Agriculture Reference
In-Depth Information
Major differences were evident (
Table
18.1
) between broiler and laying strains and
between sexes in body weight gain, and this
relates to the rates of growth of BP and BL.
Despite differences in the lipid weight at maturity,
the growth of BP was greater than that of BL in
all strains. From a nutritional perspective, the
rate of protein deposition directly affects the
amino acid requirements, and when associ-
ated with a lower rate of lipid deposition, the
protein requirements tend to increase at a
greater rate than do the energy requirements.
Likewise, it is expected that strains with
higher lipid weight at maturity will have a
greater energy requirement to meet the re-
quirement for body lipid deposition.
However, the lipid content of the bird re-
flects not only the genotype of that bird but
also the environmental conditions and feed to
which it has been subjected (Hancock
et al
.,
1995; Emmans and Kyriazakis, 1997). There-
fore, the calculated growth parameters can be
interpreted as fitting a certain desired pattern
for lipid growth. Moreover, the animal will at-
tempt to correct a deviation from the desired
rate of lipid deposition as soon as the limiting
condition is removed (Ferguson, 2006).
The fitted parameters for BWA and BA
(
Table 18.2)
indicate that water and ash ex-
hibited higher growth rates than the other
components, but this is contrary to the defin-
ition of allometry, which implies that all
chemical components of the body that are
allometrically related share the same rate of
maturity. These differences are likely to be
the result of terminating the growth trial with
broilers at a very early stage of maturity and
then extrapolating to maturity to obtain esti-
mates of the parameters. The values obtained
for broilers and laying hens of 2.9 for WPRm
and 0.2 for APRm are close to the coefficients
that have been suggested for all species (Em-
mans and Kyriazakis, 1995; Ferguson, 2006),
and these similarities indicate that these val-
ues can be considered to be nearly constant
(Emmans and Kyriazakis, 1995).
The growth rate of allometrically related
components (
dC
/
dt
) can be estimated from
protein growth rate (
dBP
/
dt
) using
dC
/
dt
=
dC
/
dP
×
dBP
/
dt
(Martin
et al
., 1994). In this
equation,
dC
/
dP
represents the relationship
between protein and water (WPRm and
b
W
),
lipid (LPRm and
b
L
) or ash (APRm and
b
A
);
therefore, EFFW weight gain can be obtained
by summing the gains in protein, lipid, water
and ash.
The slopes of the logarithmic relationship
between protein weight and water (
b
W
), lipid
(
b
L
) and ash (
b
A
) components showed simi-
lar values for broilers and laying hens. The
largest difference observed was 0.104 for ash,
whereas the water and lipid slopes were
0.061 and 0.68, respectively. It was expected
that the values obtained for
b
L
would vary
between genotypes (Emmans and Fisher,
1986), but the observed variation was simi-
lar to those of the other components. These
results are supported by other findings in the
literature (Martin
et al
., 1994).
In general, the allometric coefficients in-
dicate that as protein weight increases there
is an increase in the proportion of lipid and a
decrease in the proportions of water and ash
in the body. These relationships explain the
sequence of nutrient deposition in the body
of the bird; ash and water are deposited to
a greater extent during the immediate post-
hatching phase (Marcato
et al
., 2008).
The amino acid composition of feather
protein differs from that of body protein so
the growth of these two components should
therefore be separately modelled if their
requirements for amino acids are to be calcu-
lated. The differences in feather weight be-
tween the various genetic groups of broiler
and laying birds may be related to the natural
loss of feathers that occurs during the growth
period, which seems to be specific to the
genotype, and also a consequence of damage
to the feathers (Hancock
et al
., 1995). These
factors affect the actual feather weight and
are consequently reflected in the adjustment
to the growth parameters.
FFc describes feather growth according
to the various rate values of the Gompertz
function (Gous
et al
., 1999). The average val-
ues of 0.343 and 0.303 for broilers and lay-
ing hens, respectively, indicate that broilers
undergo a more rapid rate of feathering than
do laying hens. For broilers, the FFc rates
were higher relative to other findings in the
literature (Hruby
et al
., 1994; Gous
et al
.,
1999), indicating precocity in the feathering
observed in this study.
