FACTORS AFFECTING SELECTION PROGRESS FORSHELL STRENGTH

K. W. Washburn

Department of PoultryScienceThe University of Georgia

Athens, GA 30602

INTRODUCTION

The per capita egg consumption has decreased considerably over the last 15

years. A major reason for this decrease is the concern over cholesterol and changing

eating habits of today's family. However, I feel that some of this decline may be due

to consumer dissatisfaction with the shell quality and subsequent rejection of eggs

as a food item they wish to purchase. This negative effect on consumption added

to the approximately 7% loss of eggs from the point of lay to the consumer, provides

an enormous opportunity for dramatic improvement in economic return by improving

the shell quality of commercial layers.

In previous talks on the subject of egg shell strength, it has been stated that the

average egg breakage (including cracks) from point of lay to consumers' use was 7%.

Recent estimates indicate that this value has changed very little. Breeders of

commercial egg layers are all placing selection pressure on improving shell strength.

Thus, the question needs to be asked as to why so many of our eggs are lost because

of weak shells. A number of questions, summarized in Table 1, come to mind in

evaluating the genetic approach to improving shell strength.

136

The first question to be addressed is, "What is the status of the genetic

Variation in shell strength?" A discussion of genetic variation can be divided into 5

categories:

1. Differences between brown and white shell color populations.

2. Differences between genetic groups.

3. Heritability estimates.

4. Long term selection experiments.

5. Specific long term selection experiments which used a commercial-type

population, assessed the extent of diherences in different

environments, and assessed changes in other important traits.

Differences between brown and white shell color populations:

A number of studies have shown differences in shell strength between brown

and white shell color populations. These can be divided into pure breed

comparisons and comparisons between commercial strains. Taylor and Martin

(1928) found that the percent shell of brown eggs (from Barred Plymouth Rocks)

was lower than that of white shelled eggs from White Leghorns. Tyler and Geake

(1958) reported that the white shelled eggs from White Leghorns had thicker shells

than the brown shelled eggs from Rhode Island Reds. Eggs from Brown Leghorns

(white shell) was similar to those from the Rhode Island Red breed. Perek and

Snapir (1970) reported that the mg per cm shell was greater in White Leghorns than

in White Plymouth Rock (brown shell). Rodda (1972 found that a White Leghorn

strain had better shell strength than a Rhode Island Red strain. Shell strength of

commercial white egg strains were in general better than that of brown egg strains

137

(Potts and Washburn, 1974) and eggs from white egg commercial strains in random

sample tests had better shell strength than those from brown egg strains (Hunton,

1980).

Differences in shell strength between genetic groups (non-commercial) have

been shown in a number of studies. Significant differences among dam families in

percent shell ash were reported by Munro (1938). Genetic variation in shell

thickness in a White Leghorn population was observed by Taylor and Lerner (1941).

Significant differences in shell thickness between various breeds and lines were found

by Farnsworth and Nordskog (1955). Significant differences in specific gravity in

three White Leghorn strains were reported by Nagai and Gowe (1969). Difference

between strains in tensile strength were observed by Carter (1970). Significant

difference in percent broken eggs in White Plymouth Rock lines selected for

divergence in body size were reported by Siegel et al. (1978). A desert inhabiting

breed (Sinai) were found to have shells that were less porous and thicker than that

of commercial White Leghorns (Arad and Marder, 1982).

In contrast to these studies using non-commercial genetic groups, there are few

published studies on differences between genetic groups which would be considered

as commercial layers. Significant differences were shown in breaking strength,

thickness and percent cracks in eight commercial strains (Bowman and Challender,

1963). Significant differences in shell strength among three white and three brown

egg strains were reported by Potts and Washburn (1974). In this study shell quality

was assessed by a number of methods and the deterioration in shell strength started

earlier in brown egg strains. Differences were observed between 10 commercial

138

strains and crosses in shell strength measured by percent shell, shell weight, and

relative changes with age (Hamilton, 1979).

Heritability estimates (h2):

A summary of h2 estimates for various genetic groups and methods of

measuring is presented in Table 2. These estimates show a wide range in values.

