Agriculture Reference
In-Depth Information
who stick rigidly to the 'requirements' irre-
spective of changes in ingredient prices or
the way in which their broilers are marketed,
forego considerable opportunities that are
otherwise available to them.
Many indices are used to measure prof-
itability on a farm and with which to com-
pare farms, such as net farm income, rate of
return on farm assets, operating profit mar-
gin ratio and asset turnover ratio (Olsen,
2004). Of particular importance in a broiler
operation in which capital (fixed) costs are
high is the number of livestock units that
can be passed through the facility in a unit
of time. So, whereas many objective func-
tions could be used when deciding on the
objectives of a business, for example, min-
imizing feed conversion ratio or fat content,
maximizing breast meat yield or margin
over feed cost, the factors to be considered
should include both fixed and variable
costs, the revenue generated, the length of
time that the broiler houses are occupied,
which includes the clean-out or down-time,
and the number of livestock units occupy-
ing a unit of space. For this reason, maxi-
mizing margin/m
2
/year would seem to be
a sensible objective function in the case of
growing broilers, and from a nutritional point
of view this objective could be achieved with
the use of an optimum feeding programme.
Determining the optimum nutrient density,
the optimum concentrations of amino acids
relative to energy in each feed and the opti-
mum length of time (or amount) that each
feed should be fed is therefore both a nutri-
tional and an economic decision.
The information required for optimiza-
tion consists of feed costs at different levels
of amino acid provision, a description of all
the relevant animal responses, both fixed
and variable costs affecting the production
system and details of revenue. The com-
plexity of the information required would
depend on the level of organization at which
the optimization is to be made. If profit of
the broiler grower is to be maximized at the
farm gate, then responses in liveability,
growth and feed conversion ratio will prob-
ably suffice. However, and more realistic-
ally, a wider view will be required, and the
effect of broiler nutrition on slaughterhouse
variables (eviscerated yield, rejects, etc.) and
further processing (carcass composition) will
need to be defined. Mack
et al
. (2000) em-
phasized the importance of broiler compan-
ies considering all aspects of the production
cycle when making nutritional decisions.
Feed costs for any nutritional specifi-
cation are readily calculated by linear pro-
gramming. This will take account of feed
ingredient availability, analysis and costs.
Processing and transport costs may be added.
Broiler production costs are complex but
will usually be specified by each company.
So the only persistent problem in optimiza-
tion lies in the definition of animal re-
sponse, the accuracy of prediction hinging
on the prediction of voluntary food intake,
which leads to the prediction of the rates of
growth of the body components and hence
revenue. Only with the use of an accurate
simulation model could such an optimiza-
tion be contemplated.
An Overview of a Theory of Growth
and Voluntary Food Intake
Describing the genotype
In order to predict food intake it is neces-
sary to have some view on what a bird or an
animal is attempting to achieve when faced
with a given food. Emmans (1981) suggested
that broilers attempt to grow as fast as pos-
sible, and in such a way as to start the repro-
ductive process as early as possible and to
produce as many chicks as possible. Based
on this premise, it is necessary to know the
potential growth rate of a broiler before a
theory of food intake can be applied. But
predicting the performance of animals is a
general problem in animal production, the
solution depending, in part, on being able to
describe the animals adequately (Emmans
and Fisher, 1986). In the past there has been
neither consensus nor any general discus-
sion in the literature on methods of defining
genotypes that would allow similarities and
differences between animals to be com-
pared. However, with the advent of simula-
tion models for describing the growth and
