PAVLIDIS ET AL: ASSOCIATION OF EGG PRODUCTION AND CLUTCH LENGTH
305
In a subsequent experiment [4] the correlation
between length of the prime sequence and rate of
lay in broiler breeders was 0.45. Before imple-
mentation of this tool in a breeding program, it
is necessary to make comparisons among a range
of stocks to determine if the criterion to be em-
ployed is population specific or general. The ex-
periment reported here was undertaken to deter-
mine if the proposed model for the association
between the prime sequence and laying rate [3,
4] was consistent among several diverse genetic
lines of chickens reared in a common envi-
ronment.
between 14 h 43 min to 14 h 44 min during the
8-d period when the pullets were between 103
and 111 d of age. At 133 d, the chickens were
moved to single-bird cages with a 14 L:10 D
period.
Traits and Statistical Analyses
A pullet was considered sexually mature when
at least two eggs were laid within a 10-day period
with the age that the first egg was laid being
considered the age at sexual maturity [8]. Data
for ovipositions were obtained daily for each pul-
let from the date of first egg until the last trap
day, which was the second Friday in December
(283 d of age). Each oviposition was classified as
normal or defective with defective eggs including
double yolk, membrane, soft shell, extra calcified,
and compressed or slab sided [9].
Data were summarized as sequence 1–10, 1–
15, and 1–30 and defined as the maximum number
of consecutive days in which there were oviposi-
tions (clutch length) during the first 10, 15, and
30 d of egg production, respectively. Maximum
sequence was the longest number of consecutive
days of lay from the onset of lay until the last
trap day. Percentage hen-day and percentage hen-
day normal egg production were calculated for
each individual by dividing the number of ovula-
tions and normal eggs by days from first egg to
the last trap day. Data were analyzed within years
by analysis of variance with line as the main
effect. Prior to analysis, percentages were trans-
formed to arc sine square roots. When line effects
were significant, multiple means were compared
using Duncan’s multiple range test. Within each
year and line, phenotypic correlations were calcu-
lated between total hen-day production with se-
quences 1–10, 1–15, 1–30, and maximum se-
quence. Significance for all tests was considered
as P ≤ 0.05.
MATERIALS AND METHODS
Genetic Lines and Husbandry
The chickens used in this experiment were
from lines of White Leghorns divergently selected
for high (HAS) or low (LAS) antibody response
to sheep red blood cells [5, 6] and White Plymouth
Rocks divergently selected for high (HWS) or
low (LWS) 8-wk body weight [7]. Eggs from
age contemporary hens of the four lines were
incubated in the same machine. Temperature and
relative humidity were set at 37.5°C and 55%,
respectively. On Day 18, eggs containing viable
embryos were transferred to a hatcher with a tem-
perature of 37°C and relative humidity increasing
from 65% to about 75% at time of hatch. At hatch
(the first Tuesday in March for each year), chicks
were wing-banded, vaccinated for Marek’s dis-
ease, and placed in floor pens with pine wood
shavings as litter. Data were evaluated in 1997
for generations 24 and 40 and in 1999 for genera-
tions 26 and 42 of the antibody and weight lines,
respectively. A mash diet of 20% CP and 2,685
kcal ME/kg was provided until 56 d of age. From
56 to 126 d of age, the diet fed contained 14%
CP and 2,827 kcal ME/kg. Thereafter, the diet
fed contained 16% CP and 2,752 kcal ME/kg. To
avoid obesity, beginning at 56 d of age, daily feed
intake was restricted for HWS pullets. Allocations
were 60, 70, 80, and 100 g/pullet to 77, 98, 119,
and 140 d of age, respectively. Thereafter, there
was an increase of 5 g/wk for the next 8 wk.
Lighting was continuous to Day 7, and from
0500 to 2000 h (15 L:9 D period) to 56 d of age
when the chickens were transferred to the grower
pens with natural day length. This meant that at
our latitude the time from sunrise to sunset ranged
RESULTS AND DISCUSSION
There were considerable differences among
lines for percentage hen-day ovulations and per-
centage hen-day normal egg production with
means consistently greater for the White Leghorn
than the White Rock lines (Table 1). Within
breeds, the incidence of defective eggs was greater
for line HAS than LAS and for line HWS than
LWS; these results were consistent with those