Hatchability is of considerable economic importance for all hatcheries and therefore must be given
appropriate attention in breeding programs for commercial layers. The variability between and within
strains will tell us whether reproductive performance can be improved by selection within specific
lines. The aim of this study was to analyse the genetic variation of hatchability and correlations with
production traits. Data collected from two pure-bred female lines (LSL) at 45-46 weeks of age were
used. Estimated heritabilities were low (h² = 0.13 and 0.15) for fertility, but moderately high (h² = 0.27
and 0.30) for hatch of ‘fertile’ eggs (eggs transferred at 18 days). Hatchability was negatively correlated
with egg weight (rg = -0.43 to -0.52) and albumen height (rg = -0.25 to -0.42). Favourable correlations
were found with egg production (rg = -0.01 to +0.28), percentage yolk (rg= +0.08 to +0.39) and some
shell quality traits like shape index and breaking strength (rg = +0.14 to +0.32), but not with dynamic
stiffness (rg = -0.08 to -0.17).
Hatch of fertile eggs has sufficient genetic variation in these White Leghorn lines to expect improvement by within-line selection, especially if supported by a reduction in total egg weight and selection
for higher egg production, yolk percentage and shell breaking strength.
The avian egg is a biological system intended to ensure the well-being of the embryo and its successful
hatching into a fully developed chick (Narushin and Romanov, 2002). Predictable reproductive
performance of parent stock is essential for every commercial hatchery to produce as many saleable
chicks at the lowest possible cost, and uniform chicks set the basis for a successful rearing and
subsequent production. If the conditions in the setters and hatchers are not optimized, the uniformity
of chicks will suffer and the hatching time will spread out. Many factors can affect hatchability, especially
egg size and age of breeders, season of the year and nutrition, egg handling and storage, temperature
and humidity throughout the incubation and hatching period (Wilson 1997).
Reproductive traits usually reach a peak close to peak production and decline with increasing age
toward the end of the laying cycle. The decrease of hatchability in older flocks is well-known and may
be explained as a result of lower fertility of males and females, but probably more important is the
reduction in eggshell quality with increasing egg weight. Bamelis (2003) suggested that low hatchability
of fertile eggs at the beginning of the breeding season and the decline in hatchability with increasing
age is also due to improper egg water loss, which should be avoided by adjusting incubation conditions
in accordance with water vapour eggshell conductance. In management guides for commercial parents,
52 to 68 g may be specified as optimal weight range for hatching eggs. The critical question is how
many small eggs from young flocks and how many large eggs from old flocks are discarded to
guarantee the best possible chick quality and satisfied customers.
Breeds and lines of the same breed differ in reproductive traits, but relatively little within-line selection
has been practiced for hatchability in commercial breeding programs focused on efficient egg production
of the final cross (Flock, 1995). Reproductive traits usually benefit from heterosis in cross-line parents.
Natural selection should always favour families with above average reproductive rates, because they
contribute with more progeny to the next generation. Hatchability is a typical fitness trait with low
heritability, which suggests that optimization of breeder farm and hatchery management is the most
promising route for improvement (Förster et al., 1992). However, low heritability does not exclude
improvement by selection; it only takes a long time to see measurable results. Estimates of heritability
for hatch of fertile eggs in the literature ranges from 0.02 to 0.24 (i.e. Förster, 1993; Beaumont et al.,
1997; Szwaczkowski et al., 2000; Sapp et al., 2004; Bennewitz et al., 2007; Rozempolska-Rucinska
et al., 2009; Sharifi et al., 2010; Wolc et al., 2010).
There is a positive correlation between egg weight and incubation time, but the incubation time is
affected much more by pre-incubation storage time than by initial egg weight (Wilson, 1991). As a
rule of thumb, the lengthening of the incubation time with increasing storage time is estimated to be
0.5 to 1 h of incubation for each additional storage day (Förster, 1994). Moreover, the moisture loss
of the eggs during storage should be kept at a minimum; this can be reached with a high relative
humidity in the air, 75-85%. Furthermore a positive effect in the hatch results after long storage is
reported when the eggs are stored upside down, with the pointed end up (Mayes and Takeballi, 1984;
Förster, 1994). Regularly turning of eggs during incubation is important to ensure the nutrition and
fluid balance of the embryo and improves hatchability. In commercial hatcheries, eggs are turned in
a 45° angle each hour thorough the setter period, although it seems to be no need for further turning
after twelve days of incubation. Wilson (1991) concluded that maximum hatchability was achieved
with a turning frequency of 96 times per day, but 24 times per day was a practical frequency.