It appears that the h2 estimates obtained from brown egg populations is higher than

that obtained from white egg populations. Many of these values were obtained on

pure breeds and may not be representative of the amount of genetic variation in

commercial layers. In the study of Potts and Washburn (1985) in which a single

commercial leghorn type parent line Wasused, the h2 ranged from .18 to .31. The

values of .39 and .31 were obtained as a composite value of many h2 estimates. It

is probable that more emphasis should be placed on these estimates. Another study

of importance is the relatively recent one by Grunder (1986) in which the h2of intact

eggs were studied. These h2 estimates were relatively low (.10 to .26).

Several of these studies measured shell strength in the same population at

different ages. The data presented in Table 3 indicate that h2 of shell strength

decreases with age. This may have an impact on the age at which shell strength is

measured for selection studies. The reasons for this decline needs to be studied.

Four conclusions can be obtained from the h2 estimates:

1. The h2 of shell strength should be considered moderate (about .3).

2. The h2 estimates obtained from white shelled eggs tend to be lower than

those obtained from brown shelled populations.

139

3. The h2 of shell strength appears to decrease with age.

4. h2 estimates for current commercial laying strains are needed.

In evaluating genetic variation, the most important consideration is progress

in selection experiments. A summary of selection experiments for shell strength is

presented in Table 4. These studies used a variety of genetic populations and

methods of measuring shell strength. Most of these studies show that genetic

progress can be made in improving shell strength. However, none of these studies

utilized populations comparable to the current commercial lines.

The genetic variation in shell strength can be summarized by the following

points:

1. There is sufficient genetic variation to develop an effective selection

program.

2. Genetic variation differs between populations.

3. The amount of measurable variation decreases with age.

4. Information is needed on heritabilities, selection progress, and correlated

changes in current commercial laying strains.

Basis for genetic variation in shell strength:

There are a number of areas that provide the basis for genetic variation in

shell strength. Two of these will be discussed: 1) Egg characteristics and 2)

Physiological response. Selection for these traits could interfere with selection

progress for shell strength if there were negative correlations between them and shell

strength.

140

Possible changes in egg characteristic in response to selection for shell strength

include 1) shape, 2) shell color, 3) membrane mass, and 4) egg weight.

Some of the older data using pure breeds have shown a degree of correlation

of egg shape and shell strength, the eggs with rounder shape having better shell

strength. However, in commercial strains we have found little correlation between

shape and strength (Table 5). The data in Table 5 is a composite of three brown

egg and three white egg strains in which the shell strength was determined by a

number of methods. Only for breaking strength of brown eggs does the correlation

approach a magnitude of importance.

Some of the older data, mostly using pure breeds, have also shown a

relationship between egg shell tint and strength. However, in commercial brown egg

strains tint does not appear to be an important factor in shell strength (Table 6). In

this study, shell tint of eggs from two commercial brown egg strains was visually

scored on a scale of 1 to 5 and these values correlated with shell strength measured

by deformation, thickness, specific gravity, and breaking strength.

One egg characteristic often overlooked does have an effect on shell strength.

That characteristic is the shell membrane. In the study presented in Table 7, three

commercial white and three commercial brown egg strains were divided into low

deformation (high shell strength) and high deformation (low shell strength) groups.

After determination of shell strength, the weight of the shell membrane of these eggs

were then determined. This study shows that the shell membrane weight was

associated with shell strength both in groups selected for differences in deformation

and between brown and white shell groups which differed in shell strength.

141

The characteristic of the egg that is most often associated with shell strength_.

is egg weight. As the hen becomes older the size of the egg increases and the shell

strength decreases, however, the amount of shell deposited remains relatively

constant or is only slightly increased, thus there is a relative decrease in the amount

of shell. This relationship, utilizing deformation as the measure of shell strength, is

shown in Figure 1. It is clear that the egg weight increased throughout the 77 week

period and shell strength decreased (higher deformation = lower strength) with age

after 37 weeks. This type of relationship has resulted in an association of increased

egg weight with poorer shell strength.

In the study shown in Table 8, the association of egg weight "andshell strength

was studied in different strains utilizing four different methods to measure shell

strength within specific age periods. In general, correlations between weight and

strength within ages were low:" However, in strain 1 there was a moderatea

correlation regardless of the method used to assess shell strength. Egg weight was

also measured in the deformation studyutilizing a commercial grand-parent leghorn

line to obtain information about the association of shell strength and egg weight.

