Agriculture Reference
In-Depth Information
egg and body component weights over time
and, ultimately, of the physiological and en-
vironmental factors that may prevent each
bird from consuming sufficient of a given feed
to meet its potential performance each day.
Without a comprehensive simulation model
that incorporates all these concepts it is un-
likely that the consequences of offering feeds
of different quality to flocks of laying hens or
broiler breeders kept in different environ-
ments on the rate of egg production and the
weight and composition of the eggs produced
could be accurately predicted. Nor is there a
better way of optimizing the feeding of these
birds than being able to predict these conse-
quences before the feed is offered to the flock.
Much of the information required to de-
velop such a model has been published and
many of the relevant publications will be re-
ferred to in this chapter. But there are still
some concepts that have not been researched,
where further information would be valu-
able. Thus, by developing a simulation model
for predicting responses in laying hens and
broiler breeders, the relevant information
from the literature has been synthesized
into a workable theory for each system, the
various systems have been integrated and
gaps in our knowledge of these systems
have been identified.
Important considerations in predicting
sexual maturity in hens are that gonadal de-
velopment advances whatever lighting pro-
gramme is used, that lighting modifies the
age at sexual maturity, that changing photo-
periods have a greater influence than do
constant photoperiods (Lewis and Morris,
2006), and that the response of a broiler
breeder to light differs from that of a com-
mercial laying hen because broiler breeders,
unlike commercial laying hens, exhibit
photorefractoriness (Lewis
et al
.,
2003). The
attainment of sexual maturity is therefore
under both genetic and environmental in-
fluences, with broiler breeders still exhibit-
ing photorefractoriness, while this has been
eliminated in laying pullets by selection.
Commercial laying pullets
In full-fed commercial pullets, lighting is the
most important environmental factor influen-
cing age at first egg (AFE) (Lewis
et al
.,
2002).
When pullets are reared under constant day-
lengths the length of the photoperiod used
can influence AFE (Lewis
et al
., 1998), and
when one or two changes are made to the
daylength during rearing, the length of each
photoperiod also has an influence (Lewis and
Perry, 1994; Lewis
et al
., 1996). While the ini-
tial and final photoperiods are the principal
components of a lighting programme influen-
cing AFE in full-fed pullets, the effects of a
given change in photoperiod are not the same
at all ages. Also, the advance in AFE for birds
started on
8-
h photoperiods and given a sin-
gle increment in photoperiod at a defined age
is proportional to the size of the increment up
to about
13
h (Lewis
et al
.,
1998), but not for
longer final photoperiods (Lewis
et al
., 1996).
Lewis
et al
. (2002) proposed a model to
predict AFE of full-fed pullets when changes
were made to the photoperiod during rear-
ing. The four components of the empirical
model, each of which is calculated separ-
ately, deal with: (i) the genetic differences
in AFE in birds maintained on constant
photoperiods from hatching; (ii) the change
in AFE as a function of age at transfer to the
final photoperiod; (iii) the acquisition of
Predicting the Age at Sexual Maturity
The age and body weight of a bird on the day
it lays its first egg has a very strong influence
on future egg weight and the number of eggs
laid, which are important considerations for
both the layer and broiler breeding indus-
tries. These characteristics can be modified
by lighting and/or the nutritional control of
growth: in full-fed, egg-type hens, a
10-
day
delay in sexual maturity that has been
achieved through a lighting programme re-
sults in an increase of 1.3 g in mean egg
weight and a reduction of seven eggs over
52
weeks lay, but the total egg output will
be similar (Lewis and Morris, 2006). Clearly,
to predict the laying performance of a hen, her
age at sexual maturity (ASM) must first be
defined, and this can be predicted.
