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Correlated Response in Shell and Albumen Quality with Selection for Increased Egg Production
BREEDING AND GENETICS Correlated Response in Shell and Albumen Quality with Selection for Increased Egg Production D. G. POGGENPOEL Department of Poultry Science, University of Stellenbosch, Stellenbosch 7600, South Africa (Received for publication July 1, 1985) ABSTRACT Egg shell and albumen quality at the age of 34 weeks were measured over a period of 6 years in a closed White Leghorn flock selected for increased half-year egg production. Heritabilities estimated from the sire plus dam components were: albumen height, .48 ± .07; Haugh units, .40 ± .06; specific gravity score, .43_± .06; and shell thickness, .37 ± .06. Estimates from the sire component were consistently higher than estimates from the dam component. Heritabilities of measurements at the age of 57 weeks were lower than at 34 weeks, but the genetic correlations between measurements at these two ages were .7 and .8. With repeatabilities ranging from .50 to .58 for these traits, 7 to 9 eggs/pullet would be required for an accuracy of measurement of .90. The shell and albumen quality traits had genetic correlations of the order of .10 with sexual maturity, —.20 with egg production, and .10 with egg weight. Specific gravity score measured with the flotatiorfmethod had a genetic correlation of .87 ± .03 with shell thickness. Ten generations after a genetic control line was established from the selection line, the average differences between these two lines remained relatively stable over a period of 6 years and were: half-year egg production, 28.1 eggs, albumen height —.54, Haugh units —2.22, specific gravity score —.62, and shell thickness —2.22 X .01 mm. The genetic correlations between half-year egg production and egg quality traits calculated from the direct and correlated selection responses were: albumen height —.41, Haugh units —.25, specific gravity score —.29, and shell thickness —.51. The average correlated selection differentials over 5 years were: albumen height —.07 mm, Haught units —.52, specific gravity score —.06, and egg shell thickness —.2 X .01 mm. It is suggested that small selection pressures for these traits would maintain their original levels. (Key words.- genetics, shell quality, albumen quality, egg production) 1986 Poultry Science 65:1633-1641 INTRODUCTION Egg breakage causes substantial financial losses t o t h e egg p r o d u c e r and t h e industry. Estimates for t h e incidence of b r o k e n eggs for t h e United Kingdom, G e r m a n y , and t h e United States range from 6 t o 8% of all eggs p r o d u c e d (Washburn, 1 9 8 2 ) . A loss of a p p r o x i m a t e l y $667 million annually for t h e United States and Canada was estimated by H u n t o n ( 1 9 8 2 ) . T o limit these losses, egg shell strength is receiving considerable a t t e n t i o n b y breeders and producers. Specific gravity was found t o be closely related t o shell breaking strength (Holder and Bradford, 1 9 7 9 ; P o t t s a n d Washburn, 1 9 8 3 ) with a correlation of nearly .8 with shell thickness (Foster and Weatherup, 1979). Because of these relationships and also because it is easy t o measure and is a n o n d e s t r u c t i v e m e t h o d , t h e m e a s u r e m e n t of specific gravity is p r o b a b l y t h e m o s t widely u s e d m e a s u r e m e n t of shell strength b y breeders ( G a r w o o d et al., 1979).
from a review of t h e literature b y V a n Tijen and Kuit ( 1 9 7 0 ) , it is believed t h a t this trait can be influenced b y breeding. This is s u p p o r t e d b y positive responses for shell quality in a n u m b e r of selection e x p e r i m e n t s . T a y l o r and Lerner ( 1 9 3 9 ) selected for and against shell thickness; Quinn et al. ( 1 9 4 5 ) selected for increased a n d decreased egg weight loss; Pevzner et al. ( 1 9 7 6 ) selected for low shell d e f o r m a t i o n ; Buss et al. ( 1 9 7 7 ) established good-shell a n d poor-shell lines on specific gravity m e a s u r e m e n t s ; a n d V a n Tijen ( 1 9 7 7 ) selected for an index of shell thickness, d e f o r m a t i o n , and specific gravity. Parsons and C o m b s ( 1 9 7 8 ) m e n t i o n e d selection for differing egg shell b r e a k i n g strengths; G a r w o o d et al. ( 1 9 7 9 ) selected bidirectionally for egg breakage r a t e ; Poggenpoel ( 1 9 8 0 ) selected for increased and decreased specific gravity; H a r t m a n n et al. ( 1 9 8 1 ) practiced bidirectional selection for egg specific gravity, shell d e f o r m a t i o n , a n d b r e a k i n g strength, respectively in different lines; and McPhee et al. ( 1 9 8 2 ) selected for high specific gravity.
Because of t h e reasonably high average heritability of .39 for shell quality, estimated
With inclusion of shell quality in a selection program, t h e genetic correlation of this trait
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1634
POGGENPOEL
with egg production is of importance. Van Tijen and Kuit (1970) estimated an average value of —.12 from the literature, and Rodda (1972) found values of —.06 and .13 in two flocks. It is generally accepted that consumers prefer an egg with a large proportion of firm albumen that will stand up high around the yolk of the broken-out egg (Wells, 1968). A large proportion of thick egg white is also deisrable for foaming and heat coagulation of the white protein for cake making (Shenstone, 1968). The most widely used criterion of albumen quality is the Haugh unit (Haugh, 1937), which is the logarithm of albumen height corrected for egg weight. Van Tijen and Kuit (1970) estimated the average heritability of internal quality from the literature as .38. A few selection experiments confirm that albumen quality can be changed with selection. Lorenz et al. (1934) selected for high and low percentage of firm albumen, and Lorenz and Taylor (1940) reported on further response in these lines. Knox and Godfrey (1940) selected for high and low percentage of thick albumen and Poggenpoel (1982) for increased and decreased Haugh units. A feature of all these experiments was that the response in the high lines was about four times more than in the low lines, as measured on the natural scale. In their review of the literature, Van Tijen and Kuit (1970) estimated the average genetic correlation between albumen quality and egg production as —.077. In this study, the correlated response in shell and albumen quality over 6 years (1971 to 1976) in a line selected for increased egg production will be given, as well as the estimated genetic parameters of these and other egg production traits.