Phenotypic correlations between deformation and egg weight were near 0 and

genetic correlations of these variables ranged from - 0.29 to -0.57 (Table 9).

These correlations indicate that selection for increased shell strength should

not have a deleterious effect on egg weight. However, none of these studies used

the current commercial layer which is smaller in size and probably lays an egg whose

size is proportionally larger than those on which these data were obtained.

142

P_h_hysioloNealrespo_nscs:

There are a number of possible physiological responses to selection for shell

strength. These include:

1. Length of time egg is in shell gland

2. Rate of shell deposition

3. Uterine environment

4. Changes in acid-base balance

*5. Efficiency in metabolizing calcium

These are all important, but I would like to focus on the efficiency in metabolizing

Ca.

In discussing the efficiency of metabolizable Ca, I will focus on two points:

1. Feeding excess Ca to improve shell strength

2. Effects of selection emphasis on Ca efficiency

The importance of Ca metabolism in shell formation is well documented and

increasing Ca level to a certain level may result in improved shell strength. Thus,

one might hypothesize that differences in response to dietary Ca may explain genetic

differences in shell strength.

In the study shown in Table 10, commercial layers were divided into high and

low deformation groups. Each deformation group was divided into two treatment

groups. The control group received a diet containing 2.9% Ca while the Ca

treatment group received this diet plus free access to oyster shells. After a 3 week

period, shell strength was assessed by deformation, breaking strength, and shell

thickness. There were no consistent trends to suggest that the strength of eggs from

143

low shell strength groups receiving additional Ca was improved any more than that

of the high strength groups.

Some of our current problems with shell strength may be the result of the

selection pressure to produce a layer which provides the opportunity for maximum

economic returns. There is selection pressure to decrease body size to improve

efficiency and increased egg numbers without changing egg size.

In the pullet, the formation of new bone stays active only to the beginning of

lay at which time it appears to shut down. Activity then shifts from the conical bone

to the medullary bone. The hen uses dietary Ca and Ca from the medullary bone

for shell formation. As the body size and bone mass is genetically decreased with

expectations of more eggs and no decrease in egg size it may become more difficult

to maintain sufficient Ca deposition to maintain good shell strength. This may

become especially critical with combinations of aging and hot environmental

temperature. Studies need to be conducted using the modern layer and measuring

shell strength over the production period and in different environmental conditions.

Individual Variation

Considerable variation in shell strength is observed between individuals within

a flock. This may or may not be all genetic variation but it is important from two

aspects: 1) the differences seem to be predetermined at an early age, 2) the

differences remain consistent at least over short periods. These differences observed

when the birds first come into lay continue and are amplified throughout the period

of lay (Figure 2).

144

Figure 3 shows the deformation values over a 7 week period of two individuals

differing in basal deformation values. These values were consistent over the 7 week

period. However, it is interesting to note that the fluctuations in the poor shell

strength individual (high deformation) was greater over the period than was that of

the individual with better shell strength. Studies need to be conducted to determine

if factors which result in changes in shell strength have a greater effect on those

individuals with inherently poorer shell strength.

Methods of Measuring Shell Strength

Important aspects of measuring shell quality include: 1) physical method, 2)

age, and 3) the effects of environmental conditions.

Physical method:

The strength of the shell has most often been measured by various lab

estimates of shell strength. Many studies have been done to determine which of

these methods are the best. The results of our studies (Table 11) show a very high

phenotypic correlation between the most used different methods of measuring shell

strength indicating that any of these methods would be acceptable in lab-type

evaluations of shell strength. However, this relationship may not be so clear-cut

when comparisons are made over time. In the study presented in Table 12, 31-37

week deformation was used to classify sire families into low or high deformation

groups. At 40, 60, 68, and 77 weeks of age the breaking strength and percent shell,

in addition to deformation, was obtained. Differences between deformation groups

remained significant throughout the period. Differences in breaking strength were

145

significant for most of the ages. However, percent shell differences were significant

only for the 41 week period.

