SELECTION FOR EGG PRODUCTION AND CORRELATED RESPONSES IN THE FOWL

A. W. NORDSKOG, M. FESTING AND MARGRITH WEHRLI VERGHESE

Department of Poultry Science, Iowa State Uniuersity, Ames, Iowa 50010

Received August 2, 1966

HE effectiveness of selection for genetic improvement of egg production in the fowl remains a controversial issue. Mass selection on annual egg records

has long been regarded as ineffective as demonstrated by the early work of PEARL (1915), while selection based on family records and progeny testing has seemed to be the key to success as demonstrated by the long-time selection experiments of DEMPSTER and LERNER (1947) and HUTT and COLE (1955). All these studies, however. lacked adequate controls to properly assess time trends.

Compared with controls, selection in an “old’ Leghorn strain was ineffective after the first generation, but effective in a “new” population starting from a cross-line foundation ( GOWE, JOHNSON, DOWNS, GIBSON, MOUNTAIN, STRAIN and TINNEY 1959).

From an intensive study of records of a commercial Leghorn flock coupled with repeat mating controls, DICKERSON (1955, 1961, 1963) deduced that continuous artificial sclection for overall performance in a domesticated population simulates natural selection for reproductive fitness in a wild-type population. Selection leads to a plateau because of fluctuating year-to-year environments, negative genetic correlations between components o i performance and to random loss of useful genes from inbreeding.

Slow progress in breeding for egg production has been attributed, in part, to the time required to obtain a complete annual egg record. DEMPSTER and LERNER (1947) concluded that greater genetic progress is possible by selecting only on the early part of the annual egg record because the generation time interval is halved. MORRIS (1963, 1964) and GOWE and STRAIN (1963) found that selection on early record egg number may not necessarily increase egg production in the total period.

OLIVER, BOHREN and ANDERSON (1957) concluded that early record percentage egg production is a more efficient selection criterion than is the actual egg count in the early period. Total egg number was more highly correlated genetically with early rate than with early period egg count.

Using early rate as the primary selection criterion in a 10-year selection ex- periment with White Leghorns, YAMADA, BOHREN and CRITTENDEN (1958) reached a plateau by the fifth selection year. This was attributed to a progressive decline in additive genetic variance. Their study, however, lacked controls to

Journal Paper KO 5-5450 of the Iowa Agncultural and Home Economics Expemnent Station, h e s , Iowa, Project No 13X 111 cooperation with the North Central Regronal Poultry Breedmg Project NC-47.

Genetics 5 5 : li9-191 February 1967.

180 A. w. NORDSKOG et al.

assess time trends. Also some attention was paid to egg size and other economic traits.

In the present study, early rate was the exclusive selection criterion. The results of eight generations of selection in two populations are summarized. This is part of a more extensive selection experiment involving, in addition, lines selected for body size and egg size. A preliminary report (NORDSKOG and FEST- ING 1962) covers the results of the first four generations of the overall selection experiment.

MATERIALS A N D METHODS

Selected lines: Two breeds were used: the White Leghorn and the Egyptian Fayoumi. Leghorn Line A, selected for high egg production, originated from a cross-line foundation of four commercial outbred strains in 1955. The Fayoumi line J originated from a flock maintained at Iowa State University from hatching eggs imported from the Ministry of Agriculture, Cairo, Egypt, in 19W. In 1952, further importations from Egypt were incorporated into the flock which each year was maintained with approximately eight males and 100 females mated under panmixia. First selections were made in 1956.

Control lines: Randombred control Leghorns (RCC) were obtained each year, commencing in 1958, as hatching eggs from the Regional Cornel1 Control stock maintained by the North Central Poultry Breeding Project, Purdue University (KING, CARSON and DOOLITIU 1959).

Repeat mating controls (RM2) were produced in the third year of the experiment from Leghorn Line A. Eight to ten single-male pen matings were remated a second year. The general procedure corresponded to that proposed by GOODWIN, DICKERSON and LAMOREAUX (1960). The progeny of the repeat matings are designated RM1. In the following year, RM2 progeny were produced from a restricted group of the RM1 individuals whose parents produced progeny the previous year in the regular A line matings. Thus, RM1 progeny have 2 year old dams while RM2 progeny have yearling dams. Only the RM2 progeny were used in this study.