MATERIALS AND METHODS
The two flocks used in this study were the closed White Leghorn flock of the University of Stellenbosch, selected for increased egg production since 1953, and the genetic control flock that originated as a random sample from the selection line in 1962 (Poggenpoel and Erasmus, 1978). These two flocks are hatched at the same date annually, housed in individual laying cages in the same house, and receive the same feeding and management treatment. The criterion of selection was half-year egg produc-
tion index per hen-housed to the age of 275 days. Females were selected on an index that combined individuals, full sisters' and half sisters' performances (Osborne, 1957a), whereas males were selected on an index of full and half sisters' performance (Osborne, 1957b). Egg production traits measured on a routine basis were: sexual maturity as age in days at first egg, half-year egg production as number of eggs produced to the age of 275 days, and total egg production as eggs produced to the age of 500 days. Total egg production was not measured in 1972, and for 1974, no production measurements were available. In addition, measurements of egg quality were taken for 5 consecutive days on an average of 3.2 eggs/pullet at the age of about 34 weeks for pullets of the selection line hatched in 1971 through 1976. For progreny of the two hatching years, 1973 and 1976, these measurements were repeated at the age of 57 weeks. In the control flock, egg quality was measured concurrently on one egg per pullet from about 70 pullets in 1972, 1974, and 1976. All egg quality measurements were taken on the day after being laid. Specific gravity (SG) of the egg was measured by the flotation method with 10 salt solutions ranging in specific gravity from 1.074 to 1.110 with increments of .004. The SG of the solutions were checked every morning before the measuring period of about 2 hr. Eggs were assigned a SG score (1 to 10), corresponding to the solution in which they were first observed to float. All calculations were done on the SG score. True specific gravity can be estimated as 1.074 + (SG score X .004). Albumen height was measured with a tripod screw micrometer to .1 mm immediately after breaking and Haugh units determined with the internal quality calculator as proposed by Brant et al. (1951). Shells of the broken eggs were rinsed in clean water, air dried at room temperature for 1 day, and shell thickness, with the membranes included, was measured by means of a spring-loaded thickness gauge to the nearest .01 mm. The thickness of the shell was measured at the equatorial region, but in the first year, measurements were also taken at the blunt and sharp ends of the shell. In the first year, the dried shells including membranes were also weighed, and shell weight per unit surface area was calculated from the relationship reported by Mueller and Scott (1940) where surface area (cm 2 ) = 4.67 (eggweight) 2 ' 3 .
EGG PRODUCTION AND EGG QUALITY
1635
Egg weight was the weight recorded at the time of the measurement for egg quality. Repeatabilities were calculated on measurements of successive eggs from the same pullet for hatching years 1971 and 1976, for hens with three measurements per trait. All other estimates of shell and egg quality were based on hen means of an average of 3.2 eggs/hen. Heritabilities and genetic correlations were estimated through unbalanced halfand full-sib analysis (Becker, 1967). Pooled estimates were calculated by pooling degrees of freedom and weighted mean sums of squares and products over years. V
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RESULTS AND DISCUSSION
Means. The means and standard deviations of traits measured in the selection flock, together with the number of sires, dams, and progeny measured in each hatching year, are presented in Table 1. Egg quality values are all based on pullet means. The average numbers per line in the control line was about 50 sires, 150 dams, and 250 pullets housed. The second values for egg quality traits in years 1973 and 1976 are the means at 57 weeks of age, while all the rest are for 34 weeks of age. There was a decrease of about 1.0 mm in albumen height and a decrease of 10 Haught units when hen age increased from 34 to 57 weeks. This is in agreement with results of Nagai and Gowe (1969) and Orr (1973), although the decline in the present study tends to be steeper and more like that of Rodda (1972). The SG score tended to decrease with about one score (.004) over this period, which corresponds to decreases found by Nagai and Gowe (1969), Rodda (1972), Potts and Washburn (1974), and Hamilton (1978). Egg shell thickness showed no decline. This result is in contrast to expectancy (Rodda, 1972; Hamilton, 1978), particularly because mean egg weight increased over this period from about 50 to 55 g, and Roland (1980) found proportional less increase in shell deposition than in egg size with drastic increases in egg size. A similar result where shell weight per unit of surface area remained relatively unchanged in one line over a period of 7 months was found by Perek and Snapir (1970). For the pullets of Hatching Year 1971, the mean egg shell weight was 4.88 ± .43 (SD), and the mean egg shell thickness was 74.6 ± 5.3 mg/cm 2
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1636
POGGENPOEL
Repeatabilities. The pooled repeatabilities for shell measurements at 34 weeks of age for pullets of hatching year 1971 and 1976 are presented in Table 2. Only pullets with three measurements for a trait were included, and the total number for the various traits varied from 313 to 321 for 1971 and from 350 to 363 for 1976 hatching year. The within-year estimates for the different traits were: albumen height (.52 and .63), Haugh units (.36 and .74), SG score (.59 and .56) egg shell thickness (.38 and .57). Repeatability estimates of the present study tend to be lower than most other estimates reported in the literature. The pooled estimate of .58 for albumen height is lower than the value of about .79 of Nagai and Gowe (1969). The value of .53 for Haugh units is lower than the estimate of about .78 of Nagai and Gowe (1969), but falls within the range of .40 to .70 found by Rodda (1972). The present estimate of .58 for shell thickness tends to be slightly lower than the values of .66 by Nagai and Gowe (1969), .57 to .64 by Rodda (1972), and .60 by McPhee et al. (1982). The estimate of .50 for egg shell thickness is also lower than the estimate of .69 by Rodda (1972). There was a tendency for measurements to be more accurate at the later stage in 1976 after more experience had been gained. Heritabilities. Pooled estimates of heritabilities were based on data of 122 sires, 637 dams, and 2354 progeny and are presented in Table 2, together with heritabilities of egg production traits. The heritabilities of albumen quality and Haugh units are in agreement with the average
of .38 for internal egg quality from Van Tijen and Kuit (1970). The estimates of heritability for egg specific gravity and shell thickness correspond closely to the average of .39 from the literature by Van Tijen and Kuit (1970). The estimate for SG score is also in line with the estimates of the Rhode Island Red flock of Rodda (1972) but is higher than the values found for his White Leghorn flock. It is evident from the values in Table 2 that the heritabilities for egg quality traits are reasonably high in this flock. This was proved by two two-way selection experiments that were started with samples of the progeny of Hatching Year 1971. After six generations of selection for high and low SG score, the difference between the two lines was .013 in specific gravity (Poggenpoel, 1980). The difference between two lines selected for increased and decreased Haugh units was 17.1 units (Poggenpoel, 1982). For all egg quality traits, the heritabilities estimated from the sire components of variance are larger than the estimates from the dam components, which reflected sex-linked effects of .2 to .3 for all these traits. The practical implication is that the sire has more influence than the dam on the egg quality of their daughters. In the flocks of Rodda (1972), there were only small differences between heritability estimates from the sire and dam components. In the three White Leghorn flocks of Nagai and Gowe (1969), the average heritabilities from the dam components appear to be larger than from the sire components in most cases. From the results of these three studies, there is apparently no general trend for the size of
TABLE 2. Pooled heritabilities (± SE) and repeatabilities of traits in the selection line
S+D Albumen height Haugh units SG 2 score Shell thickness Egg weight Half year production Total production Sexual maturity 1 2
S = Sire, D = dam. SG = Specific gravity.