Lab tests such as these would be of limited value unless they have a reasonable

degree of relationship to the proportion of eggs cracked or broken under commercial

conditions. A number of studies have attempted to determine the association of

laboratory shell strength measures with shell breakage. Two points make it difficult

to assess this:

1. Considering all the points at which shells can receive an insult sufficient to

cause breakage, all eggs do not receive the same insult under commercial

conditions so that the degree of insult a particular egg receives is a matter

of chance.

2. If the degree of the insult is not controlled it may be so great that it

exceeds the degree of variation in shell strength of the samples being

tested.

However, a number of studies have shown there is a good degree of association

between shell strength measured under laboratory conditions and breakage under

commercial conditions (these are summarized in Table 13).

In addition, Grunder et al. (1989) reported that the phenotypic correlation

between a number of shell quality traits and % intact shells ranged from .19 to .47.

Genetic correlations ranged from .23 to 1.04. However, genetic correlations between

shell strength measures at 42 weeks with intact eggs at 68 weeks were low except for

shell weight.

146

Effects of age:

In addition to the physical method used for measuring, the age of the bird at

which determinations are made is important. Since the major problem in shell

strength is with older hens the question arises - should the genetic selection be based

on data obtained on older hens? This has two obvious disadvantages: 1) h2 of shell

strength decreases with age, 2) generation interval is increased if data for selection

is obtained at later ages.

In the study shown in Figure 1, sire families were categorized as high or low

shell deformation families on the basis of values obtained from 31-37 weeks of age.

Deformation values were then obtained through 77 weeks of age. The differences

between the groups were maintained throughout the 77 week period and became

greater from 41 weeks on as the shell strength of the high deformation families

deteriorated at a faster rate than the low deformation families. This indicates that

selection based on determination made at an earlier age of the production cycle

should be effective in improving shell strength at older ages when the shell strength

is in general poorer. However, at very early age there appears to be considerable

variation in shell strength of individual birds. Figure 2 shows an example of the shell

deformation of a typical individual from beginning of lay through 44 weeks of age.

The shell deformation values were quite variable during the earlier ages. This

suggests that in this particular population egg strength data obtained before 30 weeks

of age may not be reliable.

147

Environment:

A number of non-genetic environmental factors contribute to the variability in

shell strength. These include such known things as age, season, ambient

temperature, diet and disease. In the assessment of shell strength for use in a

breeding program, these factors Contributing to shell strength must be carefully

separated from the genetic contribution to the variation.

In addition to these known factors that contribute to variation in shell strength,

unknown factors may result in temporarily altered shell strength. The disturbing

thing about these changes in shell strength is that the temporarily altered values may

not be representative of the true shell strength of those individuals (Table 14). Shell

strength values obtained during week 1 (where the shell strength was altered) were

not well correlated with values obtained after the shell strength had returned to

normal (week 2).

To summarize, I would like to focus on what I think the future genetic

approach to solving the shell strength problem should be:

1. Determine the basis of the individual variation in shell strength and its

relationship to genetic variation.

2. Conduct selection experiments using current commercial type layers as base

populations and measure correlated changes.

3. Evaluate various combinations of methods of measuring shell strength, age,

and temperature.

4. Evaluate the inter-relationships of selection pressures for decreased body

weight, increased egg numbers, and maintaining egg weight on the birds

ability to maintain shell strength.

148

i

REFERENCES

Arad, A., and J. Marder, 1982. Differences in egg shell quality among the Sinai

Bedouin fowl, the commercial White Leghorn and their crossbreds. Br. Poult.

Sci. 23:107-112.

Bowman, J. C., and H. I. Challender, 1963. Egg shell strength. A comparison of two

laboratory tests and field results. Br. Poult. Sci. 4:103-116.

Buss, E. G., 1982. Genetic differences in avian egg shell formation. Poultry Sci.

61:2048-2055.

Buss, E. G., R. M. Leach, Jr., and J. T. Stout, 1977. Eggshell quality for chickens in

selected lines, Fl's. and F2's. Poultry Sci. 56:1699-1700.

Carter, T. C., 1970. The hen's egg: some factors affecting deformation in statically

loaded shell. Br. Poult. Sci. 11:15-38.

