Effects of Sex-Linked Imperfect Albinism (s?l's) in the Chicken on
the Relationships of Plasma Concentrations of
Progesterone and 17/3-Estradiol with Egg Production
H. SU and F. G. SILVERSIDES
Department des sciences animates, Universite Laval, Ste-Foy, Quebec, Canada, G1K 7P4
P. VILLENEUVE
Ministere de YAgriculture, des Pecheries et de VAlimentation du Quebec, Service de la zootechnie,
120-A Chemin du Roy, Deschambault, Quebec, Canada, GOA ISO
ABSTRACT Effects of the s*l-s gene for sex-linked
imperfect albinism on the relationships of plasma
concentrations of progesterone (P4) and estradiol (E2)
with egg production were investigated during the laying
period. Egg production of 17 albino and 16 nonalbino
hens was recorded from 19 to 60 wk of age. Blood
samples of these hens were taken between 1330 and
1500 h at 19 and 20 wk of age and every 4 wk until 60
wk of age. At 61 wk, blood samples were taken 6, 5, and
4 h before a midsequence ovulation. Plasma P4 and E2
were measured by RIA. There was no difference
between genotypes in days to first egg (157.8 vs 158.1 d).
Hen-day egg production of al binos was greater (P <
0.05) than that of nonalbinos in the 4-wk periods
between 52 and 56 wk (83.8 vs 69.2%) and 56 and 60 wk
(81.3 vs 64.3%). Egg production for the entire laying
cycle was not different between genotypes (81.0 vs
73.0%, P = 0.08). Plasma P4 and E2 concentrations were
not different between albino and nonalbino hens. From
28 to 60 wk of age, partial correlation coefficients
between P4 and egg production, and E2 and egg weight
were significant for albino hens (r = 0.15 and 0.16,
respectively) but not for nonalbinos (r = -0.03 and -0.1,
respectively), and age and P4 concentrations were
negatively correlated for both albinos (r = -0.22, P <
0.01) and nonalbinos (r = -0.32, P < 0.01). Preovulatory
levels of P4 in albino hens were higher (P < 0.05) than
those in nonalbinos. Plasma E2 was higher in albinos
than in nonalbinos 5 h before ovulation (P < 0.05). These
data suggest that increased egg production of albino
hens is associated with differences in P4 and E2
metabolism.
(Key words: sal_s, sex-linked albino, progesterone, estradiol, egg production)
1996 Poultry Science 75:13-19
been found to weigh less than those produced by
nonalbinos (Merat et al, 1986; Silversides and Crawford,
1991; Bordas and Merat, 1992; Su et al, 1994). In order to
partially explain these differences, Su et al. (1995) used
data on ovarian morphology to show that the hierarchical ovarian follicles of albino hens had greater growth
intensity than those of nonalbinos.
Hormonal events are important in reproduction.
Silversides et al. (1993) increased daylength for albino
and nonalbino hens at 15 or 19 wk in order to stimulate
sexual maturity. They measured plasma luteinizing
hormone (LH) levels between 12 and 33 wk of age and
found that LH concentrations at 17 wk and overall were
lower for albinos than were those for nonalbinos. Egg
production of albino hens in the early maturing group
was higher than that of nonalbinos and Silversides et al.
(1993) suggested that'this may be a consequence of
differences in the control of LH secretion. Merat et al.
(1994) measured plasma levels of triiodothyronine (T3),
thyroxine (T4), growth hormone (GH), and corticoster-
INTRODUCTION
The mutation to sex-linked imperfect albinism (sal) in
the chicken was first described by Mueller and Hutt
(1941). Other mutations have been described, including
one that occurred at the University of Saskatchewan
(sal-s, s representing Saskatchewan). All mutations to sal
appear to produce the same phenotype (Silversides and
Crawford, 1990). Hutt and Cole (1973) suggested the use
of this gene for determining the sex of day-old chicks
because it is sex-linked and it produces an easily
distinguished phenotype. Several studies on the albino
gene have shown that the gene is associated with an
increase in egg production and a delay in sexual
maturity (Merat and Bordas, 1989; Silversides and
Crawford, 1991; Bordas and Merat, 1992; Su et al, 1994).
Eggs laid by albinos and the yolks of these eggs have
Received for publication April 10, 1995.
Accepted for publication August 31, 1995.
