Agriculture Reference
In-Depth Information
food intake of animals, an adequate descrip-
tion of the genotype has become essential.
The approach suggested by Emmans
(1989) to describe and evaluate different
genotypes begins with a definition of poten-
tial protein growth using a Gompertz growth
curve, and the live weight of the animal is
built up from this, using the allometric rela-
tionships that exist between protein, water,
ash and lipid; that is a bottom-up approach.
He has shown that a few simple assump-
tions can lead to a description of an animal
that is sufficient for predicting its perform-
ance in non-limiting conditions and for cal-
culating what these conditions are. It seems
sensible to be able to predict performance in
non-limiting conditions before the more dif-
ficult question is tackled, namely, that of de-
fining growth in limiting conditions.
Values for the genetic parameters that
define an animal can be measured by rearing
animals in environmental conditions that
are as near to ideal as possible. Under these
conditions, growth curves are obtained that
represent the genetic potential for a particu-
lar genotype. The growth curves obtained in
this way allow comparisons to be made be-
tween breeds and strains. Examples of such
investigations are in Hancock
et al
. (1995)
and Gous
et al
. (1999).
Hence, where the requirements for protein
and fat growth are defined in terms of the
digestible amino acid and effective energy
contents (Emmans, 1994) of the tissues being
formed, these same units must be used to de-
fine the nutrient content of the feed on offer.
Predicting voluntary food intake
The implication from the above is that an
animal requires certain resources in order to
maintain its current state and to grow ac-
cording to its growth plan. Because the bird
is motivated to grow at this potential rate, the
acquisition of food as a means of obtaining
the required resources becomes a priority.
Appetite can be seen to be dependent on
the nutrient requirement of the animal and
the content of those nutrients in the food
(Emmans and Fisher, 1986).
An animal would need to eat a given
amount of a given food, termed the desired
food intake by Emmans and Fisher (1986),
if if its potential growth rate is to be achieved.
The actual food intake would be expected to
deviate from the desired when the food is un-
balanced in some way or if the animal were
placed in an unfavourable environment where
it is unable to lose the additional heat that would
be produced if more food were consumed.
Where a feed marginally deficient in an amino
acid is fed, the bird will overconsume energy
in an attempt to obtain sufficient of the limit-
ing resource, and this energy will be depos-
ited as lipid. It has been shown that broilers
exhibit higher feed conversion efficiencies and
lower lipid contents when higher concentra-
tions of amino acids than are conventionally
used in the broiler industry are included in
the feed (Gous
et al
., 1990).
Predicting nutrient requirements
For a model of growth to be successful it
must be able to calculate the nutritional and
environmental requirements of the bird that
are needed for potential growth, and it must
be able to predict the consequences of de-
viations from these optimum conditions.
A growing animal needs to be supplied with
nutrients in order to meet the requirements
for maintenance of the body and for the
growth of all other components of the body,
including feathers. The resources needed to
meet these requirements can be determined
from the growth rate and composition of the
various components of the body. The resources
available for supplying these requirements,
which are present in various feedstuffs, need
to be described in the same terms as are those
used to describe the nutrient requirements.
A Novel Approach to Optimization
Given that food intake can be accurately
predicted by the method described above,
this enables the accurate prediction of growth
and carcass composition of a given geno-
type in a given environment on a given feed
and feeding programme, which leads on to
the possibility of being able to optimize the