.65 .51 .62 .45 .71 .29 .06 .29
± .12 ± .10 ± .11 ± .09 ± .15 ± .07 ± .05 ± .07
.31 .28 .25 .29 .35 .12 .37 .13
± .09 + .09 ± .09 ± .09 ± .11 ± .09 ± .12 ± .09
.48 .40 .43 .37 .53 .21 .21 .21
± .07 ± .06 t .06 + .06 ± .09 ± .05 ± .05 ± .05
Repeatability .58 .53 .58 .50
EGG PRODUCTION AND EGG QUALITY
heritabilities estimated from the different components for egg quality traits. Of the hens of Hatching Year 1971, additional measurements of albumen height and Haugh units were taken on eggs after they were stored for a period of 2 weeks at room temperature. Means of these eggs were lower than that of the fresh eggs. For albumen height, it was 2.6 mm vs. 5.2 mm and for Haugh units, 45.8 vs. 74.3. The heritabilities ( h | + D ) estimated on these stored eggs were for both traits very similar to the estimates on the fresh eggs, viz. .51 ± .18 (.38 ± .13) for albumen height and .52 ± .18 (.43 ± .13) for Haugh units. The genetic correlations between hen means of fresh eggs and stored eggs were .86 for both traits. According to these results, eggs need not necessarily be fresh when measuring albumen quality for selection purposes. From hens of the 2 hatching years, 1973 and 1976, egg quality measurements were again taken at the age of 57 weeks. The number of hens measured was slightly less than that at 34 weeks of age (Table 1), but the number of eggs per hen was about the same. Heritabilities for these 2 years were pooled for measurements at 57 weeks of age. Heritabilities for the measurements at 34 weeks of age for these 2 years were also pooled and given in brackets. The estimates, h | + ^ were: albumen height, .23 ± .11 (.44 ± .12); Haugh units, .31 ± .10 (.42 ± .11); specific gravity, .19 ± .09 (.31 ± .09); and shell thickness, .11 ± .09 (.29 ± .09). Heritabilities for all traits declined over the laying year, indicating greater accuracy of selection early in the laying year as was also found by Nagai and Gowe (1969) and Rodda (1972). Genetic correlations between measurement at 34 and 57 weeks of age were between .7 and .8 for all the traits and support the suggestion that breeding values over the whole year are closely related (Nagai and Gowe, 1969; Rodda, 1972). Correlations. For pullets of Hatching Year 1971, the genetic and phenotypic correlations of measurements of shell thickness at the equator with measurements at the blunt end were .95 and .87, respectively, and with measurements at the small end, .93 and .81. Correlations between measurements correspond to the results of Tyler and Geake (1965) who found phenotypic correlations generally greater than .80 between measurements in different areas of the shell. The phenotypic correlations between milligrams shell per square centimeter for 1971 pullets was estimated as .82 with
1637
SG score as well as with measured shell thickness at the equator. With these high correlations, measurement of shell thickness at a convenient area like the equator seems sufficient as an indication of overall shell thickness. The pooled genetic, phenotypic, and environmental correlations between the different traits are presented in Table 3. The large standard errors of the genetic correlations, in spite of the relatively large numbers in the analysis, underline the general problem of obtaining reliable estimates of genetic correlations. Genetic correlations from the sire plus dam components, environmental, and phenotypic correlations do not differ much, indicating that genetic and environmental factors generally influence these traits in the same direction. Sexual maturity has a low genetic correlation in the order of .10 with all egg quality traits. This is in line with the average of two estimates of .305 between sexual maturity and shell quality given by Van Tijen and Kuit (1970). Half-year production as well as total egg production show a genetic correlation in the order of —.20 with albumen and shell quality. This is close to the values of —.2 to —.3 found in an extensive investigation with large numbers by Emsley et al. (1977). It is also in reasonable agreement with the average correlation values of —.116 with shell quality and —.077 with internal quality as estimated by Van Tijen and Kuit (1970) and with the values of about —.10 for shell thickness in the Rhode Island Red flock of Rodda (1972). The conclusion from these estimates is that positive selection for shell and albumen quality will, in most flocks, have an inhibiting effect on progress in number of eggs. The genetic correlations of egg weight with the two measurements of albumen quality (.27 and .03), as well as the two measurements of egg shell quality (.05 and .22), are variable. On average, the estimates seem to agree with the low values of Van Tijen and Kuit (1970) of .100 between egg weight and shell quality and —.031 between egg weight and internal quality. The genetic correlations between egg weight and shell thickness of —.20 to .47 reported by Rodda (1972) appear to be higher than in these other studies. As Haugh units is a measurement of albumen height corrected for egg weight, the high genetic correlation of .95 between Haugh units
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POGGENPOEL
T A B L E 3. Pooled correlations
between
the different r
A(S + D)1
traits in the selection r
P2
line 'E3
Sexual m a t u r i t y
X X X X
a l b u m e n height Haugh units SG score shell thickness
.14 .08 .09 .15
± .13 ± .13 ± .13 ± .14
.11 .08 .06 .09
.03 .02 .03 .11
Half year p r o d .
X X X X
a l b u m e n height Haugh units SG 4 score shell thickness
.29 .22 .16 .22
± .13 + .13 ± .13 + .13
-.16 -.13 -.12 -.12
-.10 -.12 -.12 -.13
Total production
X X X X
a l b u m e n height Haugh units SG score shell thickness
.36 .29 .13 .16
± ± ± ±
.12 .12 .13 .13
-.09 -.06 -.08 -.09
.11 .08 -.21 -.22
Egg eight
X X X X
a l b u m e n height Haugh u n i t s SG score shell thickness
.27 .03 .05 .22
+ ± ± +
.11 .10 .11 .11
.20 -.03 .01 .21
.30 .06 .01 .20
A l b u m e n height
X Haugh units X SG score X shell thickness
.95 ± .01 .04 ± .10 .14 ± .11
.83 .04 .04
.80
.09 .10
Haugh units
X SG score X shell thickness
.12 ± .10 .13 ± .11
.03 .01
.04 .02
SG score
X shell thickness
.87 ± .03
.77
.78
1
*MS + D) = Genetic correlation from sire plus d a m c o m p o n e n t s .
2
rp = P h e n o t y p i c correlation.
3
r g = Environmental correlation.
