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Genetic Variation and Correlated Responses in Chickens Selected for Part-Year Rate of Egg Production1,2
Genetic Variation and Correlated Responses in Chickens Selected for Part-Year Rate of Egg Production1'2 J. V. C R A I G , D . K. B I S W A S AND H . K.
SAADEH 3
Department of Dairy and Poultry Science, Kansas State University, Manhattan, Kansas 66502 (Received for publication January 25, 1969)
INTRODUCTION
R
There is some concern t h a t commercial egg-production strains of chickens m a y be approaching or h a v e already reached plateaus of response to selection for total performance (see Dickerson, 1955; and Clayton, 1968). T h e random sample test
1 This investigation was part of the Kansas contribution to the NC-47 Regional Poultry Breeding Project. 2 Contribution No. 727, Department of Dairy and Poultry Science, Kansas Agricultural Experiment Station, Manhattan, Kansas 66S02. 8 Present address: Institut de Recherches Agronomiques, Terbol par Rayak, Lebanon.
results cited b y Clayton m a y be interpreted in an alternate way, which he considers less likely, viz. t h a t a widespread negative environmental trend occurred as indicated b y the control population and t h a t the leading commercial strains had therefore, in fact, improved genetically. T h e present study utilized previously unreported d a t a collected during the selection experiment of Saadeh et al. (1968). Analyses were carried out to investigate two possible reasons for the apparent nonresponsiveness of certain egg production strains to selection: (a) t h a t additive genetic variance m a y be significantly reduced after a few generations of selection; and (b) t h a t negative genetic correlations m a y exist between components of total performance. MATERIALS AND METHODS All selected strains in this study were derived from the heterogeneous Cornell R a n d o m b r e d White Leghorn (CCc) and NC-47 Regional R e d R a n d o m b r e d (RRc) base populations. These foundation stocks originated from multiple strain crosses of commercial egg production chickens and were reproduced annually, at the NC-47 Regional Poultry Breeding Laboratory, b y a plan designed to minimize inbreeding and maintain relatively stable gene frequencies. T h e CCc and R R c populations were crossed and first generation females were backcrossed to males of b o t h parental strains to provide the "crossbred" foundation for the RCids strain (described below).
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E C E N T experiments involving selec• tion for improved performance in egg production strains of chickens have produced inconsistent results. Saadeh et al. (1968) increased part-year rate of lay b y selection on t h a t criterion only. T h e y also found t h a t rate of lay over longer periods was increased. Morris (1963) and Gowe and Strain (1963) selected successfully for increased egg number during the early p a r t of the laying year. After the first few generations, however, it appeared t h a t these gains were largely offset b y losses in egg number during the remainder of the year. A clear-cut inconsistency of results is provided by the experiment of Nordskog et al. (1967) which though similar in all major procedural aspects to the intrapopulation selection scheme of Saadeh et al. failed to provide any direct evidence of increase in part-year rate of egg production.
PART-YEAR EGG PRODUCTION
Strain or Cross CCc RRc RcCc CcRc CCr RRr RrCr CrRr CCids RRids RidsCids CidsRids RCids
Breeding System Randombred control Randombred control Randombred control Randombred control Reciprocal recurrent Reciprocal recurrent Reciprocal recurrent Reciprocal recurrent Individual, dam and Individual, dam and Individual, dam and Individual, dam and Individual, dam and
(c) (c) (c) (c) (r) (r) (r) (r) sire sire sire sire sire
family family family family family
(ids) (ids) (ids) (ids) (ids)
Direct and correlated responses to selection were estimated in all generations except the 5 th. All strain combinations were represented within each of 5 complete replications per generation, with 3 at the NC-47 Laboratory (NC-47) and 2 at Kansas State University (KSU). Thirty pullets were housed per strain combin-
ation within each replication at NC-47 and 25 or more at KSU. Selection responses were estimated by calculating deviations of selected strains from the unselected randombred control strain(s) from which they were derived. Selected strain crosses were similarly compared with control strain crosses. Thus, as examples, CCids was compared with CCc, CrRr with CcRc and RCids with (CCc+RRc)/2. Selection was restricted to rate of egg production from first egg to 260 days of age, based on 3 day per week trapnest records. Traits measured, but not used as selection criteria, were: Age at first egg. The week of age when a bird started laying. 32- and 55-week body weights. Recorded to the nearest 0.1 of a pound and subsequently converted to kilograms. 32- and 55-week egg weights. Eggs were collected during the week following 32 and 55 weeks of age and weighed to the nearest gram. One egg per hen at each age was weighed at KSU and an average based on 1, 2 or 3 eggs was obtained at NC-47. Albumen quality. A visual score of the albumen was obtained on broken out eggs used for 55-week egg weights and specific gravity score. Albumen quality scores ranged from 1 to 12, based on a USDA albumen quality chart. AA albumen quality was given a score of 1, medium AA a score of 2, . . . . , medium C was given a score of 11 and low C a score of 12. Specific gravity score. Shell thickness largely influences the specific gravity of an egg. Eggs were scored from 1 to 9 according to specific gravity when tested in 9 salt solutions ranging from 1.060 to 1.100 with .005 intervals. A score of 1 corresponded to 1.060 and the score of 9 with specific gravity of 1.100.