13
14
SU ET AL.
one. They found that the T3:T4 ratio was higher in albino
hens than in nonalbinos before sexual maturity and
during the laying period, that GH in albinos was higher
than that in nonalbinos during the laying period, and
that the corticosterone level did not differ between
genotypes. These results suggested differences in the
general hormone balance between the genotypes, which
M6rat et al. (1994) suggest may help explain the
difference observed for egg production.
Progesterone (P4) and 17/3-estradiol (E2) are steroid
hormones that are important for reproduction in
chickens. The preovulatory level of P4 is directly
associated with the ovulation of a mature follicle
(Johnson, 1984). The preovulatory rise of plasma P4
precedes and stimulates the rise in LH, and there is a
positive feedback reaction between P4 and LH that
induces ovulation. Tanabe et al. (1983) found positive
correlations between circulating levels of P4 and egg
production in laying hens in a pure line. They also
found that crossbred hens had botii higher egg production and higher circulating levels of P4 than did
purebreds during the period of lay. Estradiol is
associated with the regulation of yolk formation (Johnson, 1986; Palmer and Bahr, 1992) and egg weight
(Whitehead et al, 1993), but it is not correlated to egg
production (Tanabe et al, 1983; Johnson and van
Tienhoven, 1984). Estradiol may increase the responsiveness of the hen's hypothalamus to P4 (Kawashima et al,
1981).
This study was intended to compare the basal and
preovulatory levels of plasma P4 and E2 in albino and
nonalbino hens throughout the laying period and
during the ovulatory cycle, and to investigate the
relationship between circulating levels of plasma P4 and
E2 and egg production of hens carrying s al_s .
MATERIALS AND METHODS
Animals and Preparation of Plasmas
Seventeen albino and 16 nonalbino pullets were
randomly selected at 19 wk of age from an experimental
population of pullets produced by artificial insemination
of nonalbino females of the Deschambault Line 8 (Boukila
et al, 1987) with mixed semen from heterozygous albino
males carrying the sex-linked imperfect gene (sal_s) (Su et
al, 1994). At hatching, all chicks were vent-sexed and
albino and nonalbino females were separated by their eye
color. Chicks were vaccinated for Marek's disease and
their beaks were trimmed. The subsequent vaccination
1
Le Couvoir Jolibec, Inc., Ft-Felix de Valois, QC, Canada, JOK 2M0.
Marcel Berard Ltee. Quebec, QC, Canada, GOX 3L0.
3
Gossen, Erlangen, Germany.
4
Product P-5289, Sigma Chemical Co., St. Louis, MO
63178-9916.
5
Progesterone-3H, NET-381, EstradioPH, NET-517, Du Pont Canada
Inc., Markham, ON, Canada, L3R 1A9.
program followed was that used by Le Couvoir Jolibec,
Inc. 1 for their commercial flocks. All birds were reared in
floor pens and received 24 h of light/d for the first 3 d
followed by 8 h of light/d from 4 d to 19 wk of age. In the
brooding and growing periods, a commercial pullet
starter and two grower diets 2 (containing 20,18, and 16%
CP, respectively) were provided at 1 d and 5 and 8 wk of
age, respectively. At the age of 19 wk, the 33 pullets were
housed in individual cages on two levels. At the same
time, a layer diet (containing 18% CP and 2,700 kcal ME/
kg) was provided and the day length was set at 14 h light:
10 h dark (lights on from 0600 to 2000 h). Light intensity
was set at an average of 12 lx for the two cage levels
measured at the cage door with a Gossen 3 luxmeter.
During the entire experiment, feed and water were
provided for ad libitum consumption.
Egg production was recorded daily from the onset of
lay until the age of 60 wk, which allowed the calculation of
sexual maturity as the age at first egg, and the rate of egg
production. One egg per hen was weighed at 24 wk of age
and every 4 wk until 60 wk of age. Body weight of the
birds was recorded at 19 wk of age and at sexual maturity.
Thereafter, the hens were weighed every 4 wk until 60 wk
of age. Blood samples were taken from the brachial vein
into heparinized tubes for all birds between 1330 and 1500
h at 19 and 20 wk of age and every 4 wk until 60 wk of age.
