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The Heritability of Accumulative Monthly and Annual Egg Production
The Heritability of Accumulative Monthly and Annual Egg Production I. M I C H A E L L E R N E R AND D O R O T H Y M .
CRUDEN
University of California, Berkeley (Received for publication August 9, 1947)
T
HE breeding of domestic animals for economic purposes has for centuries proceeded on the basis of trial and error methods, and by and large is still carried on in the same manner today. Although Wright (1921) more than a quarter century ago laid down most of the genetic principles upon which breeding schemes with predictable rates of gain could be established, it is only recently that, through the efforts of Lush (e.g., 1945) and of his students, the descriptive data which have been accumulating for many years have been subjected to analyses necessary for the formulation of efficient breeding plans. The validation by empirical tests of the theoretical principles deduced by Wright has been slow, largely because of the necessity of collecting data for a long period of time and of the laborious statistical analysis required. With respect to egg production in the domestic fowl such verification has been recently made available by Lerner and Hazel (1947), and further confirmation on the same material though using different methods was also obtained by Dempster and Lerner (1947). These investigators demonstrated that the principle originally enunciated by Dickerson and Hazel (1944) is empirically verifiable. This principle states that the rate of progress expected under selection is equal to the intensity of selection multiplied by heritability and divided by the interval between generations. An extremely close agreement was obtained between calculated and observed gains in both cases.
Thus it is now possible to proceed with the investigation of the consequences of the demonstrated dependence of the rate of genetic progress on the factors mentioned, and the present paper in part is intended as a contribution to that end. However, the main purpose of this study is the analysis of the heritabilities and genetic correlations of egg production records for varying periods of time, as a preliminary to the eventual goal of determining the most efficient selection criteria and mating plans. THE ACCUMULATIVE EGG PRODUCTION OF SURVIVORS
The primary object of the breeder of poultry for egg production is to improve the genetic characteristics of his stock with respect to the number and quality of eggs. There are many relatively independent traits which enter into the final criterion of worth. Eventually, it should be possible by applying the principles of index construction elaborated by Hazel (1943) to establish the most efficient "total score" indexes of selection (see Hazel and Lush, 1942). It is first necessary, however, to analyze the single characters themselves. As is ultimately the case with all traits of economic value, these "single" characters can be resolved into a number of components of varying degrees of independence and value from the breeder's standpoint. Thus with respect to the annual record notable attempts have been made by Goodale (1918) and others to resolve it into such con67
68
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
tributing traits as sexual maturity, intensity, broodiness, persistency of production, and winter pause. Another approach has been utilized by many workers, such as Pearl and Surface (1911) who considered the production during different seasons of the year as such components. It is not possible to evaluate the relative merits of these two types of analysis, until actual selection indexes are constructed on the different bases and expected rates of gain computed. It is only too likely that a combination of the two methods may prove to be more efficient than either. The fact is that both require investigation. The present study deals with the second of the mentioned approaches with a slight modification. Instead of considering egg production in the different periods or months of the year, accumulative records from the beginning of production'to the end of successive calendar months are considered here and correlated with the annual production (defined as the number of eggs laid from the onset of production to September 30 of the year following the year of hatch). Although such correlations are spurious in the sense that the whole and its parts are correlated, they are valid for the purpose of predicting the complete record from the part record, sinqe in breeding practice accumulative production records are routinely computed. The material used in the present study consists of the production records of birds which survived their first laying year. Mortality must be considered by a breeder as a component of the annual flock record. It however, forms another "single" character which needs a separate investigation. MATERIAL
The flock of Single Comb White Leghorns used in this study consists of two groups which were hatched, raised and
maintained indiscriminately together. One group constitutes the production-bred fraction of the flock, the general characteristics of which have been previously described by Lerner and Hazel (1947). The other contains birds selected for various purposes and includes a number of relatively non-interbreeding populations of common origin. These lines were selected for high and low shell thickness, body size, winter pause, the tendency to produce blood spots, persistency, and susceptibility to lymphomatosis. Because of the small number of mating pens devoted to each purpose they are considerably more inbred than the production line. Furthermore, while a relatively high degree of inter-se relationship prevails among the production birds, the other lines are related to each other only slightly, diverging at different points in time from the common stem of descent. Because of these differences separate analyses were undertaken for each of the two parts of the flock, as well as for the combined flock. Two series of birds (S and T) hatched in successive years were included in the study separately and in combination. The populations selected contained all of the birds in the designated lines which survived till the end of their first laying year (no culling being practiced during that period) which met the requirement of family size. This was arbitrarily set at a minimum of three full sisters per family with a minimum of three families per sire. The number of birds involved and their mean records are shown in Table 1 and the respective standard deviations in Table 2. Some variation in family size occurred but estimates of standard deviations of family size (1.93 for full-sister families and 9.26 for half-sister families) indicate that the effects of the correction for this factor in the analysis of variance
69
HERITABILITY OF EGG PRODUCTION TABLE 1.—Mean egg production to the end of the designated month Production line Series 5 Series T Number of birds October November December January February March April May June
July
August September
111 30.6 50.9 71.1 89.0 107.0 130.6 154.4 178.2 198.6 216.6 231.9 244.0
226 36.6 59.8 80.1 99.3 116.9 140.3 164.1 188.6 211.5 233.8 253.3 268.1
Total
Other lines All
337 34.6 56.9 77.2 95.9 113.7 137.1 160.9 185.2 207.3 228.1 246.2 260.1
Series 5 Series T 235 29.7 47.6 65.8 82.5 100.7 124.4 148.5 172.4 193.4 213.4 232.4 246.7
(Winsor and Clarke, 1940) would be negligible., ANALYSIS OF HERITABILITY
The determination of the heritability of egg production was based on the design shown for the combined series of birds in
225 29.3 50.8 70.4 89.0 105.6 128.6 152.8 176.9 199.2 220.8 238.6 252.0
All
Series 5 Series T
460 29.6 49.2 68.0 85.7 103.1 126.4 150.6 174.6 196.2 217.0 235.4 249.3
346 30.0 48.7 67.5 84.6 102.7 126.3 150.4 174.3 195.0 214.5 232.2 245.9
451 33.0 55.3 75.3 94.2 111.3 134.5 158.5 182.8 205.4 227.3 246.0 260.0
All 797 31.7 52.4 71.9 90.0 107.6 130.9 155.0 179.1 200.9 221.7 240.0 253.9
Table 3. The components of variance can be isolated from such analyses and heritability estimated by considering that component A contains all of the environmental variance and half of the genetic variance, while components B and C each contain one-quarter of the genetic variance. (This