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The breeding systems, selection, management and analytical procedures used in developing and comparing the selected strains and their crosses were described in detail by Saadeh et al. (1968). Selection based on individual, dam and sire family indexes (ids) produced the CCids, RRids and RCids strains from the CCc, RRc and crossbred foundation stocks, respectively. Reciprocal recurrent selection (r) involving the CCc and RRc stocks initially, resulted in the production of the CCr and RRr strains. Crosses were made between strains within breeding systems each generation for comparative purposes. Such crosses were designated to indicate the male parental strain first and the female parental strain second. Thus the cross of RRids malesXCCids females is indicated by RidsCids and the reciprocal cross by CidsRids. In summary, the following thirteen strain combinations were compared each generation:
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J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH RESULTS
Inbreeding was consciously minimized by avoidance of full- and half-sib matings throughout the study. Average inbreeding coefficients of .055 and .06 were calculated from pedigrees on all selected strain pullets tested at KSU in generations 6 and 7, respectively. Mean coefficients were .05 for the ids and .07 for the r selected strains in generation 7. The effects of inbreeding on the traits considered in this study were estimated by regression coefficients. Sums of squares and cross products were calculated within strains and generations and then pooled to yield the following estimates of change in each of the production traits for each increase of .01 of inbreeding coefficient: Regression + SE -.4 +2.7 .007+ .005 .007+ .004 - . 0 0 6 + .006 .074+ .075 - . 0 4 4 + .096 - . 0 0 2 + .027 .018+ .183
Trait Rate of lay to 260 days, % Age at first egg, wks. 32-wk. body wt., kg. 55-wk. body wt., kg. 32-wk. egg wt., gm. 55-wk. egg wt., gm. Albumen score Specific gravity score
TABLE 1.—Sire components of variance for part-year rate of lay, expressed as percentage of phenotypic variance, and regressions of this statistic on generation number
crosses
Regression1 on
Generations
Selected strains 0
1
2
3
4
number 0.53 -0.52
3.15 8.29
4.88 -0.09
0.11 5.46
-0.17 0.25
-3.58 1.08
1.71 1.84
6.14 3.99
2.71 1.30
-0.42 0.10
-1.11
6.50
12.10
-4.82
1.79
-0.32
1.53
2.58
4.96
1.61
1.47
-0.082
-0.18 -2.96
3.69 0.79
9.83 -- 2 . 2 1
RrCr CrRr
1.22 3.50
4.05 5.71
6.17 -4.52
12.47 -2.89
CCids RRids
4.57 4.23
5.15 -2.66
9.75 3.59
RCids
3.93
3.04
Unwtd. Mean
2.21
3.28
1
6
-0.01 1.20
1.56 6.14
CCr RRr
5
-0.60 -0.47
-3.28 1.30
None of the regression coefficients was significant. The mean regression coefficient was obtained by pooling sums of squares and crossproducts used to calculate within strain regressions after failure to detect heterogeneity among them. 2
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Further examination of regression coefficients by strains and generations revealed considerable inconsistency of estimates. It was concluded that adjustments for inbreeding were not warranted due to these inconsistencies, the lack of significance of the pooled regression coefficients and the relatively low levels of inbreeding attained. Sire components of variance for rate of lay from first egg to 260 days of age were estimated generation-by-generation within strains. They were converted to percentages of phenotypic variance and regressions on generation of selection were then computed, Table 1. Variance estimates were based on data from females trapnested for selection purposes and did not include records on RidsCids and CidsRids females, produced for comparative purposes only. Approximately 400 to 500 pullets produced by 20 sires and 100 dams were trapnested per strain each generation. There was no indication of a depletion of additive genetic variance. Table 2 presents variance component and heritability estimates based on rec-
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PART-YEAR EGG PRODUCTION
TABLE 2.—Variance component and heritability estimates for eight productivity trails Source of variancei
R a t e of lay to 260 days
Age a t first egg, wks.
32-week
55-week
32-week
55-week
Albumen quality, score
Sires Dams Within Progenies Total
2.500 7.900 92.400 102.800
0.270 0.312 1.792 2.374
0.003 0.004 0.015 0.022
0.005 0.005 0.005 0.015
0.589 1.759 3.690 6.038
1.059 1.563 2.850 5.472
0.027 0.085 0.366 0.721
0.10±0.16
0.40+0.15
0.44±0.15
0.47±0.16
0.22 + 0.19
0.30±0.18
0.15±0.19
Heritability 2
%
Body wt., kg.
Egg wt., gm.