This was considered to give a measure of basal levels of P4
and E2. At 61 wk of age, blood samples were taken 6, 5,
and 4 h before a midsequence ovulation as measured by
the previous oviposition. Plasma was obtained from the
blood samples by centrifugation for 15 min at 2,000 xg at 4
C and was stored at -20 C until the day of the RIA.
Hormonal Assay
The RIA for P4 was performed with commercially
available antiserum 4 according to the suppliers' instructions and slight modifications described by Guilbault et al
(1988). The RIA for E2 was performed using antiserum to
E2 provided by A. B61anger, Centre Hospitalier de
l'Universit6 Laval (2705, Boulevard Laurier, Quebec, QC,
Canada, G1V 4G2), following the methods of Guilbault et
al. (1988). Plasma P 4 (200 pL) was extracted with 2 mL of a
mixture of benzene:ether (1:2 vol/vol) followed by a
second extraction using 1 mL of the mixture. Plasma E2
(100 pL) was extracted with 2 mL of a mixture of ethyl
acetate:petroleum ether (2:11 vol/vol). Recovery of
labelled tracers 5 after extraction was 93.0+2.4% for P4 and
92.3+2.7% for E2. Sensitivity of the assays was 15.6 pg P 4 /
mL and 3.1 pg E2/mL. Intra- and interassay coefficients of
variation were 13.5 and 4.0% for P 4 and 12.1 and 9.0% for
E2, respectively.
Statistical Analysis
2
Data on sexual maturity were treated by analysis of
variance using the General Linear Models (GLM) procedure of SAS® (SAS Institute, 1985) and the following
statistical model:
EFFECTS OF s«'-s ON PROGESTERONE AND ESTRADIOL
15
TABLE 1. Sexual maturity, egg production, and egg and body weights of
albino and nonalbino hens1
Genotype
Albino
Nonalbino
Days to
first egg
n
17
16
157.8 ± 2.2
158.1 ± 2.3
Hen-day
production
(%)
81.0 ± 3.0"
73.0 ± 3.0y
Body weight
Egg
weight
(g)
48.5 ± 0.7
49.4 ± 0.7
19 wk
At first egg
60 wk
1.23 ± 0.02b
1.39 ± 0.02=
(kg)
1.36 ± 0.02b
1.49 ± 0.02=
1.66 ± 0.02b
1.81 ± 0.02=
ab
' Means within a column with no common superscript differ significantly (P < 0.05).
MMeans within a column with no common superscript differ significantly (P = 0.08).
Values are 5c ± SEM.
Yij =
+ Gi + ey
where Yjj = the value observed; ^ = the general mean; Gj =
the effect of genotype (i = 1,2); and e^ = the error term that
was used to test the significance of the effect of genotype.
Data on hen-day egg production, egg weight, body
weight, and basal and preovulatory concentrations of P4
and E2 were analyzed with the GLM procedure using the
following statistical model:
Yijk = H + Q + H (i)j + P k + (GP) ik + e ijk
where Y;jk = the value observed; /x = the general mean; Gj =
the effect of genotype; H(j)j = the random effect of hen
nested within the genotype; P^ = the effect of period (k =
1 , 2 , . . . 12 for egg production and basal levels of steroids,
and k = 1, 2, and 3 for preovulatory levels of steroids);
(GP)ik = the interaction between genotype and period; and
ejjk = the general error term. The effect of genotype was
tested using the effect of the individual hen as the error
term, other effects were tested using the general error
term.
Differences between means were separated using a
least squares analysis of the GLM procedure. Percentage
data were subjected to arcsine transformation before
analysis and original data were used to express the results.
The relationships among egg production, hormone
levels and bird age from 28 to 60 wk of age were analyzed
with the REG procedure of SAS® (SAS Institute, 1985)
using the following multiple regression model:
Y = 00 + ftXi + /32X2 + e
where, Y = the hen-day egg production or egg weight; X^
= the P4 or E 2 concentration; X2 = age; e = the general error
term, and /3o, 0i, and /32 = the parameters in the model.
Regression and partial correlation coefficients were
calculated.
Unless otherwise noted, all tests were considered
significant at P < 0.05.
first egg (157.8 vs 158.1 d). Hen-day egg production by
albino hens was greater than that by nonalbinos (81.0 vs
73.0%, P = 0.08). There was no difference between albino
and nonalbino hens in average egg weight (48.5 vs 49.4
g, P > 0.05). Body weights of albino hens at Week 19, at
the laying of their first egg and at Week 60 were lower
than those of nonalbinos (P < 0.05). In addition, there
was no mortality for either genotype during the entire
period.
