- Email: [email protected]
Ultrasonic eggshell thickness measurement for selection of layers1
Ultrasonic eggshell thickness measurement for selection of layers1 Lucyna Kibala,∗ Iwona Rozempolska-Rucinska,† Kornel Kasperek,† Grzegorz Zieba,† and Marek Lukaszewicz‡,2 ∗
Center for Nucleus Breeding MESSA Ltd., Mienia 100, 05–319 Ceglow, Poland; † Chair for Biological Bases of Animal Production, University of Life Sciences in Lublin, Akademicka 13, 20–950 Lublin, Poland; and ‡ Institute of Genetics and Animal Breeding, Jastrzebiec, Postepu 36A, 05–552 Magdalenka, Poland micrometer predominantly at the wider end of eggs from 2,397 RIR and 4,447 RIW hens. A multiple-trait statistical model fit the fixed effect of year-of-hatch × hatch-within-year, and random effects due to repeated measurements (except EMM) and an animal’s additive genetic component. The shell was thinnest in the region where chicks break it upon hatching (USG0, USG45). Heritabilities of shell thickness in different regions of the shell ranged from 0.09 to 0.19 (EMM) in RIW and from 0.12 to 0.23 (EMM) in RIR and were highest for USG45 and USG0. Because the measurement repeatabilities were all above 0.90, our recommendation for balancing egg strength against hatching ease is to take a single measurement of USG45. Due to high positive genetic correlations between shell thickness in different regions of the shell its thickness in the pointed end region will be modified accordingly, in response to selection for USG45.
Key words: eggshell thickness, USG measurement, heritability, genetic correlation 2015 Poultry Science 94:2360–2363 http://dx.doi.org/10.3382/ps/pev254
INTRODUCTION The eggshell is a barrier that is designed to protect the developing embryo, ensure proper gas exchange during its development, and provide it with an optimal environment. The relationship between a shell’s thickness, porosity, and strength (all often described by a single correlated measure of the egg’s specific gravity) and poultry hatchability has been demonstrated in many reports (e.g., Sahan et al., 2003 in ostriches and Rozempolska-Ruci´ nska et al., 2011 in chickens). Evident cracks or microcracks weaken protection against egg pathogen penetration, and thus eggs have reduced hatching and marketing qualities. The propensity to crack, conditioned by both genetic and environmental factors, is associated with an C 2015 Poultry Science Association Inc. Received March 8, 2015. Accepted July 14, 2015. 1 Supported by the National Center for Research and Development, Poland. Grant no. PBS2/B8/8/201. 2 Corresponding author: [email protected]
egg’s strength as determined indirectly by shell thickness (Amer Eissa, 2009). Genetic correlations between eggshell strength and its thickness are very high, exceeding 0.8 in some cases (Zhang et al., 2005), which makes eggshell thickness a valuable selection criterion. Yet, the most reliable method of measuring shell thickness is direct destructive measurement with a micrometer. However, modern technology allows for direct measurement without destroying the eggs, thus suitably maintaining them for further handling steps. One such technology is ultrasonic devices that are used, inter alia, to analyze egg content quality (Abdallah et al., 1993; Lin et al., 2009; Aboonajmi et al., 2014; Yan et al., 2014), although it is not used on a large scale (de Ketelaere et al., 2004). The use of such devices in genetic improvement of laying hens requires eggshell thickness measurement methodology, to provide reliable estimates obtained with the least effort. This study aimed to develop a methodology for using ultrasonic technology (USG) to record eggshell thickness for selection of layers.
