Comparison of Three Lines of Broiler Breeders Differing by qzl20249



        Comparison of Three Lines of Broiler Breeders Differing in Ascites
         Susceptibility or Growth Rate. 1. Relationship Between Acoustic
            Resonance Data and Embryonic or Hatching Parameters.

              K. Tona,* B. Kemps,† V. Bruggeman,* F. Bamelis,† L. De Smit,* O. Onagbesan,*,1
                                 J. De Baerdemaeker,† and E. Decuypere*

     *Laboratory for Physiology and Immunology of Domestic Animals, and †Department of Agricultural Engineering,
                       Kasteelpark Arenberg 30, Katholieke Universiteit Leuven, Heverlee, Belgium

ABSTRACT Ascites is a prevalent cardiovascular dis-            albumen pH, Haugh units (HU) at setting, and embryo
ease among modern broilers with negative impacts on            weights at d 11 and 18, at internal pipping (IP), and at
production and animal welfare. The peak of mortality           hatch. The durations of IP, external pipping (EP), and
due to ascites occurs at the end of the growing period, but    hatching were also determined. At 2 hourly periods dur-
the etiology of this problem may start during embryonic        ing incubation, egg RF and damping were also measured.
development. A few recent reports have demonstrated            There were differences in egg weights between DAR and
that the signs of ascites susceptibility are manifested dur-   SASL vs. DAS, but albumen HU, albumen pH, and the
ing the late stages of incubation. In the current study,       ratio of yolk weight to egg weight were similar. There
we used a nondestructive method based on egg acoustic          were differences in RF, damping, embryonic growth rates,
resonance parameters [resonant frequency (RF) and              and hatching events. Changes in resonant frequency and
damping] to establish a relationship between embryo            damping, which certainly suggest eggshell differences
physiological events during early development in broiler       among lines, were not totally related to variations in phys-
eggs and susceptibility to ascites. The hatching eggs of 3     iological events during early and late embryonic develop-
broiler lines differing in ascites susceptibility were used    ment. A comparison between DAR and DAS, between
for this study: ascites-resistant dam line (DAR), ascites-     DAS and SASL, or DAR and SASL indicates that sensitiv-
sensitive dam line (DAS), and ascites-sensitive sire line      ity to ascites and selection for rapid growth rate in ascites-
(SASL). These lines were selected on the basis of fast         sensitive lines have different effects on embryonic param-
growth, high breast meat yield, and ascites induction at       eters. We concluded that the sensitivity of broiler breeders
low temperatures such that the order of ascites suscepti-      to ascites does not influence egg internal quality, but the
bility in terms of mortality was SASL >> DAS > DAR.            occurrence of ascites sensitivity in broilers could not be
Eggs were incubated under standard conditions in forced-       reliably predicted by early in ovo acoustic resonance pa-
draft incubators. We measured egg weights at setting,          rameters and hatching events.
        (Key words: broiler, hatching egg, resonance parameter, incubation parameter, ascites susceptibility)
                                                                                            2005 Poultry Science 84:1439–1445

                   INTRODUCTION                                of which ascites and sudden death syndrome are the most
                                                               prevalent cardiovascular diseases among modern broiler
   Intensive and focused selection over many years has         flocks (Olkowski and Classen, 1998; Korte et al., 1999)
significantly improved broiler growth rate. Long-term se-       with negative impacts on production and animal welfare.
lection for production traits has also led to an increase in   In a review, Balog (2003) reported that ascites syndrome
the ratio of demand organs, such as skeletal muscles and       results from physiological (e.g., vascular damage,
fat mass, to supply organs, such as heart, lungs, spleen,      blockage of the lymphatic system, decreased plasma on-
and liver (Ricklefs, 1985; Nestor et al., 1995; Gavin and      cotic pressure, and increase in vascular hydraulic pres-
McDevitt, 1999). This selection leads to a number of meta-     sure), environmental (e.g., altitude, ventilation or air qual-
bolic problems such as disturbances in energy metabolism       ity, and cold stress) parameters or nutritional factors. Ac-

  2005 Poultry Science Association, Inc.
  Received for publication January 11, 2005.                      Abbreviation Key: DAR = ascites-resistant dam line; DAS = ascites-
  Accepted for publication May 5, 2005.                        sensitive dam line; ED11 = embryonic day 11; ED18 = embryonic day
   To whom correspondence should be addressed: okanlawon.      18; EP = external pipping; IP = internal pipping; RF = resonant frequency;                                  SASL = ascites-sensitive sire line.