Engstr6m, G., C. Weyde, and L. E. Liljedahl, 1986. Genetic correlations and

heritabilities for frequency of cracked eggs, egg number and egg weight in

laying hens. Br. Poult. Sci. 27:55-61.

Farnsworth, G. M., Jr., and A. W. Nordskog, 1955. Breeding for egg quality.

3. Genetic differences in shell characteristics and other egg quality factors.

Poultry Sci. 34:16-26.

Gowe, R. S., and R. W. Fairfield, 1986. Long term selection for egg production in

chickens. 3rd World Congress on genetics applied to livestock production.

XII. Biotechnology, selection experiments, parameter estimation, design of

breeding system management of genetic resources. Long term selection for

egg production in chickens, pp. 152-167.

149

Grunder, A.A., R. M. G. Hamilton, R. W. Fairfield, and B, K. Thompson, 1989.

Genetic parameters of egg shell quality traits and percentage of eggs remaining

intact between oviposition and grading. Poultry Sci. 68:46-54.

Hamilton, R. M. G., K. G. Hollands, P. W. Voisey, and A. A. Grunder, 1979.

Relationship between egg shell quality and shell breakage and factors that

affect shell breakage in the field - a review. World's Poultry Sci. J. 35:177-190.

Hunton P., 1982. Genetic factors affecting egg shell quality. World's Poultry Sci.

J. 38:75-84.

Jaff6, W. P., 1966. Egg production, body weight and egg quality characters, their

heritability and the correlations between them. Br. Poult. Sci. 7:91-98.

Johnson, A. S., and E. S. Merritt, 1955. Heritability of albumen weight and specific

gravity of eggs from White Leghorns and Barred Rocks and the correlations

of these traits with egg production. Poultry Sci. 34:578-587.

McNally, E. H., 1965. The relationship of egg shell weight to cracked eggs. Poultry

Sci. 44:1513-1518.

Munro, S. S. 1938. Effects of heredity on interior egg quality and shell composition.

Poultry Sci. 17:17-27.

Nagai, J., and R.S. Gowe, 1969. Genetic control of egg quality. 1. Source of

variation. Br. Poult. Sci. 10:337-350.

Perek, M., and N. Snapir, 1970. Interrelationships between shell quality and egg

production and egg and shell weights in Wfiite Leghorn and White Rock hens.

Br. Poult. Sci. 11:133-145.

150

Pevzner, I. Y., G. W. Friars, H. L Orr, and B. S. Reinhart, 1976. The use of

selections and strain crossing to reduce egg shell breakage. Br. Poult. Sci.

17:185-194.

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, Sr., and K. W. Washburn, 1985. Genetic variation in shell strength and

its relationship to egg size. Poultry Sci. 64:1249-1256.

Quinn, J. P., C. D. Gordon, and A. B. Godfrey, 1945. Breeding for egg shell quality

as indicated by egg weight loss. Poultry Sci. 24:399-403.

Rodda, D. D., 1972. Breeding for late egg shell quality in the domestic hens. Br.

Poult. Sci. 13:45-60.

Siegel, P. B., J. H. van Middelkoop, and P. R. K. Reddy, 1978. Comparisons of

frequencies and egg shell characteristics of broken and intact eggs within

diverse populations of chickens. Br. Poult. Sci. 19:411-416.

Strong, C. F., 1988. Relationship between several measures of shell quality and egg

breakage in commercial processing plants. Poultry Sci. 67:162.

Taylor, L. W., and J. H. Martin, 1928. Factors influencing thickness of egg shell.

Poultry Sci. 8:39-41.

Taylor, L. W., and I. M. Lerner, 1939. Inheritance of eggshell thickness in White

Leghorn Pullets. J. Agri. Res. 58:383-396.

Taylor, L. W., and I. M. Lerner, 1941. Inheritance of shell finish in Single Comb

White Leghorns. J. Hered. 32:33-36.

151

Tyler, C., and F. H. Geake, 1958. Studies on egg shells. IX. The influence of

individuality, breed, and season on certain characteristics of egg shell from

pullets. J. Sci. Food Agric. 9:473-483.

Van Tijen, W. F., 1977. Shell quality in poultry asseen from the breeder's

viewpoint. 3. Heritabilities: Expected versus accomplished response. Poultry

Sci. 56:1121-1126.