2360
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
ABSTRACT This study aimed to develop a methodology for using ultrasonic technology (USG) to record eggshell thickness for selection of layers. Genetic correlations between eggshell strength and its thickness have been reported to be around 0.8, making shell thickness a selection index candidate element. Applying ultrasonic devices to measure shell thickness leaves an egg intact for further handling. In this study, eggs from 2 purebred populations of Rhode Island White (RIW) and Rhode Island Red (RIR) hens were collected on a single day in the 33rd week of the farm laying calendar from 2,414 RIR and 4,525 RIW hens. Beginning from the large end of the egg, measurements were taken at 5 latitudes: 0o (USG0), 45o (USG45), 90o (USG90), 135o (USG135), and 180o (USG180). To estimate the repeatability of readings, measurements were repeated at each parallel on 3 meridians. Electronic micrometer measurement (EMM) were taken with an electronic
2361
EGGSHELL THICKNESS FOR LAYER SELECTION
MATERIAL AND METHODS
RESULTS AND DISCUSSION Eggshell thickness is a trait correlated, among others, with egg weight, shell strength, and hatchability (Nirasawa et al., 1998; Sahan et al., 2003; Zhang et al., 2005; Rozempolska-Ruci´ nska et al., 2011). Shells that are too thick nor too thin may lead to impaired gas exchange and hatchability or to egg breakage (Bennett, 1992; Sahan et al., 2003). Mean thicknesses at different shell regions are presented in Table 2. The difference of more than 100 μm between EMM and the equivalent measurement of USG90 must have resulted from the specificity of tools used. While the micrometer screw can crush the egg membranes and return smaller readings of the thickness, the USG measurement can include the thickness of the inner and outer membranes in the overall reading. To ensure EMM and USG45 are the same trait from the selection point of view, we computed ad hoc Spearman rank correlations between the breeding values (Best Linear Unbiased Prediction) for EMM and USG45, which resulted in correlation coefficients of 0.97 and 0.99 in RIW and RIR, respectively. Also, the heritabilities of these traits (Table 3) are of similar magnitude, indicating that we are dealing with essentially the same trait, in any case.
Table 1. Effects fit (x) for particular measurements in the multiple-trait approach. Measurement: effect/type2
USG0, USG45, USG90, USG135, USG1801
EMM
x x x
x – x
year-of-hatch × hatch-within-year/F repetition-of-measurement-at-given-parallel/R additive genetic of hen/A
1 EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 F, fixed; R, random, diagonal; A, random, associated with the relationship matrix.
Table 2. Across-breed1 means, standard deviations, and range of eggshell thickness measurements.2 Trait
EMM USG0 USG45 USG90 USG135 USG180 1
RIW
RIR
Mean (μ m)
SD (μ m)
Min. (μ m)
Max. (μ m)
Mean (μ m)
SD (μ m)
Min. (μ m)
Max. (μ m)
315 407 412 421 423 420
31 43 35 37 41 44
178 211 246 213 176 229
399 742 635 748 757 756
312 403 403 415 414 409
31 38 33 36 36 43
192 249 256 260 250 236
398 817 645 631 618 636
RIW, Rhode Island White; RIR, Rhode Island Red. EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
Eggs were obtained from 2 purebred populations of Rhode Island White (RIW) and Rhode Island Red (RIR) laying hens maintained at a commercial breeding farm. The eggs were collected on the same day in the 33rd week of farm laying from 2,414 RIR and 4,525 RIW hens. Depending on the hatch number within a hatching season, the actual age of hens at measurement could be spread over 4 wk. Ultrasonic measurements were collected with a commercially available ultrasonic device (ORKA ESTG-1, ORKA Technology Ltd, Israel) designated for measuring the thickness of different materials, including eggshells. The manufacturer’s preset settings for the eggshell thickness option were applied. Beginning from the large end of the egg, the measurements were taken at 5 latitudes: 0o (USG0), 45o (USG45), 90o (USG90), 135o (USG135), and 180o (USG180). To estimate the repeatability of readings, measurements were repeated at each parallel on 3 meridians, which gave a total of 104,085 single measurements. Single electronic micrometer measurement (EMM) were taken with an electronic micrometer at random points over the equator toward the wide end (close to USG45) on eggs from 2,397 RIR and 4,447 RIW hens. The decision on which factors should be fit in the models for (co)variance component estimation followed introductory runs of ANOVA that aimed to find effects that could significantly affect the measurements. We eventually excluded effects of cage battery and floor in the battery. The effects fit in the multiple-trait (6 traits at a time) models are shown in Table 1.