1440                                                     TONA ET AL.

cording to Decuypere and Verstegen (1999), almost 50%             were stored for 5 d at 15°C and 70% relative humidity
of normal, noninfectious mortality among broiler chicks           before incubation. Before the beginning of incubation, all
is due to ascites. Although the peak of mortality due to          the eggs were numbered and weighed individually. Also,
ascites occurs at the end of the growing period (Mogha-           samples of eggs were used to determine resonant fre-
dam et al., 2001), it is reported that the etiology of these      quency and to measure albumen Haugh units (HU) and
problems may start during embryonic development                   pH. In this study, each incubation setting consisted of 2
(Coleman and Coleman, 1991; Dewil et al., 1996). How-             replications of 150 eggs per line. Eggs were set for incuba-
ever, most of these reports concern the late stage of incu-       tion in a forced-draft incubator3 at specific dry bulb tem-
bation and are based on destructive methods. To our               perature of 37.6°C and wet bulb temperature of 29°C. At
knowledge, there is no information about the relationship         embryonic stages of 11 d (ED11), 18 d (ED18), or internal
between embryonic parameters during early stages of               pipping (IP), a sample of 15 eggs per line and per incuba-
incubation and ascites sensitivity etiological parameters.        tion setting was opened to measure embryo weights.
Coucke et al. (1997) and Coucke (1999) reported a relation-
ship between acoustic resonance parameters [e.g., reso-           Albumen Characteristics
nant frequency (RF) and damping] and embryo physio-
logical events during early development in chicken eggs.            Before setting for incubation, samples of 30 eggs were
Kemps et al. (2003) also pointed out a positive correlation       weighed and broken to measure yolk weight, albumen,
between the timing of decrease of RF during early stage           HU, and pH of the albumen. For each broken egg, the
of incubation and total incubation duration. Chineme et           Egg Quality Measurement System4 was used to record
al. (1995) reported a relationship between eggshell con-          the egg weight and the height of thick albumen. After
ductance during incubation and the occurrence of ascites          calibration of the albumen height gauge, height of the
in later life. Because sensitivity to ascites results in longer   albumen was measured (±0.25 mm) with a vertically
incubation (Dewil et al., 1996), acoustic resonance analysis      mounted micrometer2 with an electronic path. Between
at an early stage of incubation may be a tool to detect           eggs, the micrometer was cleaned with wet and then dry
posthatch sensitivity to ascites. Differential embryonic          absorbing paper. The standard software program version
development, as hypothesized to occur between lines dif-          2A2 calculates the albumen HU with the egg weight and
fering in ascites susceptibility, may lead to differential        albumen height. After HU measurement of each egg, the
changes in early acoustic resonance parameters. There-            yolk and albumen were separated for, respectively,
fore, the aim of this study was to investigate the relation-      weighing or pH measurement as quickly as possible. For
ships between acoustic resonance parameters during                pH measurement, the probe was cleaned with distilled
early stages of incubation, embryonic growth, and hatch-          water between eggs.
ing parameters of 3 lines differing in susceptibility to
ascites syndrome. The lines were ascites-resistant dam            Acoustic Resonance Analysis
line (DAR), ascites-sensitive dam line (DAS), and ascites-
sensitive sire line (SASL). These lines were selected for            For each incubation setting, a sample of 40 eggs per line
fast growth on the basis of breast meat yield, and their          was randomly selected for the determination of acoustic
susceptibility to ascites was based on mortality due to           properties. Because acoustic resonance parameters are
ascites induction at low temperatures. The DAS and the            linked to egg weights (Coucke et al., 1997), egg weights
SASL have higher breast meat yield than the DAR line.             were recorded to correct for their influence on RF and
Under normal rearing conditions, mortality due to ascites         damping. The measurements were done first at 4 h after
was not more than 1%, but at low temperatures or at high          the beginning of incubation (data not shown) and were
altitude mortality due to ascites among lines was DAR <           repeated every 2 h between 68 and 122 h of incubation.
DAS << SASL.                                                      Acoustic resonance analysis was performed using the
                                                                  technique described by Coucke et al. (1997), involving
           MATERIAL AND METHODS                                   mechanical excitation of the egg by a mechanical im-
                                                                  pactor. The impactor hit the egg at its equator, and noise
Experimental Design                                               of the vibrating egg was recorded by a microphone, situ-
                                                                  ated at the equator at 90° to the impactor. The recorded
   We used 2 incubation settings each with 900 eggs pro-          signal was sent to a data acquisition card and transformed
vided by Hybro2 in our study. Eggs were produced by 3             by fast Fourier transformation to obtain the resonant fre-
lines differing in ascites sensitivity or growth rate: DAR,       quency for the first spherical mode of the vibrating egg.
DAS, and SASL. The DAR and DAS are slow feathering                Four instantaneous excitations, with a phase shift of 90°,
with relatively low 6-wk broiler body weight, whereas             were applied at the equator zone as previously described
SASL is fast feathering with high body weight. The eggs           Coucke (1999).