The difference between albino and nonalbino hens in
egg production over the course of the production period
is shown in Figure 1. Production by albinos was higher
than that of nonalbinos from the peak of production at
28 wk until the end of the experiment. However, it was
significantly higher only near the end of the cycle, for
the 4-wk periods including 53 to 56 wk (83.8 vs 69.2%
for albinos and nonalbinos, respectively) and 57 to 60
wk (81.3 vs 64.3% for albinos and nonalbinos).
Figure 2 shows basal plasma levels of P4 between 19
and 60 wk. There were no differences between albino
and nonalbino hens in average P4 levels over the entire
period (1.09 vs 1.15 n g / m L , P > 0.05). Plasma P 4 was
low before the increase in day length and rose rapidly to
100
a
o
w
9
80
60
"8
u
a. 40
w>
eo
•
a
X
Albinos
Nonalbinos
20
— 1
1
1
1
1
1
1
1
1
1
20 24 28 32 36 40 44 48 52 56 60
RESULTS
Sexual maturity, egg production, egg weight, and
body weights for albino and nonalbino hens during the
experimental period are shown in Table 1. There was no
difference between albino and nonalbino hens in days to
Age (wk)
FIGURE 1. Hen-day egg production of albino and nonalbino hens
between 24 and 60 wk of age. The production corresponds to the
4-wk period preceding that age. Asterisks indicate that means of the
genotypes at each time differ significantly (P < 0.05).
16
SU ET AL.
2
1.8
a
1.6
°
Albinos
•
Nonalbinos
« 1.4
s
1.2
2
.8
"a,
a 1
c
.6
Ml
O
.4
16 20 24 28 32 36 40 44 48 52 56 60
Age (wk)
genotypes at 19 wk, rose rapidly after the increase in
day length, and reached a peak about 2 wk before the
onset of lay. By 28 wk of age, E2 levels for albino and
nonalbino hens had declined to 246 and 278 p g / m L
plasma, respectively. Thereafter, there was no difference
between genotypes or significant variation over time in
the level of E2.
Preovulatory concentrations of plasma P4 and E2 for
the two genotypes were shown in Figures 4a and 4b.
Progesterone levels in albino hens were higher (P < 0.05)
than those in nonalbinos at all three times (4.62 vs 3.17,
5.13 vs 3.16, and 4.77 vs 3.52 n g / m L at 6, 5, and 4 h
before ovulation). The plasma level of E2 of albinos was
higher than that of nonalbinos 5 h before ovulation (381
vs 302 p g / m L , P < 0.05), but the differences at 6 and 4 h
before ovulation were not significant.
The relationships between egg production, hormone
levels, and age from 28 to 60 wk of age are shown in
FIGURE 2. Concentration of progesterone in plasma of albino and
nonalbino hens between 19 and 60 wk of age.
C Albinos
H Nonalbinos
N-l
>5
a peak corresponding to the peak in egg production.
Thereafter, P4 underwent a general decline until the end
of the experiment. Between 28 wk and the end of the
experiment, age and P4 concentrations were negatively
correlated for both albinos (r = -0.22) and nonalbinos (r
= -0.32).
Concentrations of plasma estradiol (E2) over the
period of the experiment are shown in Figure 3. Over
the entire period of the experiment, E2 level in albinos
did not differ significantly from that in nonalbinos (274
vs 307 pg/mL). However, plasma E2 concentrations for
albino hens were significantly lower than those for
nonalbinos at Week 19 (137 vs 229 pg/mL) and at Week
20 (345 vs 437 pg/mL). Estradiol was low for both
B
£4
"a
s3
i
c
u
a>
*^
©
u
CM
Time before ovulation (hours)
450
i
450
Albinos
400
Nonalbinos
.0* 350
i
300
i
Albinos
Nonalbinos
400
3> 350
«
§ 300
"5.
B
"a, 250 -
•
250
a
200
«
I 200
100
100
S3
•mm
u
% 150 1
is 150
(A
16 20 24 28 32 36 40 44 48 52 56 60
Age (wk)
FIGURE 3. Concentration of estradiol in plasma of albino and
nonalbino hens between 19 and 60 wk of age. Double asterisks indicate
that means of the genotypes at each time differ significantly (P < 0.01).