Variance-covariance components were estimated with the AIREMLF90 software of Misztal et al. (2002). Convergence was set to
2362
KIBALA ET AL. Table 3. Heritability (h2 ) and repeatability (r2 ) of the eggshell thickness measurements.1 RIW2 2
EMM USG0 USG45 USG90 USG135 USG180
RIR2 2
2
h
SE
r
SE
h
SE
r2
SE
0.19 0.12 0.17 0.09 0.09 0.10
0.003 0.003 0.004 0.002 0.002 0.003
– 0.93 0.94 0.94 0.92 0.95
– 0.001 0.001 0.001 0.001 0.001
0.23 0.15 0.21 0.13 0.15 0.12
0.004 0.005 0.006 0.004 0.005 0.004
– 0.96 0.98 0.98 0.97 0.97
– 0.001 0.001 < 0.001 0.001 0.001
1 EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 RIW, Rhode Island White; RIR, Rhode Island Red.
EMM
EMM USG0 USG45 USG90 USG135 USG180
rg
SE
0.90 0.92 0.89 0.90 0.84
0.003 0.002 0.003 0.003 0.004
USG0
USG45
USG90
USG135
USG180
rg
SE
rg
SE
rg
SE
rg
SE
rg
SE
0.91
0.003
0.94 0.86
0.002 0.005
0.87 0.77 0.78 0.73
0.003 0.006 0.005 0.007
0.91 0.83 0.87
0.003 0.006 0.005
0.87 0.83 0.71
0.003 0.004 0.007
0.91 0.80 0.90 0.85
0.003 0.007 0.004 0.005
0.85 0.69
0.004 0.007
0.88 0.82 0.81 0.76 0.75
0.004 0.006 0.007 0.008 0.008
0.77
0.006
1
EMM, electronic micrometer measurement; USG0, at 0 ; USG45, at 45 ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 Rhode Island White, beneath diagonal; Rhode Island Red, above diagonal.
Shell thickness at 0 and 45o , regions belonging to the thinner part of eggshell, is clearly less than shell thickness toward the pointed end of the egg, and this differentiation of shell thickness is a well-known phenomenon (e.g., Sun et al., 2012). In the current study, heritability of eggshell thickness ranged from 9 to 19% in RIW and from 12 to 23% in RIR (Table 3). It has been documented that shell thickness can vary along an egg’s parallel (Tyler and Geake, 1964), which we accounted for by fitting measurement repetition in the model and including the resulting variance component in the phenotypic variance. The variation of eggshell thickness appears to be even higher along the egg’s meridian (Tyler and Geake, 1964), so we treated shell thicknesses at different latitudes as different, though correlated, traits. The highest heritability in both RIR and RIW was found for EMM, immediately followed by USG45 and then by USG0. Both USG0 and USG45 measurements were taken within thinner shell regions (Table 2), where hatching chicks break the shell. Thus, this region has to balance protecting egg contents and ease of hatching. Repeatability coefficients (Table 3), all of well above 0.90 hint at the recommendation of a single measurement upon thickness recording, should eggshell thickness be a selection criterion. Genetic correlations between thickness measurements (Table 4) can be considered high and very high, with the majority of the estimates exceeding 0.80. Nonetheless, some of them drop below 0.80, which is
o
o
considered the threshold value for differentiating traits (Robertson, 1959). Sun et al. (2012) proposed thickness at the pointed end as a representative measurement of eggshell thickness because it is the closest to the average thickness of the whole eggshell. While this approach could well suit a terminal egg production enterprise, we would recommend the USG45 measurement for selection purposes because it relates to a very crucial area of the eggshell that is important from both handling and hatchability points of view. The genetic correlation between USG45 and USG180 (pointed end) was found to be 0.71 and 0.81 in RIW and RIR, respectively, which is below and at the brink of the threshold for considering these measurements for different traits. Hence, direct selection for USG45 may enable more efficient balancing of hatchability against thickness; positive genetic correlations will cause an adequate reaction to selection for USG45 in other regions of the eggshell. This recommendation is further supported by the higher heritability of shell thickness in the larger end area. Moreover, because the genetic correlations fall well below unity, there is some room for independent selection of shell thickness in the pointed end region of eggshell, if such a need emerges.