Specific gravity, score 0.110 -0.028 0.834 0.916 0.34+0.19
1
Variance components were estimated separately for each strain combination within generation and replication and pooled. Invariances of the estimates were used as the weighting factors. 2 Heritability was estimated as four times the ratio of the sire component of variance to the total phenotypic variance. Estimates of heritabilities were obtained within strain combination, generation and replication and pooled by weighting the estimates with the invariances.
these same populations for rate of lay from first egg to 385 or 500 days of age indicated significant responses in all ids strain combinations except the RCids (Saadeh et al., 1968). I t was hypothesized that the RCids represented a special case because of the recent origin of the crossbred foundation from which it was derived. Evidence is lacking of an increase in rate of lay to 260 days for the CCr and RRr strains, selected for reciprocal combining ability. There is some indication that their crosses may have improved, e.g. the regression of the RrCr response on generation approached significance ( P < 0.10). Likewise, Saadeh et al. found highly significant responses for the r crosses when
TABLE 3.—Direct and correlated responses resulting from seven generations of selection for rate of lay from first egg to 260 days of age1 Selected strains R a t e of lay and their to 260 days crosses
32-week
5 5-week
32-week
55-week
Albumen quality, score
Specific gravity, score
0.08 0.55" 0.32**
-0.02* -0.01 -0.02*
-0.03* 0.00 -0.02*
-0.19* 0.10 -0.05
-0.50** 0.29 -0.11
-0.06** -0.17** -0.12"
-0.02 -0.02* -0.02
0.21* 0.00 0.12
-0.02* -0.02* -0.02*
-0.01 -0.02* -0.02*
0.16 -0.09 0.04
0.14 -0.43* -0.15
-0.12* -0.11** -0.12**
-0.04* 0.02 -0.01 0.01 0.03 0.02
Age a t first egg, wks.
%
CCr RRr Av.
1
0.27 -0.20 0.03
Body wt,. , k g .
Egg wt. , gm.
RrCr CrRr Av.
0.97 0.50 0.73
CCids RRids Av.
1.30** 0.42 0.86**
-0.06 0.32** 0.13
-0.03 -0.03** -0.03**
-0.04* -0.04" -0.04**
-0.44** -0.24 -0.34**
-0.82** -0.37 -0.60"
0.04 0.09* 0.07
RidsCids CidsRids Av.
0.98** 1.14** 1.06**
0.07 -0.10 -0.02
-0.02* -0.02* -0.02*
-0.03" -0.03** -0.03"
0.05 -0.13 -0.04
-0.11 -0.47** -0.29
0.01 0.02 0.02
-0.05 0.00 -0.03
RCids
0.29
0.12
-0.02*
-0.03**
-0.46*
-0.58**
-0.01
0.03
Regressions of selection response on generation number. Responses estimated as deviations of selected strains from control strains and deviation of selected strain crosses from control strain crosses. * P<0.05; **=P<0.01
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ords obtained at KSU from 75 pullets produced by 10 sires and 40 to 50 dams per selected strain or cross per generation. These were calculated within strain combination-replication-generation subclasses for the first 4 selected generations and pooled with weighting of each subclass estimate by its invariance (Fisher, 1949). Direct and correlated responses obtained are presented graphically in Figure 1 and as regressions on generation of selection in Table 3. Selection based on efficient use of additive genetic variance, i.e. the ids breeding system, was clearly effective in increasing rate of lay to 260 days of age within 3 of the 5 ids populations. Comparisons of
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J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
to selection was lacking. Use of the near zero response estimates for part-year rate of lay in Falconer's (1954) formula for AG would presumably inflate the associated genetic correlation estimates. Genetic correlations were also estimated by means of sire components of covariances and variances. Data for these estimates were the same as used in calculating the variance components and heritabilities presented in Table 2. Pooled values were obtained from subclass estimates using invariances as weighting factors, as before. Standard errors are presented for pooled estimates only. Cursory examination of the genetic correlation estimates in Table 4A and 4B indicates their relatively low reliability. Thus, estimates derived from the correlated responses often exceed unity and those derived by use of covariances and variances vary widely even when based on selected strains recently developed from the same base population. Lack of precision is also indicated by the very large standard errors associated with the weighted averages. DISCUSSION
Intrapopulation selection based on part-year records was effective in increasing rate of egg production in the CCids and RRids strains and their crosses over a 7-generation interval. Neither response curves nor regressions of sire components of variance on generation suggested depletion of additive genetic variance. Egg production records from pullets tested as candidates for selection and from those used in comparisons of breeding systems yielded heritability estimates of 0.10 + 0.05 and 0.10+ 0.16, respectively, for partyear rate. These estimates are in essential agreement with previous estimates of 0.06 and 0.18 for the randombred CCc,
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rate of lay was measured from first egg to 385 or 500 days of age. Age at sexual maturity was largely excluded in selecting for rate of lay as the number of eggs and number of trap days used in calculating rate were counted beginning with the first egg recorded for each hen. However, birds with unusually late sexual maturity were excluded from consideration, as production of at least 10 eggs on trap days prior to 260 days of age was required before rate was considered as estimated reliably enough for selection purposes. Changes in age at first egg over the 7 generations of study occurred in some strain combinations (Table 3). Significant positive regressions were found for the RRr, RRids and RrCr strain combinations. In contrast no trend was apparent for the selected strain derived from the CCc foundation. Graphic representation of changes in age at first egg (Figure 1) suggests a reversal of trend from earlier to later sexual maturity for the first 3 and the last 4 generations, respectively. Body and egg weights at both 32 and 55 weeks of age tended to decrease with advancing generations. The downward trend was more apparent under the ids than under the r breeding system. Albumen quality and specific gravity scores tended to decrease under the r breeding system. These changes indicate better albumen quality, but thinner egg shells. Changes under the ids breeding system were generally of opposite sign but nonsignificant for the same characteristics. Genetic correlations based on estimates of heritabilities, phenotypic variances and correlated responses obtained in this study are presented in Table 4A. Such estimates were not calculated for the CCr, RRr and RCids populations, as statistical evidence of response of the primary trait
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P A R T - Y E A R E G G PRODUCTION
AGE AT FIRST EGG, WK.