A Fayoumi control population (P) consisting of a quasi-randombred flock maintained by matings of approximately ten males and 100 females in each year was used as a check against the Fayoumi selected line J.

Selection procedures: Selections in the A and J lines were based on rate of egg production to approximately 9 months of age. From the early record, a selection index was constructed for each bird, based on optimal weightings of the individual’s record, its dam family and its sire family average following the procedure of OSBORNE (1957a, b) . Sires were selected on their dam family and sire family average with optimal weightings. As applied to the conditions of the present experiment, OSBORNE’S index should theoretically increase selection efficiency for additive genetic effects by k = 1.3 and k = 1.5 for pullet and cockerel selection, respectively, over mass selection, or, for the combined sexes, by k = 1.4.

Full-sib and half-sib matings were avoided. Records of egg production were corrected each year for hatch effects.

The plan was to select about one third of the superior progeny as breeders in each line. This has deviated considerably from generation to generation, depending upon the level of fertility, hatchability, egg production and viability. The Fayoumi J line was handled separately on a different farm (Ankeny) from the Leghorn A line (Ames). However, the same breeding pens at the Ames farm were used for the selected breeders of both breeds. The Leghorns were hatched in March and April and the Fayoumis in April and May.

For economic reasons, approximately half of the cockerel chicks were randomly discarded at hatching time.

Chicks were produced in two or three hatches. A hatch consisted of eggs collected over a 2-week interval. Chicks were brooded intermingled with other lines of the same breed to 8 weeks of age for the Leghorns and to 6 or 7 weeks of age for the Fayoumis. After brooding, the chicks were placed on a pasture range and fed a standard ration.

SELECTION IN THE FOWL 181

In each year, beginning with the second generation of 1959, a sample case of 360 hatching eggs from Leghorn Line A was sent to the North Central Regional Poultry Breeding Laboratory, Lafayette, Indiana, for comparative tests. Thus, Leghorn Line A was tested in two environments.

Birds were housed at approximately 5 months of age. Hatches were penned separately on conventional litter. The Leghorn pullets at Ames were trapnested 2 days per week and the Fayoumis at the Ankeny farm were trapnested 3 days per week. The difference in procedure was due to available labor.

In each year, 16 single-male breeding pens were used for each line. Typically, ten female mates were placed in each, but the number varied from 9 to 15.

Statistical methods: Heritabilities and genetic correlations were estimated from sire and dam variance and covariance components by using a hierarchial analysis of variance (KING and HENDERSON 1954). This is also known as the full-sib correlation method, and because it includes unknown amounts of dominance and sex-linkage variance (FALCONER 1960), the computed heritabilities overestimate the additive genetic fraction of the variance.

Traits, Definitions and Symbols

X-Early record percentage egg production, i.e., the fraction of trapnest days that eggs were

Y-Full-record percentage egg production from first egg to approximately 15 months of age

W-Body weight in pounds at approximately 9 months of age. %Egg weight, in grams, at approximately 9 months of age.

are secondary truits, but, in addition, Y is the basic trait we wish to improve genetically.

for convenience: oZ,-Phenotypic variance of trait i. u2 (G) ,-Additive genetic variance of trait i. h2,-Heritability of trait i = u ~ G ~ / u ~ ~ . rG-Genetic correlation between two traits. A,-Selection differential in trait i and Ai/ui = Si. Direct selection-the direct response ( D R ) from selection on the primary trait, X. R (DR)-the realized direct response observed from the experiment. b,-the regression of X on generation number which is the linear estimator of R(DR). E(DR)-the expected response from mass selection based on the parameter estimates, E ( D R ) =

Indirect selection-the correlated response ( C R ) per generation in Y, W or Z from selection on X. R(CR) i-the realized correlated response in i from selection on X. b, - R(CR)i, the linear estimator from the observed regression of i on generation number. E(CR)i-the expected correlated response in i from mass selection based on the parameter esti-

laid froni first egg to approximately 9 months of age.

(or to age at death).

In this study, X is the sole criterion of selection and is called the primary trait. Y, W and Z

The following additional definitions follow FALCONER (1960), but with some modifications

h*, 8, U, per generation.

mates, E(CR)i = h,h,rG8,u, per generation.