DEFINITION OF TERMS AND SYMBOLS Symbol Definition X The egg production from the beginning of lay till September 30 of the year following the year of hatch. The complete or the full record. Xx The egg production from beginning of lay to the end of the month X. Part record The initial letter of the month is used as a subscript. Thus, for the part record ending with December, X=D. Xs is equivalent to the annual record. A Component of variance between full sisters. Variance Components B ' Component of variance between dams. C Component of variance between sires. Heritability hx The additively genetic fraction of the variance of X. The square of the correlation coefficient between phenotype and genotype. Phenotypic correlation ra The observed correlation between characters 1 and 2. Genetic correlation r c ^ j The correlation between the genotypes for characters 1 and 2. Environmental correlation fE^t The proportion of the correlation between characters 1 and 2 that is due to common environment. Amount of genetic gain AGX The fraction of each standard deviation of the selection differential realized as a gain in annual production per generation when selection is based on character X. In table 8 the figures represent twice the gain expected from the selection of females. H The weighted sum of genotypes for a complex of characters. Aggregate genotype Economic weight Cx The relative economic value of a unit of character X. Selection Index I The multiple regression equation maximizing rmMultiple regression coefficients The regression coefficient for the complete record selected so as to maximize miThe regression coefficient for the part record selected as above. Term Annual record
70
I. MICHAEL LERNER AND DOROTHY M. CRUDEN TABLE 2. —Standard deviations of egg production to the end of the designated month
Production line October November December January February
March April May June July August September
Series 5 Series T 18.08 16.07 21.69 17.87 24.12 20.05 26.75 23.68 28.58 25.84 29.82 27.04 31.25 28.53 33.64 29.94 36.36 32.06 40.66 34.58 45.09 38.25 51.57 42.70
Other lines
All 16.75 19.21 21.47 24.73 26.77 27.98 29.45 31.20 33.53 36 ..68 40.99 45.80
Series 5 Series T 17.13 15.41 17.92 20.69 23.85 20.44 26.44 23.70 27.44 26.67 28.56 28.48 30.03 30.25 31.89 31.85 35.01 34.18 38.83 36.86 40.42 43.06 47.46 45.74
partitioning is not rigorously accurate, since in the other lines C contains line differences and hence represents somewhat more than a quarter of the genetic variance.) The coefficients of B and C in Table 3 represent the average number of daughters per dam and per sire respectively. It is obvious that three different estimates of heritability are possible. \B (1) (3)
4C
(2)
A+B+C
A + B+C
2{B+C) A+B+C
Some sampling variation may be expected between these estimates, but in the absence of any significant trend with respect to the relative magnitudes of the first and the second estimates, the third one may be accepted as the most reliable one. Table 4 lists all three of the estimates for the various sub-groups of birds,
Total All 16.31 19.38 22.25 25.14 27.07 28.52 30.14 31.87 34.61 37.88 41.79 46.63
Series 5 Series T 17.44 15.74 21.01 17.89 23.94 20.25 26.54 23.69 27.81 26.26 28.97 27.77 30.43 29.40 32.46 30.91 35.44 33.13 39.42 35.74 44.06 39.34 48.81 44.24
All 16.50 19.31 21.92 24.97 26.94 28.29 29.85 31.59 34.16 37.38 41.45 46.28
while Table 5 shows the variance ratios to indicate the relative reliability of the different estimates made. The columns under the heading of "Heritability" in Table 6 give the estimates obtained from pooling the data for the two series on the basis of the third type of estimate. It may be seen from the latter figures that the heritabilities in the production line are lower in general than those of the other lines. This was to be expected since the latter include several non-interbreeding sub-populations. However, the combination of all lines gives remarkably uniform heritability values for each of the periods investigated, the complete range of estimates being from about 29 to 37 percent. These figures show a very good agreement with the only other comparable heritability determination made for annual egg production of survivors, where the value of 34 percent was presented by Shoffner (1946). Lines 9 and 18 of Table 5, which give the lvalues cor-
TABLE 3.—Experimental design Dther Lines
Production Line Source of variance
Total Between sires Between dams within sires Between full sisters
Degrees of freedom 335 14 53 268
Composition of mean square A+i.9B+2l.lC A+4.9B A
Degrees of freedom 458 20 74 364
Composition of mean square A+t. SB+20.9C A+i.SB A
Total Degrees of freedom 793 34 127 632
Composition of mean square A+i.8B+21.0C A+i.SB A
71
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HERITABILITY OF EGG PRODUCTION
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73
HERITABILITY OF EGG PRODUCTION
responding to the heritability of the total population in Table 6 show significance to be in each case below the one percent probability level, so that these estimates may be viewed with considerable confidence. It seems then that the heritability of egg production throughout the year is reasonably uniform and can be considered to have a value approximating 33 percent. In a closed flock which has been selected for high production and inbred to some extent this value may be approximately cut in half. There is one additional point here that may be mentioned in view of the discussion on selection indexes to follow. It may be noted that in the production flock the heritabilities of the early records are in general higher than those for more extended periods of time. This is most readily explained by the fact that the selection practiced in the productionbred flock was based to a great degree on complete and not on partial records. Consequently, the reduction in heritability was greater in the former than in the latter. THE CORRELATIONS BETWEEN PART AND FULL RECORDS There has been a great amount of data presented by various investigators
on the correlation coefficients between records for part of the year and the annual record. Most of them, however, did not deal with egg production in an accumulative manner, nor did they consider any but the phenotypic correlations. The values reported by different investigators working with different breeds are, however, on the whole rather uniform. For instance, a sampling of the correlation coefficients between winter production (from beginning of lay to March 1, or from November 1 to March 1) and annual production produces values between 0.62 and 0.69 (Ball and Alder, 1917; Card, 1917; Hays, Sanborn and James, 1924; Hervey, 1924; Thompson and Jeffrey, 1936). Similar phenotypic correlations were computed for the data utilized in this paper for the production line, the other lines and the total population. The coefficients comparable to the ones cited above are somewhat higher in our case but not unreasonably so (Table 6). The technique used by Hazel, Baker and Reinmiller (1943) for isolating the genetic and environmental parts of the phenotypic correlation was extended to the analysis, and the values obtained for the estimates from both sires and dams are also listed in Table 6. Estimates of genetic correlation
TABLE 6.—Estimates of heritability and of correlations between annual production and production to the end of the month designated Production Line Heritability October November December January February March April May June July August September
0.342 0.308 0.231 0.221 0.204 0.170 0.158 0.145 0.141 0.117 0.133 0.147
Pheno- Environtypic mental .458 .555 .656 .713 .739 .788 .830 .876 .916 .951 .987 1.000
.442 .532 .623 .690 .754 .773 .818 .866 .906 .949 .986 1.000
Total
Other Lines
Correlation
Correlation Genetic 0.574 0.695 0.819 0.839 0.780 0.874 0.898 0.939 0.976 0.973 0.991 1.000
Heritability 0.256 0.273 0.351 0.479 0.481 0.461 0.450 0.463 0.470 0.477 0.489 0.500
Pheno- Environtypic mental .423 .563 .666 .733 .780 .856 .861 .899 .933 .959 .982 1.000
.551 .607 .631 .661 .713 .850 .798 .846 .894 .913 .972 1.000
Correlation Genetic 0.239 0.534 0.732 0.813 0.853 0.866 0.936 0.960 0.978 0.989 0.993 1.000
Heritability 0.294 ' 0.288 0.304 0.373 0.353 0.342 0.332 0.335 0.339 0.333 0.345 0.356
Pheno- Environtypic mental .438 .560 .662 .725 .763 .828 .848 .890 .926 .956 .984 1.000
.495 .566 .624 .676 .718 .808 .808 .857 .900 .941 .980 1.000
Genetic 0.319 0.553 0.742 0.815 0.849 0.867 0.928 0.954 0.978 0.986 0.993 1.000
74
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