Conclusion The repeatability of the measurement of USG45, which had the highest heritability of all USG measurements, and its biologically vital location on the eggshell that enables balancing shell strength and hatching ease,
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
Table 4. Genetic correlations (rg ) between thickness measurements1 in studied breeds.2
EGGSHELL THICKNESS FOR LAYER SELECTION
provide a basis for our recommendation to include USG45 as a selection criterion. Furthermore, due to high positive genetic correlations between shell thickness in different regions of the shell, its thickness at the pointed end of the egg will be influenced accordingly, in response to selection for USG45, although some space for independent genetic selection on the pointed end thickness exists.
REFERENCES
Misztal, I., S. Tsuruta, T. Strabel, B. Auvray, T. Druet, and D. H. Lee. 2002. Blupf90 and related programs (bgf90). Proc. 7th World Congr. Genet. Appl. Livest. Prod., Montpellier, France. CD-ROM communication 28:07. Nirasawa, K., H. Takahashi, H. Takeda, T. Furukawa, T. Takeda, and Y. Nagamine. 1998. Restricted maximum likelihood estimates of genetic parameters and genetic trends of chickens divergently selected for eggshell strength. J. Anim. Breed. Genet. 115: 375–381. Robertson, A. 1959. The sampling variance of the genetic correlation coefficient. Biometrics. 15:469–485. Rozempolska-Ruci´ nska, I., G. Zieba, M. L ukaszewicz, M. ´ Ciecho´ nska, A. Witkowski, and B. Slaska. 2011. Egg specific gravity in improvement of hatchability in laying hens. J. Anim. Feed. Sci. 20:84–92. ¨ Altan, A. Ipek, and B. Yilmaz. 2003. Effects of some Sahan, U., O. egg characteristics on the mass loss and hatchability of ostrich (Struthio camelus) eggs. Br. Poult. Sci. 44:380–385. Sun, C. J., S. R. Chen, G. Y. Xu, X. M. Liu, and N. Yang. 2012. Global variation and uniformity of eggshell thickness for chicken eggs. Poult. Sci. 91:2718–2721. Tyler, C., and F. H. Geake. 1964. The testing of methods for cracking egg shells, based on paired readings from individual eggs and the measurement of some effects of various treatments. Br. Poult. Sci. 5:19–28. Yan, Y.-Y., C.-J. Sun, L. Lian, J.-X. Zheng, G.-Y. Xu, and N. Yang. 2014. Effect of uniformity of eggshell thickness on eggshell quality in chickens. J. Poult. Sci. 51:338–342. Zhang, L.-C., Z.-H. Ning, G.-Y. Xu, Z.-C. Hou, and N. Yang 2005. Heritabilities and genetic and phenotypic correlations of egg quality traits in brown-egg dwarf layers. Poult. Sci. 84: 1209–1213.
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
Aboonajmi, M., S. K. Setarehdan, A. Akram, T. Nishizu, and N. Kondo. 2014. Prediction of poultry egg freshness using ultrasound. Int. J. Food Prop. 17:1889–1899. Abdallah, A.G, R. H. Harms, and O. El-Husseiny. 1993. Various methods of measuring shell quality in relation to percentage of cracked eggs. Poult. Sci. 72:2038–2043. Amer Eissa, A. H. 2009. Comparative eggshell stability assessment using three different non-destructive sensing instruments and breakage force strength. J. Food Eng. 93:444–452. Bennett, C. D. 1992. The influence of shell thickness on hatchability in commercial broiler breeder flocks. J. Appl. Poul. Res. 1:61–65. de Ketelaere, B., F. Bamelis, B. Kemps, E. Decuypere, and J. de Baerdemaeker. 2004. Non-destructive measurements of the egg quality. Worlds. Poult. Sci. J. 60:289–302. Lin, H., J.-W. Zhao, Q.-S. Chen, J.-R. Cai, and P. Zhou. 2009. Eggshell crack detection on acoustic impulse response and supervised pattern recognition. Czech Journal of Food Sciences. 27:393–402.