Hatching time from the first to last chick on a tray often spreads over a period of 24-48 hours, and all
chicks are kept in the hatcher until most chicks have hatched. Chick processing in the hatchery and
transport to the destination farm may involve up to 72 hours without water and feed for some chicks,
which will have negative effects on survival and later performance. The spread in individual hatch
causes variability in biological age and chick quality (Tona et al., 2003; Willemsen et al., 2008). As
Decuypere and Bruggeman (2007) pointed out, the spread of hatch varies due to pre-incubation
factors (e.g. age of parent flock and duration and temperature during egg storage, egg turning, the
gaseous environment), only some of which can be controlled with optimal management. If hatching
eggs from different parent flocks are used to supply large orders, these effects should be known and
taken into account.
Hatchability declines with storage duration of hatching eggs over an extended period even under
optimum storage conditions (Mayes and Takeballi, 1984). Although commercial hatcheries avoid
prolonged storage of hatching eggs, sometimes there is no other option to fill large orders. Tona et
al. (2003) concluded that long egg storage time increases incubation duration, which affects negatively the quality of the chicks. One of the methods to reduce the negative effect of long-term storage
has been to pre-incubate eggs for short periods before storage (Meijerhof, 1992).
Different lines may also respond differently to longer storage periods, i.e. some lines maintain acceptable hatchability longer than others (Förster, 1994). A good management practise to optimize hatchability is to pre-heat the eggs before they are introduced in the setters, thus reducing the temperature difference between storage and setters. With comparable humidity in the setter, large eggs lose
relatively less water than small eggs, which should be taken into account by lowering humidity in the
setter so that the desired weight loss is achieved.
Egg characteristics greatly influence the process of incubation and are responsible for its success
(Narushin and Romanov, 2002). The egg shell has an important role during embryonic development,
isolating the embryo from the external environment while allowing the proper gas exchange through
the shell. Barnett et al. (2004) reported that eggs with hair-cracks showed increased bacterial exposure and weight loss, with significantly lower hatchability (56.4% vs. 80.9%) compared with intact
shells. Bennet (1992) compared thin and thick shells based on specific gravity measurements and
reported a reduction in hatchability of 3 to 9%, which he attributed to increased cracks, moisture loss
and bacterial contamination of eggs with thin shells.
To support hatchery management in producing high quality chicks within a reduced hatch window,
even from older flocks and/or prolonged egg storage, primary breeders select families with persistent hatchability under these conditions (Förster, 1994). The aim of the present study is to estimate
genetic parameters for reproductive traits and to evaluate genetic relationships with egg quality and
production traits so that genetic improvement of total performance including hatchability can be optimized.
Material and methods
Data of 6,226 and 6,516 fully pedigreed hens of two pure-bred female lines C and D of a commercial
White Leghorn breeding program (LSL) were analyzed. The average number of daughters per sire
and per dam was 48.3 and 6.4 and 50.5, and 6.6 for the lines C and D respectively. Each hen was tested
twice at the age of 45 and 46 weeks. Hatching eggs were collected for a period of 7 days each time.
Double-yolk eggs, misshaped eggs, dirty eggs, eggs without shell or with abnormal shell were excluded.
Prior to setting, the hatching eggs were subjected to long storage challenging conditions, which
explains the low average hatchability (Table 1). Prolonged storage was expected to increase the
apparent variation between families, thus improving the basis for selection. All eggs were stored in
the hatchery for 7 days after the last collection day at 16°C and 60-70% HR, i.e. the eggs were
between 8 and 15 days old when set. Furthermore, the time of incubation was limited to exactly 508
hours (21 days + 4 h), 8 hours less than commercial hatcheries would plan for these lines. Pooled
semen was used from several males to eliminate the influence of male fertility.
Trays for 150 eggs were used, allowing 2 hens per row (max. 7 eggs per hen and a gap in the middle),
i.e. eggs of 20 hens per tray. The trays were randomly distributed in the incubators, and possible
environmental differences within the machines were treated as part of the error variance.
Table 1: Phenotypic statistics for the reproductive traits analysed (data from two years)
The eggs were candled on the 18th day of incubation and transferred to the hatchers after elimination
of ‘clear’ eggs (infertile eggs and early embryonic mortality). True fertility was probably around 95%,
based on data reported by Sharifi et al. (2010), who found fertility ranging between 94% and 97% in
a similar test of data from these lines. Families with less than 3 fertile eggs were completely removed.