coefficients from the combined sires and dams are presented in Table 7. Although some sampling variation leads to impossible estimates above unity in a few instances, the generally uniform trend inspires confidence in the validity of the last column of Table 6 (repeated as the last row in Table 7). Thus about 50 percent of the genetic variance in annual record is accounted for by the part record to
method of selection practiced. However the relative efficiency of selection on the basis of part records may be established very readily. For each standard deviation in the selection differential the gain expected per generation is h in the limit. If the . criterion of selection is not the character itself but a correlated character the rate becomes hxrasGx where the last term is the genetic correlation between
TABLE 7.—Estimates of genetic correlation from combined sires and dams between annual production and production to the end of the designated month Decem- Janu- Febru- March April October November ber ary ary
Line
Series
May
June
July
August
Production
S T S+T
0.822 0.457 0.574
0.923 0.552 0.695
0.970 0.690 0.819
0.948 0.750 0.839
0.925 0.797 0.780
0.914 0.879 0.874
0.900 0.952 0.898
0.926 1.004 0.939
0.972 0.984 0.976
0.997 0.990 0.973
1.011 0.981 0.991
Others
S T S+T
-0.112 0.523 0.239
0.323 0.708 0.534
0.631 0.840 0.732
0.729 0.930 0.813
0.766 0.942 0.853
0.732 0.952 0.866
0.893 0.969 0.936
0.930 0.983 0.960
0.960 0.991 0.978
0.986 0.991 0.989
0.995 0.992 0.993
Total
S T S+T
0.110 0.450 0.319
0.460 0.617 0.553
0.699 0.790 0.742
0.772 0.887 0.815
0.796 0.917 0.849
0.788 0.938 0.867
0.890 0.964 0.928
0.924 0.983 0.954
0.962 0.992 0.978
0.988 0.985 0.986
0.998 0.989 0.993
the end of December, and about 90 percent by the part record to the end of May. The genetic correlations shown in Table 6 are higher than the corresponding phenotypic correlations in the production line in all of the first eleven months and in nine of the eleven months in the other lines. This illustrates that the genes which influence egg production are more persistent in their effects than are the environmental factors, the effects of the latter being more transient in nature. This same persistency of genie effects has been demonstrated for rate of gain by Hazel and co-workers (1943) for swine and by Knapp and Clark (1947) for beef cattle. THE EXPECTED GENETIC GAIN FROM MASS SELECTION The constants computed to this point permit prediction of the rates of gain expected in the character under selection. The actual rate will, of course, depend on the intensity of selection as well as on the
the character selected and the trait desired. These values are presented in Table 8 and apply to selection on basis of individual performance. Selection based on family records will favor the part records more than does mass selection. However, the actual gains realized under field conditions will probably be considerably less TABLE 8.—Comparative amounts of genetic gain
Per generation in annual egg production from mass selection based on the record to the end of the designated month
Month October November December January February March
April May June July August September
AG*
AG* (hjaxai)
AG,
0.173 0.297 0.401 0.498 0.504 0.506 0.535 0.552 0.569 0.569 0.583 0.597
0.290 0.497 0.672 0.834 0.844 0.848 0.896 0.925 0.953 0.953 0.977 1.000
I
HERITABILITY OF EGG PRODUCTION
than might be assumed from the first column of Table 8, because the heritability estimates used here are for the combined lines, and because selection for other characters which may be genetically antagonistic to egg production must be also practiced. In order to evaluate the relative efficiency of selection for the annual record by using the part records as selection criteria, the ratios of these values to the gains expected when selection is based on the complete record itself were computed, and are also presented in Table 8. SOME PRACTICAL CONSEQUENCES
The following considerations apply only to the production records of surviving birds. The interrelation of mortality and egg production is not considered in this study but information is available that the general trend of the conclusions presented here is not invalidated by considering mortality and egg production simultaneously. This information appears in the paper by Dempster and Lerner to which reference is made on several occasions below. The amount of genetic gain per generation when selection is practiced on the basis of part records to the end of December is about two-thirds of that expected if selection were based on the full annual record. Since it is the customary practice in breeding to select breeding birds about January, the breeder may either base his selections on the part records to that date and use one-year old birds for mating, or if the complete records are used, mate the birds up when they are two years old. Since in the latter case the interval between generations will be doubled, it is clear that greater rate of improvement can be maintained if the first procedure is followed. Thus the analysis presented indicates that the use of younger birds with
75
part records rather than older birds with full records leads to more rapid gains. The exact rates of efficiency expected depend on the selection scheme practiced (individual performance, sister test, progeny test), as well as on the age of the male birds mated. The whole problem of the optimum age composition of the breeding flock is analyzed in detail by Dempster and Lerner, but the general trend may be clearly observed in the present data. The cost of trap-nesting constitutes one of the heaviest expenses in breeding operations. The use of part-time trap-nesting could reduce this cost materially. If the breeder follows the practice of mating oneyear old birds he would need to continue trap-nesting after January 1 only the birds selected for breeding and their full sisters (since the full records of families are needed for the purpose of a second selection at two years of age, as is shown by Dempster and Lerner) and thus effect considerable savings. The remaining birds could be maintained without trap-nesting for the purpose of obtaining flock production averages. A previous report by Lerner and Taylor (1940) anticipated this conclusion. It was shown there that Tschuproff's coefficient of association between the part records of the dams and their sisters with the complete records of the daughters (including mortality effects) did not differ greatly from the coefficient of association of the latter with the full records of dams and aunts. A variation of this scheme would be to suspend trap-nesting on the whole flock in the summer of their first year of laying. It may be seen from Table 6 that the genetic correlation between production to the end of May, for instance, and the annual production is about 0.95. This means that the efficiency of selection on the basis of such a part record is about 92 percent of that obtained from selecting from com-
76
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
plete records (Table 8). It is quite likely that the saving effected by eliminating trap-nesting for the remaining five months more than compensates for the eight per cent loss in efficiency. A somewhat different facet of practical significance lies in the application of these findings to the construction of selection indexes. Hazel (1943) presented methods of determining the proper weights to be assigned to each of several characters for which selection is practiced. In this particular case we may consider annual production as the prime desideratum or the aggregate genotype in Hazel's terminology. The two component characters here are production to the end of December and production following January 1st. A "total score" index which would take in account the heritabilities of these components and the genetic correlation between them would as shown by Hazel and Lush (1942) lead to greater efficiency in improvement of the annual record than if some other criterion of selection for these components is used. We may first construct an index using the part record to the end of December as our first character (XD) and the full record as the second (X), with the selection index being defined as
I = aXD+bX, a and b being the respective weights assigned to characters XD and X. Following Hazel's procedure the regression coefficients a and b may be obtained by solving the simultaneous equations (rxDa-\-rxDxaxb =*rxDH0H, and rxDx
record. The products rxDHGH and rxWH may be solved in terms of CXQ and Cx from TXDH(TH = hxD{hxD
rhx'JxfGx
QxCx),
and rxH
(X-XD).