Time before ovulation (hours)
FIGURE 4. Concentrations of progesterone (a) and estradiol (b) in
plasma of albino and nonalbino hens 4, 5, and 6 h before a
midsequence ovulation at 61 wk of age. Different letters indicate that
means of the genotypes at each time differ significantly (P < 0.05).
17
EFFECTS OF s"ls ON PROGESTERONE AND ESTRADIOL
TABLE Z Partial regression coefficients (b) of egg production or egg weight on age and
progesterone (P4, model 1) or estradiol (Ej, model 2), and partial correlation coefficients (r)
between egg production or egg weight and age and P4 or E2 in albino and
nonalbino hens between 28 and 60 wk of age1
Hen-day egg production
Model
Albinos
Model 1
Intercept
P4, n g / m L
Age, wk
R2
Model 2
Intercept
E 2/ p g / m L
Age, wk
R2
Egg weight
Nonalbinos
Albinos
Nonalbinos
(b)
(r)
(b)
(r)
(b)
(r)
(b)
1.0396"
0.0206*
-0.0041"
0.20
0.15*
-0.42**
1.1284**
-0.0153
-0.0078**
0.08
-0.03
-0.29**
38.9672**
0.1834
0.2359**
0.23
0.02
0.55**
41.4529**
0.6615
0.1970**
0.26
0.12
0.59**
1.0426**
0.0001
-0.0045**
0.18
0.10
-0.46**
1.1761**
-0.0003
-0.0071**
0.10
-0.09
-0.30**
37.0870**
0.0082*
0.2343**
0.26
0.16*
0.56**
43.8286**
-0.0038
0.1870**
0.25
-0.10
0.58**
(r)
*n = 122 to 151 for each parameter estimated.
*P < 0.05.
**P < 0.01.
Table 2. In albino hens, plasma P4 was significantly
correlated with egg production (r = 0.15) and plasma E2
was correlated with egg weight (r = 0.16). These
correlations were not significant in nonalbinos (r = -0.03
and -0.10, respectively). In albino hens, 1 ng P4/mL
plasma contributed 0.0206% to hen-day egg production
and 1 pg E2/mL contributed 0.0082 g to egg weight.
Correlations (r) between egg production and age using
the two models were -0.42 and -0.46 for albinos and
-0.29 to -0.30 for nonalbinos. Those between egg weight
and age were 0.55 and 0.56 for albinos and 0.59 and 0.58
for nonalbinos. Hen-day egg production was reduced
0.0041 and 0.0045%/wk (using the two models) for
albinos and 0.0078 and 0.0071%/wk for nonalbinos. Egg
weight increased 0.2343 and 0.2359 g / w k for albinos
and 0.1870 and 0.1970 g / w k for nonalbinos.
DISCUSSION
Several publications have described an association of
the s al_s gene with a delay in sexual maturity, increased
egg production, and decreased egg weight (M6rat et al,
1986; Silversides and Crawford, 1991; Bordas and M6rat,
1992; Su et al, 1994). In this experiment, an advantage in
egg production was seen for old albino hens, as was
described by Silversides and Crawford (1991) and Su et
al. (1994). There was no significant difference between
genotypes in sexual maturity or egg weight, probably
because of the large sampling variance associated with
the small sample size. Body weight of albino hens was
always lower than that of nonalbinos, which is consistent with the findings of M6rat et al. (1986) and Su et al.
(1994).
No difference between genotypes was found in basal
concentration of P4. Prior to sexual maturity, P4
concentration was lower than that after sexual maturity.
Similar results were reported by Tixier-Boichard et al.
(1990) in a study of dwarf (dw) birds. After sexual
maturity, P4 concentration and egg production reached a
peak at about the same age (28 to 36 wk) and declined
afterwards. Mashaly and Wentworth (1974) described
similar results in turkeys, as did Mashaly et al. (1982) in
pheasants. Mashaly and Wentworth (1974) suggested
that this may indicate that ovarian tissue reaches its
maximum activity in secreting P4 5 or 6 wk after
exposure to stimulatory photoperiods.
The prepubertal peak of E2 in both genotypes,
observed about 2 wk before the first egg, is in agreement
with other reports (Senior, 1974; Tanabe et al, 1981;
Gilbert and Wells, 1984; Tixier-Boichard et al, 1990).
