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Effect of Heredity on Interior Egg Quality and Shell Composition
Effect of Heredity on Interior Egg Quality and Shell Composition S. S. M U N R O
Poultry Division, Experimental Farm, Ottawa, Canada, and Institute of Animal Genetics, University of Edinburgh, Scotland (Received for publication July 17, 1937)
D
quite justified. This definition, however, may require modification in certain respects since St. John and Flor (1931) and Henry and Barbour (1933) have shown that the thin portion of the white gives a larger volume when whipped than the firm portion. Hunt and St. John (1931) have further shown that a given quantity of thin white produced a larger volume of angel food cake than a similar quantity of firm white. However much, these and similar findings may modify our conception of egg quality, such internal measurements must remain, positively or negatively, associated with quality. Therefore, factors affecting the physical structure of the new-laid eggs are of economic as well as biological importance. Hoist and Almquist (1931) showed that the percentage of firm white is a characteristic dependent on the individual hen. Lorenz, Taylor and Almquist (1934) claim that this individuality is inherited. By selecting as a foundation, two families, the majority of whose members were producers of high and low percentages of firm white respectively, they succeeded in producing from the high line, offspring whose eggs contained an average of 66.7 percent firm white as compared with 56.4 percent from an unselected lot. Offspring from the low line, however, were characterized by 57.2 percent firm white which is actually slightly higher than the unselected lot. Further, while selection and inbreeding tended to increase percentage firm white in the high line, such increases were small and were
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storage or as they age, eggs of the domestic fowl undergo changes with respect to certain of their interior measurements. In addition to the general loss of moisture and accompanying shrinkage in weight, these changes have been shown by Hoist and Almquist (1931) to include gradual liquefaction of the firm white, with a corresponding decrease in its percentage and an increase in the size of the yolk. Sharp and Powell (1930) showed that a change in the yolk index (ratio of height to width) occurred during storage, the yolk becoming broader and natter when placed on a plane surface. These structural differences between fresh and storage eggs also exist between individual eggs when fresh and the author has recently shown (Munro, 1936) that eggs which possess larger than average percentages of firm white when fresh not only retain their advantage on holding but tend to break down or liquefy less rapidly than poorer eggs. Further, it has been shown (Munro, loc. cit.) that greater viscosity of the firm white as measured by its height (upstanding quality) and strength of the vitelline membrane are also greater in fresh eggs and that eggs which are superior in respect of these measurements when fresh also hold their superiority in storage. Assuming that the changes which occur during storage are associated with loss in quality from the consumer's point of view, then the use of these measurements as indices of quality in the new-laid egg seems URING
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S. S. MUNEO
EXPERIMENTAL MATERIAL
Approximately 180 individually pedigreed Barred Plymouth Rock pullets were used for this purpose. The birds had been reared on the range and were brought in and placed in individual laying batteries just before sexual maturity. All birds chosen were
members of full-sister families of four or more, with at least one other such halfsister group out of the same sire. They were distributed at random through the battery units and supplied with an all-mash standard laying ration and oyster shell ad libitum. After a period of two months in the battery, during which time the birds adjusted themselves to the close confinement and came into normal production, representative eggs were gathered and submitted to various physical measurements in the laboratory within two hours following oviposition. After being weighed, the egg was broken open in a flat dish and the score of the condition of firm white recorded according to the standards of Van Wagenen and Wilgus (1936). The color of the yolk was scored according to a series of 12 color gradations ranging from very light yellow to deep orange and especially prepared for the purpose. The yolk was then released from the firm white envelope by snipping the latter with scissors and removed to the apparatus described by Munro and Robertson (1935) where the breaking strength of the vitelline membrane was measured by atmospheric pressure. The white was separated into firm and thin portions by the sieve method of Lorenz and Almquist (1934) and the percentage of thick white computed by means of a ready reckoner. The shell, in two halves, was wiped dry of adhering white and weighed together with the attached membranes. It was set aside to partially dry being later broken down into fine pieces with mortar and pestle and dried in an electric oven to constant weight. The dry weight was then recorded and the shell was burnt to a white ash in a muffle furnace. The ash determinations were made on the advice of Dr. Carl Shroeder of Larrowe Research Farms who, in a private communication, expressed the opinion that it was less variable and more reliable as a guide to the commercially important
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largely negatived by nearly similar trends away from the direction of selection in the high line. The authors explain the apparent discrepancies by suggesting that factors for low percentage firm white may be dominant to those for high percentage. If such is the case one would expect the distribution curves to be at least slightly skewed. No skewness is noticeable, however, in either line nor in the unselected population. It appears probable that while the California workers may have succeeded in isolating lines which are fundamentally different in respect to certain genes which exercise some control over the proportion of firm white, the effectiveness of this genetic control in the presence of non-genetic variables is largely limited. This view is supported by the work of Hunter, Van Wagenen, and Hall (1936) who showed that the percentage thin albumen as well as the yolk index and score of the condition of firm white (upstanding quality) have distinct seasonal trends. Furthermore, Van Wagenen and Hall (1936) secured but little evidence for the inheritance of percentage outer thin white, percentage firm white, or yolk index by daughter-dam or daughtersire's dam correlations. Following the original announcement by Lorenz, Taylor, and Almquist (1934) concerning the heritable nature of the percentage of firm white, an experiment was planned for the purpose of examining further their content and of extending the observations to other physical measurements on the broken out egg. This work was carried out during the winter of 1934-35.
EFFECT OF HEREDITY ON EGG QUALITY
the effects of possible differences between the periods were removed, there were differences between individual females in the various characteristics measured; (2) whether, when this individuality was found to exist, there were significant differences between the inter and intra family variance or in other words whether the individuality with respect to the characteristics in question was determined by heredity. In this manner seven different egg quality measurements were analyzed, viz., percentage firm white, observed condition of firm white, yolk weight, yolk color, breaking strength of vitelline membrane, percentage ash of dry weight of shell, percentage ash of total egg weight. The results of the first analysis of variance, primarily designed to measure the differences between individual hens but also showing the effects of periods and of the interaction between hens and periods are shown in Table 1. The significance of the difference between the mean square was measured by Snedecor's (1934) " F " test. In all cases the " F " value for the difference between hens exceeds the 1 percent error point. Thus all seven characteristics are clearly individual traits; each hen produces an. egg, which, in respect of any single measure, is clearly and distinctly the product of her own soma. These data confirm those of Hoist and Almquist (1931) and of Van Wagenen and Hall (1936) with respect to percentage firm white and those of the latter authors in respect of yolk color and extend the factor of individuality to cover yolk weight, breaking strength of yolk membrane, and the percentage ash of dry shell as well as of total egg weight. With individuality an established fact it becomes of immediate interest to seek the cause of variability between individuals. Presumably this may be caused by inheritance, by environment, or what is more likely by a combination of these. That environment must be responsible for part of
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characteristic of breaking strength than were machine measurements of crushing strengths. The ash was converted to a percentage of total egg weight as well as dry weight of shell, since it was felt that not only was the percentage of shell important but the proportion of ash to total volume or weight of egg or in other words the amount of ash (chiefly Ca) surrounding a given volume of egg material might be an even better index of breaking strength. Furthermore, Morgan (1932) has shown that the higher the proportion of shell, the greater the breaking strength. The records covered three periods of four weeks each beginning December 27 and ending March 20. The number of eggs so examined each period from any one bird varied with her rate of production but in any case were chosen so as to be uniformly distributed with regard to time within the period. In cases where such a uniform sampling throughout the period was impossible such as when the bird ceased to lay or paused for a time such records were not included in the analysis. Data for the condition of firm white were taken in the last period only, since the scale for measuring this quality was not available earlier. It was found that the shell was particularly difficult to reduce to white ash. Survey of the original data collected during the periods mentioned above revealed large discrepancies that could only be accounted for by technic. Consequently these data were discarded and new data collected from the same birds during September. In the repetition it was found necessary to burn the shell for eight hours at a temperature of 2,000°F. to completely oxidize the organic material. The new data were found to be much less variable and to show highly significant individual differences which were not revealed by the original figures. The data were analyzed by the method of variance to determine: (1) whether, when
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S. S. MUNRO
the variability is clearly evident by the fact that a distinct difference between periods exists with respect to the four characteristics in Table 1 which were measured throughout three periods. This variability between periods is not, however, completely
same degree or necessarily of the same order in all birds. While much of the individual variability must therefore be non-genetic, it is of considerable scientific importance to determine whether heredity plays any appreciable part. Previous students who inthe
TABLE 1.—Analyses of variance for seven egg quality characteristics effect of the individual and the period {season)
Values of F for error points of Character
Variance due to
D.F.