At hatch, only first-quality chicks were taken into account, which means, that chicks which had physical
abnormalities, unhealed navels or were too wet or too weak were not counted and not considered to
Data of two generations were used for this study. In each generation, three different houses were
tested with two hatches. The traits recorded in this test are: fertility rate (F), hatch of eggs set (HoS)
and hatch of fertile eggs (HoF). Information about egg production and egg quality of these pedigree
birds was available as well. Egg quality was measured on other eggs of the same hens before and after
the hatching test.
In breeding practice there are often situations in which individual performance can be measured
repeatedly, and the average of individual records can be used as selection criterion. The collection
of repeated measurements and subsequent use of average performance as selection criterion can
be especially advantageous in traits with low heritability but good repeatability. For the estimation of
the heritability of hatchability and fertility, the mean value of the two hatches was calculated for each
hen. A multicode was created combining generations (2 generations), houses (3 houses), and tierbatteries where the hens were allocated (4 batteries with 3 tiers per house). The genetic parameters
for the average F, HoS and HoF were estimated based on the following linear multi-trait animal model:
y = Xb + Za + e
Where: y = vector of observations on t traits; b = vector of fixed effects of the multicode (year, house
and battery/tier); a = vector of random additive genetic effects; e = vector of random errors; X and Z
= known design matrix of fixed effects and random additive effects, respectively.
Variance and covariance components were estimated using the REML-method of the software VCE4
(Neumeier and Groeneveld, 1998). Although the distribution of reproductive traits is not normal, the
percentage data of hatchability and fertility were not converted to arcsine, because the benefit of
transformation is relatively small (Förster et al., 1993). Moreover, the breeding values on a transformed
scale have no biological meaning and are difficult to interpret (Savas et al., 1999).
Breeding values were calculated by adding the line mean to the BLUP values estimated with the
software PEST (Groeneveld et al., 1990).
Results and discussion
Heritabilities and genetic correlations using model 1 are shown in Table 2. As expected, the estimated
heritabilities in this study were higher for hatchability compared to fertility, which is in accordance with
other studies (Förster, 1993; Szwaczkowski et al., 2000; Bennewitz et al., 2007). On the contrary,
other authors have found lower heritabilities for hatchability compared to fertility (Beaumont et al.,
1997; Hartmann et al., 2002; Sharifi et al., 2010; Wolc et al., 2010). High genetic correlations were
found between the reproductive traits (F, HoS and HoF), as has been consistently reported in the
literature. That means that hens laying a high proportion of fertile eggs also tend to have a high
hatchability. Since the genetic correlation between HoF and HoS is very close to one, it would be
sufficient to use one of these traits or total number of chicks during the hatching egg saving period
as basis for selection.
Table 2: Heritabilities (diagonal) and genetic correlations for fertility rate (F), hatch of eggs
set (HoS) and hatch of fertile eggs (HoF)
The estimated heritabilities were similar for both lines and in the case of hatchability slightly higher
than values reported in the literature, where the heritability ranged from 0.02 to 0.24 by applying
different statistical methods (Förster, 1993; Beaumont et al., 1997; Szwaczkowski et al., 2000; Sapp
et al., 2004; Bennewitz et al. 2007, Rozempolska et al., 2009, Sharifi et al., 2010; Wolc et al., 2010).
Estimates from different studies in the literature are difficult to compare, due to different definition of
traits, data collection and structure of the data and statistical models used in the analysis. Using a
cumulative model may overestimate the heritability (Sapp et al., 2004; Swalve, 1995). Higher heritability and accuracy of selection can be obtained by averaging fertility over several weeks, either by
pooling all weeks or by calculating average fertility (Wolc et al., 2009).
Genetic correlations were estimated between reproductive traits and other economically important
- Egg Production: at the start of the laying cycle (P1), around peak production (P2) and at the end
of the laying cycle (P3).
- Egg Quality: Egg Weight (EW), Shell Strength (SS), Dynamic Stiffness (DS), Egg Shape (ES,
length/width), Albumen Height (AH), Yolk Proportion (YP).
- Egg Quality: Egg Weight (EW), Shell Strength (SS), Dynamic Stiffness (DS), Egg Shape (ES,
length/width), Albumen Height (AH), Yolk Proportion (YP).
Since the genetic correlations between fertility and different egg quality and production traits are
generally low (Table 3), and no distinction could be made between true infertility and early embryonic
mortality, we will focus on the genetic correlations involving hatch of fertile eggs (HoF).