4
SG = Specific gravity.
and albumen height is not surprising. Albumen height, as well as Haugh units, have genetic correlations in the order of .10 with shell quality, which is in accordance with the average value of .116 of Van Tijen and Kuit (1970). The genetic correlation between specific gravity and shell thickness is important as it gives an indication of the accuracy of the easy nondestructure flotation method as a measurement of real shell thickness. The value of .87 shows a close relationship between these two traits in this flock and corresponds to values of around .80 and higher in the flocks of Rodda (1972). The relationship between these two traits in this flock is confirmed by the response with two-way selection for SG score. After six generations of selection, the difference of .013 in SG between the two lines was accompanied by a correlated difference of .067 mm in shell thickness (Poggenpoel, 1980). Selection Response. Deviations of means of the selection line from means of the control
line for half-year egg production (the criterion of selection) and for the egg quality traits are presented in Table 4. Deviations of half-year production suggest that there was no further selection response over the period 1972 to 1976. This situation (1972) was reached 10 years after establishment of the control line and after 10 generations of selection on half-year egg production in the selection line (Poggenpoel and Erasmus, 1978). The selection response of 28.10 eggs in half-year production was accompanied by consistent small negative responses in albumen and shell quality. According to the estimates of negative genetic correlations, a negative correlated response in these traits was expected. Van Tijen (1977) also found in his selection lines that stronger shells were obtained at the expense of number of eggs. With the available data, it was possible to calculate from direct and correlated selection responses, genetic correlations between half-
EGG PRODUCTION AND EGG QUALITY
1639
TABLE 4. Deviations of means of the selection line from means of the control line Half-year production eggs
Year of hatch
Albumen height
Haugh units
SG1 score
Shell thickness X .01 mm
(mm) 1972 1973 1974 1975 1976 Mean % of Control mean 1
29.3 29.9 27.3 25.9 28.10 50.2
-.69
-3.76
-.59
-2.96
-.25
-.18
-.47
-2.10
-.68 -.54 -8.6
-2.72 -2.22 -2.8
-.80 -.62 -13.4
-1.6 -2.22 -5.9
SG = Specific gravity.
year egg production and the egg quality traits from the relationship: rA
CR, RX
J
AX
>AY that is obtained with rearrangement from Falconer (1981, p. 286), where r^. is the genetic correlation between two characters, R x is the selection response in character X, CRy the correlated response in character Y, and 6 A is the genetic standard deviation. The different values of R are obtained from Table 4, and values of 6 A are calculated from the respective heritabilities (sire plus dam components) and the standard deviations. The calculated genetic correlations between half-year egg production and the different egg quality traits, with the comparable genetic correlations estimated from the sire plus dam components in brackets, are as follows: albumen height, —.41 (—.29); Haugh units, - . 2 5 ( - . 2 2 ) ; SG score, - . 2 9 (-.16); shell thickness-.51 (-.22). All the genetic correlations calculated from selection response have greater numeric values than those estimated from the sib analysis. Predicted correlated responses would consequently be less than the responses obtained. Falconer (1981) mentioned that close agreement between observed and predicted correlated responses is not often found because of two reasons, viz. the low precision of estimates of genetic correlations and their sensitivity to gene frequency changes. The genetic correlations estimated from the selection responses confirm the negative association between egg numbers and egg quality traits in this flock. However, from the relatively
small correlated responses after 10 generations of selection (Table 4), it appears as if the original levels of these traits could have been maintained with slight selection pressure. It is possible that the negative correlated response in egg and shell quality, with selection for increased egg production, may be smaller, or even absent, in other lines. Washburn et al. (1981) found that differences in shell strength between their lines were not associated with differences in daily egg mass produced from 154 to 294 days of age, which was the trait of selection. Because of the small negative phenotypic correlations of the order of —.10, between half-year egg production and the egg quality traits (Table 3), a correlated negative selection differential in these traits is expected with selection for half-year production. These correlated selection differentials were calculated for the 5 years of 1971 to 1976 with 1974 not included. The mean selection differentials, with the range of values in brackets, are: half-year egg production, 11.12 eggs (8.4 to 15.6); albumen height, —.07 mm (—.19 to - . 0 4 ) ; Haugh units, - . 5 2 (-.92 to .04); specific gravity score, —.06 (—.38 to —.18); and egg shell thickness, - . 2 0 X .01 mm ( - . 8 to .2 X .01 mm). The relatively small correlated selection differentials in comparison to that of egg numbers strengthens the belief that the negative responses in the egg quality traits could have been prevented with only small selection pressures for these traits. An indication of this necessary correlated selection differential (Sy) can be calculated from the relationship C R y /
1640
POGGENPOEL
S
Y = r A h X h Y / r P ( F a l c o n e r , 1 9 8 1 , p. 2 8 8 ) . F r o m the available parameters, it was estimated t h a t t o have obtained a positive correlated response of a b o u t 1% in each of t h e egg quality traits, t h e following correlated selection differentials would have been required: albumen height, .03 m m ; Haugh units, . 5 0 ; SG score, . 0 2 ; shell thickness, .19 X .01 m m . H u n t o n ( 1 9 8 2 ) presented results of seven commercial flocks believed t o be present in summaries of t h e 1 9 6 5 as well as 1978 American r a n d o m sample tests. On average, t h e SG score of these flocks over this period of 13 years, as deviation from t h e Kentville control, did not change. This gives an indication of how shell quality in commercial flocks is maintained, or even increased, t o g e t h e r with select i o n response for increased egg p r o d u c t i o n . T h e estimated correlations as well as t h e selection responses of this s t u d y prove a negative genetic association b e t w e e n n u m b e r of eggs p r o d u c e d and shell and a l b u m e n quality, as was f o u n d in most o t h e r flocks investigated. T h e size of t h e correlated responses and correlated selection differentials however, suggests t h a t these egg quality traits can be maintained at a satisfactory level with slight selection pressure and a small sacrifice of egg n u m b e r s .
ACKNOWLEDGMENTS The a u t h o r gratefully acknowledges the technical assistance of W. R o d e , M. R u n c i m a n n , J. R o b y n , and D. J. van P a p e n d o r p .
REFERENCES Becker, W. A., 1967. Manual of Procedures in Quantitative Genetics. Washington State Univ. Press, Pullman, WA. Brant, A. W., A. W. Otte, and K. H. Norris, 1951. Recommended standards for scoring and measuring open egg quality. Food Technol. 5: 356-361. Buss, E. G., R. M. Leach, Jr., and J. T. Stout, 1977. Eggshell quality for chickens in selected lines, F, 's, and F 2 's. Poultry Sci. 56:1699-1700. Emsley, A., G. E. Dickerson, and T. S. Kashyap, 1977. Genetic parameters in progeny-test selection for field performance of strain-cross layers. Poultry Sci. 56:121-146. Falconer, D. S., 1981. Introduction to Quantitative Genetics. Longman, London, England and New York, NY. Foster, W. H., and S.T.C. Weatherup, 1979. The use of specific gravity of the egg to estimate shell thickness. Br. Poult. Sci. 2 0 : 4 3 9 - 4 4 3 . Garwood, V. A., P. C. Lowe, and C. G. Haugh, 1979. Method for improving egg-shell strength by selection. Br. Poult. Sci. 20:289-295.