Mean Square
F 1%
10.88 6.57 1.41
1.59 3.21 1.48
1.92 5.11 1.75
3.46
1.23
1.35
% Firm White
Between hens Between periods Interaction Remainder Total
23 2 46 144 215
Condition of Firm White
Between hens Remainder Total
56 114 170
Yolk Weight
Between hens Between periods Interaction Remainder Total
28 2 56 174 260
5.65 83.53 1.68 0.92 2.23
6.14 90.79 1.83
1.17 3.05 1.16
1.26 4.72 1.24
Yolk Color
Between hens Between periods Interaction Remainder Total
28 2 56 174 260
1.43 6.90 0.53 0.37 0.57
3.86 18.65 1.43
1.25 3.05 1.16
1.40 4.72 1.24
Breaking Strength Vitelline Membrane
Between hens Between periods Interaction Remainder Total
16 2 32 102 152
10.33 9.22 2.06
1.79 3.09 1.26
2.24 4.82 1.39
% Ash of. Dry Shell
Between hens Remainder Total
34 140 174
1.232 0.210 0.410
5.87
1.19
1.29
% Ash of Total Egg
Between hens Remainder Total
34 140 174
1.238 0.097 0.312
12.76
1.19
1.29
198.60 120.05 12.97 18.25 37.36 0.457 0.132 0.239
543.5 484.8 108.2 52.6 121.6
governed by the periods, i.e., by seasonal differences, because there is a highly significant degree of interaction between hens and periods. This is true for yolk weight, yolk color, and breaking strength of vitelline membrane. Thus, while, with respect to a single trait, there are differences from month to month in the general mean for the entire population, the trends are not of the
vestigated the inheritance of certain of these egg quality points did so either by measuring the effectiveness of several generations of selection in establishing high and low lines (Lorenz, Taylor and Almquist, 1934) or by the degree of correlation between daughter and dam and between daughters and their sire's dam (Van Wagenen and Hall, 1936). While admittedly legitimate technics, such
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5%
EFFECT OF HEREDITY ON EGG QUALITY
methods, when dealing with traits which are influenced by non-genetic causes which vary from season to season and year to year and especially when the records of dam's are acquired one or more years' previous to those of their daughters, are not as critical as one would desire. In the selection method a degree of control can be established when a check flock, either in the form of a negatively selected or unselected (random breeding line), is simultaneously maintained. In such cases, however, it is of extreme importance to have the two lines hatched,
Variance due to Between Dams Within Dams Total
sire in respect of the strength of the vitelline membrane. In this case the mean square for between dams is only slightly larger than that within dams, the ratio of the two (F value) being only 1.02. The necessary ratio which must be reached for 95 percent accuracy, considering the particular numbers of daughters and dams involved, is 3.29. The data for each period was then pooled by adding the degrees of freedom and sum of squares from each sire and an analysis of variance was thus secured for each period. This procedure was followed
of variance of strength of vitelline membrane. Progeny of sire 722 in period 4
D.F.
Sum of sq.
Mean square
3 IS 18
346.6 1695.0 2041.6
115.5 113.0 113.4
reared, and housed together, the individuals from each preferably being mixed at random and competing together within each husbandry unit. In the absence of such desirable measures of control, it would seem that the method used in assessing heredity in this study is more critical. For this purpose the mean of each of the measured characteristics for each pullet was calculated for each of the four-week periods. The data were then tabulated separately for each period, within each period the pullet records were grouped according to sire and within each sire according to dam. The data from each such sire group, therefore, represents a case of single classifications (according to dam) with unequal numbers of observations in the classes. The total variance of the offspring of each sire was analyzed into a portion representing differences in the average production of the daughters of different dams and a remainder representing the variation between daughters of the same dam. Table 2 shows the case of a typical
F. 1.02
Value of F for error point of 5% 3.29
for each of the measured characteristics and the summarized analyses of variance showing the variability within full sisters as compared to that between half-sisters with a common sire are shown for each characteristic by periods (Table 3). The ratio, within family variance: between family variance, is shown in the " F " column followed by the necessary level which this ratio must reach before we can ascribe to the dams an effect upon their daughters with an accuracy of 95 percent and 99 percent. In cases where the F value does not reach the 5 percent error point, the corresponding 1 percent point is not listed. It can be seen that four of the measured characteristics, viz., percentage firm white, condition of the firm white, yolk color, and breaking strength of vitelline membrane show no significant differences between the transmitting ability of dams. In fact, in two of the three periods, the variability with respect to firm white, was actually greater within full sisters than between half sisters. This is also the case in one period for both
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TABLE 2 . --Analysis
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yolk color and membrane strength. Such cases are marked with an asterisk in Table 3. Obviously, unless inheritance is entirely through the sire, a possibility which it seems unnecessary to entertain, genetics plays but a minor part as a casual agent of variability in respect of these traits. On the other hand, yolk weight, percentage ash of dry shell and
respect to yolk size and percentage ash in shell the pullets investigated can be grouped into sensibly distinct classes according to dam. Since these pullets were not in any way grouped into families during the rearing stage and since their distribution throughout the cages in the laying battery was entirely at random, it seems justified
TABLE 3.—Summarized analyses of variance for each of the measured characteristics, showing the effect of female parentage (variance due to sire is not included in the total) Variance due to Character
Period
Total
Between dams
Within dams
F value
38.62 49.71 41.16
18 24 22
36.94 48.28 46.64
5%
1%
1.06 1.03 1.16
1.92 1.73 1.65
—
0.227
1.15
1.74
—
88 128 118
2.52 2.31 2.57
1.73 1.97 1.15
1.70 1.60 1.65
2.10 1.94
90 125 115
0.410 0.319 0.295
1.00 1.32 1.46
1.69 1.64 1.67
—
1.13 1.48 1.46
1.78 1.60 1.67
—
0.249
2.78
1.96
2.62
SS
0.191
2.21
1.92
2.62
S
%Firm White
3 4 5
99 152 139
Condition of Firm White
5
126
0.232
19
0.263
107
Yolk Weight
3 4 5
109 152 140
2.87 2.67 2.63
21 24 22
4.36 4.56 2.95
Yolk Color
3 4 5
111 147 136
0.411 0.308 0.316
21 22 21
0.412 0.243 0.431
Breaking Strength Vitelline Membrane
3 4 5
96 152 137
% Ash of Dry Shell
12
56
0.352
13
0.691
43
% Ash of Total Egg
12
56
0.244
13
0.423
43
119.27 98.53 101.91
17 24 21
81 128 117
107.47 135.32 138.64
percentage ash of total egg, show highly significant differences between dams. This is particularly true with respect to the two last mentioned traits. To facilitate reference to Table 3, cases showing significant differences between dams, i.e., cases where the odds for the existence of true differences are greater than 19 in 20, have been marked in the last column with an "S" (significant). In cases where the odds are 99 in 100, or greater, the designation is "SS" (highly significant). It is clearly apparent that with
79 128 116
39.00 49.98 40.13
121.81 91.64 95.27
* *
s SS
* *
to ascribe the differences between damdaughter groups to inheritance. Having thus shown that the genotype governs the proportion of mineral within the shell and thus, presumably, the ability of the egg to withstand external pressure, the question naturally arises as to how this effect is brought about. According to Wickie, as quoted by Stewart (1935), calcium carbonate comprises 97 percent of the minerals of the egg shell. The chief source of this material in the present case was oyster
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D.F. Mean sq. D.F. Mean sq. D.F. Mean sq.