32-WK. BODY WT, KG.
55-WK. BODY WT, KG.
32-WK. EGG WT, GM.
55-WK. EGG WT, GM.
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RATE OF LAY TO 2 6 0 DAYS, %
o en O O
o en to
z O
>
^
^
LU Q
SPECIFIC GRAVITY, SCORE
ALBUMEN QUALITY, SCORE
-.8
0
1
2
3
4
6
7
GENERATIONS
6 OF
1
2
3
4
6
7
SELECTION
FIG. 1. Direct and correlated responses to selection shown graphically on a generation-by-generation basis. Responses were estimated by deviations of selected strains from control strams and by deviations of crosses of selected strains from crosses of control strains.
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J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
TABLE 4.—Genetic correlations of rate of lay from first egg to 260 days of age with other productivity trails Selected strains and their crosses
Age at first egg 0.73 0.00
RrCr CrRr
Body ' Wt. 32-week
55-week
Egg wt. 32-week
55-week
Albumen quality
Specific gravity
A. Estimated from correlated responses1 0.35 0.45 -0.39 -0.66 -0.51 -1.57 -1.31 -2.17
-1.17 -2.15
-0.23 0.23
-0.17 2.54
-0.66 -2.29
-1.18 -3.92
-0.96 -1.60
-1.59 -2.22
0.29 2.06
0.05 0.41
RidsCids CidsRids
0.23 -0.30
-0.66 -0.66
-1.18 -1.18
-0.14 -0.31
-0.28 -1.03
0.10 0.20
-0.29 0.00
0.51
-1.04
-1.57
-0.51
-1.16
-0.11
0.03
Unwtd. Av.
2
CCr RRr
B. Estimated from sire components. of covariance and variances -0.13 -0.29 0.01 1.10 1.16 -0.08 0.20 -0.54 -0.26 -0.68 -0.71 0.65
RrCr CrRr
0.96 -0.56
0.50 0.68
0.39 0.58
0.28 -0.55
0.20 -0.21
0.31 0.25
0.78 0.04
CCids RRids
-0.32 -0.34
-0.24 0.20
-0.42 0.03
-0.69 -0.21
-1.27 -0.06
-0.20
-0.01 -0.54
RidsCids CidsRids
-0.08 0.21
0.58 0.02
-0.19 -0.47
-0.30 -0.42
-0.58 -0.59
0.20 -0.14
-1.06 0.29
0.25
-0.42
-0.50
-0.18
-1.16
0.06
-0.58
0.04 + 0.55
0.44 + 0.66
-.17 + 0.37
-.42 + 0.80
-.41 + 2.14
0.15 + 0.42
0.10 + 0.43
RCids Wtd. Av.
3
-0.49 0.53
1 Based on the Falconer (1954) method of estimation, i.e. genetic correlation=AG'hP/AGh'p'; where AG, h and P are the regressions of selection response on generation number, square root of heritability estimates and phenotypic standard deviations, respectively. The prime indicates the statistic for the trait not under direct selection. 2 Estimates were obtained within generation and replication and pooled by weighting the estimates with their invariance. 3 Invariances were used as the weighting factors.