In other words the most efficient index for selection places one and three-quarters times more emphasis on the production before January than on the production after January. Since the actual economic worth of the eggs produced in the first period is less than that of the eggs produced in the second period (due to their smaller size) the factor 1.74 is probably an overestimate. It is possible to approximate the more correct coefficient by setting Cx>CxD- A sample of 69 birds from the flock under study was investigated with respect to the distribution of egg size in the periods corresponding to XD and X and their respective values were computed by using the market price of eggs (obtained from average San Francisco wholesale quotations for the ten-year period 1936-1945). The ratio CxD/Cx in this manner was found to be 0.867, and the index recomputed to I = 0.66XD+X,orI
=
1.66XD+(X-XD).
All of the estimates presented in this section err on the conservative side with
HERITABILITY OF EGG PRODUCTION
respect to the production-bred flock. As noted previously, the ratios of the heritabilities of the part records to that of the full record are higher in the production line than in the others. Hence, if the AG's are computed for the production line separately, higher ratios of AGx/AGs are obtained than those presented in Table 8. For instance, AGD/AGS rises above unity in this case as compared to 0.672 in Table 8. Hence the weights assigned to the early record in the indexes computed are, if anything, underestimates. It thus appears that in the flock considered the selection of chickens for improved annual egg production more weight should be placed on the early than on the late production. The greatest economic efficiency will be obtained if birds selected on the basis of their part records are mated at one year of age. When two-year old birds are selected on the basis of their complete records, the criterion of selection should give more emphasis to their production before January of their first laying year than to the production beyond that date. SUMMARY
The heritability of accumulative egg production in a White Leghorn flock was found to be nearly constant throughout the year and in the neighborhood of 33 percent. In the part of the flock which had previously been subjected to selection for egg production the heritability was lower. However, less decrease was shown in the early phases of production than in the later, verifying the fact that more emphasis in selection had been placed on the full than on the part record. Genetic correlations between part and full production were found to be high, leading to the possibility of efficient selection on the basis of part records more economically than on the basis of full
77
records. The decrease in interval between generations incident on the possible use of younger birds in breeding when partrecord selection is used suggests modification of standard breeding procedures, a point more fully explored in another publication. Even if selection is based on complete records, the analysis indicates that production prior to January should be given at least 1.66 times the weight assigned to subsequent production. LITERATURE CITED
Ball, E. D., and B. Alder, 1917. Breeding for egg production. Part II. Utah Agr. Exp. Sta. Bull. 149. Card, L. C , 1917. A study of egg production in the White Leghorn. Storrs Agr. Exp. Sta. Bull. 91. Dempster, E. R. and I. M. Lerner, 1947. The optimum structure of breeding flocks. Genetics 32 (in press). Dickerson, G. E. and L. N. Hazel, 1944. Effectiveness of selection on progeny testing performance as a supplement to earlier culling in livestock. J. Agric. Res. 69:459^76. Goodale, H. D., 1918. Internal factors influencing egg production in the Rhode Island Red breed of domestic fowl. Am. Nat. 52: 65-94, 209-232, 301-321. Hays, F. A., R. Sanborn, and L. L. James, 1924. Correlation studies on winter fecundity. Mass. Agr. Exp. Sta. Bull. 220. Hazel, L. N., 1943. The genetic basis for constructing selection indexes. Genetics 28:267-490. — — , M. L. Baker and C. F. Reinmiller, 1943. Genetic and environmental correlations between the growth rates of pigs at different ages. J. Animal Sci. 2:118-128. , and J. L. Lush, 1942. The efficiency of three methods of selection. J. Hered. 33:393-399. Hervey, G. W., 1924. Seasonal and annual eggproduction-correlation tables. N. J. Agr. Exp. Sta. Bull. 402. Knapp, B., Jr. and R. T. Clark, 1947. Genetic and environmental correlations between growth rates of beef cattle at different ages. J. Animal Sci. 6: 174-181. Lerner, I. M. and L. N. Hazel, 1947. Population genetics of a poultry flock under artificial selection. Genetics 32:325-339. , and L. W. Taylor, 1940. A note on the use of short-time trap-nesting in breeding selection. Poultry Sci. 19: 187-190.
78
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
Lush, J. L., 1945. Animal breeding plans. 3-rd ed. Ames, Iowa: Collegiate Press. Pearl, R. and F. M. Surface, 1911. A biometrical study of egg production in the domestic fowl. II. U. S. Dept. Agr., Bur. Anim. Ind. Bull. 110. Shoffner, R. N., 1946. The heritability of egg production. Poultry Sci. 25:412. Thompson, W. C. and F. P. Jeffrey, 1936. The use-
fulness of winter and summer-fall egg yield records as criteria of poultry breeder selection. N. J. Agr. Exp. Sta. Bull. 612. Winsor, C. P. and G. L. Clarke, 1940. A statistical study of variation in the catch of plankton nets. J. Marine Res. 3:1-34. Wright, S., 1921. Systems of mating. Genetics 6: 111-178.
News and Notes {continued from page 66)
the food inspection service. He will be engaged in poultry research projects. Dr. G. M. Briggs, formerly of the Poultry Department, University of Maryland, is now at the University of Minnesota. P. E. Bernier, formerly Chief R.O.P. Inspector for the Dominion of Canada, is now Associate Professor of Poultry Husbandry, Oregon State College, Corvallis. Prof. Dr. Miecszyslaw Cena, The Institution for Breeding Small Animals, University and School of Technology, Wroclaw, Poland, would appreciate receiving reprints of recent research for the library of the Institute. They are particularly interested in any literature on breeding, disease, and management of poultry. The Delaware Substation at Georgetown, Delaware, has been chosen as the site for the 1948 Chicken-of-Tomorrow contest. Mr. Karl Seeger and Professor A. E. Tomhave will assist in conducting the contest. Forty contestants have been notified by Mr. H. L. Shrader, Chairman of the Procedure and Awards Committee, of the acceptance of their entries by the National Committee. From the two cases of hatching eggs of each entry it is planned to start 400 chicks for a 12-week growing period. Any suggestions regarding
methods for conducting the test or collecting and analyzing the records should be sent to Mr. H. L. Shrader, Poultry Extension Service, U.S.D.A., Washington, D. C. The Eighth Annual Conference of the Institute of Food Technologists will be held in Philadelphia, Pa., June 6-10, 1948, at the Benjamin Franklin Hotel. A number of foreign delegates are expected to attend the Conference. The American Economic Committee for Palestine, 250 West 57th Street, New York 19, have prepared a 3-reel silent 16 mm. moving picture and a 144-page cardboard-covered book entitled "The American Poultry Industry as Applied to Palestine" by Simon Bornstein. The film may be borrowed from the U.S.D.A., Cornell or Rutgers Universities. The book may be purchased for $2.00. A National Egg Products Association with headquarters at 110 North Franklin Street, Chicago 6, was established October 1, 1947 by 90 commercial firms engaged in processing frozen and dried eggs. Dr. O. J. Kahlenberg is director and has announced that the first project will be to determine whether the breed of laying hens is a factor in improving the quality of frozen egg whites.