In general, the hypotheses we wish to test are R ( D R ) = k X E(DR) - d , ( F ) , and R(CR)i = k X E(CR)i - d , ( F ) ,

where k is the expected relative gain in selection efficiency from using OSBORNE'S selection index over mass selection, and d , ( F ) or d , ( F ) is the inbreeding depression effect in trait X (or i) per generation from inbreeding increase, F.

RESULTS

Eflectiue population number and inbreeding: The number of pullets housed at approximately 5 months of age was typically 400 to 500 in each generation for the selected lines and 100 to 200 for the control lines (Table 1). After 1961, for

182 A. w. NORDSKOG et al.

TABLE 1

Number of females in the selected and control lines

Line 1956' 1957 1958 1959 1960 1961 1962 1963 1964

Leghorn A 1902 828 891 1218 894 537 545 662 476 Control RCC 114 293 125 143 107 186 162 Control RM2 177 294 116 157 149 191

Fayoumi J 937 373 588 922 970 723 502f 127 99 Control P 50 54 103 156

* Generation zero. t Selection suspended.

TABLE 2

Effective number of sires, dams and parents and ezpected inbreeding for birds reaching breeding age

Line Year ("1 (N,) ( N e ) (F) Sires Dams Parents Inbreeding

Leghorn A 1957 10 61 34.4 ,014 1958 9 60 31.3 ,016 1959 10 55 33.8 ,015 1960 15 70 49.4 ,010 1961 15 54 47.0 ,010 1962 15 60 48.0 ,010 1963 11 4.F 35.2 ,014

AV. 12.1 57.7 39.9 ,013 Cumulative F .089

1957 13 53 41.7 ,012 1958 9 4.0 29.4 ,017 1959 13 41 39.5 .013 1960 14 54 44.5 .011 1961 14 58 4.1 .011 1962 16 55 49.6 ,010

AV. 13.2 50.2 41.6 .012 Cumulative F .074

Fayoumi J

reasons of economy, the number of birds in the experiment was substantially reduced.

The effective number of parents in each generation averaged about 40 and the expected inbreeding increased about 1.2% per generation (Table 2). The effec- tive number of sires and dams ( N , and Nf) is defined as the number with progeny surviving to breeding age. The effective number of parents is Ne=4N,N, / (N,+Nf) (WRIGHT 1940). The expected inbreeding from WRIGHT'S formula, 1/(8N,) f 1/(8Nf), is an overestimate because full-sib and half-sib matings were avoided.

The limited population numbers undoubtedly lead to some inbreeding depres- sion effects. STEPHENSON, WYATT, and NORDSKOG (1953) estimated that the egg

SELECTION IN THE FOWL 183

production traits, X and Y, decline 0.30 and 0.43% for each increase of 1% in inbreeding. Using these regressions, the estimated decline in X and Y per gen- eration is 0.39 and 0.56'%, respectively.

Observed genetic chunges: The results of eight generations of selection in the Leghorn A line tested at Ames are shown in Figure 1 . The trend in percentage egg production was upward except for the 8th generation. Body weight and egg weight show a declining trend. The rather close correspondence between the se- lected lines and the controls demonstrates that most of the generation differences are yearly environmental effects.

The test of the A line at the North Central Regional Poultry Breeding Labora- tory at Purdue University, Lofayette, Indiana, shows no improvement in egg production (Figure 2). In fact, the trends in egg production, body weight and egg weight are all downward.

The regressions of the difference between corresponding traits of the selected and control lines on generation number are presented in Table 3. Because the effects of selection are cumulative over generations these are estimates of R(DR)

A

RCC

3 5 t

t 1 1 1 1 1 1 I I 0 1 2 3 4 5 6 7 6 0 1 2 3 4 5 6 7 8

Generation GENERATION

FIGURE 1.-Results of selection for early FIGURE 2.-Results of selection for early record rate of egg production compared with record rate of egg production compared with a randombred control population (RCC) and a randombred control population (RCC) tested a repeat mating control (RM2) tested at the at the Lafayette Station. Ames Station.

184 A. w. NORDSKOG et al.

FIGURE 3.-Observed and control-corrected full record rate of egg production of two samples of the A line tested at Ames and Lafayette.