0.18 for the RRc and the average of 0.18 from a literature survey (see Kinney et ak, 1968; and Kinney and Lowe, 1968). Additive genetic variance for other productivity traits was also indicated from estimates of this study and those of Kinney et al. (1968) and Kinney and Lowe (1968). The smaller heritability estimates for egg weight presented here may be due to the restriction that only one egg per pullet was weighed at each age. Correlated responses in age at first egg, albumen quality and shell thickness as indicated by specific gravity were in some cases significant. They were not, however,
consistent over the various selected strains and crosses and in the case of age at first egg an apparent reversal of response occurred midway through the experiment. There was a general tendency for the selected strains and their crosses to decrease in body and egg weight at 32 and 55 weeks of age. The effect was greater at 55 weeks for both traits. These correlated responses were larger and more often significant under the ids breeding system where clear-cut changes in part-year rate of lay were obtained by selection. Other studies in which strains were se-
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CCids RRids
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PART-YEAR EGG PRODUCTION
ity and persistence of egg laying. Some selection pressure was used to increase egg size in advanced generations of both studies and to improve fertility, hatchability and livability by Gowe and Strain. Morris (1963) did not report any adjustments of his data for increasing levels of inbreeding, although deleterious effects on productivity were clearly evident in his strains (Morris, 1962) and are therefore confounded with selection effects. Conclusions from the foregoing considerations are: (a) usable genetic variance was available for major productivity traits within each of two egg production populations of chickens based on crosses of commercial strains, (b) exhaustion of additive genetic variation did not accompany 7 generations of selection for the lowly heritable trait of part-year rate of egg production and (c) negative genetic correlations exist between rate of lay and body and egg weight. It appears likely that undesirable genetic associations between components of total productivity are more likely responsible for lack of improvement in egg production strains at this time than is exhaustion of genetic variation. SUMMARY
Data from strains of chickens which responded to selection for part-year rate of egg production were analyzed for evidence of change in additive genetic variance and for correlated responses in traits not under direct selection. There was no suggestion of depletion of additive genetic variance resulting from 7 generations of selection. No consistent changes in age at first egg, albumen quality and egg shell thickness were detected. However, body and egg weights at 32 and 55 weeks of age were generally reduced and the response was more striking in those strains (and their crosses) showing greater response to se-
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lected primarily for high egg production early in the year indicated decreases in body and egg weights similar to those we observed. Thus, Nordskog et al. (1967), though unable to detect increased rate of lay due to selection for that trait, found significant negative changes in these secondary characteristics. We suggested (Saadeh et al., 1968) that such changes in traits not under direct selection might be interpreted as indicative of a cryptic gain in the primary trait. Morris (1963) observed downward trends in body and egg weight and Gowe's results (cited by Clayton, 1968) concur in terms of egg weight response. The absence of correlated responses for age at first egg in the White Leghorn strains of this study are in contrast with the downward trend in 2 other selection experiments with this breed (Morris, 1963; Gowe and Strain, 1963; and Gowe's later results cited by Clayton, 1968). Such a response obtained in the other experiments is not surprising, however, since age at sexual maturity is considerably influenced by additive genetic variance and is a major component of number of eggs per hen housed for the part-year record, used as their primary selection criterion. A discrepancy exists between the results of this selection experiment (see also Saadeh et al., 1968) and those of Morris (1963) and Gowe and Strain (1963; see also Clayton, 1968). As previously noted, we found that rate of lay over longer periods was increased by selection for part-year rate, while they reported a plateauing of response in annual egg production when selection was for part-year number of eggs per pullet housed. Differences in selection criteria may account for these divergent results. Their lack of improvement for full-year productivity could be associated with a negative genetic correlation between early sexual matur-
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J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
lection for part-year rate of lay. It was concluded that undesirable genetic associations between components of total productivity are more likely responsible for lack of improvement in egg production strains at this time than is exhaustion of genetic variation. ACKNOWLEDGMENT
REFERENCES Clayton, G. A. 1968. Some implications of selection results in poultry. World's Poultry Sci. J. 24:3757. Dickerson, G. E., 1955. Genetic slippage in response to selection for multiple objectives. Cold Spring
NEWS AND
NOTES
(Continued from page 1275) volume. The present supplement is the third to be issued on an annual basis. The bibliography regards farm buildings as part of farm equipment and therefore gives reference to any matter affecting their design, construction, economics, and use. It includes the findings of scientific research, of such systematic but less precise forms of investigations as surveys, and of field trials and similar developmental phases of new techniques. It includes information from any country where conditions are relevant to Great Britian. It gives abstracts of the original material, classified by farm enterprise, and a guide to the contents. The titles and prices of previous reports in the series are given below. All are published by the Agricultural Research Council and obtainable from Her Majesty's Stationery Office.
(a) Bibliography of Farm Buildings Research {194558) Part 1. Buildings for Pigs. (2/6) Part 2. Buildings for Potato Storage. (2/-) Part 3. Buildings for Poultry. (4/6) Part 4. Buildings for Cattle. (4/6) Part 5. Buildings for Drying and Storage of Grain. (3/-) Part 6. Buildings for the Processing and Storage of Fodder. (4/-) Part 7. Miscellaneous Items. (2/-) (b) First Supplement (1958-61) Part 1. Buildings for Pigs. (4/-) Part 3. Buildings for Poultry. (4/-) Part 4. Buildings for Cattle. (6/-) Part 5. Buildings for Drying and Storage of Grain. (2/6)
(Continued on page 1364)
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We thank S. C. King, J. R. Carson, S. P. Wilson, T. B. Kinney, P. C. Lowe and B. T. Weinland of the NC-47 Regional Laboratory Staff for supplying samples of the control populations and for collection of data at the Regional Poultry Breeding Laboratory. We are also grateful to L.T. Smith and S. Wearden for their assistance at the Kansas Experiment Station during the earlier years of the study.
