Agriculture Reference
In-Depth Information
Modelling Egg Weight and Composition
the external cycle length will enable the hen
to lay long sequences with oviposition occur-
ring at a similar time each day (Morris, 1978).
Internal cycle lengths are under genetic con-
trol and can be manipulated (Foster, 1981),
thus the constraining effect of the external
cycle length on potential rate of lay may be
reduced either by reducing the internal cycle
length or making use of ahemeral cycles
greater in length than
24
h (Morris, 1978).
External cycle lengths longer or shorter than
24
h can be accommodated when such an
approach is used. When the ovulation curves
of individuals in the flock are integrated,
the characteristic laying curve is faithfully
reproduced. The slope of the initial rise in
flock egg production to peak rate of lay is
influenced by the distribution of ages at sex-
ual maturity and by the lengths of the indi-
vidual prime sequences. The incidence of
internal laying at onset of maturity plays a
role in modifying rate of lay but not ovula-
tion rate. The persistency of lay after peak
will be determined by the rate at which
sequence lengths of individual hens shorten
over time, as well as by the number of pause
days. Hence the prediction of sequence
length is a logical step in predicting the per-
formance of a flock of laying hens over an
entire laying cycle.
The reproductive rates of flocks of com-
mercial laying hens and broiler breeders
may be simulated by making use of the
Monte Carlo simulation method, which re-
quires the choice of appropriate values for
the means and standard errors of the param-
eters in the various equations used to simu-
late ovulation rate, the rate of decay in in-
ternal cycle length and the incidence of
pause days, internal laying and soft shelled
eggs (Johnston and Gous, 2006, 2007a,b,c).
The potential performance of each hen in
the population is simulated in this way,
thereby producing information necessary
for predicting the nutrients required by
each hen on each day of lay. For more preci-
sion in determining these nutrient require-
ments, the weight of the egg and the propor-
tions of yolk and albumen in the egg need to
be known, and these can be modelled as de-
scribed below.
When modelling the nutrient requirements
of a hen over a production cycle, based on
the daily outputs of each nutrient, egg
weight needs to be predicted as the sum of
the three components, since each has a
unique chemical composition, and these
proportional changes will therefore influ-
ence the nutrient requirements of the hen.
Egg weight increases as hens age, but the
eggs contain proportionally more yolk and
less albumen and shell. However, at a given
age, larger eggs contain proportionally more
albumen (Johnston and Gous, 2007b). Yolk
weight is dependent mainly on the geno-
type, but within a strain because it is related
to hen age it may be calculated using an
appropriate (logistic) function. Allometric
functions may then be used to predict albu-
men weight from yolk weight and shell
weight from the weight of the egg contents.
The methods described by Johnston and
Gous (2007b) for this purpose appear also
to work satisfactorily for broiler breeders
(Gous and Nonis, 2010) as long as appropri-
ate functions are used to describe the rela-
tionships between age and yolk weight,
albumen and yolk weight, and shell and egg
content weight. These relationships differ
not only between laying hens and broiler
breeders, but also between strains. Examples
of the coefficients that may be used to pre-
dict yolk weight from age of hen, albumen
weight from yolk weight and shell weight
from yolk + shell weight for an egg-laying
and two broiler breeder strains are given in
Table 3.1.
The only difference between the
two broiler breeder strains is in their allo-
metric relationship between albumen and
yolk weight.
The position of an egg in the sequence
also influences the proportions of the weight
of the egg and its components. The weight
of consecutive eggs within a sequence grad-
ually decreases (Belyavin
et al
.,
1987; Mi-
yoshi
et al
.,
1997) and similar patterns have
been observed with yolk weights (Bastian
and Zarrow, 1955; Zakaria
et al
.,
1984;
Zakaria, 1999) with the heaviest yolks oc-
curring more frequently in the first two places