Nece ssary F for error of
EFFECT OF HEREDITY ON EGG QUALITY
found that the contents of the egg are reduced by lack of calcium, the reduction being equally divided between the yolk and the white, although the percentage of protein or of calcium oxide in the contents of the egg was not altered. It appears, therefore, that the percentage constituents of the egg tend to remain essentially the same and that the lowering of the intake of a particular nutrient responsible for a specific component in the egg is not reflected in a lowered percentage of that component within the egg but the rate of egg manufacture is retarded to keep pace with the available quantity of the diminished essential. Such a law would explain the failure of Sowell and Morgan (1936) to induce measurable differences in percentage yolk and. percentage firm albumen by varying levels and kinds of protein supplements. It seems most likely, therefore, that the gene must govern the mineral composition of the shell by regulating the efficiency with which this mineral element is utilized, largely irrespective of the calcium intake. Utilization of calcium for the purpose of egg shell formation involves assimilation from the intestinal tract, transportation in the blood stream, mobilization within the shell glands of the uterus, and deposition on the shell membrane. Warren and Scott (1935) showed that the egg passes rapidly through the anterior portions of the oviduct and reaches the uterus between four and five hours after ovulation. By this time it is fully formed but lacks the shell. It spends about 20 hours in the uterus, during which time the shell is deposited over the membranes. The normal shell contains about 5 gms. of calcium carbonate, which equals an excretion of about 42 mgs. per minute. A gene control over calcium metabolism may conceivably be exercised by regulating: (1) intestinal absorption and consequent mobili-
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shell which was fed ad libitum. Does egg shell strength depend on the efficiency with which a given quantity of oyster shell is utilized or does it depend on the amount of oyster shell consumed? In other words, is it efficiency of utilization or appetite which is inherited? Data published by Bird (1937) and, in part, drawn from the same group of pullets yielding the present egg quality data, show considerable variation in individual oyster shell consumption. In fact many of the birds consumed insufficient amounts of oyster shell to provide calcium for the egg shells which they produced. In such cases, Common (1933) has suggested that calcium reserves in the bones are drawn upon for the manufacture of egg shell. This has been confirmed by Deobald, Lease, Hart, and Halpin (1936) who found that the ash content of the bones of hens in negative calcium balance to average 10 percent less than those consuming sufficient calcium. This depletion of bone continues for a short time until egg laying either ceases or slows to the point where ingested calcium maintains a positive balance. During this period of physiological stress within the organism, although the ability to deposit calcium over the shell membranes is limited and is partly reflected in thinner egg shells, the most noticeable reaction of the organism to the calcium deficiency is the curtailment or complete cessation of egg laying. Thus, Buckner, Martin, and Peter (1924) found that although a restricted calcium diet decreased the weight of the shell, the percentage of CaO in the shell was not diminished. Egg production ceased or was retarded but shelless eggs were not produced. This agrees with the data of Deobald, Lease, Hart, and Halpin (1936) who found that although thin egg shells are produced by suddenly reducing the calcium intake, no eggs completely without shells could be obtained. Buckner, Martin, and Peter (1925) also
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S. S. MUNEO
zation within the tissues; (2) efficiency with which mobilized calcium is secreted through the shell glands. When the ash (CaO) is expressed as a percentage of dry shell weight, the measurement may also be affected by a gene control over the proportion of calcium within the shell irrespective of its absolute weight. That is to say a bird which normally secretes a higher than average weight of calcium during the 20 hours of egg shell formation may produce
the amount of oyster shell ingested has no effect on the differences between families either in respect of absolute or percentage of calcium in the shell. Possible genetic control of calcium metabolism, therefore, does not function by way of governing the appetite. On the other hand, there is a rather close relationship between absolute ash weight and the percentage of ash as indicated by the highly significant simple correlation coefficient of .690 and partial
Sums of squares Source D.F. oj'ster .of. variation shell (1) Total Between Sires Within Sires
perwt. of cent ash ash (2) (3)
Sums of products
1.2
1.3
Simple correlation coefficients
Necessary .Partial correlevels at lation error points coefficients
2.3 .05
.01 12.3 13.2 23.1
5.3 1 299 197 - . 0 3 0 690 .532
661 .135 - . 2 3 4 .698
12
23
23
17 35334.0 1.1040 4.00 24.57 - 7 1 . 2 1.331 .124 - . 1 8 9 .634 4 25701.2 0.0161 0.74 13
4.41 - 6 5 . 9 0.032 .217 - . 4 7 8 .294
9632.8 1.0879 3.26 20 16 -
a correspondingly higher proportion of organic constituents within the shell thus leaving the percentage of ash essentially unaltered. Fortunately weekly measurements of individual bird oyster shell consumption, as well as absolute and percentage ash weights for individual eggs, are available. Table 4 shows the analysis of variance and covariance for oyster shell consumption, absolute weight of ash in egg shell and percentage of ash in egg shell. This table was calculated from the average measurements of full sister families, and shows the net simple and partial correlation coefficients with the effect of the sire removed. This table reveals interesting facts concerning the mechanism of calcium utilization. Very briefly, it is plainly evident that
coefficient of .698. There are two possible explanations for this, viz., either the family ability to extract and secrete a larger than average proportion of ingested calcium is the causal agent of the increased percentage of shell ash in the same family or these characteristics are both controlled by, and are expressions of, the genotype and have no direct causal association. This point can be settled by investigating the relationship between the variations in absolute ash weight and percentage of ash between eggs from the same bird. For this purpose the data from 23 representative hens, laying a total of 131 analyzed eggs during the experimental period, were subjected to an analysis of variance and covariance. The summarized data in Table 5 clearly show that the correlation coefficient of
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TABLE 4.—Analysis of variance and covanance, showing the relationship between oyster shell consumption, absolute weight of egg shell ash, and the percentage of ash in the shell. Both net simple and partial correlation coefficients are shown
EFFECT OF HEREDITY ON EGG QUALITY
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.569 representing the association of the characters over the entire 131 eggs is reduced to .290 when the effect of differences between individual hens are removed. The net correlation (.290) while much reduced is nevertheless highly significant, and indicates that the percentage ash in the shell of an individual egg is governed, to a small degree, by the amount of calcium deposited by the uterine glands during the period of shell formation. In this connection, however, the fact that the shell membranes were not cleared from the shell before ashing must be considered. Since these membranes are al-
and secretes calcium, also, but largely independently, decree the proportion of calcium in her egg shells. In view of the indications of genetic control previously shown in Table 3, it is but logical to assume that the genotype of the individual plays a large part in this control. DISCUSSION With respect to the various points of egg quality under investigation it seems clear that, while each and every trait is distinctly influenced by the individual hen (Table 1), only yolk weight and the quantity of the