Table 3: Estimated genetic correlations between reproductive traits and production and egg
The negative correlation between egg weight and hatchability (rg = -0.46 and -0.52) in both lines
(Table 3) confirms earlier results of Förster et al. (1992), who found correlations ranging from rg = -0.50
to -0.54 in two brown-egg pure lines. In a breeding program for layers the male lines can be selected
for higher egg weight if the female lines lose egg weight as a result of more emphasis on reproductive
traits. In this way, it is possible to overcome the negative correlations and maintain a balanced
performance profile in the commercial layers. Heterosis effects are also utilized for fitness traits
(Förster et al., 1992).
Consumers in Europe prefer eggs of size M or L, i.e. between 53 and 73 g, but the average egg size
would be under this economical optimum if this trait were not selected for, due to negative correlations
with fitness traits (Stöve-Schimmelpfening and Flock, 1982). Large eggs tend to have relatively less
shell surface, and that can be an obstacle for normal gas exchange for the embryo (Narushin and
The higher negative correlation between egg weight and hatchability of fertile eggs (HoF) compared
to hatchability of eggs set (HoS) and Fertility (FER) suggests that higher egg weight might mainly
affect late embryonic mortality and may prolong incubation time, which is in accordance with practical
experience. Egg weight at start of incubation seems to have an effect on the time of hatch within the
hatch window, thus, early hatchers had a significant lower starting egg weight as compared to late
hatchers (Careghi et al., 2005). Therefore in this study, where the incubation time was shorted, eggs
with a high egg weight might be additionally penalised in hatchability results.
It has been suggested that it may be a nonlinear relationship between egg weight and hatchability
(Wolc et al., 2010). As shown in Figure 1, that is not the case in this study, thus a clear linear negative
effect of egg weight can be distinguished.
Figure 1: Breeding values for hatch of fertile eggs (HoF) plotted against breeding values for
A positive correlation was found between egg production at the end of the laying period and hatchability and fertility, especially in line D, which was also shown during peak production. The values are
according to the values indicated by Förster et al. (1993), who gave a possible explanation based on
the negative correlation existing between egg production and egg weight. This author further argued
that the first egg in a sequence shows a lower hatchability, and that hens with lower productivity have
a higher number of first eggs. Förster et al. (1992) reported a decrease in hatchability ranging from 4
to 9% comparing first egg and middle egg in a sequence, this drop was even bigger for single egg
sequences (12-14%). Robinson et al. (1991) found that the eggs in the first position of the laying
series had lower fertility compared to the following eggs. However, other authors did not find significant differences in hatchability relative to time of oviposition (Zakaria et al., 2005), where early laid
eggs were associated to first-in-sequence eggs.
The eggshell performs a double function during embryo development. It has to be strong enough to
protect the embryo from external influences and penetration of pathogens, but at the same time it
should have enough porosity to allow gas exchange for embryonic development (Narushin and
Romanov, 2002). These authors also indicated that hatchability tends to increase with increasing
shell thickness and length-to-width ratio. The positive effect of egg shell quality on hatchability has
been confirmed in several studies (i.e. Bennet, 1992; Barnett et al., 2004). The correlations estimated
in our study range from rg = +0.19 to +0.29 (Table 3), which is in agreement with the positive genetic
correlation reported by Wolc et al. (2010) between specific gravity and hatchability (rg = + 0.53). The
relationship between shell strength and hatchability is plotted in Figure 2.
Figure 2: Breeding values for hatch of fertile eggs (HoF) plotted against breeding values for
Coucke et al. (1999) proposed that acoustic resonance frequency analysis could measure the mechanical stiffness of intact eggs and defined a novel eggshell parameter, dynamic stiffness (Kdyn). De
Ketelaere et al. (2003) propose that Kdyn might provide a better indicator of an egg’s ability to withstand
insult because most forces leading to breakage are dynamic and not static. Dunn et al. (2005) and
Icken et al. (2006) found that Kdyn has a higher heritability than breaking strength, while the correlation between these traits was significantly below 1 (rg = +0.49 and +0.40). This suggests that Kdyn
measures different aspects of the mechanical properties of the egg, in particular structural strength (Dunn
et al., 2005). Although the negative genetic correlations with hatchability are low (rg = -0.09 and
-0.17) strong selection on Kdyn may not help to improve hatchability, especially in line D, as shown
in Figure 3.