                                                                  Hatching and Pipping Events
   Hybro BV, Boxmeer, The Netherlands.
   PasReform, Zeddam, The Netherlands.                             Between 472 and 510 h of incubation, transferred eggs
   Futura, Lohne, Germany.                                        were checked individually every 2 h for IP, EP, and hatch-
                             EARLY SIGNS FOR ASCITES SUSCEPTIBILITY DURING INCUBATION                                                  1441
                                     TABLE 1. Egg weights and egg internal quality according to line

               Egg quality characteristic                            DAR                       DAS                    SASL
               Egg weights (g)                                   64.93   ±   0.38a         62.38   ±   0.31b       65.36   ±   0.36a
               Albumen HU                                        69.40   ±   2.07          68.34   ±   2.32        66.94   ±   1.36
               Albumen pH                                         9.49   ±   0.02           9.49   ±   0.03         9.54   ±   0.01
               Yolk weight (g)                                   20.80   ±   0.68          20.57   ±   0.72        20.60   ±   0.45
               Ratio: yolk weight/egg weight (%)                 30.79   ±   0.85          32.30   ±   0.73        31.14   ±   0.50

                   Data not sharing a common letter are different (P < 0.05).
                  HU = Haugh unit; DAR = ascites-resistant dam line; DAS = ascites-susceptible dam line; SASL = ascites-
               susceptible sire line.

ing event occurrences. At IP, EP, or hatching stage, incu-                     in Table 2. At ED11, embryos of the SASL had lower body
bation duration was defined as the time between setting                         weight (P < 0.05) and lower body weight/egg weight
and the occurrences of these events for an egg. Then                           ratio (P < 0.05) than those of DAS. At this incubation
the times of occurrence of hatching events were used                           stage, embryo weights and embryo body weight/egg
to calculate their durations as: IP duration = duration                        weight ratios of DAR were comparable with those of
between occurrences IP and EP, EP duration = duration                          embryos from SASL and DAS. At ED18, embryo weights
between EP and hatching, and hatching duration = dura-                         and embryo weight/egg weight ratios were similar
tion between IP and hatching. The spreads of IP, EP, or                        among lines but at IP, embryos from SASL were heavier (P
hatch were defined as the dispersion around the average                         < 0.01) than those from DAS and DAR, although embryo
incubation duration.                                                           weight/egg weight ratios were similar among the 3 lines.
                                                                               At hatch, chick weights were significantly heavier but
Statistical Analysis                                                           similar for DAR and SASL compared with DAS.