Hamilton, R.M.G., 1978. Observations on the changes in physical characters that influence egg shell quality in ten strains of White Leghorns. Poultry Sci. 57:1192-1197. Hartmann, W., G. Heil, and H.-W. Rauch, 1981. Formation of lines differing in egg shell quality traits to investigate their relevance for the frequency of broken eggs. In Quality of Eggs. Proc. 1st Eur. Symp., Apeldoorn. Haugh, R. R., 1937. The Haugh unit for measuring egg quality. US Egg and Poult. Magazine 43:522— 555 and 572. Holder, D. P., and M. V. Bradford, 1979. Relationship of specific gravity of chicken eggs to number of cracked eggs observed and percent shell. Poultry Sci. 5 8 : 2 5 0 - 2 5 1 . Hunton, P., 1982. Genetic factors affecting egg shell quality. World's Poult. Sci. J. 38:75-84. Knox, C. W., and A. B. Godfrey, 1940. Five years of breeding for high and low percentage of thick albumen in the eggs of Rhode Island Reds. Poultry Sci. 19:291-294. Lorenz, F. W., and L. W. Taylor, 1940. The inheritance of an albumen quality characteristic of chicken eggs. J. Agric. Res. 6 1 : 2 9 3 - 3 0 1 . Lorenz, F. W., L. W. Taylor, and H. J. Almquist, 1934. Firmness of albumen as an inherited characteristic. Poultry Sci. 13:14-17. McPhee, C. P., H. W. Burton, and D. E. Fuelling, 1982. Selection for high specific gravity of eggs in a flock of Australorp hens. Br. Poult. Sci. 23: 215-223. Mueller, C. D., and H. M. Scott, 1940. The porosity of eggshell in relation to hatchability. Poultry Sci. 19:163-166. Nagai, J., and R. S. Gowe, 1969. Genetic control of egg quality. 1. Sources of variation. Br. Poult. Sci. 10:337-350. Orr, H. L., 1973. Eggs and Egg Products. Publication 1498, Canada Dept. Agric. Osborne, R., 1957a. The use of sire and dam family averages in increasing the efficiency of selective breeding under a hierarchical mating system. Heredity 11:93-116. Osborne, R., 1957b. Family selection in poultry: the use of sire and dam family averages in choosing male parents. Proc. Royal Soc. Edinburgh, Vol. 66(4): 374-393. Parsons, A. H., and G. F. Combs, Jr., 1978. Differences in calcium metabolism in two SCWL strains producing high (HES) or low (LES) breaking strength eggs. Poultry Sci. 57:1178. (Abstr.) Perek, M., and N. Snapir, 1970. Interrelationships between shell quality and egg production and egg and shell weight in White Leghorn and White Rock hens. Br. Poult. Sci. 11:133-145. Pevzner, I. Y., G. W. Friars, H. L. Orr, and B. S. Basehart, 1976. The use of selection and strain crossing to reduce egg shell breakage. Br. Poult. Sci. 17:185-194. Poggenpoel, D. G., 1980. Two-way selection for egg specific gravity. Proc. 6th Eur. Poult. Conf., Hamburg, Vol. 2:122-129. Poggenpoel, D. G., 1982. Two-way selection for egg albumen quality. S. Afr. J. Anim. Sci. 12:389— 391.
EGG PRODUCTION AND EGG QUALITY Poggenpoel, D. G., and J. E. Erasmus, 1978. Longterm selection for increased egg production. Br. Poult. Sci. 19:111-123. Potts, P. L., and K. W. Washburn, 1974. Shell evaluation of white and brown egg strains by deformation, breaking strength, shell thickness and specific gravity. Poultry Sci. 53:1123-1128. Potts, P. L. and K. W. Washburn, 1983. The relationship of age, method of measuring and strain on variation in shell strength. Poultry Sci. 62: 239-246. Quinn, J. P., C. D. Gordon, and A. G. Godfrey, 1945. Breeding for egg shell quality as indicated by weight loss. Poultry Sci. 24:399-403. Rodda, D. D., 1972. Breeding for late egg shell quality in the domestic hen. Br. Poult. Sci. 13:45-60. Roland, D. A., Sr., 1980. The ability of young and old hens to change shell deposition with sudden natural drastic changes in egg size. Poultry Sci. 59:924-926. Shenstone, F. S., 1968. The gross composition, chemistry and physico-chemical basis of organization of the yolk and white. In Egg Quality. T. C. Carter, ed. Oliver and Boyd, Edinburgh, Scotland.
1641
Taylor, L. W., and I. M. Lerner, 1939. Inheritance of shell thickness in White Leghorn pullets. J. Agric. Res. 58:383-396. Tyler, C , and F. H. Geake, 1965. Egg shell thickness patterns as shown by individual domestic hens. Br. Poult. Sci. 6:235-243. Van Tijen, W. F., 1977. Shell quality as seen from the breeder's viewpoint. 1. Improvement reached after four years of selection and the effect on productivity. Poultry Sci. 56:211—218. Van Tijen, W. F., and A. R. Kuit, 1970. The heritability of characteristics of egg quality, their mutual correlation and the relationship with productivity. Arch. Gefliigelkd. 34:201-210. Washburn, K. W., 1982. Incidence, cause and prevention of egg shell breakage in commercial production. Poultry Sci. 61:2005-2012. Washburn, K. W., G. S. Brah, and H. L. Marks, 1981. Selection for egg mass in the domestic fowl. 3 Changes in shell strength. Poultry Sci. 6 0 : 1 7 8 8 1791. Wells, R. G., 1968. The measurement of certain egg quality characters: A review. In Egg Quality. T. C. Carter, ed. Oliver and Boyd, Edinburgh, Scotland.
from a review of t h e literature b y V a n Tijen and Kuit ( 1 9 7 0 ) , it is believed t h a t this trait can be influenced b y breeding. This is s u p p o r t e d b y positive responses for shell quality in a n u m b e r of selection e x p e r i m e n t s . T a y l o r and Lerner ( 1 9 3 9 ) selected for and against shell thickness; Quinn et al. ( 1 9 4 5 ) selected for increased a n d decreased egg weight loss; Pevzner et al. ( 1 9 7 6 ) selected for low shell d e f o r m a t i o n ; Buss et al. ( 1 9 7 7 ) established good-shell a n d poor-shell lines on specific gravity m e a s u r e m e n t s ; a n d V a n Tijen ( 1 9 7 7 ) selected for an index of shell thickness, d e f o r m a t i o n , and specific gravity. Parsons and C o m b s ( 1 9 7 8 ) m e n t i o n e d selection for differing egg shell b r e a k i n g strengths; G a r w o o d et al. ( 1 9 7 9 ) selected bidirectionally for egg breakage r a t e ; Poggenpoel ( 1 9 8 0 ) selected for increased and decreased specific gravity; H a r t m a n n et al. ( 1 9 8 1 ) practiced bidirectional selection for egg specific gravity, shell d e f o r m a t i o n , a n d b r e a k i n g strength, respectively in different lines; and McPhee et al. ( 1 9 8 2 ) selected for high specific gravity.