Figure 3: Breeding values for hatch of fertile eggs (HoF) plotted against breeding values for
dynamic stiffness (Kdyn)
A possible explanation is that Kdyn has a lower negative correlation with egg weight than breaking
strength (Dunn et al., 2005; Icken et al., 2006). A negative effect of increased dynamic stiffness may
also be the result of reduced shell porosity, which is essential for gas exchange and for embryo development. The number and size of pores influences the rate of moisture loss and heat conductance
across the eggshell (Hulet et al., 2007). Further research is desirable to substantiate this tentative
The genetic correlation between albumen height and hatchability (rg = -0.26 and -0.42) would perhaps
have been less highly negative, if albumen height had been transformed to Haugh Units to remove the
effect of the egg weight. Nevertheless, the negative correlation between albumen quality and hatchability
is real (Flock et al., 2007) and may be due to limited nutrient availability in eggs with higher albumen
and lower yolk percentage. Wolc et al. (2010) reported also a negative correlation between Haugh
Units and hatchability (rg = -0.25). Practical experience has also shown that the hatchability of very fresh
eggs, which have high Haugh Units, is lower than after a few days of storage (Förster et al., 1992), but
that is not relevant here. Albumen consistency is mainly an issue in connection with storage conditions
for shell eggs in retail stores rather than for hatcheries.
Round eggs often have lower hatchability. The positive correlations between egg shape (length/with)
and hatchability (rg = +0.19 and +0.32) confirm that “longer” eggs tend to hatch better. Most eggs
used in this study had shapes within the normal range, and eggs with abnormal shapes were already
sorted out before setting.
Figure 4: Breeding values for hatchability of fertile eggs (HoF) plotted against the breeding
values for egg shape
A high proportion of yolk, i.e. a high dry matter content of the egg, is appreciated by the egg-processing
industry. The breeding goal is therefore at least 30% yolk in these white egg lines (Flock et al., 2007),
not only for the egg-processing industry, but also for optimal hatchability and chick quality. We confirmed
positive correlations with hatchability (rg = +0.32 and +0.39 for HoS and HoF, respectively). Similar
results in a White Leghorn line were also reported by Hartmann et al. (2002), who found positive
correlations between hatch of fertile eggs and yolk weight, yolk proportion and albumen dry matter
of rg = +0.28, +0.52 and +0.26, respectively. Narushin and Romanov (2002) concluded from the values
obtained in the literature that hatchability decreased when the liquid content of the egg increased.
Milisits et al. (2010) used electrical conductivity measurements to demonstrate that the hatchability of
eggs benefits from a high egg yolk ratio.
In our study, this relation can be partly explained by the negative correlation between yolk proportion
and egg weight (rg = -0.60 and -0.65). The composition of the egg changes with egg size: yolk
proportion decreases with increasing egg weight. This decrease in yolk proportion may have a negative
effect on the nutrient supply of the embryo and consequently on the hatchability of larger eggs (Förster
et al., 1992). Wilson (1997) emphasized the crucial role of maternal nutrients for the developing avian
embryo and concluded that inadequate, excessive or imbalanced levels of nutrients could even have
In contrast to Förster et al. (1993), who reported negative correlations in two brown layer lines ranging
from rg = -0.12 to -0.25, no significant correlation was found between body weight and hatchability
in these White Leghorn lines.
Predictable hatchability of first quality chicks within a narrow time window is a common objective for
commercial hatcheries. To improve reproductive traits by genetic selection, they must be included in
the selection index, with proper attention to all genetic correlations to other traits. In view of the
negative correlation with egg weight, focus on hatchability should be limited to female lines, while
selection for desirable egg weight is practiced in male lines. With this strategy, egg weight could
decline in the female lines toward a level determined by chick quality standards. A possibly negative
relationship between dynamic stiffness of egg shells and hatchability can also be taken care of with
appropriate choice of shell quality criteria in male and female lines. Persistent egg production and
high yolk percentage are positively related to hatchability, and only the negative correlation with
albumen height indicates a conflict with traditional breeding goals in White Leghorns.
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Züchterische Verbesserung der Schlupfrate bei Weißen Leghorn (LSL)
Pedigree-Daten von zwei LSL Reinzuchtlinien wurden ausgewertet, um genetische Parameter für
Fruchtbarkeit und Schlupfrate und Korrelationen zu Eiqualitätskriterien zu bestimmen. Die Schlupfrate
befruchteter Eier hatte mit 0,27-0,30 eine deutlich höhere Heritabilität als die Fruchtbarkeit mit
0,13-0,15 und ist deshalb als Selektionsmerkmal geeigneter. Genetische Korrelationen zum Eigewicht
und zum Eiklaranteil waren deutlich negativ, während positive Beziehungen zum Dotteranteil und zur
Schalenstabilität bestehen. Daraus ergibt sich die Empfehlung, bei Hennenlinien auf höheren Dotteranteil
zu selektieren und eine Reduzierung des Eigewichts durch natürliche Selektion in Kauf zu nehmen.
Ein marktgerechtes Eigewicht kann durch entsprechende Selektion auf Eigewicht in den Hahnenlinien