Ash weight Source of variation
Total Between hens Within hens
Degrees of freedom
130 22 108
Sum of squares 18.837 15.371 3.466
Mean square 0.1449 0.6987 0.0321
Covariance
Percent ash Sum of squares
Mean square
Sum of products
53.180 30.221 22.959
0.4091 1.3737 0.2126
18.0174 15.4323 2.5851
most entirely organic in composition it is clear that the deposition of a thicker shell, i.e., the secretion of a large absolute amount of calcium, will result in an increase in the percentage of ash because of the comparative decrease of organic membrane material even if the organic constituents of the shell proper remain proportionally undisturbed. The inclusion of this relatively constant organic membrane in the dry weight of the shell is probably sufficient to account for the net correlation coefficient of .290. Thus, irrespective of the absolute amount of calcium secreted in forming the shell of the individual egg, the percentage of mineral remains essentially unaltered. The very high correlation of .716 between the mean measurements of individual hens shows that the factors which determine the efficiency with which the individual bird absorbs, mobilizes,
r
.569 .716 .290
Values of r at error points .05
.01
.171 .413 .188
.224 .526 .246
inorganic shell material are affected by the coincidence of parentage (Table 3). Since total egg weight is definitely known to be controlled, in part, by heredity, it is to be expected that yolk weight, being but a component part of the whole, would also be so controlled. On the other hand, the idea of a genetic control over the mineral content of the shell is a new and fascinating one. This becomes all the more interesting because the solid matter of the shell is almost entirely calcium carbonate and the possibility of a gene control over the complex process of calcium metabolism is of fundamental biologic importance. In this study, sufficient data are available to indicate in a general way the time and manner of this control. Thus, it is shown that individual differences are expressed in two largely independent functional phases; "efficiency" genes present
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T A B I E 5.—Analysis of variance and covariance showing the relation between the absolute and the percentage ash weight of 131 eggs from 23 hens
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S. S. MUNRO
SUMMARY
1. The following seven egg quality characteristics were found to be markedly a function of the individuality of the hen:
Percentage firm white, condition of firm white, yolk weight, yolk color, breaking strength of vitelline membrane, percentage ash of dry shell, percentage ash of total egg. 2. Environmental factors are responsible for much of the variability in percentage firm white, yolk weight, yolk color, and breaking strength of vitelline membrane since these traits were measured over fourweek periods and show significant differences between the periods. 3. Three of the characteristics, viz., yolk weight, percentage ash of dry shell, and percentage ash of total egg, are definitely family characteristics and thus are probably influenced by the genotype. 4. Since the ash is composed almost entirely of calcium oxide, there must be a gene control over calcium utilization. 5. By the method of partial correlation it is shown that the amount of oyster shell consumed has no effect on either the amount of calcium secreted or on the percentage calcium in the shell. The genes concerned with calcium utilization, therefore, produce their effect by regulating the efficiency with which a given intake of calcium is absorbed, mobilized in the tissues, and secreted in the form of egg shell. 6. It is further shown that families and individuals which secrete a large quantity of calcium in the form of egg shell also produce shells containing a high percentage of calcium. However, the absolute amount of calcium in the shell and its percentage occurrence therein are not causally related since there is little or no correlation between these characteristics in different eggs from the same hen. 7. It is argued that the effect of the gene on calcium metabolism is exercised on at least two independent functional phases, viz., (1) consisting of absorption and mobilization and culminating in secretion and (2) concerned with the regulation of the proportionate deposition in the egg shell
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in certain individuals determine: first, that these individuals shall transform into egg shell a large proportion of ingested calcium arid, second, that they shall also produce an egg shell containing a high and relatively constant percentage of calcium regardless of minor daily variations in their control over the first phase. It has been shown by Deobald, Lease, Hart, and Halpin (1936) that increased levels of parathormone will produce a marked rise of the blood calcium in the actively laying bird. Gene control of the first functional phase may, therefore, be a control over the activity of the parathyroid. At any rate, the above authors have shown that individual laying birds are characterized by constant differences in the serum calcium level. This difference must be brought about by a differential rate of intestinal absorption. On the other hand this differential in intestinal absorption might be due to a gene control over cell absorption ability on a constant parathormone level. In any case, it is clear that the second functional phase, in which the percentage constituents in the shell are determined, cannot be due to parathormone control since it varies independently of the first and the two phases occur simultaneously. The second phase must be due, therefore, to a fixed cellular control. In view of this it seems likely that both phases are controlled by a common fixed cellular (genie) control but that variations in parathormone level from day to day provide an element of variability in the first phase only. Greater light can be shed on the control mechanism by a study of the relationships between serum calcium level, rate of calcium secretion, and percentage of calcium in the egg shell.
EFFECT OF HEREDITY ON EGG QUALITY
of organic and inorganic materials. 8. The possible manner of gene control is discussed and it is suggested that both phases are simultaneously controlled within the individual at a fixed level by a cellular complement of "efficiency" genes. The first phase, however, is also subject to an independent physiological variability, possibly under the control of parathyroid activity. REFERENCES
of single comb White Leghorn hens. Poul. Sci., 15:115-118. Lorenz, F. W., and Almquist, H. J., 1934. Determination of percentage of firm white. U. S. Egg and Poul. Mag., 40:30-33. Lorenz, F. W., Taylor, L. W., and Almquist, H. J., 1934. Firmness of albumen as an inherited characteristic. Poul. Sci., 13:14-17. Morgan, C. L., 1932. Relationship between breaking strength and the percent of egg shell. Poul. Sci., 11:172-175. Munro, S. S., 1936. Correlations between certain fresh and storage measurements of egg yolk and white. Proc. VI World's Poul. Cong. BerlinLeipzig, pp. 224-226. Munro, S. S., and Robertson, G., 1935. A method of measuring the strength of the yolk membrane. U. S. Egg and Poul. Mag., 4 1 : 48-50. Sharp, P. F., and Powell, C. K., 1930. Decrease in interior quality of hens' eggs during storage as indicated by the yolk. Ind. Eng. Chem., 22: 908-1010. Snedecor, G. W., 1934. Calculation and interpretation of analysis of variance and covariance. pp. 1-96, Collegiate Press, Ames, Iowa. Sowell, D. F., and Morgan, C. L., 1936. The relationship of protein supplement to interior egg quality. Poul. Sci., 15:219-222. Stewart, G. F., 1935. The structure of the hens' egg shell. Poul. Sci., 14:24-32. St. John, J. L., and Flor, I. H., 1931. A study of whipping and coagulation of eggs of varying quality. Poul. Sci., 10:71-82. Van Wagenen, A., and Hall, G. O., 1936. The inheritance of certain characters affecting egg quality. Poul. Sci., 15:405-410. Van Wagenen, A., and Wilgus, H. S., 1936. The determination and importance of the condition of the firm albumen in studies of egg white quality. Jour. Agr. Res., 51:1129-1137. Warren, D. C , and Scott, H. M., 1935. The time factor in egg formation. Poul. Sci., 14:195-207.