Because of t h e reasonably high average heritability of .39 for shell quality, estimated
With inclusion of shell quality in a selection program, t h e genetic correlation of this trait
1633
1634
POGGENPOEL
with egg production is of importance. Van Tijen and Kuit (1970) estimated an average value of —.12 from the literature, and Rodda (1972) found values of —.06 and .13 in two flocks. It is generally accepted that consumers prefer an egg with a large proportion of firm albumen that will stand up high around the yolk of the broken-out egg (Wells, 1968). A large proportion of thick egg white is also deisrable for foaming and heat coagulation of the white protein for cake making (Shenstone, 1968). The most widely used criterion of albumen quality is the Haugh unit (Haugh, 1937), which is the logarithm of albumen height corrected for egg weight. Van Tijen and Kuit (1970) estimated the average heritability of internal quality from the literature as .38. A few selection experiments confirm that albumen quality can be changed with selection. Lorenz et al. (1934) selected for high and low percentage of firm albumen, and Lorenz and Taylor (1940) reported on further response in these lines. Knox and Godfrey (1940) selected for high and low percentage of thick albumen and Poggenpoel (1982) for increased and decreased Haugh units. A feature of all these experiments was that the response in the high lines was about four times more than in the low lines, as measured on the natural scale. In their review of the literature, Van Tijen and Kuit (1970) estimated the average genetic correlation between albumen quality and egg production as —.077. In this study, the correlated response in shell and albumen quality over 6 years (1971 to 1976) in a line selected for increased egg production will be given, as well as the estimated genetic parameters of these and other egg production traits.
MATERIALS AND METHODS
The two flocks used in this study were the closed White Leghorn flock of the University of Stellenbosch, selected for increased egg production since 1953, and the genetic control flock that originated as a random sample from the selection line in 1962 (Poggenpoel and Erasmus, 1978). These two flocks are hatched at the same date annually, housed in individual laying cages in the same house, and receive the same feeding and management treatment. The criterion of selection was half-year egg produc-
tion index per hen-housed to the age of 275 days. Females were selected on an index that combined individuals, full sisters' and half sisters' performances (Osborne, 1957a), whereas males were selected on an index of full and half sisters' performance (Osborne, 1957b). Egg production traits measured on a routine basis were: sexual maturity as age in days at first egg, half-year egg production as number of eggs produced to the age of 275 days, and total egg production as eggs produced to the age of 500 days. Total egg production was not measured in 1972, and for 1974, no production measurements were available. In addition, measurements of egg quality were taken for 5 consecutive days on an average of 3.2 eggs/pullet at the age of about 34 weeks for pullets of the selection line hatched in 1971 through 1976. For progreny of the two hatching years, 1973 and 1976, these measurements were repeated at the age of 57 weeks. In the control flock, egg quality was measured concurrently on one egg per pullet from about 70 pullets in 1972, 1974, and 1976. All egg quality measurements were taken on the day after being laid. Specific gravity (SG) of the egg was measured by the flotation method with 10 salt solutions ranging in specific gravity from 1.074 to 1.110 with increments of .004. The SG of the solutions were checked every morning before the measuring period of about 2 hr. Eggs were assigned a SG score (1 to 10), corresponding to the solution in which they were first observed to float. All calculations were done on the SG score. True specific gravity can be estimated as 1.074 + (SG score X .004). Albumen height was measured with a tripod screw micrometer to .1 mm immediately after breaking and Haugh units determined with the internal quality calculator as proposed by Brant et al. (1951). Shells of the broken eggs were rinsed in clean water, air dried at room temperature for 1 day, and shell thickness, with the membranes included, was measured by means of a spring-loaded thickness gauge to the nearest .01 mm. The thickness of the shell was measured at the equatorial region, but in the first year, measurements were also taken at the blunt and sharp ends of the shell. In the first year, the dried shells including membranes were also weighed, and shell weight per unit surface area was calculated from the relationship reported by Mueller and Scott (1940) where surface area (cm 2 ) = 4.67 (eggweight) 2 ' 3 .
EGG PRODUCTION AND EGG QUALITY
1635
Egg weight was the weight recorded at the time of the measurement for egg quality. Repeatabilities were calculated on measurements of successive eggs from the same pullet for hatching years 1971 and 1976, for hens with three measurements per trait. All other estimates of shell and egg quality were based on hen means of an average of 3.2 eggs/hen. Heritabilities and genetic correlations were estimated through unbalanced halfand full-sib analysis (Becker, 1967). Pooled estimates were calculated by pooling degrees of freedom and weighted mean sums of squares and products over years. V
~
C3
RESULTS AND DISCUSSION
Means. The means and standard deviations of traits measured in the selection flock, together with the number of sires, dams, and progeny measured in each hatching year, are presented in Table 1. Egg quality values are all based on pullet means. The average numbers per line in the control line was about 50 sires, 150 dams, and 250 pullets housed. The second values for egg quality traits in years 1973 and 1976 are the means at 57 weeks of age, while all the rest are for 34 weeks of age. There was a decrease of about 1.0 mm in albumen height and a decrease of 10 Haught units when hen age increased from 34 to 57 weeks. This is in agreement with results of Nagai and Gowe (1969) and Orr (1973), although the decline in the present study tends to be steeper and more like that of Rodda (1972). The SG score tended to decrease with about one score (.004) over this period, which corresponds to decreases found by Nagai and Gowe (1969), Rodda (1972), Potts and Washburn (1974), and Hamilton (1978). Egg shell thickness showed no decline. This result is in contrast to expectancy (Rodda, 1972; Hamilton, 1978), particularly because mean egg weight increased over this period from about 50 to 55 g, and Roland (1980) found proportional less increase in shell deposition than in egg size with drastic increases in egg size. A similar result where shell weight per unit of surface area remained relatively unchanged in one line over a period of 7 months was found by Perek and Snapir (1970). For the pullets of Hatching Year 1971, the mean egg shell weight was 4.88 ± .43 (SD), and the mean egg shell thickness was 74.6 ± 5.3 mg/cm 2
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1636
POGGENPOEL
Repeatabilities. The pooled repeatabilities for shell measurements at 34 weeks of age for pullets of hatching year 1971 and 1976 are presented in Table 2. Only pullets with three measurements for a trait were included, and the total number for the various traits varied from 313 to 321 for 1971 and from 350 to 363 for 1976 hatching year. The within-year estimates for the different traits were: albumen height (.52 and .63), Haugh units (.36 and .74), SG score (.59 and .56) egg shell thickness (.38 and .57). Repeatability estimates of the present study tend to be lower than most other estimates reported in the literature. The pooled estimate of .58 for albumen height is lower than the value of about .79 of Nagai and Gowe (1969). The value of .53 for Haugh units is lower than the estimate of about .78 of Nagai and Gowe (1969), but falls within the range of .40 to .70 found by Rodda (1972). The present estimate of .58 for shell thickness tends to be slightly lower than the values of .66 by Nagai and Gowe (1969), .57 to .64 by Rodda (1972), and .60 by McPhee et al. (1982). The estimate of .50 for egg shell thickness is also lower than the estimate of .69 by Rodda (1972). There was a tendency for measurements to be more accurate at the later stage in 1976 after more experience had been gained. Heritabilities. Pooled estimates of heritabilities were based on data of 122 sires, 637 dams, and 2354 progeny and are presented in Table 2, together with heritabilities of egg production traits. The heritabilities of albumen quality and Haugh units are in agreement with the average
of .38 for internal egg quality from Van Tijen and Kuit (1970). The estimates of heritability for egg specific gravity and shell thickness correspond closely to the average of .39 from the literature by Van Tijen and Kuit (1970). The estimate for SG score is also in line with the estimates of the Rhode Island Red flock of Rodda (1972) but is higher than the values found for his White Leghorn flock. It is evident from the values in Table 2 that the heritabilities for egg quality traits are reasonably high in this flock. This was proved by two two-way selection experiments that were started with samples of the progeny of Hatching Year 1971. After six generations of selection for high and low SG score, the difference between the two lines was .013 in specific gravity (Poggenpoel, 1980). The difference between two lines selected for increased and decreased Haugh units was 17.1 units (Poggenpoel, 1982). For all egg quality traits, the heritabilities estimated from the sire components of variance are larger than the estimates from the dam components, which reflected sex-linked effects of .2 to .3 for all these traits. The practical implication is that the sire has more influence than the dam on the egg quality of their daughters. In the flocks of Rodda (1972), there were only small differences between heritability estimates from the sire and dam components. In the three White Leghorn flocks of Nagai and Gowe (1969), the average heritabilities from the dam components appear to be larger than from the sire components in most cases. From the results of these three studies, there is apparently no general trend for the size of
TABLE 2. Pooled heritabilities (± SE) and repeatabilities of traits in the selection line
S+D Albumen height Haugh units SG 2 score Shell thickness Egg weight Half year production Total production Sexual maturity 1 2
S = Sire, D = dam. SG = Specific gravity.