CRUDEN
University of California, Berkeley (Received for publication August 9, 1947)
T
HE breeding of domestic animals for economic purposes has for centuries proceeded on the basis of trial and error methods, and by and large is still carried on in the same manner today. Although Wright (1921) more than a quarter century ago laid down most of the genetic principles upon which breeding schemes with predictable rates of gain could be established, it is only recently that, through the efforts of Lush (e.g., 1945) and of his students, the descriptive data which have been accumulating for many years have been subjected to analyses necessary for the formulation of efficient breeding plans. The validation by empirical tests of the theoretical principles deduced by Wright has been slow, largely because of the necessity of collecting data for a long period of time and of the laborious statistical analysis required. With respect to egg production in the domestic fowl such verification has been recently made available by Lerner and Hazel (1947), and further confirmation on the same material though using different methods was also obtained by Dempster and Lerner (1947). These investigators demonstrated that the principle originally enunciated by Dickerson and Hazel (1944) is empirically verifiable. This principle states that the rate of progress expected under selection is equal to the intensity of selection multiplied by heritability and divided by the interval between generations. An extremely close agreement was obtained between calculated and observed gains in both cases.
Thus it is now possible to proceed with the investigation of the consequences of the demonstrated dependence of the rate of genetic progress on the factors mentioned, and the present paper in part is intended as a contribution to that end. However, the main purpose of this study is the analysis of the heritabilities and genetic correlations of egg production records for varying periods of time, as a preliminary to the eventual goal of determining the most efficient selection criteria and mating plans. THE ACCUMULATIVE EGG PRODUCTION OF SURVIVORS
The primary object of the breeder of poultry for egg production is to improve the genetic characteristics of his stock with respect to the number and quality of eggs. There are many relatively independent traits which enter into the final criterion of worth. Eventually, it should be possible by applying the principles of index construction elaborated by Hazel (1943) to establish the most efficient "total score" indexes of selection (see Hazel and Lush, 1942). It is first necessary, however, to analyze the single characters themselves. As is ultimately the case with all traits of economic value, these "single" characters can be resolved into a number of components of varying degrees of independence and value from the breeder's standpoint. Thus with respect to the annual record notable attempts have been made by Goodale (1918) and others to resolve it into such con67
68
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
tributing traits as sexual maturity, intensity, broodiness, persistency of production, and winter pause. Another approach has been utilized by many workers, such as Pearl and Surface (1911) who considered the production during different seasons of the year as such components. It is not possible to evaluate the relative merits of these two types of analysis, until actual selection indexes are constructed on the different bases and expected rates of gain computed. It is only too likely that a combination of the two methods may prove to be more efficient than either. The fact is that both require investigation. The present study deals with the second of the mentioned approaches with a slight modification. Instead of considering egg production in the different periods or months of the year, accumulative records from the beginning of production'to the end of successive calendar months are considered here and correlated with the annual production (defined as the number of eggs laid from the onset of production to September 30 of the year following the year of hatch). Although such correlations are spurious in the sense that the whole and its parts are correlated, they are valid for the purpose of predicting the complete record from the part record, sinqe in breeding practice accumulative production records are routinely computed. The material used in the present study consists of the production records of birds which survived their first laying year. Mortality must be considered by a breeder as a component of the annual flock record. It however, forms another "single" character which needs a separate investigation. MATERIAL
The flock of Single Comb White Leghorns used in this study consists of two groups which were hatched, raised and
maintained indiscriminately together. One group constitutes the production-bred fraction of the flock, the general characteristics of which have been previously described by Lerner and Hazel (1947). The other contains birds selected for various purposes and includes a number of relatively non-interbreeding populations of common origin. These lines were selected for high and low shell thickness, body size, winter pause, the tendency to produce blood spots, persistency, and susceptibility to lymphomatosis. Because of the small number of mating pens devoted to each purpose they are considerably more inbred than the production line. Furthermore, while a relatively high degree of inter-se relationship prevails among the production birds, the other lines are related to each other only slightly, diverging at different points in time from the common stem of descent. Because of these differences separate analyses were undertaken for each of the two parts of the flock, as well as for the combined flock. Two series of birds (S and T) hatched in successive years were included in the study separately and in combination. The populations selected contained all of the birds in the designated lines which survived till the end of their first laying year (no culling being practiced during that period) which met the requirement of family size. This was arbitrarily set at a minimum of three full sisters per family with a minimum of three families per sire. The number of birds involved and their mean records are shown in Table 1 and the respective standard deviations in Table 2. Some variation in family size occurred but estimates of standard deviations of family size (1.93 for full-sister families and 9.26 for half-sister families) indicate that the effects of the correction for this factor in the analysis of variance
69
HERITABILITY OF EGG PRODUCTION TABLE 1.—Mean egg production to the end of the designated month Production line Series 5 Series T Number of birds October November December January February March April May June
July
August September
111 30.6 50.9 71.1 89.0 107.0 130.6 154.4 178.2 198.6 216.6 231.9 244.0
226 36.6 59.8 80.1 99.3 116.9 140.3 164.1 188.6 211.5 233.8 253.3 268.1
Total
Other lines All
337 34.6 56.9 77.2 95.9 113.7 137.1 160.9 185.2 207.3 228.1 246.2 260.1
Series 5 Series T 235 29.7 47.6 65.8 82.5 100.7 124.4 148.5 172.4 193.4 213.4 232.4 246.7
(Winsor and Clarke, 1940) would be negligible., ANALYSIS OF HERITABILITY
The determination of the heritability of egg production was based on the design shown for the combined series of birds in
225 29.3 50.8 70.4 89.0 105.6 128.6 152.8 176.9 199.2 220.8 238.6 252.0
All
Series 5 Series T
460 29.6 49.2 68.0 85.7 103.1 126.4 150.6 174.6 196.2 217.0 235.4 249.3
346 30.0 48.7 67.5 84.6 102.7 126.3 150.4 174.3 195.0 214.5 232.2 245.9
451 33.0 55.3 75.3 94.2 111.3 134.5 158.5 182.8 205.4 227.3 246.0 260.0
All 797 31.7 52.4 71.9 90.0 107.6 130.9 155.0 179.1 200.9 221.7 240.0 253.9
Table 3. The components of variance can be isolated from such analyses and heritability estimated by considering that component A contains all of the environmental variance and half of the genetic variance, while components B and C each contain one-quarter of the genetic variance. (This