(AMESi

8 %

U

W

a 0 a 70-FAYOUMI J 0 0 W a

0 60

%

-

_-_- CONTROL CORRECTED - 0 1 2 3 4 5 6 7 8

GENERATION

and R(CR)i . In all cases, the signs of the regression coefficients are negative but only the bw = -.08 * .01 for body weight at Ames was statistically significant. From the combined results, the decline in egg production of -0.75% per genera- tion was significant between .05<P<.10 while the decline in body weight and egg weight were each statistically significant at P<.05. If we assume that egg production in the A line has plateaued, then the decline in X is readily explained in terms of inbreeding depression.

The changes in the full record, Y, for both Leghorn A and Fayoumi J are

TABLE 3

Estimated genetic change per generation in the A line corrected for the RCC control i.e., regression (b) of (Line A-RCC control) on generation number

Trait Ames Lafayette Combined Statistical

(b) (b) (b) sianificance

Egg rate, X ( % ) -.53+.46 -1.09rt.47 -0.75k.344 .05<P<.1 Egg rate, Y ( % 1 -.18rt.15 - .03k.72 - .12f .31 not significant

Egg wt.9 (9) -.25 f . 1 7 - .42 2.22 -.3 15 f .I3 7 .O% < P< .05 Body wt., W (lbs) - .08f.01 - . 0 6 . 0 3 --.073rt.015 P<.OOl

Note The standard errors are based on the deviations from linear regression. Because the variance of the change in mean genetic value from generation to generahon due to random drift is not taken into account, the standard errors are underestimates of the true values.

SELECTION I N THE FOWL

TABLE 4

Heritability estimates from full-sib correlations

185

1958 1959 1960 1961 1962 1963 Average

Early egg rate (X) Leg. A Fay. J

Full egg rate (Y) Leg. A Fay. J

Body weight (W) Leg. A Fay. J

Egg weight (Z) Leg. A Fay. J

.I4

.17

.38

.31

.44

.52

.65 ,48

.I4

.02 .23 .2l .71 .61 .60 .36

.09

.I8

.39

.27

.87

.46

.61

.56

.03

.03

.I6

.I9

.60

.54

.56

.56

.05

.01

.09

.27

.65

.37

.49

.53

.05

.I7

.35

.22

.56

.66

.62

.83

.083 f .02

.097 t .04

.267 f .05 ,245 f .02 .638 t .02 .527 +- .04 .588 +- .02 .553 f .06

presented in Figure 3. When the trends are adjusted for yearly differences in controls, Y shows no statistically significant response to selection. For the Fayoumi line the regression of Y per generation is -1.0 f 2.8. The regressions of the Leghorn A line are given in Table 3.

Parameter estimates and expected genetic changes: The heritability estimates (Table 4) are consistent over the generations and are in close agreement between breeds. The average heritability of the early record is .09O t .03 and of the full record is 0.256 f .04. The difference is highly significant which suggests that selection on the full record might have been a more effective selection criterion. The heritability estimates for body weight and egg weight are moderately high as expected.

The phenotypic and genetic correlations for the different traits are given in Table 5. The genetic correlations between the early (X) and full production (U) records are considerably higher (.88 and .79) than the phenotypic correlations (.53 and .48) for the two breeds. The correlations between body weight (W) and egg weight (Z) are moderately high, as expected, but the correlations of these traits with X and Y, are low and not statistically significant. If we take these correlations at €ace value, selection on X should change Y substantially but not W or Z.

TABLE 5

Phenotypic and genetic correlations

Phenotypic Genetic Correlation* Leahorn A Fayoumi J Leahom A Favoumi J

xw .02 f .03 .06 f .04 -.03 t .06 -.04 f .I4 XY .53 f .05 .48 f .05 .88 f .08 .79 f .03 xz .OO f .02 -,05 f .02 -.15 f .20 -.31 f .I5

YW YZ

.05 k .05 .05 i: .05 .08 t .IO .02 f .03

.05 rt .02 .06 t .04 . I O f .I3 -.I3 f .M

wz .38 f .02 .36 f .02 .64 f .03 .46 t .04

' W-Body weight. X-Early remrd egg production. Y-Full record egg production. &Egg weight. , Note: The standard errors were ernpirically calculated from the separate generation means.