.65 .51 .62 .45 .71 .29 .06 .29
± .12 ± .10 ± .11 ± .09 ± .15 ± .07 ± .05 ± .07
.31 .28 .25 .29 .35 .12 .37 .13
± .09 + .09 ± .09 ± .09 ± .11 ± .09 ± .12 ± .09
.48 .40 .43 .37 .53 .21 .21 .21
± .07 ± .06 t .06 + .06 ± .09 ± .05 ± .05 ± .05
Repeatability .58 .53 .58 .50
EGG PRODUCTION AND EGG QUALITY
heritabilities estimated from the different components for egg quality traits. Of the hens of Hatching Year 1971, additional measurements of albumen height and Haugh units were taken on eggs after they were stored for a period of 2 weeks at room temperature. Means of these eggs were lower than that of the fresh eggs. For albumen height, it was 2.6 mm vs. 5.2 mm and for Haugh units, 45.8 vs. 74.3. The heritabilities ( h | + D ) estimated on these stored eggs were for both traits very similar to the estimates on the fresh eggs, viz. .51 ± .18 (.38 ± .13) for albumen height and .52 ± .18 (.43 ± .13) for Haugh units. The genetic correlations between hen means of fresh eggs and stored eggs were .86 for both traits. According to these results, eggs need not necessarily be fresh when measuring albumen quality for selection purposes. From hens of the 2 hatching years, 1973 and 1976, egg quality measurements were again taken at the age of 57 weeks. The number of hens measured was slightly less than that at 34 weeks of age (Table 1), but the number of eggs per hen was about the same. Heritabilities for these 2 years were pooled for measurements at 57 weeks of age. Heritabilities for the measurements at 34 weeks of age for these 2 years were also pooled and given in brackets. The estimates, h | + ^ were: albumen height, .23 ± .11 (.44 ± .12); Haugh units, .31 ± .10 (.42 ± .11); specific gravity, .19 ± .09 (.31 ± .09); and shell thickness, .11 ± .09 (.29 ± .09). Heritabilities for all traits declined over the laying year, indicating greater accuracy of selection early in the laying year as was also found by Nagai and Gowe (1969) and Rodda (1972). Genetic correlations between measurement at 34 and 57 weeks of age were between .7 and .8 for all the traits and support the suggestion that breeding values over the whole year are closely related (Nagai and Gowe, 1969; Rodda, 1972). Correlations. For pullets of Hatching Year 1971, the genetic and phenotypic correlations of measurements of shell thickness at the equator with measurements at the blunt end were .95 and .87, respectively, and with measurements at the small end, .93 and .81. Correlations between measurements correspond to the results of Tyler and Geake (1965) who found phenotypic correlations generally greater than .80 between measurements in different areas of the shell. The phenotypic correlations between milligrams shell per square centimeter for 1971 pullets was estimated as .82 with
1637
SG score as well as with measured shell thickness at the equator. With these high correlations, measurement of shell thickness at a convenient area like the equator seems sufficient as an indication of overall shell thickness. The pooled genetic, phenotypic, and environmental correlations between the different traits are presented in Table 3. The large standard errors of the genetic correlations, in spite of the relatively large numbers in the analysis, underline the general problem of obtaining reliable estimates of genetic correlations. Genetic correlations from the sire plus dam components, environmental, and phenotypic correlations do not differ much, indicating that genetic and environmental factors generally influence these traits in the same direction. Sexual maturity has a low genetic correlation in the order of .10 with all egg quality traits. This is in line with the average of two estimates of .305 between sexual maturity and shell quality given by Van Tijen and Kuit (1970). Half-year production as well as total egg production show a genetic correlation in the order of —.20 with albumen and shell quality. This is close to the values of —.2 to —.3 found in an extensive investigation with large numbers by Emsley et al. (1977). It is also in reasonable agreement with the average correlation values of —.116 with shell quality and —.077 with internal quality as estimated by Van Tijen and Kuit (1970) and with the values of about —.10 for shell thickness in the Rhode Island Red flock of Rodda (1972). The conclusion from these estimates is that positive selection for shell and albumen quality will, in most flocks, have an inhibiting effect on progress in number of eggs. The genetic correlations of egg weight with the two measurements of albumen quality (.27 and .03), as well as the two measurements of egg shell quality (.05 and .22), are variable. On average, the estimates seem to agree with the low values of Van Tijen and Kuit (1970) of .100 between egg weight and shell quality and —.031 between egg weight and internal quality. The genetic correlations between egg weight and shell thickness of —.20 to .47 reported by Rodda (1972) appear to be higher than in these other studies. As Haugh units is a measurement of albumen height corrected for egg weight, the high genetic correlation of .95 between Haugh units
1638
POGGENPOEL
T A B L E 3. Pooled correlations
between
the different r
A(S + D)1
traits in the selection r
P2
line 'E3
Sexual m a t u r i t y
X X X X
a l b u m e n height Haugh units SG score shell thickness
.14 .08 .09 .15
± .13 ± .13 ± .13 ± .14
.11 .08 .06 .09
.03 .02 .03 .11
Half year p r o d .