DEFINITION OF TERMS AND SYMBOLS Symbol Definition X The egg production from the beginning of lay till September 30 of the year following the year of hatch. The complete or the full record. Xx The egg production from beginning of lay to the end of the month X. Part record The initial letter of the month is used as a subscript. Thus, for the part record ending with December, X=D. Xs is equivalent to the annual record. A Component of variance between full sisters. Variance Components B ' Component of variance between dams. C Component of variance between sires. Heritability hx The additively genetic fraction of the variance of X. The square of the correlation coefficient between phenotype and genotype. Phenotypic correlation ra The observed correlation between characters 1 and 2. Genetic correlation r c ^ j The correlation between the genotypes for characters 1 and 2. Environmental correlation fE^t The proportion of the correlation between characters 1 and 2 that is due to common environment. Amount of genetic gain AGX The fraction of each standard deviation of the selection differential realized as a gain in annual production per generation when selection is based on character X. In table 8 the figures represent twice the gain expected from the selection of females. H The weighted sum of genotypes for a complex of characters. Aggregate genotype Economic weight Cx The relative economic value of a unit of character X. Selection Index I The multiple regression equation maximizing rmMultiple regression coefficients The regression coefficient for the complete record selected so as to maximize miThe regression coefficient for the part record selected as above. Term Annual record
70
I. MICHAEL LERNER AND DOROTHY M. CRUDEN TABLE 2. —Standard deviations of egg production to the end of the designated month
Production line October November December January February
March April May June July August September
Series 5 Series T 18.08 16.07 21.69 17.87 24.12 20.05 26.75 23.68 28.58 25.84 29.82 27.04 31.25 28.53 33.64 29.94 36.36 32.06 40.66 34.58 45.09 38.25 51.57 42.70
Other lines
All 16.75 19.21 21.47 24.73 26.77 27.98 29.45 31.20 33.53 36 ..68 40.99 45.80
Series 5 Series T 17.13 15.41 17.92 20.69 23.85 20.44 26.44 23.70 27.44 26.67 28.56 28.48 30.03 30.25 31.89 31.85 35.01 34.18 38.83 36.86 40.42 43.06 47.46 45.74
partitioning is not rigorously accurate, since in the other lines C contains line differences and hence represents somewhat more than a quarter of the genetic variance.) The coefficients of B and C in Table 3 represent the average number of daughters per dam and per sire respectively. It is obvious that three different estimates of heritability are possible. \B (1) (3)
4C
(2)
A+B+C
A + B+C
2{B+C) A+B+C
Some sampling variation may be expected between these estimates, but in the absence of any significant trend with respect to the relative magnitudes of the first and the second estimates, the third one may be accepted as the most reliable one. Table 4 lists all three of the estimates for the various sub-groups of birds,
Total All 16.31 19.38 22.25 25.14 27.07 28.52 30.14 31.87 34.61 37.88 41.79 46.63
Series 5 Series T 17.44 15.74 21.01 17.89 23.94 20.25 26.54 23.69 27.81 26.26 28.97 27.77 30.43 29.40 32.46 30.91 35.44 33.13 39.42 35.74 44.06 39.34 48.81 44.24
All 16.50 19.31 21.92 24.97 26.94 28.29 29.85 31.59 34.16 37.38 41.45 46.28
while Table 5 shows the variance ratios to indicate the relative reliability of the different estimates made. The columns under the heading of "Heritability" in Table 6 give the estimates obtained from pooling the data for the two series on the basis of the third type of estimate. It may be seen from the latter figures that the heritabilities in the production line are lower in general than those of the other lines. This was to be expected since the latter include several non-interbreeding sub-populations. However, the combination of all lines gives remarkably uniform heritability values for each of the periods investigated, the complete range of estimates being from about 29 to 37 percent. These figures show a very good agreement with the only other comparable heritability determination made for annual egg production of survivors, where the value of 34 percent was presented by Shoffner (1946). Lines 9 and 18 of Table 5, which give the lvalues cor-
TABLE 3.—Experimental design Dther Lines
Production Line Source of variance
Total Between sires Between dams within sires Between full sisters
Degrees of freedom 335 14 53 268
Composition of mean square A+i.9B+2l.lC A+4.9B A
Degrees of freedom 458 20 74 364
Composition of mean square A+t. SB+20.9C A+i.SB A
Total Degrees of freedom 793 34 127 632
Composition of mean square A+i.8B+21.0C A+i.SB A
71
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73
HERITABILITY OF EGG PRODUCTION
responding to the heritability of the total population in Table 6 show significance to be in each case below the one percent probability level, so that these estimates may be viewed with considerable confidence. It seems then that the heritability of egg production throughout the year is reasonably uniform and can be considered to have a value approximating 33 percent. In a closed flock which has been selected for high production and inbred to some extent this value may be approximately cut in half. There is one additional point here that may be mentioned in view of the discussion on selection indexes to follow. It may be noted that in the production flock the heritabilities of the early records are in general higher than those for more extended periods of time. This is most readily explained by the fact that the selection practiced in the productionbred flock was based to a great degree on complete and not on partial records. Consequently, the reduction in heritability was greater in the former than in the latter. THE CORRELATIONS BETWEEN PART AND FULL RECORDS There has been a great amount of data presented by various investigators
on the correlation coefficients between records for part of the year and the annual record. Most of them, however, did not deal with egg production in an accumulative manner, nor did they consider any but the phenotypic correlations. The values reported by different investigators working with different breeds are, however, on the whole rather uniform. For instance, a sampling of the correlation coefficients between winter production (from beginning of lay to March 1, or from November 1 to March 1) and annual production produces values between 0.62 and 0.69 (Ball and Alder, 1917; Card, 1917; Hays, Sanborn and James, 1924; Hervey, 1924; Thompson and Jeffrey, 1936). Similar phenotypic correlations were computed for the data utilized in this paper for the production line, the other lines and the total population. The coefficients comparable to the ones cited above are somewhat higher in our case but not unreasonably so (Table 6). The technique used by Hazel, Baker and Reinmiller (1943) for isolating the genetic and environmental parts of the phenotypic correlation was extended to the analysis, and the values obtained for the estimates from both sires and dams are also listed in Table 6. Estimates of genetic correlation
TABLE 6.—Estimates of heritability and of correlations between annual production and production to the end of the month designated Production Line Heritability October November December January February March April May June July August September
0.342 0.308 0.231 0.221 0.204 0.170 0.158 0.145 0.141 0.117 0.133 0.147
Pheno- Environtypic mental .458 .555 .656 .713 .739 .788 .830 .876 .916 .951 .987 1.000
.442 .532 .623 .690 .754 .773 .818 .866 .906 .949 .986 1.000
Total
Other Lines
Correlation
Correlation Genetic 0.574 0.695 0.819 0.839 0.780 0.874 0.898 0.939 0.976 0.973 0.991 1.000
Heritability 0.256 0.273 0.351 0.479 0.481 0.461 0.450 0.463 0.470 0.477 0.489 0.500
Pheno- Environtypic mental .423 .563 .666 .733 .780 .856 .861 .899 .933 .959 .982 1.000
.551 .607 .631 .661 .713 .850 .798 .846 .894 .913 .972 1.000
Correlation Genetic 0.239 0.534 0.732 0.813 0.853 0.866 0.936 0.960 0.978 0.989 0.993 1.000
Heritability 0.294 ' 0.288 0.304 0.373 0.353 0.342 0.332 0.335 0.339 0.333 0.345 0.356
Pheno- Environtypic mental .438 .560 .662 .725 .763 .828 .848 .890 .926 .956 .984 1.000
.495 .566 .624 .676 .718 .808 .808 .857 .900 .941 .980 1.000
Genetic 0.319 0.553 0.742 0.815 0.849 0.867 0.928 0.954 0.978 0.986 0.993 1.000
74
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
coefficients from the combined sires and dams are presented in Table 7. Although some sampling variation leads to impossible estimates above unity in a few instances, the generally uniform trend inspires confidence in the validity of the last column of Table 6 (repeated as the last row in Table 7). Thus about 50 percent of the genetic variance in annual record is accounted for by the part record to