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Bird, S., 1937. Fecundity and reproductive ability in closely confined fowl. Sci. Agr., 17:359-375. Buckner, G. D., Martin, J. H., and Peter, A. M., 1924. Calcium metabolism in the laying hen. Kentucky Agr. Exp. Sta. Bui., 252 :3-36. , 1925. Effect of calcium carbonate supplement in the diet of hens, on the weight, protein content, and calcium content of the white and yolk of their eggs. Amer. Jour. Physiol., 72 :458-463. Common, R. H., 1933. Observations on the mineral metabolism of pullets. Jour. Agr. Sci., 23 :5SS-569. Deobald, H. J., Lease, E. J., Hart, E. B., and Halpin, J. G., 1936. Studies on the calcium metabolism of laying hens. Poul. Sci., 15:179185. Henry, W. C , and Barbour, A. D., 1933. Beating properties of egg white. Indus. Eng. Chem., 25 :1054. Hoist, W. F., and Almquist, H. J., 1931. Measurement of deterioration in the stored hen's egg. Hilgardia, 6:49-60. Hunt, L. W., and St. John, J. L., 1931. Angel food cake from the thick and thin portions of egg white. Jour. Home Econ., 23:1151. Hunter, J. A., Van Wagenen, A., and Hall, G. O., 1936. Seasonal changes in interior egg quality
27
Poultry Division, Experimental Farm, Ottawa, Canada, and Institute of Animal Genetics, University of Edinburgh, Scotland (Received for publication July 17, 1937)
D
quite justified. This definition, however, may require modification in certain respects since St. John and Flor (1931) and Henry and Barbour (1933) have shown that the thin portion of the white gives a larger volume when whipped than the firm portion. Hunt and St. John (1931) have further shown that a given quantity of thin white produced a larger volume of angel food cake than a similar quantity of firm white. However much, these and similar findings may modify our conception of egg quality, such internal measurements must remain, positively or negatively, associated with quality. Therefore, factors affecting the physical structure of the new-laid eggs are of economic as well as biological importance. Hoist and Almquist (1931) showed that the percentage of firm white is a characteristic dependent on the individual hen. Lorenz, Taylor and Almquist (1934) claim that this individuality is inherited. By selecting as a foundation, two families, the majority of whose members were producers of high and low percentages of firm white respectively, they succeeded in producing from the high line, offspring whose eggs contained an average of 66.7 percent firm white as compared with 56.4 percent from an unselected lot. Offspring from the low line, however, were characterized by 57.2 percent firm white which is actually slightly higher than the unselected lot. Further, while selection and inbreeding tended to increase percentage firm white in the high line, such increases were small and were
[17]
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storage or as they age, eggs of the domestic fowl undergo changes with respect to certain of their interior measurements. In addition to the general loss of moisture and accompanying shrinkage in weight, these changes have been shown by Hoist and Almquist (1931) to include gradual liquefaction of the firm white, with a corresponding decrease in its percentage and an increase in the size of the yolk. Sharp and Powell (1930) showed that a change in the yolk index (ratio of height to width) occurred during storage, the yolk becoming broader and natter when placed on a plane surface. These structural differences between fresh and storage eggs also exist between individual eggs when fresh and the author has recently shown (Munro, 1936) that eggs which possess larger than average percentages of firm white when fresh not only retain their advantage on holding but tend to break down or liquefy less rapidly than poorer eggs. Further, it has been shown (Munro, loc. cit.) that greater viscosity of the firm white as measured by its height (upstanding quality) and strength of the vitelline membrane are also greater in fresh eggs and that eggs which are superior in respect of these measurements when fresh also hold their superiority in storage. Assuming that the changes which occur during storage are associated with loss in quality from the consumer's point of view, then the use of these measurements as indices of quality in the new-laid egg seems URING
18
S. S. MUNEO
EXPERIMENTAL MATERIAL
Approximately 180 individually pedigreed Barred Plymouth Rock pullets were used for this purpose. The birds had been reared on the range and were brought in and placed in individual laying batteries just before sexual maturity. All birds chosen were
members of full-sister families of four or more, with at least one other such halfsister group out of the same sire. They were distributed at random through the battery units and supplied with an all-mash standard laying ration and oyster shell ad libitum. After a period of two months in the battery, during which time the birds adjusted themselves to the close confinement and came into normal production, representative eggs were gathered and submitted to various physical measurements in the laboratory within two hours following oviposition. After being weighed, the egg was broken open in a flat dish and the score of the condition of firm white recorded according to the standards of Van Wagenen and Wilgus (1936). The color of the yolk was scored according to a series of 12 color gradations ranging from very light yellow to deep orange and especially prepared for the purpose. The yolk was then released from the firm white envelope by snipping the latter with scissors and removed to the apparatus described by Munro and Robertson (1935) where the breaking strength of the vitelline membrane was measured by atmospheric pressure. The white was separated into firm and thin portions by the sieve method of Lorenz and Almquist (1934) and the percentage of thick white computed by means of a ready reckoner. The shell, in two halves, was wiped dry of adhering white and weighed together with the attached membranes. It was set aside to partially dry being later broken down into fine pieces with mortar and pestle and dried in an electric oven to constant weight. The dry weight was then recorded and the shell was burnt to a white ash in a muffle furnace. The ash determinations were made on the advice of Dr. Carl Shroeder of Larrowe Research Farms who, in a private communication, expressed the opinion that it was less variable and more reliable as a guide to the commercially important
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largely negatived by nearly similar trends away from the direction of selection in the high line. The authors explain the apparent discrepancies by suggesting that factors for low percentage firm white may be dominant to those for high percentage. If such is the case one would expect the distribution curves to be at least slightly skewed. No skewness is noticeable, however, in either line nor in the unselected population. It appears probable that while the California workers may have succeeded in isolating lines which are fundamentally different in respect to certain genes which exercise some control over the proportion of firm white, the effectiveness of this genetic control in the presence of non-genetic variables is largely limited. This view is supported by the work of Hunter, Van Wagenen, and Hall (1936) who showed that the percentage thin albumen as well as the yolk index and score of the condition of firm white (upstanding quality) have distinct seasonal trends. Furthermore, Van Wagenen and Hall (1936) secured but little evidence for the inheritance of percentage outer thin white, percentage firm white, or yolk index by daughter-dam or daughtersire's dam correlations. Following the original announcement by Lorenz, Taylor, and Almquist (1934) concerning the heritable nature of the percentage of firm white, an experiment was planned for the purpose of examining further their content and of extending the observations to other physical measurements on the broken out egg. This work was carried out during the winter of 1934-35.
