Agriculture Reference
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proposed by Whittemore and Fawcett
(1974). It is difficult to summarize the main
features of these different models in a com-
parative way, but some points may be noted.
In particular it is interesting to note the
driving element in each model.
The theory of growth and food intake first
proposed by Emmans (1981) has been exten-
sively described and discussed in the litera-
ture. It forms the basis of the software devel-
oped by EFG Software (1995) and has also
been followed fairly directly by Roan and
Wang (1996). The same ideas have been ap-
plied more widely in pig modelling, and Em-
mans (1989) discusses their application to the
turkey. Gous and Brand (2008) demonstrate
the application of this theory to ostriches.
In this theory growth is driven by the
idea that animals have a purpose; that is to
achieve their potential growth of the feather-
free body protein, and that they will con-
tinue to eat food to achieve this purpose un-
less prevented from doing so by other
factors. The amount of a given food that will
meet the nutrient requirements of potential
growth is expressed as a 'desired' food in-
take that the bird is seeking to fulfil. This
idea is wrongly seen by some commentators
as a theory of feed intake driven by energy
transactions, but this is not so; the theory is
held to apply whichever nutrient is limiting,
although in practice only energy and amino
acids are considered. A second important
element is that animal characteristics (geno-
typic values) are seen as only being mean-
ingful if they are defined under non-limiting
conditions, i.e. the animal's potential is de-
fined by its genotype, as discussed above.
The approach proposed by Emmans is
the only one used in poultry modelling that
contains a true theory of food intake. The
ideas are challenging but they have the ad-
vantage of being open-ended and being cap-
able of further development. A limitation is
that the method of describing the growth of
body components other than protein, which
uses allometry, restricts the basic growth
curve (for potential feather-free body pro-
tein against time) to the Gompertz curve.
Isariyodom et al . (1988) describe the
main framework of their model as being
based on Whittemore and Fawcett (1974).
The essential driver of growth is metaboliz-
able energy (ME) intake, which is predicted
from body weight and corrected for various
factors. Energy and protein partition and
utilization closely follow the pig models,
but a correction of ME intake for photo-
period shows an interesting combination of
empirical and mechanistic modelling. The
equations used were:
MEI = C 2 ((1165 − 4.73T) ( W /1000) 0.75 )) + C 1
(1.1)
and
C 2 = (96.54035767 + 1.44628213 TH
- 0.0512548 TH 2 - 1.36290206 LH
+ 0.00114968 LH 4
- 0.00004401 LH 5 )/ 100 (1.2)
Where MEI = ME intake (kJ/day); T = tempera-
ture, ºC; W = body weight, g; C 1 = correction for
birds less than 500 g ( C 1 = ( 60 - 0.12 W ); TH =
total hours in one light:dark cycle; and LH =
the photoperiod (light hours) in one light:dark
cycle.
This second equation is described as
'based on 14 literature sources', but no evalu-
ation of such a complex function is provided.
For 24- h values of TH , C 2 varies between 0.93
when LH = 8 h and 1.16 when LH = 20 h. It is
difficult to see how such an approach can be
developed further.
Finally Isariyodom et al . (1988) provide
some comparisons of predicted growth and
feed intake to 56 days of age with published
data from the literature. These involved
fully fed birds and the ability to distinguish
between different conditions was not re-
ported. The authors allude to further devel-
opments but these have not been found.
Similar mechanistic elements were
used by Muramatsu et al . (1989) in a model
for growth prediction in replacement pul-
lets. Again the driving element is the pre-
diction of energy intake from body weight
and weight gain. In this model more empir-
ical elements were introduced to deal with
a slower growing animal and a production
system in which some controlled feeding
is  used. First, the equations for energy in-
take were derived empirically from a set of
production data described by the National
Research Council (1994). Second, a large set
 
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