Harbor Symposia on Quantitative Biol. 20: 213— 224. Falconer, D. S., 1954. Validity of the theory of genetic correlation. J. Heredity, 45:42-44. Fisher, R. A., 1949. The Design of Experiments. 5th edition. Oliver and Boyd, Edinburgh. Gowe, R. S., and J. H. Strain, 1963. Effect of selection for increased egg production based on partyear records in two strains of White Leghorns. Canad. J. Genet. Cyt. 5: 99-100. Kinney, T. B., Jr., and P. C. Lowe, 1968. Genetic and phenotypic variation in the Regional Red Controls over nine years. Poultry Sci. 47: 11051110. Kinney, T. B., Jr., P. C. Lowe, B. B. Bohren and S. P. Wilson, 1968. Genetic and phenotypic variation in randombred White Leghorn controls over several generations. Poultry Sci. 47:113-123. Morris, J. A., 1962. The effect of mild inbreeding in two lines of White Leghorn. Australian J. Agr. Res. 13:362-375. Morris, J. A., 1963. Continuous selection for egg production using short-term records. Australian J. Biol. Sci. 14: 909-925. Nordskog, A. W., A. Festing and M. W. Verghese, 1967. Selection for egg production and correlated responses in the fowl. Genetics, 55:179-191. Saadeh, H. K., J. V. Craig, L. T. Smith and S. Wearden, 1968. Effectiveness of alternative breeding systems for increasing rate of egg production in chickens. Poultry Sci. 47: 1057-1072.
SAADEH 3
Department of Dairy and Poultry Science, Kansas State University, Manhattan, Kansas 66502 (Received for publication January 25, 1969)
INTRODUCTION
R
There is some concern t h a t commercial egg-production strains of chickens m a y be approaching or h a v e already reached plateaus of response to selection for total performance (see Dickerson, 1955; and Clayton, 1968). T h e random sample test
1 This investigation was part of the Kansas contribution to the NC-47 Regional Poultry Breeding Project. 2 Contribution No. 727, Department of Dairy and Poultry Science, Kansas Agricultural Experiment Station, Manhattan, Kansas 66S02. 8 Present address: Institut de Recherches Agronomiques, Terbol par Rayak, Lebanon.
results cited b y Clayton m a y be interpreted in an alternate way, which he considers less likely, viz. t h a t a widespread negative environmental trend occurred as indicated b y the control population and t h a t the leading commercial strains had therefore, in fact, improved genetically. T h e present study utilized previously unreported d a t a collected during the selection experiment of Saadeh et al. (1968). Analyses were carried out to investigate two possible reasons for the apparent nonresponsiveness of certain egg production strains to selection: (a) t h a t additive genetic variance m a y be significantly reduced after a few generations of selection; and (b) t h a t negative genetic correlations m a y exist between components of total performance. MATERIALS AND METHODS All selected strains in this study were derived from the heterogeneous Cornell R a n d o m b r e d White Leghorn (CCc) and NC-47 Regional R e d R a n d o m b r e d (RRc) base populations. These foundation stocks originated from multiple strain crosses of commercial egg production chickens and were reproduced annually, at the NC-47 Regional Poultry Breeding Laboratory, b y a plan designed to minimize inbreeding and maintain relatively stable gene frequencies. T h e CCc and R R c populations were crossed and first generation females were backcrossed to males of b o t h parental strains to provide the "crossbred" foundation for the RCids strain (described below).
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E C E N T experiments involving selec• tion for improved performance in egg production strains of chickens have produced inconsistent results. Saadeh et al. (1968) increased part-year rate of lay b y selection on t h a t criterion only. T h e y also found t h a t rate of lay over longer periods was increased. Morris (1963) and Gowe and Strain (1963) selected successfully for increased egg number during the early p a r t of the laying year. After the first few generations, however, it appeared t h a t these gains were largely offset b y losses in egg number during the remainder of the year. A clear-cut inconsistency of results is provided by the experiment of Nordskog et al. (1967) which though similar in all major procedural aspects to the intrapopulation selection scheme of Saadeh et al. failed to provide any direct evidence of increase in part-year rate of egg production.