EFFECT OF HEREDITY ON EGG QUALITY
the effects of possible differences between the periods were removed, there were differences between individual females in the various characteristics measured; (2) whether, when this individuality was found to exist, there were significant differences between the inter and intra family variance or in other words whether the individuality with respect to the characteristics in question was determined by heredity. In this manner seven different egg quality measurements were analyzed, viz., percentage firm white, observed condition of firm white, yolk weight, yolk color, breaking strength of vitelline membrane, percentage ash of dry weight of shell, percentage ash of total egg weight. The results of the first analysis of variance, primarily designed to measure the differences between individual hens but also showing the effects of periods and of the interaction between hens and periods are shown in Table 1. The significance of the difference between the mean square was measured by Snedecor's (1934) " F " test. In all cases the " F " value for the difference between hens exceeds the 1 percent error point. Thus all seven characteristics are clearly individual traits; each hen produces an. egg, which, in respect of any single measure, is clearly and distinctly the product of her own soma. These data confirm those of Hoist and Almquist (1931) and of Van Wagenen and Hall (1936) with respect to percentage firm white and those of the latter authors in respect of yolk color and extend the factor of individuality to cover yolk weight, breaking strength of yolk membrane, and the percentage ash of dry shell as well as of total egg weight. With individuality an established fact it becomes of immediate interest to seek the cause of variability between individuals. Presumably this may be caused by inheritance, by environment, or what is more likely by a combination of these. That environment must be responsible for part of
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characteristic of breaking strength than were machine measurements of crushing strengths. The ash was converted to a percentage of total egg weight as well as dry weight of shell, since it was felt that not only was the percentage of shell important but the proportion of ash to total volume or weight of egg or in other words the amount of ash (chiefly Ca) surrounding a given volume of egg material might be an even better index of breaking strength. Furthermore, Morgan (1932) has shown that the higher the proportion of shell, the greater the breaking strength. The records covered three periods of four weeks each beginning December 27 and ending March 20. The number of eggs so examined each period from any one bird varied with her rate of production but in any case were chosen so as to be uniformly distributed with regard to time within the period. In cases where such a uniform sampling throughout the period was impossible such as when the bird ceased to lay or paused for a time such records were not included in the analysis. Data for the condition of firm white were taken in the last period only, since the scale for measuring this quality was not available earlier. It was found that the shell was particularly difficult to reduce to white ash. Survey of the original data collected during the periods mentioned above revealed large discrepancies that could only be accounted for by technic. Consequently these data were discarded and new data collected from the same birds during September. In the repetition it was found necessary to burn the shell for eight hours at a temperature of 2,000°F. to completely oxidize the organic material. The new data were found to be much less variable and to show highly significant individual differences which were not revealed by the original figures. The data were analyzed by the method of variance to determine: (1) whether, when
19
20
S. S. MUNRO
the variability is clearly evident by the fact that a distinct difference between periods exists with respect to the four characteristics in Table 1 which were measured throughout three periods. This variability between periods is not, however, completely
same degree or necessarily of the same order in all birds. While much of the individual variability must therefore be non-genetic, it is of considerable scientific importance to determine whether heredity plays any appreciable part. Previous students who inthe
TABLE 1.—Analyses of variance for seven egg quality characteristics effect of the individual and the period {season)
Values of F for error points of Character
Variance due to
D.F.
Mean Square
F 1%
10.88 6.57 1.41
1.59 3.21 1.48
1.92 5.11 1.75
3.46
1.23
1.35
% Firm White
Between hens Between periods Interaction Remainder Total
23 2 46 144 215
Condition of Firm White
Between hens Remainder Total
56 114 170
Yolk Weight
Between hens Between periods Interaction Remainder Total
28 2 56 174 260
5.65 83.53 1.68 0.92 2.23
6.14 90.79 1.83
1.17 3.05 1.16
1.26 4.72 1.24
Yolk Color
Between hens Between periods Interaction Remainder Total
28 2 56 174 260
1.43 6.90 0.53 0.37 0.57
3.86 18.65 1.43
1.25 3.05 1.16
1.40 4.72 1.24
Breaking Strength Vitelline Membrane
Between hens Between periods Interaction Remainder Total
16 2 32 102 152
10.33 9.22 2.06
1.79 3.09 1.26
2.24 4.82 1.39
% Ash of. Dry Shell
Between hens Remainder Total
34 140 174
1.232 0.210 0.410
5.87
1.19
1.29
% Ash of Total Egg
Between hens Remainder Total
34 140 174
1.238 0.097 0.312
12.76
1.19
1.29
198.60 120.05 12.97 18.25 37.36 0.457 0.132 0.239
543.5 484.8 108.2 52.6 121.6
governed by the periods, i.e., by seasonal differences, because there is a highly significant degree of interaction between hens and periods. This is true for yolk weight, yolk color, and breaking strength of vitelline membrane. Thus, while, with respect to a single trait, there are differences from month to month in the general mean for the entire population, the trends are not of the
vestigated the inheritance of certain of these egg quality points did so either by measuring the effectiveness of several generations of selection in establishing high and low lines (Lorenz, Taylor and Almquist, 1934) or by the degree of correlation between daughter and dam and between daughters and their sire's dam (Van Wagenen and Hall, 1936). While admittedly legitimate technics, such
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5%
EFFECT OF HEREDITY ON EGG QUALITY
methods, when dealing with traits which are influenced by non-genetic causes which vary from season to season and year to year and especially when the records of dam's are acquired one or more years' previous to those of their daughters, are not as critical as one would desire. In the selection method a degree of control can be established when a check flock, either in the form of a negatively selected or unselected (random breeding line), is simultaneously maintained. In such cases, however, it is of extreme importance to have the two lines hatched,
Variance due to Between Dams Within Dams Total
sire in respect of the strength of the vitelline membrane. In this case the mean square for between dams is only slightly larger than that within dams, the ratio of the two (F value) being only 1.02. The necessary ratio which must be reached for 95 percent accuracy, considering the particular numbers of daughters and dams involved, is 3.29. The data for each period was then pooled by adding the degrees of freedom and sum of squares from each sire and an analysis of variance was thus secured for each period. This procedure was followed
of variance of strength of vitelline membrane. Progeny of sire 722 in period 4
D.F.
Sum of sq.