2363
Center for Nucleus Breeding MESSA Ltd., Mienia 100, 05–319 Ceglow, Poland; † Chair for Biological Bases of Animal Production, University of Life Sciences in Lublin, Akademicka 13, 20–950 Lublin, Poland; and ‡ Institute of Genetics and Animal Breeding, Jastrzebiec, Postepu 36A, 05–552 Magdalenka, Poland micrometer predominantly at the wider end of eggs from 2,397 RIR and 4,447 RIW hens. A multiple-trait statistical model fit the fixed effect of year-of-hatch × hatch-within-year, and random effects due to repeated measurements (except EMM) and an animal’s additive genetic component. The shell was thinnest in the region where chicks break it upon hatching (USG0, USG45). Heritabilities of shell thickness in different regions of the shell ranged from 0.09 to 0.19 (EMM) in RIW and from 0.12 to 0.23 (EMM) in RIR and were highest for USG45 and USG0. Because the measurement repeatabilities were all above 0.90, our recommendation for balancing egg strength against hatching ease is to take a single measurement of USG45. Due to high positive genetic correlations between shell thickness in different regions of the shell its thickness in the pointed end region will be modified accordingly, in response to selection for USG45.
Key words: eggshell thickness, USG measurement, heritability, genetic correlation 2015 Poultry Science 94:2360–2363 http://dx.doi.org/10.3382/ps/pev254
INTRODUCTION The eggshell is a barrier that is designed to protect the developing embryo, ensure proper gas exchange during its development, and provide it with an optimal environment. The relationship between a shell’s thickness, porosity, and strength (all often described by a single correlated measure of the egg’s specific gravity) and poultry hatchability has been demonstrated in many reports (e.g., Sahan et al., 2003 in ostriches and Rozempolska-Ruci´ nska et al., 2011 in chickens). Evident cracks or microcracks weaken protection against egg pathogen penetration, and thus eggs have reduced hatching and marketing qualities. The propensity to crack, conditioned by both genetic and environmental factors, is associated with an C 2015 Poultry Science Association Inc. Received March 8, 2015. Accepted July 14, 2015. 1 Supported by the National Center for Research and Development, Poland. Grant no. PBS2/B8/8/201. 2 Corresponding author: [email protected]
egg’s strength as determined indirectly by shell thickness (Amer Eissa, 2009). Genetic correlations between eggshell strength and its thickness are very high, exceeding 0.8 in some cases (Zhang et al., 2005), which makes eggshell thickness a valuable selection criterion. Yet, the most reliable method of measuring shell thickness is direct destructive measurement with a micrometer. However, modern technology allows for direct measurement without destroying the eggs, thus suitably maintaining them for further handling steps. One such technology is ultrasonic devices that are used, inter alia, to analyze egg content quality (Abdallah et al., 1993; Lin et al., 2009; Aboonajmi et al., 2014; Yan et al., 2014), although it is not used on a large scale (de Ketelaere et al., 2004). The use of such devices in genetic improvement of laying hens requires eggshell thickness measurement methodology, to provide reliable estimates obtained with the least effort. This study aimed to develop a methodology for using ultrasonic technology (USG) to record eggshell thickness for selection of layers.
2360
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
ABSTRACT This study aimed to develop a methodology for using ultrasonic technology (USG) to record eggshell thickness for selection of layers. Genetic correlations between eggshell strength and its thickness have been reported to be around 0.8, making shell thickness a selection index candidate element. Applying ultrasonic devices to measure shell thickness leaves an egg intact for further handling. In this study, eggs from 2 purebred populations of Rhode Island White (RIW) and Rhode Island Red (RIR) hens were collected on a single day in the 33rd week of the farm laying calendar from 2,414 RIR and 4,525 RIW hens. Beginning from the large end of the egg, measurements were taken at 5 latitudes: 0o (USG0), 45o (USG45), 90o (USG90), 135o (USG135), and 180o (USG180). To estimate the repeatability of readings, measurements were repeated at each parallel on 3 meridians. Electronic micrometer measurement (EMM) were taken with an electronic
2361
EGGSHELL THICKNESS FOR LAYER SELECTION
MATERIAL AND METHODS
RESULTS AND DISCUSSION Eggshell thickness is a trait correlated, among others, with egg weight, shell strength, and hatchability (Nirasawa et al., 1998; Sahan et al., 2003; Zhang et al., 2005; Rozempolska-Ruci´ nska et al., 2011). Shells that are too thick nor too thin may lead to impaired gas exchange and hatchability or to egg breakage (Bennett, 1992; Sahan et al., 2003). Mean thicknesses at different shell regions are presented in Table 2. The difference of more than 100 μm between EMM and the equivalent measurement of USG90 must have resulted from the specificity of tools used. While the micrometer screw can crush the egg membranes and return smaller readings of the thickness, the USG measurement can include the thickness of the inner and outer membranes in the overall reading. To ensure EMM and USG45 are the same trait from the selection point of view, we computed ad hoc Spearman rank correlations between the breeding values (Best Linear Unbiased Prediction) for EMM and USG45, which resulted in correlation coefficients of 0.97 and 0.99 in RIW and RIR, respectively. Also, the heritabilities of these traits (Table 3) are of similar magnitude, indicating that we are dealing with essentially the same trait, in any case.