  The data were processed with the statistical software                        Effects of Lines on Resonant
package SAS version 8.2.5 Because the conditions for incu-                     Frequency and Damping
bation settings were kept constant for each repeated ex-
periment, data were pooled for analysis. The GLM proce-                           Mean weights of eggs used for measurement of RF and
dure was used to analyze the effects of lines on egg and                       damping were 64.78 ± 0.58 g, 64.17 ± 0.39 g, and 64.60 ±
embryo weights, albumen pH, albumen HU, yolk weight,                           0.49 g for DAR, DAS, and SASL respectively. Figure 1
resonant frequency, spectra data, and durations of IP,                         shows RF variations, from 68 to 122 h of incubation,
EP, and hatching. In a second analysis, the Kolmogorov-                        among lines in relation to incubation duration. From 4 h
Smirnov test was used to assess the normality of the                           of incubation until 68 h, the RF of all egg lines remained
distribution of pipping and hatching events. From the                          consistently unaltered (data not shown). A similar trend
hatching events distribution, incubation duration of me-                       was maintained until 98 h but the RF in the DAS line
dian and 5, 25, 75, and 95% quartiles were calculated.                         was significantly lower (P < 0.01) than those of DAR
A 2-tailed test for comparison of variances was used to                        and SASL lines, which were similar. There was a sharp
analyze the effect of line on spread of pipping or hatch-                      decrease in RF in all lines from 98 to 106 h of incubation.
ing events.                                                                    Although the heights of decrease of RF were similar be-
                                                                               tween lines, the start of the decrease during the incubation
                         RESULTS                                               period was later (P < 0.05) for eggs from SASL (106 h)
                                                                               compared with those for eggs from DAS (99 h) and DAR
Effects of Lines on Albumen                                                    (100 h), which were similar.
Characteristics and Embryo Growth                                                 Figure 2 shows variations of damping according to the
                                                                               lines in relation to incubation duration. During incuba-
  Table 1 shows the values of albumen HU and pH,                               tion, damping increased at about 92 h for DAR and DAS
yolk weight, and egg weights at setting according to line.                     but around 100 h for SASL and reached a maximum level
Albumen HU and pH, yolk weights and ratio of yolk                              around 110 h for all the lines. Damping, from 68 h to 122
weights to egg weights were similar between lines. How-                        h of incubation of eggs, of SASL eggs was lower than
ever, eggs of DAS were lighter (P < 0.05) than those of                        that of eggs from DAS. Also, the timing of the increase
DAR and SASL.                                                                  in damping during incubation was similar to the time at
  Embryo body weights and embryo weight/egg weight                             which RF decreased.
ratios according to lines and incubation stages are shown
                                                                               Effects of Line on Hatching Events
                                                                                 Figure 3 shows the spread of hatching in relation to
  SAS User’s Guide, version 8.2, SAS Institute Inc., Cary, NC.                 incubation duration and according to line. At 472 h of
1442                                                              TONA ET AL.
                            TABLE 2. Embryo body weight and relative body weight relative to initial egg weight according
                                          to incubation stage and weights of 1-d-old chicks of the 3 lines1

                 Incubation stage                    Line                  Body weight (g)                 Relative body weight (%)

                 ED11                                DAR                    3.961 ± 0.128    ab
                                                                                                                 6.072 ± 0.205ab
                                                     DAS                    4.152 ± 0.077a                       6.663 ± 0.166a
                                                     SASL                   3.629 ± 0.122b                       5.453 ± 0.198b
                 ED18                                DAR                   28.220 ± 0.675a                      44.075 ± 1.761a
                                                     DAS                   27.485 ± 0.762a                      44.130 ± 0.795a
                                                     SASL                  27.829 ± 0.623a                      41.974 ± 1.181a
                 IP                                  DAR                   46.628   ±   0.574b                  72.466   ±   0.402a
                                                     DAS                   44.003   ±   0.749b                  72.715   ±   0.500a
                                                     SASL                  49.755   ±   0.957a                  73.149   ±   0.658a
                 Hatch                               DAR                    47.46   ±   0.37a                    73.69   ±   0.22a
                                                     DAS                    45.41   ±   0.30b                    72.57   ±   0.28b
                                                     SASL                   48.33   ±   0.36a                    74.00   ±   0.22a
                    Data not sharing a common letter are different (P < 0.05).
                   ED11 = embryonic d 11; ED18 = embryonic d 18; IP = internal pipping; DAR = ascites-resistant dam line;
                 DAS = ascites-susceptible dam line; SASL = ascites-susceptible sire line.

incubation, 27.20, 22.73, and 19.46% of eggs were at IP
for DAR, DAS, and SASL, respectively. At the same time,
9.02, 5.43, and 5.79% of eggs were at EP for DAR, DAS,
and SASL, respectively.
   Durations of IP, EP, and hatching were longer in eggs
of SASL (P < 0.05) compared with those of DAR and DAS,
which were similar (Table 3). The duration of IP or EP
of eggs of SASL was almost 2 h longer than those from
DAR and DAS. On average, 50% of IP occurred 2 h earlier
in eggs from DAR compared with those from DAS and
SASL (P < 0.05). The peak of EP (50% of EP) occurred
almost 4 h earlier in the eggs from DAR compared with
those from SASL and DAS for which the peak of EP
occurred at the same time (P < 0.05). Although 50% of
the hatch in DAR was obtained 2 h earlier than those of

                                                                                  FIGURE 2. Relationship between damping and incubation duration
                                                                               according to line. DAR = ascites-resistant dam line; DAS = ascites-
                                                                               susceptible dam line; SASL = ascites-susceptible sire line.

