we intro-
duced the concept of the 'thermostat',
which compares the thermal balance in