Mean square
3 IS 18
346.6 1695.0 2041.6
115.5 113.0 113.4
reared, and housed together, the individuals from each preferably being mixed at random and competing together within each husbandry unit. In the absence of such desirable measures of control, it would seem that the method used in assessing heredity in this study is more critical. For this purpose the mean of each of the measured characteristics for each pullet was calculated for each of the four-week periods. The data were then tabulated separately for each period, within each period the pullet records were grouped according to sire and within each sire according to dam. The data from each such sire group, therefore, represents a case of single classifications (according to dam) with unequal numbers of observations in the classes. The total variance of the offspring of each sire was analyzed into a portion representing differences in the average production of the daughters of different dams and a remainder representing the variation between daughters of the same dam. Table 2 shows the case of a typical
F. 1.02
Value of F for error point of 5% 3.29
for each of the measured characteristics and the summarized analyses of variance showing the variability within full sisters as compared to that between half-sisters with a common sire are shown for each characteristic by periods (Table 3). The ratio, within family variance: between family variance, is shown in the " F " column followed by the necessary level which this ratio must reach before we can ascribe to the dams an effect upon their daughters with an accuracy of 95 percent and 99 percent. In cases where the F value does not reach the 5 percent error point, the corresponding 1 percent point is not listed. It can be seen that four of the measured characteristics, viz., percentage firm white, condition of the firm white, yolk color, and breaking strength of vitelline membrane show no significant differences between the transmitting ability of dams. In fact, in two of the three periods, the variability with respect to firm white, was actually greater within full sisters than between half sisters. This is also the case in one period for both
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TABLE 2 . --Analysis
21
22
S. S. MUNEO
yolk color and membrane strength. Such cases are marked with an asterisk in Table 3. Obviously, unless inheritance is entirely through the sire, a possibility which it seems unnecessary to entertain, genetics plays but a minor part as a casual agent of variability in respect of these traits. On the other hand, yolk weight, percentage ash of dry shell and
respect to yolk size and percentage ash in shell the pullets investigated can be grouped into sensibly distinct classes according to dam. Since these pullets were not in any way grouped into families during the rearing stage and since their distribution throughout the cages in the laying battery was entirely at random, it seems justified
TABLE 3.—Summarized analyses of variance for each of the measured characteristics, showing the effect of female parentage (variance due to sire is not included in the total) Variance due to Character
Period
Total
Between dams
Within dams
F value
38.62 49.71 41.16
18 24 22
36.94 48.28 46.64
5%
1%
1.06 1.03 1.16
1.92 1.73 1.65
—
0.227
1.15
1.74
—
88 128 118
2.52 2.31 2.57
1.73 1.97 1.15
1.70 1.60 1.65
2.10 1.94
90 125 115
0.410 0.319 0.295
1.00 1.32 1.46
1.69 1.64 1.67
—
1.13 1.48 1.46
1.78 1.60 1.67
—
0.249
2.78
1.96
2.62
SS
0.191
2.21
1.92
2.62
S
%Firm White
3 4 5
99 152 139
Condition of Firm White
5
126
0.232
19
0.263
107
Yolk Weight
3 4 5
109 152 140
2.87 2.67 2.63
21 24 22
4.36 4.56 2.95
Yolk Color
3 4 5
111 147 136
0.411 0.308 0.316
21 22 21
0.412 0.243 0.431
Breaking Strength Vitelline Membrane
3 4 5
96 152 137
% Ash of Dry Shell
12
56
0.352
13
0.691
43
% Ash of Total Egg
12
56
0.244
13
0.423
43
119.27 98.53 101.91
17 24 21
81 128 117
107.47 135.32 138.64
percentage ash of total egg, show highly significant differences between dams. This is particularly true with respect to the two last mentioned traits. To facilitate reference to Table 3, cases showing significant differences between dams, i.e., cases where the odds for the existence of true differences are greater than 19 in 20, have been marked in the last column with an "S" (significant). In cases where the odds are 99 in 100, or greater, the designation is "SS" (highly significant). It is clearly apparent that with
79 128 116
39.00 49.98 40.13
121.81 91.64 95.27
* *
s SS
* *
to ascribe the differences between damdaughter groups to inheritance. Having thus shown that the genotype governs the proportion of mineral within the shell and thus, presumably, the ability of the egg to withstand external pressure, the question naturally arises as to how this effect is brought about. According to Wickie, as quoted by Stewart (1935), calcium carbonate comprises 97 percent of the minerals of the egg shell. The chief source of this material in the present case was oyster
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D.F. Mean sq. D.F. Mean sq. D.F. Mean sq.
Nece ssary F for error of
EFFECT OF HEREDITY ON EGG QUALITY
found that the contents of the egg are reduced by lack of calcium, the reduction being equally divided between the yolk and the white, although the percentage of protein or of calcium oxide in the contents of the egg was not altered. It appears, therefore, that the percentage constituents of the egg tend to remain essentially the same and that the lowering of the intake of a particular nutrient responsible for a specific component in the egg is not reflected in a lowered percentage of that component within the egg but the rate of egg manufacture is retarded to keep pace with the available quantity of the diminished essential. Such a law would explain the failure of Sowell and Morgan (1936) to induce measurable differences in percentage yolk and. percentage firm albumen by varying levels and kinds of protein supplements. It seems most likely, therefore, that the gene must govern the mineral composition of the shell by regulating the efficiency with which this mineral element is utilized, largely irrespective of the calcium intake. Utilization of calcium for the purpose of egg shell formation involves assimilation from the intestinal tract, transportation in the blood stream, mobilization within the shell glands of the uterus, and deposition on the shell membrane. Warren and Scott (1935) showed that the egg passes rapidly through the anterior portions of the oviduct and reaches the uterus between four and five hours after ovulation. By this time it is fully formed but lacks the shell. It spends about 20 hours in the uterus, during which time the shell is deposited over the membranes. The normal shell contains about 5 gms. of calcium carbonate, which equals an excretion of about 42 mgs. per minute. A gene control over calcium metabolism may conceivably be exercised by regulating: (1) intestinal absorption and consequent mobili-
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shell which was fed ad libitum. Does egg shell strength depend on the efficiency with which a given quantity of oyster shell is utilized or does it depend on the amount of oyster shell consumed? In other words, is it efficiency of utilization or appetite which is inherited? Data published by Bird (1937) and, in part, drawn from the same group of pullets yielding the present egg quality data, show considerable variation in individual oyster shell consumption. In fact many of the birds consumed insufficient amounts of oyster shell to provide calcium for the egg shells which they produced. In such cases, Common (1933) has suggested that calcium reserves in the bones are drawn upon for the manufacture of egg shell. This has been confirmed by Deobald, Lease, Hart, and Halpin (1936) who found that the ash content of the bones of hens in negative calcium balance to average 10 percent less than those consuming sufficient calcium. This depletion of bone continues for a short time until egg laying either ceases or slows to the point where ingested calcium maintains a positive balance. During this period of physiological stress within the organism, although the ability to deposit calcium over the shell membranes is limited and is partly reflected in thinner egg shells, the most noticeable reaction of the organism to the calcium deficiency is the curtailment or complete cessation of egg laying. Thus, Buckner, Martin, and Peter (1924) found that although a restricted calcium diet decreased the weight of the shell, the percentage of CaO in the shell was not diminished. Egg production ceased or was retarded but shelless eggs were not produced. This agrees with the data of Deobald, Lease, Hart, and Halpin (1936) who found that although thin egg shells are produced by suddenly reducing the calcium intake, no eggs completely without shells could be obtained. Buckner, Martin, and Peter (1925) also
23
24
S. S. MUNEO
zation within the tissues; (2) efficiency with which mobilized calcium is secreted through the shell glands. When the ash (CaO) is expressed as a percentage of dry shell weight, the measurement may also be affected by a gene control over the proportion of calcium within the shell irrespective of its absolute weight. That is to say a bird which normally secretes a higher than average weight of calcium during the 20 hours of egg shell formation may produce
the amount of oyster shell ingested has no effect on the differences between families either in respect of absolute or percentage of calcium in the shell. Possible genetic control of calcium metabolism, therefore, does not function by way of governing the appetite. On the other hand, there is a rather close relationship between absolute ash weight and the percentage of ash as indicated by the highly significant simple correlation coefficient of .690 and partial
Sums of squares Source D.F. oj'ster .of. variation shell (1) Total Between Sires Within Sires