   FIGURE 1. Relationship between resonance frequency and incuba-                 FIGURE 3. Number of chicks hatched according to incubation dura-
tion duration according to line. DAR = ascites-resistant dam line; DAS =       tion and line. DAR = ascites-resistant dam line; DAS = ascites-susceptible
ascites-susceptible dam line; SASL = ascites-susceptible sire line.            dam line; SASL = ascites-susceptible sire line.
                           EARLY SIGNS FOR ASCITES SUSCEPTIBILITY DURING INCUBATION                                            1443
                        TABLE 3. Effects of line on internal pipping, external pipping, and hatching duration

                                                                  Duration of hatching events (h)
              Line                            IP                                 EP                             Hatching
              DAR                         9.67 ± 0.52b                     11.43 ± 0.46b                      20.36 ± 0.36b
              DAS                        10.14 ± 0.51b                     11.35 ± 0.47b                      20.42 ± 0.46b
              SASL                       12.02 ± 0.60a                     13.17 ± 0.79a                      23.74 ± 0.76a

                  Within a column, data not sharing a common letter are different (P < 0.05).

                 DAR = ascites-resistant dam line; DAS = ascites-susceptible dam line; SASL = ascites-susceptible sire line;
              IP = internal pipping; EP = external pipping.

DAS, incubation duration was not significantly different                than ascites sensitivity. Similarly, there was no difference
between these 2 lines. Eggs from SASL had significantly                 between the DAR and SASL in RF or body weights, even
longer incubations and hatched 4 or 6 h later than those               though the SASL is supposed to be fast growing and
of DAS or DAR, respectively (P < 0.01). Overall, eggs                  extremely ascites susceptible. The changes in body
from DAR hatched earlier than those of the ascites-sensi-              weights at ED18 or at IP could not be related to the RF
tive lines. Also, the spread of IP and EP were different               data, even though SASL had a better rate of growth to
among lines (P < 0.05) and were in the following order:                IP than DAR or DAS. The only period when RF had any
DAR = DAS < SASL. Hatching spread differed between                     correlation with body weight was at hatch, when DAS
lines (P < 0.05).                                                      had the lowest RF and body weight. Again, there was no
                                                                       difference between DAR and SASL. This lack of consis-
                      DISCUSSION                                       tency between RF and embryo body weights among lines
                                                                       suggests that the values of RF may not be a useful pre-
   In this study, we attempted to characterize the parame-             dictor for ascites susceptibility. A common factor in the
ters that might be associated with ascites susceptibility              dam line was the similarity of timing of the general de-
during embryonic development. In doing so, we looked                   crease in RF after the 90th hour of incubation, whereas
for associations between egg RF, damping, embryo body                  this decline was delayed for some 6 h in the SASL geno-
weights at different ages, and hatching parameters to                  type. The decrease of RF around 100 h of incubation has
relate these with ascites susceptibility of the DAR, DAS,              been shown to occur as a result of subembryonic fluid
and SASL genotypes. The results showed that selection of               formation by Bamelis et al.(2002), and it was suggested
broiler breeders for ascites susceptibility may not directly           to be an indication of a well-defined physiological event at
influence aspects of egg parameters or early embryonic                  a chronological age, depending on incubation conditions
development or hatching events. A comparison between                   and genetic variation in speed of early development.
DAR and DAS, on the one hand, and, on the other hand,                     The damping results had lower values for SASL com-
between DAS and SASL or DAR and SASL indicated that                    pared with the dam lines during early incubation and a
sensitivity to ascites and selection for rapid growth rate             late increase that mimicked the late decrease observed
in ascites sensitive lines might have different effects on             for RF in Figure 1. In the absence of an ascites-resistant
embryonic parameters. The differences identified among                  sire line genotype in this study, it is difficult to conclude
lines seemed mainly to be due to genotypic differences                 that this finding could be an early indication of ascites
that are unrelated to ascites sensitivity.                             susceptibility. However, the clear divergence in damping
   Among genotypes, the albumen HU, albumen pH, and                    after 90 h among lines indicates significant differences
ratio of yolk weight to egg weight were not different for              between the SASL and DAS genotypes and the DAR geno-
the 3 lines studied. Therefore, the differences observed               type. These damping data from 100 h clearly suggest a
in RF, embryonic growth, and hatching events could not                 link to ED11 body weights. This trend of body weights
be explained by egg size or albumen quality parameters.                was reversed at hatch. Although damping measures have
Overall, the differences in egg weights between lines,                 not been associated with any embryo physiological pro-
although yolk weights were similar, might have been due                cesses, it certainly suggests eggshell differences between
to differences in albumen or shell weight.                             lines. Chineme et al. (1995) and Odom et al. (1995) pre-
   Coucke et al. (1997) showed that changes in RF and                  viously showed that eggshell conductance may influence
damping could be related to variations in physiological                ascites susceptibility, and, therefore, differences in egg-
events during early embryonic development. Early occur-                shell characteristics among lines as revealed by RF and
rence of the decrease in RF in DAS and DAR vs. SASL                    damping may be relevant in this respect. However, these
should have indicated a more rapid early development                   authors did not study the implications of the measured
in the embryo weights of the DAR and DAS at ED11, but                  parameters on embryonic development. At IP and hatch,
this was not the case. Surprisingly, the ascites-susceptible           embryo and chick weights were negatively correlated
genotypes were different in RF and body weights, which                 with the early acoustic data in the DAR vs. SASL and
suggests a major difference between the sire and dam                   between DAS and SASL. Accelerated embryonic growth
lines that may be ascribed to genotype differences rather              rate in the second half of incubation of the SASL genotype
1444                                                    TONA ET AL.

