Agriculture Reference
In-Depth Information
sensitivity to increases in photoperiod in
the young pullet; and (iv) the onset of spon-
taneous rapid gonadal development, that is,
the proportion of birds maturing under the
influence of the initial photoperiod, with-
out responding to a late change in photo-
period. In this latter case it is assumed that
a pullet will lay its first egg at the age ex-
pected for a long-day rearing treatment even
if the photoperiod is reduced during the
final 13 days before that egg is due, this
being the length of time taken for final rapid
development of the largest ovarian follicle
(Etches, 1996). Responses to constant photo-
periods must remain an integral part of any
model even though a change in photoperiod
exerts a greater influence on sexual matur-
ation than either the initial or final photo-
period itself.
Lewis and Morris (2008) modified their
original model to accommodate subsequent
evidence related to the effect of follicle
stimulating hormone (FSH) on the process of
sexual maturity. They found that when two
opposing changes in photoperiod are given
within an interval of <30 days, rate of sexual
maturation is determined by the change in
circulating FSH concentration achieved dur-
ing the time of the second photoperiod. If a
decrease in photoperiod is given within a
week of an increase, a period in which circu-
lating FSH concentration rises very little,
AFE will not be significantly different from
constant short-day controls.
13
h matured about 25 days later than birds
held on
10
h. Exposure to a photoperiod
longer than
13
h then resulted in a steady ad-
vance in ASM, which is in complete contrast
to the continued delay that occurred in
egg-laying strains. The relationships in the
three phases (≤10 h,
10
<
13
h, and ≥13 h) may
be described by the following equations:
P
≤
10
h:
y
= (202.5 − 1.15
P
)
− 20(
BW
− 2.1)
(3.1)
P
=
10
<
13
h:
y
=
A
10 +
p
(
A
13 −
A
10)
− 20(
BW
− 2.1) (3. 2)
P
≥
13
h:
y
= (224.7 − 0.76
P
)
− 20(
BW
− 2.1)
(3.3)
Where
y
= mean age at 50% egg production
(days);
A
10 = age at 50% egg production for
P
=
10
h (days) from Eqn 3.1; and
A
13 = age
at 50% egg production for P =
13
h (days)
from Eqn 3.3;
p
= the area under a normal
curve (mean 11.5 h,
sd
= 0.43 h) to the left of
a standard normal deviate
x
; where
x
=
(
P
- 11.5)/0.43, and
BW
= mean body weight
at
20
weeks (kg). The
sd
of 0.43 was estab-
lished by choosing the value that resulted in
the smallest residual sum of squares when the
distribution was fitted to the observed data.
Only one change in photoperiod is ac-
commodated in this model, unlike in the
laying hen model. The photosexual re-
sponse in broiler breeders is strongly influ-
enced by the feed allowance and hence the
rate of prepubertal growth. Mean age at
50% lay in birds given either a constant
photoperiod or a single increase was ad-
vanced by 2 days for every
100-
g increase
in body weight at
20
weeks (Lewis
et al
.,
2007). Because broiler breeders exhibit ju-
venile photorefractoriness they require up
to
20
weeks to dissipate this, although faster
growth allows quicker dissipation. As a con-
sequence, birds grown to a typical weight of
2.1 kg at
20
weeks do not start to be photo-
responsive until about
10
weeks and are not
uniformly responsive until
19
or
20
weeks.
A transfer to a stimulatory photoperiod before
a bird has dissipated photorefractoriness
causes a delay of about
3
weeks in its sex-
ual development, and this results in a bi-
modal distribution of ages at maturity when
Broiler breeder pullets
Following a series of trials with broiler breed-
ers, Lewis
et al
. (2007) conducted a meta-
analysis of relevant data for broiler breeder
pullets, which indicated a linear advance
in sexual maturity up to
10
h similar to that
described by Lewis and Morris (2004) for
early strains of egg-type pullets. However,
responses to photoperiods >
10
h were very
different due to the fact that broiler breeders
still exhibit photorefractory tendencies.
While egg-type pullets experienced a min-
imal delay in ASM of about 1 day between
10
and
13
h, broiler breeders maintained on