X X X X
a l b u m e n height Haugh units SG 4 score shell thickness
.29 .22 .16 .22
± .13 + .13 ± .13 + .13
-.16 -.13 -.12 -.12
-.10 -.12 -.12 -.13
Total production
X X X X
a l b u m e n height Haugh units SG score shell thickness
.36 .29 .13 .16
± ± ± ±
.12 .12 .13 .13
-.09 -.06 -.08 -.09
.11 .08 -.21 -.22
Egg eight
X X X X
a l b u m e n height Haugh u n i t s SG score shell thickness
.27 .03 .05 .22
+ ± ± +
.11 .10 .11 .11
.20 -.03 .01 .21
.30 .06 .01 .20
A l b u m e n height
X Haugh units X SG score X shell thickness
.95 ± .01 .04 ± .10 .14 ± .11
.83 .04 .04
.80
.09 .10
Haugh units
X SG score X shell thickness
.12 ± .10 .13 ± .11
.03 .01
.04 .02
SG score
X shell thickness
.87 ± .03
.77
.78
1
*MS + D) = Genetic correlation from sire plus d a m c o m p o n e n t s .
2
rp = P h e n o t y p i c correlation.
3
r g = Environmental correlation.
4
SG = Specific gravity.
and albumen height is not surprising. Albumen height, as well as Haugh units, have genetic correlations in the order of .10 with shell quality, which is in accordance with the average value of .116 of Van Tijen and Kuit (1970). The genetic correlation between specific gravity and shell thickness is important as it gives an indication of the accuracy of the easy nondestructure flotation method as a measurement of real shell thickness. The value of .87 shows a close relationship between these two traits in this flock and corresponds to values of around .80 and higher in the flocks of Rodda (1972). The relationship between these two traits in this flock is confirmed by the response with two-way selection for SG score. After six generations of selection, the difference of .013 in SG between the two lines was accompanied by a correlated difference of .067 mm in shell thickness (Poggenpoel, 1980). Selection Response. Deviations of means of the selection line from means of the control
line for half-year egg production (the criterion of selection) and for the egg quality traits are presented in Table 4. Deviations of half-year production suggest that there was no further selection response over the period 1972 to 1976. This situation (1972) was reached 10 years after establishment of the control line and after 10 generations of selection on half-year egg production in the selection line (Poggenpoel and Erasmus, 1978). The selection response of 28.10 eggs in half-year production was accompanied by consistent small negative responses in albumen and shell quality. According to the estimates of negative genetic correlations, a negative correlated response in these traits was expected. Van Tijen (1977) also found in his selection lines that stronger shells were obtained at the expense of number of eggs. With the available data, it was possible to calculate from direct and correlated selection responses, genetic correlations between half-
EGG PRODUCTION AND EGG QUALITY
1639
TABLE 4. Deviations of means of the selection line from means of the control line Half-year production eggs
Year of hatch
Albumen height
Haugh units
SG1 score
Shell thickness X .01 mm
(mm) 1972 1973 1974 1975 1976 Mean % of Control mean 1
29.3 29.9 27.3 25.9 28.10 50.2
-.69
-3.76
-.59
-2.96
-.25
-.18
-.47
-2.10
-.68 -.54 -8.6
-2.72 -2.22 -2.8
-.80 -.62 -13.4
-1.6 -2.22 -5.9
SG = Specific gravity.
year egg production and the egg quality traits from the relationship: rA
CR, RX
J
AX
>AY that is obtained with rearrangement from Falconer (1981, p. 286), where r^. is the genetic correlation between two characters, R x is the selection response in character X, CRy the correlated response in character Y, and 6 A is the genetic standard deviation. The different values of R are obtained from Table 4, and values of 6 A are calculated from the respective heritabilities (sire plus dam components) and the standard deviations. The calculated genetic correlations between half-year egg production and the different egg quality traits, with the comparable genetic correlations estimated from the sire plus dam components in brackets, are as follows: albumen height, —.41 (—.29); Haugh units, - . 2 5 ( - . 2 2 ) ; SG score, - . 2 9 (-.16); shell thickness-.51 (-.22). All the genetic correlations calculated from selection response have greater numeric values than those estimated from the sib analysis. Predicted correlated responses would consequently be less than the responses obtained. Falconer (1981) mentioned that close agreement between observed and predicted correlated responses is not often found because of two reasons, viz. the low precision of estimates of genetic correlations and their sensitivity to gene frequency changes. The genetic correlations estimated from the selection responses confirm the negative association between egg numbers and egg quality traits in this flock. However, from the relatively
small correlated responses after 10 generations of selection (Table 4), it appears as if the original levels of these traits could have been maintained with slight selection pressure. It is possible that the negative correlated response in egg and shell quality, with selection for increased egg production, may be smaller, or even absent, in other lines. Washburn et al. (1981) found that differences in shell strength between their lines were not associated with differences in daily egg mass produced from 154 to 294 days of age, which was the trait of selection. Because of the small negative phenotypic correlations of the order of —.10, between half-year egg production and the egg quality traits (Table 3), a correlated negative selection differential in these traits is expected with selection for half-year production. These correlated selection differentials were calculated for the 5 years of 1971 to 1976 with 1974 not included. The mean selection differentials, with the range of values in brackets, are: half-year egg production, 11.12 eggs (8.4 to 15.6); albumen height, —.07 mm (—.19 to - . 0 4 ) ; Haugh units, - . 5 2 (-.92 to .04); specific gravity score, —.06 (—.38 to —.18); and egg shell thickness, - . 2 0 X .01 mm ( - . 8 to .2 X .01 mm). The relatively small correlated selection differentials in comparison to that of egg numbers strengthens the belief that the negative responses in the egg quality traits could have been prevented with only small selection pressures for these traits. An indication of this necessary correlated selection differential (Sy) can be calculated from the relationship C R y /
1640
POGGENPOEL
S
Y = r A h X h Y / r P ( F a l c o n e r , 1 9 8 1 , p. 2 8 8 ) . F r o m the available parameters, it was estimated t h a t t o have obtained a positive correlated response of a b o u t 1% in each of t h e egg quality traits, t h e following correlated selection differentials would have been required: albumen height, .03 m m ; Haugh units, . 5 0 ; SG score, . 0 2 ; shell thickness, .19 X .01 m m . H u n t o n ( 1 9 8 2 ) presented results of seven commercial flocks believed t o be present in summaries of t h e 1 9 6 5 as well as 1978 American r a n d o m sample tests. On average, t h e SG score of these flocks over this period of 13 years, as deviation from t h e Kentville control, did not change. This gives an indication of how shell quality in commercial flocks is maintained, or even increased, t o g e t h e r with select i o n response for increased egg p r o d u c t i o n . T h e estimated correlations as well as t h e selection responses of this s t u d y prove a negative genetic association b e t w e e n n u m b e r of eggs p r o d u c e d and shell and a l b u m e n quality, as was f o u n d in most o t h e r flocks investigated. T h e size of t h e correlated responses and correlated selection differentials however, suggests t h a t these egg quality traits can be maintained at a satisfactory level with slight selection pressure and a small sacrifice of egg n u m b e r s .
ACKNOWLEDGMENTS The a u t h o r gratefully acknowledges the technical assistance of W. R o d e , M. R u n c i m a n n , J. R o b y n , and D. J. van P a p e n d o r p .
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