PART-YEAR EGG PRODUCTION
Strain or Cross CCc RRc RcCc CcRc CCr RRr RrCr CrRr CCids RRids RidsCids CidsRids RCids
Breeding System Randombred control Randombred control Randombred control Randombred control Reciprocal recurrent Reciprocal recurrent Reciprocal recurrent Reciprocal recurrent Individual, dam and Individual, dam and Individual, dam and Individual, dam and Individual, dam and
(c) (c) (c) (c) (r) (r) (r) (r) sire sire sire sire sire
family family family family family
(ids) (ids) (ids) (ids) (ids)
Direct and correlated responses to selection were estimated in all generations except the 5 th. All strain combinations were represented within each of 5 complete replications per generation, with 3 at the NC-47 Laboratory (NC-47) and 2 at Kansas State University (KSU). Thirty pullets were housed per strain combin-
ation within each replication at NC-47 and 25 or more at KSU. Selection responses were estimated by calculating deviations of selected strains from the unselected randombred control strain(s) from which they were derived. Selected strain crosses were similarly compared with control strain crosses. Thus, as examples, CCids was compared with CCc, CrRr with CcRc and RCids with (CCc+RRc)/2. Selection was restricted to rate of egg production from first egg to 260 days of age, based on 3 day per week trapnest records. Traits measured, but not used as selection criteria, were: Age at first egg. The week of age when a bird started laying. 32- and 55-week body weights. Recorded to the nearest 0.1 of a pound and subsequently converted to kilograms. 32- and 55-week egg weights. Eggs were collected during the week following 32 and 55 weeks of age and weighed to the nearest gram. One egg per hen at each age was weighed at KSU and an average based on 1, 2 or 3 eggs was obtained at NC-47. Albumen quality. A visual score of the albumen was obtained on broken out eggs used for 55-week egg weights and specific gravity score. Albumen quality scores ranged from 1 to 12, based on a USDA albumen quality chart. AA albumen quality was given a score of 1, medium AA a score of 2, . . . . , medium C was given a score of 11 and low C a score of 12. Specific gravity score. Shell thickness largely influences the specific gravity of an egg. Eggs were scored from 1 to 9 according to specific gravity when tested in 9 salt solutions ranging from 1.060 to 1.100 with .005 intervals. A score of 1 corresponded to 1.060 and the score of 9 with specific gravity of 1.100.
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The breeding systems, selection, management and analytical procedures used in developing and comparing the selected strains and their crosses were described in detail by Saadeh et al. (1968). Selection based on individual, dam and sire family indexes (ids) produced the CCids, RRids and RCids strains from the CCc, RRc and crossbred foundation stocks, respectively. Reciprocal recurrent selection (r) involving the CCc and RRc stocks initially, resulted in the production of the CCr and RRr strains. Crosses were made between strains within breeding systems each generation for comparative purposes. Such crosses were designated to indicate the male parental strain first and the female parental strain second. Thus the cross of RRids malesXCCids females is indicated by RidsCids and the reciprocal cross by CidsRids. In summary, the following thirteen strain combinations were compared each generation:
1289
1290
J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH RESULTS
Inbreeding was consciously minimized by avoidance of full- and half-sib matings throughout the study. Average inbreeding coefficients of .055 and .06 were calculated from pedigrees on all selected strain pullets tested at KSU in generations 6 and 7, respectively. Mean coefficients were .05 for the ids and .07 for the r selected strains in generation 7. The effects of inbreeding on the traits considered in this study were estimated by regression coefficients. Sums of squares and cross products were calculated within strains and generations and then pooled to yield the following estimates of change in each of the production traits for each increase of .01 of inbreeding coefficient: Regression + SE -.4 +2.7 .007+ .005 .007+ .004 - . 0 0 6 + .006 .074+ .075 - . 0 4 4 + .096 - . 0 0 2 + .027 .018+ .183
Trait Rate of lay to 260 days, % Age at first egg, wks. 32-wk. body wt., kg. 55-wk. body wt., kg. 32-wk. egg wt., gm. 55-wk. egg wt., gm. Albumen score Specific gravity score
TABLE 1.—Sire components of variance for part-year rate of lay, expressed as percentage of phenotypic variance, and regressions of this statistic on generation number
crosses
Regression1 on
Generations
Selected strains 0
1
2
3
4
number 0.53 -0.52
3.15 8.29
4.88 -0.09
0.11 5.46
-0.17 0.25
-3.58 1.08
1.71 1.84
6.14 3.99
2.71 1.30
-0.42 0.10
-1.11
6.50
12.10
-4.82
1.79
-0.32
1.53
2.58
4.96
1.61
1.47
-0.082
-0.18 -2.96
3.69 0.79
9.83 -- 2 . 2 1
RrCr CrRr
1.22 3.50
4.05 5.71
6.17 -4.52
12.47 -2.89
CCids RRids
4.57 4.23
5.15 -2.66
9.75 3.59
RCids
3.93
3.04
Unwtd. Mean
2.21
3.28
1
6
-0.01 1.20
1.56 6.14
CCr RRr
5
-0.60 -0.47
-3.28 1.30
None of the regression coefficients was significant. The mean regression coefficient was obtained by pooling sums of squares and crossproducts used to calculate within strain regressions after failure to detect heterogeneity among them. 2
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Further examination of regression coefficients by strains and generations revealed considerable inconsistency of estimates. It was concluded that adjustments for inbreeding were not warranted due to these inconsistencies, the lack of significance of the pooled regression coefficients and the relatively low levels of inbreeding attained. Sire components of variance for rate of lay from first egg to 260 days of age were estimated generation-by-generation within strains. They were converted to percentages of phenotypic variance and regressions on generation of selection were then computed, Table 1. Variance estimates were based on data from females trapnested for selection purposes and did not include records on RidsCids and CidsRids females, produced for comparative purposes only. Approximately 400 to 500 pullets produced by 20 sires and 100 dams were trapnested per strain each generation. There was no indication of a depletion of additive genetic variance. Table 2 presents variance component and heritability estimates based on rec-
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PART-YEAR EGG PRODUCTION
TABLE 2.—Variance component and heritability estimates for eight productivity trails Source of variancei
R a t e of lay to 260 days
Age a t first egg, wks.