186 A. w. NORDSKOG et al. TABLE 6

Selection differentials (A) of primary trait X (early egg production record)

Fayoumi J A Year U

Generation

1958 10.4 20.2 6.8 18.4 1959 13.6 19.5 11.0 19.2 1960 11.4 15.7 10.1 18.2 1961 15.1 18.7 16.5 17.2 1962 17.5 18.0 13.0 15.7 1963 10.0 17.1 11.1 15.7 I964

AV. 13.5 18.6 11.4 17.6 A/U .72 .65

. . ~ __ 16.2 21.2

~-

The selection differentials (Table 6) for the primary trait averaged 13.5% egg production in the Leghorn line and 11.4% in the Fayoumi line. These are equivalent to .72a and .650, respectively, o r to truncation selection of the top 55 to 60% of the early records in each generation. This, however, is well below the original plan of the experiment calling for selection of the top one-third of the records.

The expected correlated response E(CR) from selection on the early record, is given in Table 7. For the Leghorns and Fayoumis, the E(CR) for W and Z were small and mostly negative, but for Y the E(CR) was 1.67% and 1.23%, respectively. Thus, on the additive genetic scale, we estimate the expected im- provement in the full record (Y) by mass selection to be something over 1% per generation.

More realistically, the responses should be corrected upward first, because OSBORNE’S selection index, not mass selection, was used, and secondly downward for inbreeding depression. The adjusted responses in X and Y compared with the unadjusted responses for the Leghorn A line and Fayoumi J line are presented in Table 8. Most of the increased gain from index selection is offset by the in- breeding depression effect. Yet, if we assume a simple additive genetic model,

TABLE 7

The expected correlated response per generation, ECCR), in trait i from selection on the early egg record X

Correlated response In 1 h&,& 8, U, E(CR),

Full record (Y) Leg. A .I294 .72 17.9 1.67% Fay. J .I249 .65 15.2 1.23%

Body weight (W) Leg. A -.0068 .72 .54 -.OW6 lb. Fay. J -- .ow2 .65 .41 -.0025 lb.

Egg weight (Z) Leg. A --.0326 .72 4.5 -.IO g Fay. J -.0727 .65 3.5 -.17 g

E(CR), = h,h,rG 6, ai

SELECTION IN THE FOWL 187

TABLE 8

Expected response per generation in the primary trait X and t h basic trait Y from mass selection and from Osborne’s index selection adjusted for inbreeding

Leghorn A Fayoumi J - Trait Kind of selection Expected response’

X Mass E(DR) = 1.12% 1.11% Index (adj. F) E(DR)’= 1.18% 1.19%

Y Mass E(CR) = 1.67% 1.23% Index (adj F) E(CR)’ 1.78% 1.21%

‘E(DR) ’=l 4E(DR) - 3 0 ( F ) E(CR) ’=l 4E(CR) - 4 3 ( F )

the selection program I‘ollowed should have made a substantial improvement in the basic genetic trait, Y.

DISCUSSION

From the population parameter estimates, the expected increases in the full record rate of egg production are 1.78% and 1.21 % in the Leghorn and Fayoumi lines, respectively, after adjusting for OSBORNE’S index selection and for inbreed- ing. Thus, if genetic gains were linear, we would expect a total increase of 14.4% in Leghorn A line and 9.6% in the Fayoumi line J from seven generations of selection.

Clearly, the realized genetic gains from this experiment did not measure up to expectations. DICKERSON (1955, 1961, 1963) has discussed the question of re- fractory response to continued selection for egg production and concluded that the most likely causes are: (1) genetic “slippage” due to fluctuating yearly en- vironmental trends; (2) negative genetic correlations between components of performance; and ( 3 ) random loss 01 useful genes by inbreeding.

By “genetic slippage” DICKERSON implied that selection is mainly directed towards nonadditive genetic effects of overdominance, epistasis, and genotype X environment interactions which dissipate in the next generation; this could well account for the “non-response” in this experiment. However, the possibility of negative genetic correlations among components of performance would not apply to the present study since only one trait-rate of egg production-was selected. Inbreeding may possibly have reduced genetic gains one fourth or one third, but this would be offset by the expected increase in selection efficiency from OSBORNE’S index selection over mass selection.