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.

Geflu gelk. 34:201-210.

152

TABLE 1. Important questions in evaluating genetic approach to improvingshell strength

1. What is the status of the genetic variation?

2. What is the basis for the genetic variation and could selection for othertraits interfere with selection progress?

3. What is the influence of individual bird variation?

4. Is the method of measuring appropriate?

5. Are non-genetic factors interfering with assessment of genetic potential?

TABLE 2. A summary of heritability estimates for shell strength

Stock Method h_ Reference

7 Pure breeds Specific gravity .09 TO .24 Farnsworth & Nordskog (1955)White Leghorn Specific gravity .32 Johnson & Merritt (1955)Barred Plymouth rock Specific gravity .56 Johnson & Merritt (1955)White Leghorn Specific gravity .28 TO .47 Jaffe (1966)

Strains (Various) Specific gravity .22 TO .47 Nagai & Gowe (1969)Rhode Island Red Specific gravity .57 TO .33 Rodda (1972)White Leghorn Specific gravity .27 TO .05 Rodda (1972)

Commercial White Leghorn Deformation .30 TO .18 Potts & Washburn (1982)

Parent line

White Leghorn Cracks .11 TO .43 Engstrom et al. (1986)Various Various .39 Van Tijen & Kuit (1970)Review

Various Various .31 Buss (1982) Review

TABLE 3. Effect of age on h2 estimates

Age (wks) Stock Change Reference

24-40 White Leghorn .44 TO .21 Johnson & Merritt (1955)

33-65 Rhode Island Red .57 TO .33 Rodda (1972)

33-65 White Leghorn .27 TO .05 Rodda (1972)21-28 Commercial

White Leghorn .30 Potts & Washburn (1982)29-37 Commercial

White Leghorn .3841 Commercial

White Leghorn .1842-68 Commercial

White Leghorn .36 TO .25 Grunder et al. (1989)

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TABLE 4. Summary of selection experiments

Genetic _oup _ Generations Results Reference

White Leghorn Breed Percent Shell 5 .67% Difference Taylor & Lerner (1939)from base

population

White Leghorn Breed Egg weight loss 8 3.6% Difference Qulnn et al. (1945)in lfigh &low lines

White Leghorn Strains Deformation 3 Significant in 1 Pevzner et al. (1975)strain but not in

another

White Leghorn G shell/cm _ 8 Percent in thick Buss et al. (1977)& % shell shell line = 10.7,

percent in thinshell line = 7.7

White Leghorn Index 4 Thick t from Van tijen (lg'F0.35 mm to .37 ram,

specific gravityt from 1.089 to

1.095

CornelK BreakingStrength3 2.9kginhigh& Parsons& Combs

(1978)

2.47 kg in low line

Random-bred Breakage 1 37% breakage & Garwood (1979)234mm thickness

in high line,

29% breakage &.36 mm thickness

in lowline

- Specific gravity 4 Specific gravity McPhee et al. (1982)1".04

Egg-type Specific gravity 9 Significant Gowe (1986)

improvement in

specific gravity in3 or 4 strains

154

TABLE 5. Correlation of shape (W/L) with shell strength .

C0rr¢l_ti0n_

Specific BreakingStrain _W/L Thickness _ Deformation strength

Brown egg 72.6 .05 .22 .24 .34

White egg 71.6 .07 .15 .23 .22

TABLE 6. Correlation of shell tint with shell strength in commercial strains

Strain 1 Strain 2

Deformation .05 -.06

Thickness .08 .36

Specific gravity .06 .01

Breaking strength -.16 .34

155

TABLE 7. Relationship of shell membrane to shell strength

High vs low deformation _oups

LOw HighMembrane Membrane

Deformation weight Deformation weight

White egg 2.87 25.3a 3.09 24.1b

Brown egg 2.90 23.(P 3.68 20.4 b

White vs brown deformation groups

Membrane

Deformation weight

White egg 2.98 24.7x

Brown egg 3.29 21.7y

TABLE 8. Correlation of egg weight with shell strength

Straim

1 2 3 4 5

Thickness .65 .32 .15 .04 -.15 .20

Specific gravity .32 -.02 -.10 -.16 -.26 .04

Deformation .39 .07 -.09 -.01 -.32 .01

Breaking strength .39 .23 -.06 .26 .32 .22

.44 .15 .02 .03 -.10

156

TABLE 9. Association of shell deformation and egg weight in a commercialleghorn-type grandparentpopulation

CorrelationAge Phenotypic Genetic

21-29 x -.39

31-37 x -.29

41 .06 -.57

60 .00 x

68 .04 x

77 -.04 x

x Correlations not obtained.