Table 1. Effects fit (x) for particular measurements in the multiple-trait approach. Measurement: effect/type2
USG0, USG45, USG90, USG135, USG1801
EMM
x x x
x – x
year-of-hatch × hatch-within-year/F repetition-of-measurement-at-given-parallel/R additive genetic of hen/A
1 EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 F, fixed; R, random, diagonal; A, random, associated with the relationship matrix.
Table 2. Across-breed1 means, standard deviations, and range of eggshell thickness measurements.2 Trait
EMM USG0 USG45 USG90 USG135 USG180 1
RIW
RIR
Mean (μ m)
SD (μ m)
Min. (μ m)
Max. (μ m)
Mean (μ m)
SD (μ m)
Min. (μ m)
Max. (μ m)
315 407 412 421 423 420
31 43 35 37 41 44
178 211 246 213 176 229
399 742 635 748 757 756
312 403 403 415 414 409
31 38 33 36 36 43
192 249 256 260 250 236
398 817 645 631 618 636
RIW, Rhode Island White; RIR, Rhode Island Red. EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2
Downloaded from http://ps.oxfordjournals.org/ at Pennsylvania State University on November 15, 2015
Eggs were obtained from 2 purebred populations of Rhode Island White (RIW) and Rhode Island Red (RIR) laying hens maintained at a commercial breeding farm. The eggs were collected on the same day in the 33rd week of farm laying from 2,414 RIR and 4,525 RIW hens. Depending on the hatch number within a hatching season, the actual age of hens at measurement could be spread over 4 wk. Ultrasonic measurements were collected with a commercially available ultrasonic device (ORKA ESTG-1, ORKA Technology Ltd, Israel) designated for measuring the thickness of different materials, including eggshells. The manufacturer’s preset settings for the eggshell thickness option were applied. Beginning from the large end of the egg, the measurements were taken at 5 latitudes: 0o (USG0), 45o (USG45), 90o (USG90), 135o (USG135), and 180o (USG180). To estimate the repeatability of readings, measurements were repeated at each parallel on 3 meridians, which gave a total of 104,085 single measurements. Single electronic micrometer measurement (EMM) were taken with an electronic micrometer at random points over the equator toward the wide end (close to USG45) on eggs from 2,397 RIR and 4,447 RIW hens. The decision on which factors should be fit in the models for (co)variance component estimation followed introductory runs of ANOVA that aimed to find effects that could significantly affect the measurements. We eventually excluded effects of cage battery and floor in the battery. The effects fit in the multiple-trait (6 traits at a time) models are shown in Table 1.
Variance-covariance components were estimated with the AIREMLF90 software of Misztal et al. (2002). Convergence was set to
2362
KIBALA ET AL. Table 3. Heritability (h2 ) and repeatability (r2 ) of the eggshell thickness measurements.1 RIW2 2
EMM USG0 USG45 USG90 USG135 USG180
RIR2 2
2
h
SE
r
SE
h
SE
r2
SE
0.19 0.12 0.17 0.09 0.09 0.10
0.003 0.003 0.004 0.002 0.002 0.003
– 0.93 0.94 0.94 0.92 0.95
– 0.001 0.001 0.001 0.001 0.001
0.23 0.15 0.21 0.13 0.15 0.12
0.004 0.005 0.006 0.004 0.005 0.004
– 0.96 0.98 0.98 0.97 0.97
– 0.001 0.001 < 0.001 0.001 0.001
1 EMM, electronic micrometer measurement; USG0, at 0o ; USG45, at 45o ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 RIW, Rhode Island White; RIR, Rhode Island Red.