Other possible causes of the ineffectiveness of selection in the persent study are: (1) inadequate control populations; (2) high experimental error; and ( 3 ) inef- fective selection criterion.

Inadequate control populations: For the Fayoumi J Line, the P control popula- tion was likely not wholly adequate in design (GOWE, ROBERTSON and LATTER 1959), and furthermore, it did not cover all the test generations. On the other hand, special care has been talien to make the RCC Leghorn control an adequate population for selection experiments (KING et al. 1959). We would not expect

188 A. w. NORDSKOG et al.

egg production to have increased in either control line, but it might have decreased from a relaxed selection effect (NORDSKOG and GIESBRECHT 1964). In this case the difference between the selection line and its control would be biased upwards. We conclude that the control populations probably are not responsible for the failure of the experiment to demonstrate gains from selection for egg production.

Sampling errors: For the Leghorn A line, the observed regression of X on gen- eration number (Table 3) is -0.75 f .34, and the expected regression is 4-1.01 corrected for index selection and inbreeding depression. Hence, the (observed - expected) deviation = -1.76% is more than five times the standard error of the regression. Because the observed regression is significantly less than expected and also since the sign of the regression is negative, it seems reasonable to con- clude that selection did not genetically increase egg production.

Yet we cannot ignore the significant decline in both body weight and egg weight in the Leghorn A line (Table 3) indicating that the particular selection practiced has influenced these two traits. Furthermore, in the Lafayette test (data not presented in this report), the Leghorn A line declined 1.9 * 0.6 days in age at sexual maturity per generation compared with the RCC control (age at matur- ity was not consistently recorded in the Ames test.) Even though the decline in these traits may be real, they may not necessarily be due to selection itself. If body weight, for example, is above the optimum for maximum fitness in the initial population, then natural selection would lower it. Moreover, random drift could account for at least part of the decline. Consequently from the evidence in this study we believe that rate of egg production is highly refractive to selection and that the consistently positive estimates of hz from full-sib correlations (Table 4) are strongly influenced by maternal effects and (or) nonadditive genetic effects of dominance, gene interaction, and gene-environment interaction.

Ineffective selection criterion: In commercial breeding practice, DICKERSON and HAZEL (1944) argued that the efficiency of selection should be measured per unit of time rather than per generation. Applied to selection for egg production in poultry, DEMPSTER, and LERNER (1947) concluded that greater genetic gain is derived from selection on an early but incomplete egg record compared with the full record because the generation time interval is halved. As a consequence of these and similar studies, commercial poultry breeders in the 1950 decade promptly converted their operations to selection of pullets on a part-record basis.

From the results of the present study, we may estimate the relative selection efficiency (R.S.E.) of improving Y by selection on X (indirect selection) com- pared with selection on Y (direct selection), where R.S.E. = rG d&/,/h, (SEARLE 1965).

Since the generation time interval for the early record, X, is one year and for the full record, Y, is two years, then when R.S.E. = yz, selection on X or Y is equally efficient. For the Leghorns, R.S.E. z .48 and, for the Fayoumis, R.S.E. z 30. Thus, early record selection may be equal to, but is no more efficient than,

-

SELECTION IN THE FOWL 189

selection on the complete egg record even though the latter doubles the genera- tion time.

SEARLE (1965) has shown that the sampling error of R.S.E. may be rather high. He derived a standard error formula in terms of heritabilities and correlations of X and Y. (SEARLE’S formula is o(p) = {l/f [2/h2, (1 4- 2p2 4- R2 - 4Rp) 4- h,/2h, (1 - 2Rp) 4- ( i / z . )p (2R [l + 2p2 - Rp] - 3p)]}% where: p = R.S.E., h , hy = heritability of X and Y. R = phenotypic correlation between X and Y and f = sum of the effective number of parents over generations). Application to this present study from the combined breed results gives, R.S.E. = .49 .21. Thus, we have little basis to say that selection on the early record is superior to the full record, although this result lends little support to the currently popular pro- cedure of part-record selection used in poultry breeding practice.