However, the E2 levels measured in albino hens before
the first egg were lower than those in nonalbinos. The
majority of the plasma E2 is assumed to be derived from
immature small follicles (Senior and Furr, 1975; Robinson and Etches, 1986). A lower level of E2 in albino hens
could therefore be the result of a smaller number of
immature small follicles on the ovary. The lower level of
E2 could also be an indication of a delayed estradiol
peak. A delay in the estradiol peak could delay the days
to first egg, which has previously been observed in
albino hens (M6rat and Bordas, 1989; Silversides and
Crawford, 1991; Bordas and M<§rat, 1992; Su et al, 1994).
After the onset of laying, the difference between
genotypes in plasma E2 disappeared by 28 wk of age
and remained constant for both genotypes for the rest of
the experiment. This pattern of changes in basal E2
levels was similar to that reported by Tixier-Boichard et
al. (1990) in their study of the dwarf chicken.
The preovulatory surge of P4 plays an important role
in ovulation in chickens. Johnson (1984) suggested that
the rise in plasma P4 precedes and stimulates the rise of
luteinizing hormone (LH) and a positive feedback
between P4 and LH results in the hormone peaks that
induce ovulation. Yoshimura et al. (1992) suggested that
18
SU ET AL.
P4 may directly regulate the fundamental functions of
growing follicular cells and of ovulation via P4 receptors
on the follicle. In this study, plasma P4 levels in albino
hens 6, 5, and 4 h before ovulation were higher than
those in nonalbinos, and E2 concentration in albinos was
significantly higher at 5 h before ovulation. Progesterone
is produced mainly by the largest preovulatory follicle
(Shahabi et ah, 1975). A higher preovulatory level of P4
in albino hens suggests that the activity of the largest
preovulatory follicle of albino hens is greater than that
of nonalbinos. Plasma E2 is produced by the immature
follicles (Shahabi et ah, 1975) and, in conjunction with P4,
it primes the release of LH (Wilson and Sharp, 1976). A
higher preovulatory E2 level suggests that the activity of
the immature follicles and that of the largest preovulatory follicle are associated. The higher preovulatory P4
level supports the suggestion of Su et al. (1995) that
hierarchical follicles of albinos are precocious.
These results could indicate a relation between P4 and
E2 and events in egg production. Between 28 and 60 wk
of age, positive correlations were observed between P4
and egg production and between E2 and egg weight in
albino hens but not in nonalbinos. This may suggest that
the positive effects of the albino gene on egg production
are mediated via the metabolism of these hormones.
This also supports Johnson (1986), who stated that P4 is
associated with ovulation, and Whitehead et al. (1993),
who found that E2 is associated with controlling egg
weight. Gilbert and Wells (1984) pointed out that
circulating basal plasma P4 is mainly derived from five
to seven preovulatory follicles. As a hen ages, the rate at
which follicles enter the rapid growth phase decreases
(Williams and Sharp, 1978a), as does the response of the
positive feedback mechanism between LH and P4
(Williams and Sharp, 1978b) and the sensitivity of
follicles to LH (Moudgal and Razdan, 1985; Johnson et
al, 1986). It was not therefore surprising that plasma P4
concentration decreased with age, exhibiting a negative
correlation between P4 and age for both albino and
nonalbino hens. Similar results were reported by
Mashaly and Wentworth (1974) in turkeys and Mashaly
et al. (1982) in pheasants.
ACKNOWLEDGMENTS
The authors are indebted to V. Naud, H. Lavallee,
and the workers of the Station de Recherche de
Deschambault for care of the birds and collection of
data. This work was supported by the National Sciences
and Engineering Research Council of Canada and Le
Couvoir Jolibec, Inc.
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Gilbert, A. B., and C. G. Wells, 1984. Structure and function of
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Guilbault, L. A., G. L. Roy, F. Grasso, and P. Matton, 1988.
Influence of pregnancy on the onset of oestrus and luteal
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Hutt, F. B., and R. K. Cole, 1973. Identification of sex in chicks
by use of the gene sal. Poultry Sci. 52:2044. (Abstr.)
Johnson, A. L., 1984. Interactions of progesterone and luteinising hormone leading to ovulation in the domestic hen.
Pages 133-143 in: Reproductive Biology of Poultry. F. J.
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