perwt. of cent ash ash (2) (3)
Sums of products
1.2
1.3
Simple correlation coefficients
Necessary .Partial correlevels at lation error points coefficients
2.3 .05
.01 12.3 13.2 23.1
5.3 1 299 197 - . 0 3 0 690 .532
661 .135 - . 2 3 4 .698
12
23
23
17 35334.0 1.1040 4.00 24.57 - 7 1 . 2 1.331 .124 - . 1 8 9 .634 4 25701.2 0.0161 0.74 13
4.41 - 6 5 . 9 0.032 .217 - . 4 7 8 .294
9632.8 1.0879 3.26 20 16 -
a correspondingly higher proportion of organic constituents within the shell thus leaving the percentage of ash essentially unaltered. Fortunately weekly measurements of individual bird oyster shell consumption, as well as absolute and percentage ash weights for individual eggs, are available. Table 4 shows the analysis of variance and covariance for oyster shell consumption, absolute weight of ash in egg shell and percentage of ash in egg shell. This table was calculated from the average measurements of full sister families, and shows the net simple and partial correlation coefficients with the effect of the sire removed. This table reveals interesting facts concerning the mechanism of calcium utilization. Very briefly, it is plainly evident that
coefficient of .698. There are two possible explanations for this, viz., either the family ability to extract and secrete a larger than average proportion of ingested calcium is the causal agent of the increased percentage of shell ash in the same family or these characteristics are both controlled by, and are expressions of, the genotype and have no direct causal association. This point can be settled by investigating the relationship between the variations in absolute ash weight and percentage of ash between eggs from the same bird. For this purpose the data from 23 representative hens, laying a total of 131 analyzed eggs during the experimental period, were subjected to an analysis of variance and covariance. The summarized data in Table 5 clearly show that the correlation coefficient of
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TABLE 4.—Analysis of variance and covanance, showing the relationship between oyster shell consumption, absolute weight of egg shell ash, and the percentage of ash in the shell. Both net simple and partial correlation coefficients are shown
EFFECT OF HEREDITY ON EGG QUALITY
25
.569 representing the association of the characters over the entire 131 eggs is reduced to .290 when the effect of differences between individual hens are removed. The net correlation (.290) while much reduced is nevertheless highly significant, and indicates that the percentage ash in the shell of an individual egg is governed, to a small degree, by the amount of calcium deposited by the uterine glands during the period of shell formation. In this connection, however, the fact that the shell membranes were not cleared from the shell before ashing must be considered. Since these membranes are al-
and secretes calcium, also, but largely independently, decree the proportion of calcium in her egg shells. In view of the indications of genetic control previously shown in Table 3, it is but logical to assume that the genotype of the individual plays a large part in this control. DISCUSSION With respect to the various points of egg quality under investigation it seems clear that, while each and every trait is distinctly influenced by the individual hen (Table 1), only yolk weight and the quantity of the
Ash weight Source of variation
Total Between hens Within hens
Degrees of freedom
130 22 108
Sum of squares 18.837 15.371 3.466
Mean square 0.1449 0.6987 0.0321
Covariance
Percent ash Sum of squares
Mean square
Sum of products
53.180 30.221 22.959
0.4091 1.3737 0.2126
18.0174 15.4323 2.5851
most entirely organic in composition it is clear that the deposition of a thicker shell, i.e., the secretion of a large absolute amount of calcium, will result in an increase in the percentage of ash because of the comparative decrease of organic membrane material even if the organic constituents of the shell proper remain proportionally undisturbed. The inclusion of this relatively constant organic membrane in the dry weight of the shell is probably sufficient to account for the net correlation coefficient of .290. Thus, irrespective of the absolute amount of calcium secreted in forming the shell of the individual egg, the percentage of mineral remains essentially unaltered. The very high correlation of .716 between the mean measurements of individual hens shows that the factors which determine the efficiency with which the individual bird absorbs, mobilizes,
r
.569 .716 .290
Values of r at error points .05
.01
.171 .413 .188
.224 .526 .246
inorganic shell material are affected by the coincidence of parentage (Table 3). Since total egg weight is definitely known to be controlled, in part, by heredity, it is to be expected that yolk weight, being but a component part of the whole, would also be so controlled. On the other hand, the idea of a genetic control over the mineral content of the shell is a new and fascinating one. This becomes all the more interesting because the solid matter of the shell is almost entirely calcium carbonate and the possibility of a gene control over the complex process of calcium metabolism is of fundamental biologic importance. In this study, sufficient data are available to indicate in a general way the time and manner of this control. Thus, it is shown that individual differences are expressed in two largely independent functional phases; "efficiency" genes present
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T A B I E 5.—Analysis of variance and covariance showing the relation between the absolute and the percentage ash weight of 131 eggs from 23 hens
26
S. S. MUNRO
SUMMARY
1. The following seven egg quality characteristics were found to be markedly a function of the individuality of the hen:
Percentage firm white, condition of firm white, yolk weight, yolk color, breaking strength of vitelline membrane, percentage ash of dry shell, percentage ash of total egg. 2. Environmental factors are responsible for much of the variability in percentage firm white, yolk weight, yolk color, and breaking strength of vitelline membrane since these traits were measured over fourweek periods and show significant differences between the periods. 3. Three of the characteristics, viz., yolk weight, percentage ash of dry shell, and percentage ash of total egg, are definitely family characteristics and thus are probably influenced by the genotype. 4. Since the ash is composed almost entirely of calcium oxide, there must be a gene control over calcium utilization. 5. By the method of partial correlation it is shown that the amount of oyster shell consumed has no effect on either the amount of calcium secreted or on the percentage calcium in the shell. The genes concerned with calcium utilization, therefore, produce their effect by regulating the efficiency with which a given intake of calcium is absorbed, mobilized in the tissues, and secreted in the form of egg shell. 6. It is further shown that families and individuals which secrete a large quantity of calcium in the form of egg shell also produce shells containing a high percentage of calcium. However, the absolute amount of calcium in the shell and its percentage occurrence therein are not causally related since there is little or no correlation between these characteristics in different eggs from the same hen. 7. It is argued that the effect of the gene on calcium metabolism is exercised on at least two independent functional phases, viz., (1) consisting of absorption and mobilization and culminating in secretion and (2) concerned with the regulation of the proportionate deposition in the egg shell
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in certain individuals determine: first, that these individuals shall transform into egg shell a large proportion of ingested calcium arid, second, that they shall also produce an egg shell containing a high and relatively constant percentage of calcium regardless of minor daily variations in their control over the first phase. It has been shown by Deobald, Lease, Hart, and Halpin (1936) that increased levels of parathormone will produce a marked rise of the blood calcium in the actively laying bird. Gene control of the first functional phase may, therefore, be a control over the activity of the parathyroid. At any rate, the above authors have shown that individual laying birds are characterized by constant differences in the serum calcium level. This difference must be brought about by a differential rate of intestinal absorption. On the other hand this differential in intestinal absorption might be due to a gene control over cell absorption ability on a constant parathormone level. In any case, it is clear that the second functional phase, in which the percentage constituents in the shell are determined, cannot be due to parathormone control since it varies independently of the first and the two phases occur simultaneously. The second phase must be due, therefore, to a fixed cellular control. In view of this it seems likely that both phases are controlled by a common fixed cellular (genie) control but that variations in parathormone level from day to day provide an element of variability in the first phase only. Greater light can be shed on the control mechanism by a study of the relationships between serum calcium level, rate of calcium secretion, and percentage of calcium in the egg shell.
EFFECT OF HEREDITY ON EGG QUALITY
of organic and inorganic materials. 8. The possible manner of gene control is discussed and it is suggested that both phases are simultaneously controlled within the individual at a fixed level by a cellular complement of "efficiency" genes. The first phase, however, is also subject to an independent physiological variability, possibly under the control of parathyroid activity. REFERENCES
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