EMM
EMM USG0 USG45 USG90 USG135 USG180
rg
SE
0.90 0.92 0.89 0.90 0.84
0.003 0.002 0.003 0.003 0.004
USG0
USG45
USG90
USG135
USG180
rg
SE
rg
SE
rg
SE
rg
SE
rg
SE
0.91
0.003
0.94 0.86
0.002 0.005
0.87 0.77 0.78 0.73
0.003 0.006 0.005 0.007
0.91 0.83 0.87
0.003 0.006 0.005
0.87 0.83 0.71
0.003 0.004 0.007
0.91 0.80 0.90 0.85
0.003 0.007 0.004 0.005
0.85 0.69
0.004 0.007
0.88 0.82 0.81 0.76 0.75
0.004 0.006 0.007 0.008 0.008
0.77
0.006
1
EMM, electronic micrometer measurement; USG0, at 0 ; USG45, at 45 ; USG90, at 90o ; USG135, at 135o ; USG180, at 180o , all beginning from the large end of the egg. 2 Rhode Island White, beneath diagonal; Rhode Island Red, above diagonal.
Shell thickness at 0 and 45o , regions belonging to the thinner part of eggshell, is clearly less than shell thickness toward the pointed end of the egg, and this differentiation of shell thickness is a well-known phenomenon (e.g., Sun et al., 2012). In the current study, heritability of eggshell thickness ranged from 9 to 19% in RIW and from 12 to 23% in RIR (Table 3). It has been documented that shell thickness can vary along an egg’s parallel (Tyler and Geake, 1964), which we accounted for by fitting measurement repetition in the model and including the resulting variance component in the phenotypic variance. The variation of eggshell thickness appears to be even higher along the egg’s meridian (Tyler and Geake, 1964), so we treated shell thicknesses at different latitudes as different, though correlated, traits. The highest heritability in both RIR and RIW was found for EMM, immediately followed by USG45 and then by USG0. Both USG0 and USG45 measurements were taken within thinner shell regions (Table 2), where hatching chicks break the shell. Thus, this region has to balance protecting egg contents and ease of hatching. Repeatability coefficients (Table 3), all of well above 0.90 hint at the recommendation of a single measurement upon thickness recording, should eggshell thickness be a selection criterion. Genetic correlations between thickness measurements (Table 4) can be considered high and very high, with the majority of the estimates exceeding 0.80. Nonetheless, some of them drop below 0.80, which is
o
o
considered the threshold value for differentiating traits (Robertson, 1959). Sun et al. (2012) proposed thickness at the pointed end as a representative measurement of eggshell thickness because it is the closest to the average thickness of the whole eggshell. While this approach could well suit a terminal egg production enterprise, we would recommend the USG45 measurement for selection purposes because it relates to a very crucial area of the eggshell that is important from both handling and hatchability points of view. The genetic correlation between USG45 and USG180 (pointed end) was found to be 0.71 and 0.81 in RIW and RIR, respectively, which is below and at the brink of the threshold for considering these measurements for different traits. Hence, direct selection for USG45 may enable more efficient balancing of hatchability against thickness; positive genetic correlations will cause an adequate reaction to selection for USG45 in other regions of the eggshell. This recommendation is further supported by the higher heritability of shell thickness in the larger end area. Moreover, because the genetic correlations fall well below unity, there is some room for independent selection of shell thickness in the pointed end region of eggshell, if such a need emerges.
Conclusion The repeatability of the measurement of USG45, which had the highest heritability of all USG measurements, and its biologically vital location on the eggshell that enables balancing shell strength and hatching ease,
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Table 4. Genetic correlations (rg ) between thickness measurements1 in studied breeds.2
EGGSHELL THICKNESS FOR LAYER SELECTION
provide a basis for our recommendation to include USG45 as a selection criterion. Furthermore, due to high positive genetic correlations between shell thickness in different regions of the shell, its thickness at the pointed end of the egg will be influenced accordingly, in response to selection for USG45, although some space for independent genetic selection on the pointed end thickness exists.
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