method of selection practiced. However the relative efficiency of selection on the basis of part records may be established very readily. For each standard deviation in the selection differential the gain expected per generation is h in the limit. If the . criterion of selection is not the character itself but a correlated character the rate becomes hxrasGx where the last term is the genetic correlation between
TABLE 7.—Estimates of genetic correlation from combined sires and dams between annual production and production to the end of the designated month Decem- Janu- Febru- March April October November ber ary ary
Line
Series
May
June
July
August
Production
S T S+T
0.822 0.457 0.574
0.923 0.552 0.695
0.970 0.690 0.819
0.948 0.750 0.839
0.925 0.797 0.780
0.914 0.879 0.874
0.900 0.952 0.898
0.926 1.004 0.939
0.972 0.984 0.976
0.997 0.990 0.973
1.011 0.981 0.991
Others
S T S+T
-0.112 0.523 0.239
0.323 0.708 0.534
0.631 0.840 0.732
0.729 0.930 0.813
0.766 0.942 0.853
0.732 0.952 0.866
0.893 0.969 0.936
0.930 0.983 0.960
0.960 0.991 0.978
0.986 0.991 0.989
0.995 0.992 0.993
Total
S T S+T
0.110 0.450 0.319
0.460 0.617 0.553
0.699 0.790 0.742
0.772 0.887 0.815
0.796 0.917 0.849
0.788 0.938 0.867
0.890 0.964 0.928
0.924 0.983 0.954
0.962 0.992 0.978
0.988 0.985 0.986
0.998 0.989 0.993
the end of December, and about 90 percent by the part record to the end of May. The genetic correlations shown in Table 6 are higher than the corresponding phenotypic correlations in the production line in all of the first eleven months and in nine of the eleven months in the other lines. This illustrates that the genes which influence egg production are more persistent in their effects than are the environmental factors, the effects of the latter being more transient in nature. This same persistency of genie effects has been demonstrated for rate of gain by Hazel and co-workers (1943) for swine and by Knapp and Clark (1947) for beef cattle. THE EXPECTED GENETIC GAIN FROM MASS SELECTION The constants computed to this point permit prediction of the rates of gain expected in the character under selection. The actual rate will, of course, depend on the intensity of selection as well as on the
the character selected and the trait desired. These values are presented in Table 8 and apply to selection on basis of individual performance. Selection based on family records will favor the part records more than does mass selection. However, the actual gains realized under field conditions will probably be considerably less TABLE 8.—Comparative amounts of genetic gain
Per generation in annual egg production from mass selection based on the record to the end of the designated month
Month October November December January February March
April May June July August September
AG*
AG* (hjaxai)
AG,
0.173 0.297 0.401 0.498 0.504 0.506 0.535 0.552 0.569 0.569 0.583 0.597
0.290 0.497 0.672 0.834 0.844 0.848 0.896 0.925 0.953 0.953 0.977 1.000
I
HERITABILITY OF EGG PRODUCTION
than might be assumed from the first column of Table 8, because the heritability estimates used here are for the combined lines, and because selection for other characters which may be genetically antagonistic to egg production must be also practiced. In order to evaluate the relative efficiency of selection for the annual record by using the part records as selection criteria, the ratios of these values to the gains expected when selection is based on the complete record itself were computed, and are also presented in Table 8. SOME PRACTICAL CONSEQUENCES
The following considerations apply only to the production records of surviving birds. The interrelation of mortality and egg production is not considered in this study but information is available that the general trend of the conclusions presented here is not invalidated by considering mortality and egg production simultaneously. This information appears in the paper by Dempster and Lerner to which reference is made on several occasions below. The amount of genetic gain per generation when selection is practiced on the basis of part records to the end of December is about two-thirds of that expected if selection were based on the full annual record. Since it is the customary practice in breeding to select breeding birds about January, the breeder may either base his selections on the part records to that date and use one-year old birds for mating, or if the complete records are used, mate the birds up when they are two years old. Since in the latter case the interval between generations will be doubled, it is clear that greater rate of improvement can be maintained if the first procedure is followed. Thus the analysis presented indicates that the use of younger birds with
75
part records rather than older birds with full records leads to more rapid gains. The exact rates of efficiency expected depend on the selection scheme practiced (individual performance, sister test, progeny test), as well as on the age of the male birds mated. The whole problem of the optimum age composition of the breeding flock is analyzed in detail by Dempster and Lerner, but the general trend may be clearly observed in the present data. The cost of trap-nesting constitutes one of the heaviest expenses in breeding operations. The use of part-time trap-nesting could reduce this cost materially. If the breeder follows the practice of mating oneyear old birds he would need to continue trap-nesting after January 1 only the birds selected for breeding and their full sisters (since the full records of families are needed for the purpose of a second selection at two years of age, as is shown by Dempster and Lerner) and thus effect considerable savings. The remaining birds could be maintained without trap-nesting for the purpose of obtaining flock production averages. A previous report by Lerner and Taylor (1940) anticipated this conclusion. It was shown there that Tschuproff's coefficient of association between the part records of the dams and their sisters with the complete records of the daughters (including mortality effects) did not differ greatly from the coefficient of association of the latter with the full records of dams and aunts. A variation of this scheme would be to suspend trap-nesting on the whole flock in the summer of their first year of laying. It may be seen from Table 6 that the genetic correlation between production to the end of May, for instance, and the annual production is about 0.95. This means that the efficiency of selection on the basis of such a part record is about 92 percent of that obtained from selecting from com-
76
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
plete records (Table 8). It is quite likely that the saving effected by eliminating trap-nesting for the remaining five months more than compensates for the eight per cent loss in efficiency. A somewhat different facet of practical significance lies in the application of these findings to the construction of selection indexes. Hazel (1943) presented methods of determining the proper weights to be assigned to each of several characters for which selection is practiced. In this particular case we may consider annual production as the prime desideratum or the aggregate genotype in Hazel's terminology. The two component characters here are production to the end of December and production following January 1st. A "total score" index which would take in account the heritabilities of these components and the genetic correlation between them would as shown by Hazel and Lush (1942) lead to greater efficiency in improvement of the annual record than if some other criterion of selection for these components is used. We may first construct an index using the part record to the end of December as our first character (XD) and the full record as the second (X), with the selection index being defined as
I = aXD+bX, a and b being the respective weights assigned to characters XD and X. Following Hazel's procedure the regression coefficients a and b may be obtained by solving the simultaneous equations (rxDa-\-rxDxaxb =*rxDH0H, and rxDx
record. The products rxDHGH and rxWH may be solved in terms of CXQ and Cx from TXDH(TH = hxD{hxD
rhx'JxfGx
QxCx),
and rxH
(X-XD).