TABLE 10. Effect of supplemental Ca on shell strength variation

Ca treatment ControlHigh Low High Low

deformation deformation deformation deformation

Deformation (X 10"3) 3.72 3.16 3.31 2.71

Breaking strength(kg) 3.16 3.72 2.94 3.75

Thickness (X 10.2 mm) 30.7 33.7 30.5 33.2

High, low = groups selected for high or low shell deformation.

157

TABLE 11. Phenotypic correlations of different methods of measuringshell strength (31-37 weeks of age)

Strains SG/DEF SG/TH _ DEF/TH DEF/B$ TH/B$

1 -.80 .78 .68 -.79 -.75 .68

2 -.76 .84 .70 -.82 -.80 .76

3 -.78 .66 .73 -.82 -.80 .66

4 -.78 .74 .70 -.82 -.76 .72

5 -.74 .74 .67 -.78 -.82 .76

SG = specific gravity; DEF = deformation; TH = shell thickness; BS = breakingstrength.

TABLE 12. Deformation, breaking strength, and percent shell at later ages offamilies selected for high and low shell deformation at 31-77 weeks of age

Deformation Breaking strength Percent shellAge Low High Low High Low High

41 2.54a 2.84b 6.16a 5.66b 9.18a 8.76b

60 2.52_ 2.72b 6.05_ 5.98_ 8.94_ 8.68a

68 2.80_ 2.98b 5.65_ 5.21b 8.92_ 8.74_

77 3.1& 3.42b 5.21Y 4.88b 8.34_ 7.98a

158

TABLE 13. Association of shell strength measures with shell breakage

REFERENCE CONCLUSION

McNally (1965) Logorithimic t in cracking with # shell weight.

Wells (1967A) Almost perfect curvilinear relationship betweenpercent cracks under commercial conditions andspecific gravity measures.

Wells (1967B) Shell strength of eggs that cracked during transitwas less than for non-cracked.

Wells (1967A) Percent cracks was correlated with all methodsof measuring.

Bowman & Challender (1963) Significant correlation between shell thicknessand number of cracks produced on farm or in-transit.

Holder & Bradford (1979) As specific gravity t there was significant # innumber of cracked eggs after passing throughegg grading machine.

Shrimpton &Hann (1967) Values were not good predictors of subsequentbreakage.

Strong (1988) Specific gravity and percent shell werecorrelated with percent cracks. Breakingstrength, shell thickness, and shell weight werenot.

TABLE 14. Effects of disruptions on correlations of repeated measures

Deformati0n/Deformation Percent shell/Percent shell

Week 1 and week 2 .43 .45

Week 2 and week 3 .74 .56

Week 3 and week 4 .76 .65

159

A

_5,0_

40.-..v i_1_ mclh

_3.0 -

B_2.0 -

1.0_wQ

I 1 t I I I I

1 2 3 4 5 6 7WEEKSON TRIAL

FIGURE 3. Individual variation in shell strengthA = individual selected on the basis of high shell deformation

before trial started.B = selected on basis of low shell deformation.

160

24 25 29 33 37 41 60 6,8 77

WEEKS OF AGE

FIGURE 1. Shell deformation of high and low deformation families from 24-77weeks of age.

161

65mo-,,--,--4 W_IGM T

I- .....4 OFFOA _IATION

,_4.0

so-

" i. ',..........:.........• .......,...,/: .... ...:,_._45 -- .. .._ • \ ."

• I(' O" : ql '.. ""

• 2.0

4o I I L I l L L I l21 23 25 27 29 31 33 35 2,7 41

WEEKS OF AGE

FIGURE 2. Shell deformation of an individual from 21 to 41 weeks of age.

162