compared with the DAR indicated an early sign of high           between DAS and DAR and between DAS and SASL
growth rate potential. On the other hand, the DAS geno-         or DAR and SASL suggests that, on top of an inherent
type had a lower growth rate in the last incubation period      sensitivity to ascites, it is selection for rapid posthatch
compared with DAR. This finding suggests that predispo-          growth that increases broiler sensitivity to ascites, as is
sition to ascites is not necessarily linked to embryonic        the case for SASL.
growth speed or to growth rate at all but can be related           It is concluded that the sensitivity of broiler breeders
to completely other physiological functioning as sug-           to ascites do not influence egg internal quality as mea-
gested by Scheele et al. (2003a,b).                             sured by the pH and HU. The inconsistency in the rela-
   At median (50%) occurrence of IP, EP, or hatch, al-          tionship between the values of RF or the timing of its
though the eggs from DAS hatched 2 h later than those           decrease and embryo development did not permit a reli-
from DAR, there was no significant difference between            able association of egg acoustic data with ascites suscepti-
these lines. This finding agrees with the timing of the          bility. Although the timing of the decrease in RF or in-
RF decrease and damping increase between the 2 lines.           crease in damping could be partly associated with incuba-
Timing of the decrease in RF has also been used as an early     tion duration in the SASL, it could not be specifically
indication for predicting incubation duration. Kemps et         associated with ascites susceptibility. Therefore differ-
al. (2003) previously reported a positive correlation be-       ences observed in acoustics, body weights, or hatching
tween the timing of RF decrease during early incubation         parameters among lines might be partly due to genotype
and hatching time. The lack of difference in durations of       differences that are secondary (associated responses) to
hatching events between the DAR and DAS suggests that           the primary selection goal, which was fast growth with
although DAS is ascites sensitive, it can cope better with      high breast meat yield.
relatively mild hypoxia during the last stage of incubation
than SASL.                                                                     ACKNOWLEDGMENTS
   As for the comparison between DAR and SASL, the
results of this study are in the line with those of Dewil et      This study was supported by FWO, Belgium (grant
al. (1996), who reported a similar delay of approximately 5     number G.0286.04). K. Tona received a postdoctoral
h in hatching of eggs from SASL compared with those             award (KU Leuven) and V. Bruggeman had a postdoc-
of DAR. Although 50% of the eggs of all 3 lines were at         toral funding from Funds for Scientific Research (FWO,
IP at almost the same incubation duration, eggs of DAR          Flanders, Belgium). The authors are grateful to G. A.
hatched 6 h earlier than those from SASL due to longer          Albers of Hybro BV for the supply of experimental
durations of IP, EP, and hatching time. This delay is hy-       animals.
pothesized to be linked with the delay in early RF de-
crease and damping increase in the SASL eggs and, per-                                REFERENCES
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