In other words the most efficient index for selection places one and three-quarters times more emphasis on the production before January than on the production after January. Since the actual economic worth of the eggs produced in the first period is less than that of the eggs produced in the second period (due to their smaller size) the factor 1.74 is probably an overestimate. It is possible to approximate the more correct coefficient by setting Cx>CxD- A sample of 69 birds from the flock under study was investigated with respect to the distribution of egg size in the periods corresponding to XD and X and their respective values were computed by using the market price of eggs (obtained from average San Francisco wholesale quotations for the ten-year period 1936-1945). The ratio CxD/Cx in this manner was found to be 0.867, and the index recomputed to I = 0.66XD+X,orI
=
1.66XD+(X-XD).
All of the estimates presented in this section err on the conservative side with
HERITABILITY OF EGG PRODUCTION
respect to the production-bred flock. As noted previously, the ratios of the heritabilities of the part records to that of the full record are higher in the production line than in the others. Hence, if the AG's are computed for the production line separately, higher ratios of AGx/AGs are obtained than those presented in Table 8. For instance, AGD/AGS rises above unity in this case as compared to 0.672 in Table 8. Hence the weights assigned to the early record in the indexes computed are, if anything, underestimates. It thus appears that in the flock considered the selection of chickens for improved annual egg production more weight should be placed on the early than on the late production. The greatest economic efficiency will be obtained if birds selected on the basis of their part records are mated at one year of age. When two-year old birds are selected on the basis of their complete records, the criterion of selection should give more emphasis to their production before January of their first laying year than to the production beyond that date. SUMMARY
The heritability of accumulative egg production in a White Leghorn flock was found to be nearly constant throughout the year and in the neighborhood of 33 percent. In the part of the flock which had previously been subjected to selection for egg production the heritability was lower. However, less decrease was shown in the early phases of production than in the later, verifying the fact that more emphasis in selection had been placed on the full than on the part record. Genetic correlations between part and full production were found to be high, leading to the possibility of efficient selection on the basis of part records more economically than on the basis of full
77
records. The decrease in interval between generations incident on the possible use of younger birds in breeding when partrecord selection is used suggests modification of standard breeding procedures, a point more fully explored in another publication. Even if selection is based on complete records, the analysis indicates that production prior to January should be given at least 1.66 times the weight assigned to subsequent production. LITERATURE CITED
Ball, E. D., and B. Alder, 1917. Breeding for egg production. Part II. Utah Agr. Exp. Sta. Bull. 149. Card, L. C , 1917. A study of egg production in the White Leghorn. Storrs Agr. Exp. Sta. Bull. 91. Dempster, E. R. and I. M. Lerner, 1947. The optimum structure of breeding flocks. Genetics 32 (in press). Dickerson, G. E. and L. N. Hazel, 1944. Effectiveness of selection on progeny testing performance as a supplement to earlier culling in livestock. J. Agric. Res. 69:459^76. Goodale, H. D., 1918. Internal factors influencing egg production in the Rhode Island Red breed of domestic fowl. Am. Nat. 52: 65-94, 209-232, 301-321. Hays, F. A., R. Sanborn, and L. L. James, 1924. Correlation studies on winter fecundity. Mass. Agr. Exp. Sta. Bull. 220. Hazel, L. N., 1943. The genetic basis for constructing selection indexes. Genetics 28:267-490. — — , M. L. Baker and C. F. Reinmiller, 1943. Genetic and environmental correlations between the growth rates of pigs at different ages. J. Animal Sci. 2:118-128. , and J. L. Lush, 1942. The efficiency of three methods of selection. J. Hered. 33:393-399. Hervey, G. W., 1924. Seasonal and annual eggproduction-correlation tables. N. J. Agr. Exp. Sta. Bull. 402. Knapp, B., Jr. and R. T. Clark, 1947. Genetic and environmental correlations between growth rates of beef cattle at different ages. J. Animal Sci. 6: 174-181. Lerner, I. M. and L. N. Hazel, 1947. Population genetics of a poultry flock under artificial selection. Genetics 32:325-339. , and L. W. Taylor, 1940. A note on the use of short-time trap-nesting in breeding selection. Poultry Sci. 19: 187-190.
78
I. MICHAEL LERNER AND DOROTHY M. CRUDEN
Lush, J. L., 1945. Animal breeding plans. 3-rd ed. Ames, Iowa: Collegiate Press. Pearl, R. and F. M. Surface, 1911. A biometrical study of egg production in the domestic fowl. II. U. S. Dept. Agr., Bur. Anim. Ind. Bull. 110. Shoffner, R. N., 1946. The heritability of egg production. Poultry Sci. 25:412. Thompson, W. C. and F. P. Jeffrey, 1936. The use-
fulness of winter and summer-fall egg yield records as criteria of poultry breeder selection. N. J. Agr. Exp. Sta. Bull. 612. Winsor, C. P. and G. L. Clarke, 1940. A statistical study of variation in the catch of plankton nets. J. Marine Res. 3:1-34. Wright, S., 1921. Systems of mating. Genetics 6: 111-178.
News and Notes {continued from page 66)
the food inspection service. He will be engaged in poultry research projects. Dr. G. M. Briggs, formerly of the Poultry Department, University of Maryland, is now at the University of Minnesota. P. E. Bernier, formerly Chief R.O.P. Inspector for the Dominion of Canada, is now Associate Professor of Poultry Husbandry, Oregon State College, Corvallis. Prof. Dr. Miecszyslaw Cena, The Institution for Breeding Small Animals, University and School of Technology, Wroclaw, Poland, would appreciate receiving reprints of recent research for the library of the Institute. They are particularly interested in any literature on breeding, disease, and management of poultry. The Delaware Substation at Georgetown, Delaware, has been chosen as the site for the 1948 Chicken-of-Tomorrow contest. Mr. Karl Seeger and Professor A. E. Tomhave will assist in conducting the contest. Forty contestants have been notified by Mr. H. L. Shrader, Chairman of the Procedure and Awards Committee, of the acceptance of their entries by the National Committee. From the two cases of hatching eggs of each entry it is planned to start 400 chicks for a 12-week growing period. Any suggestions regarding
methods for conducting the test or collecting and analyzing the records should be sent to Mr. H. L. Shrader, Poultry Extension Service, U.S.D.A., Washington, D. C. The Eighth Annual Conference of the Institute of Food Technologists will be held in Philadelphia, Pa., June 6-10, 1948, at the Benjamin Franklin Hotel. A number of foreign delegates are expected to attend the Conference. The American Economic Committee for Palestine, 250 West 57th Street, New York 19, have prepared a 3-reel silent 16 mm. moving picture and a 144-page cardboard-covered book entitled "The American Poultry Industry as Applied to Palestine" by Simon Bornstein. The film may be borrowed from the U.S.D.A., Cornell or Rutgers Universities. The book may be purchased for $2.00. A National Egg Products Association with headquarters at 110 North Franklin Street, Chicago 6, was established October 1, 1947 by 90 commercial firms engaged in processing frozen and dried eggs. Dr. O. J. Kahlenberg is director and has announced that the first project will be to determine whether the breed of laying hens is a factor in improving the quality of frozen egg whites.