MORRIS (1964) has questioned the effectiveness of early record selection. He found that early record index selection (egg rate + viability) increased egg number in the early period but oiily at the expense of the residual part of the period so that the total record showed no increase. Experiments reported by GOWE and STRAIN (1963) substantiaie MORRIS’S view that continued early record selec- tion for annual egg production is ineffective in improving the full record. In fact, results of the long-time California Leghorn selection experiment ( LERNER 1958, p. 233) show that from 1933 to 1949, when selections were based on the entire production year, steady progress was obtained in both the full record and the part record. After 1949, when selections were based on the early record, the full record progressively declined ((to 1954) even though the early record continued to in- crease.

Perhaps, ironically, selection for the full record as practiced by most breeders prior to 1950 had been effective as demonstrated (without controls) by HUTT and COLE (1955) and DEMPSTER and LERNER (1947), and a major cause of our inability to demonstrate clear-cut gains in total rate of egg production in the last 15 years have been due to the ineffectiveness of indirect (early record) selection.

From the results of this study pertaining to the part record, we hypothesize that the additive genetic variance is essentially zero and that the heritability esti- mates (Table 4) are mainly reflections of maternal or other nonadditive genetic effects. Since this same nonadditive component would be present in the full record, this could account for the high phenotypic and genetic correlations between X and Y shown in Table 5. Thus, it is possible that the cause of the plateau demon- s :rated in this study is due to exhaustion of additive genetic variance in the selec- tion criterion (part record) even though the additive variance of the full record may not be exhausted. The substantially higher estimates of heritability of the full record suggest that the additive variance for this trait is not yet exhausted.

Conclusions: 1. Eight generations of selection for early record (part record) percentage egg production in two breeds of chickens (Leghorns and Fayoumis) showed no statistically detectable increase in rate of egg production. 2. Based on population parameter estimates, the expected increase in total rates of egg pro-

190 A. w. NORDSKOG et al.

duction from early record selection were 1.78% and 1.21 % respectively, in the Leghorn and Fayoumi lines after adjusting for inbreeding depression and index selection. 3. The estimates of heritability from full-sib correlations computed over six generations showed that early record egg production was significantly lower (.09) than the full record (.25). We believe that the former is mainly nonadditive genetic variance but not necessarily the latter. 4. The results of this study combined with some recently published reports, suggest that a major cause of the inability of breeders to demonstrate clear-cut gains in egg production over the past 15 years has been due to the ineffectiveness of the early record as a selec- tion criterion for the full record.

SUMMARY

Two populations of chickens (Leghorn A and Fayoumi J) were selected ex- clusively for percentage egg production from first egg to approximately 9 months of age for eighi generations, i.e. early record egg rate. The basic trait for which genetic improvement was sought was percentage egg production from first egg to age at death or approximately 500 days of age in the survivors, i.e. full record egg rate. Line A was tested at two locations (Ames and Lafayette) and Line J at one. Each population was maintained with a breeding flock of 16 sires and approximately 9 to 10 mates per sire annually. Yearly means of the selected lines wcre corrected with control populations. Selections were based on an index using the sire, dam and individual records.-For line A, the linear change of the early record per generation was -.53 t .46% egg production at Ames and -1.09 f .47 at Lafayette. The expected increase in percentage egg production in the early record after adjusting for index selection and inbreeding depression was about 1% per generation. The difference between expected and observed regressions was statistically significant.-For the full egg production record, the observed change per generation in line A was -.18 2 .35 at Ames and -.03 .72 at Lafayette. For the Fayoumi line, the control-corrected change per generation over four generations was -1.0 f 2.8% and the expected change, after adjust- ing for index selection and inbreeding, was 1.2% per generation.-The herit- ability estimates from half-sib correlations were significantly higher for the full record (.25 2 .03) than for the early record (.09 * .03). The genetic correlation between the early record and the full record was .88 and .79 in the Leghorns and Fayoumis, respectively.-The results of this study failed to demonstrate, first, that selection on early rate of egg production improved either early rate or total rate of egg production. Secondly, from the parameter estimates of the population it could not be demonstrated that indirect selection on early rate was any more efficient than direct selection on total rate of egg production when measured per unit of time.

LITERATURE CITED

DEMPSTER, E. R., and I. M. LERNER, 1947

DICKERSON, G. E., 1955

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