32-week
55-week
32-week
55-week
Albumen quality, score
Sires Dams Within Progenies Total
2.500 7.900 92.400 102.800
0.270 0.312 1.792 2.374
0.003 0.004 0.015 0.022
0.005 0.005 0.005 0.015
0.589 1.759 3.690 6.038
1.059 1.563 2.850 5.472
0.027 0.085 0.366 0.721
0.10±0.16
0.40+0.15
0.44±0.15
0.47±0.16
0.22 + 0.19
0.30±0.18
0.15±0.19
Heritability 2
%
Body wt., kg.
Egg wt., gm.
Specific gravity, score 0.110 -0.028 0.834 0.916 0.34+0.19
1
Variance components were estimated separately for each strain combination within generation and replication and pooled. Invariances of the estimates were used as the weighting factors. 2 Heritability was estimated as four times the ratio of the sire component of variance to the total phenotypic variance. Estimates of heritabilities were obtained within strain combination, generation and replication and pooled by weighting the estimates with the invariances.
these same populations for rate of lay from first egg to 385 or 500 days of age indicated significant responses in all ids strain combinations except the RCids (Saadeh et al., 1968). I t was hypothesized that the RCids represented a special case because of the recent origin of the crossbred foundation from which it was derived. Evidence is lacking of an increase in rate of lay to 260 days for the CCr and RRr strains, selected for reciprocal combining ability. There is some indication that their crosses may have improved, e.g. the regression of the RrCr response on generation approached significance ( P < 0.10). Likewise, Saadeh et al. found highly significant responses for the r crosses when
TABLE 3.—Direct and correlated responses resulting from seven generations of selection for rate of lay from first egg to 260 days of age1 Selected strains R a t e of lay and their to 260 days crosses
32-week
5 5-week
32-week
55-week
Albumen quality, score
Specific gravity, score
0.08 0.55" 0.32**
-0.02* -0.01 -0.02*
-0.03* 0.00 -0.02*
-0.19* 0.10 -0.05
-0.50** 0.29 -0.11
-0.06** -0.17** -0.12"
-0.02 -0.02* -0.02
0.21* 0.00 0.12
-0.02* -0.02* -0.02*
-0.01 -0.02* -0.02*
0.16 -0.09 0.04
0.14 -0.43* -0.15
-0.12* -0.11** -0.12**
-0.04* 0.02 -0.01 0.01 0.03 0.02
Age a t first egg, wks.
%
CCr RRr Av.
1
0.27 -0.20 0.03
Body wt,. , k g .
Egg wt. , gm.
RrCr CrRr Av.
0.97 0.50 0.73
CCids RRids Av.
1.30** 0.42 0.86**
-0.06 0.32** 0.13
-0.03 -0.03** -0.03**
-0.04* -0.04" -0.04**
-0.44** -0.24 -0.34**
-0.82** -0.37 -0.60"
0.04 0.09* 0.07
RidsCids CidsRids Av.
0.98** 1.14** 1.06**
0.07 -0.10 -0.02
-0.02* -0.02* -0.02*
-0.03" -0.03** -0.03"
0.05 -0.13 -0.04
-0.11 -0.47** -0.29
0.01 0.02 0.02
-0.05 0.00 -0.03
RCids
0.29
0.12
-0.02*
-0.03**
-0.46*
-0.58**
-0.01
0.03
Regressions of selection response on generation number. Responses estimated as deviations of selected strains from control strains and deviation of selected strain crosses from control strain crosses. * P<0.05; **=P<0.01
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ords obtained at KSU from 75 pullets produced by 10 sires and 40 to 50 dams per selected strain or cross per generation. These were calculated within strain combination-replication-generation subclasses for the first 4 selected generations and pooled with weighting of each subclass estimate by its invariance (Fisher, 1949). Direct and correlated responses obtained are presented graphically in Figure 1 and as regressions on generation of selection in Table 3. Selection based on efficient use of additive genetic variance, i.e. the ids breeding system, was clearly effective in increasing rate of lay to 260 days of age within 3 of the 5 ids populations. Comparisons of
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J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
to selection was lacking. Use of the near zero response estimates for part-year rate of lay in Falconer's (1954) formula for AG would presumably inflate the associated genetic correlation estimates. Genetic correlations were also estimated by means of sire components of covariances and variances. Data for these estimates were the same as used in calculating the variance components and heritabilities presented in Table 2. Pooled values were obtained from subclass estimates using invariances as weighting factors, as before. Standard errors are presented for pooled estimates only. Cursory examination of the genetic correlation estimates in Table 4A and 4B indicates their relatively low reliability. Thus, estimates derived from the correlated responses often exceed unity and those derived by use of covariances and variances vary widely even when based on selected strains recently developed from the same base population. Lack of precision is also indicated by the very large standard errors associated with the weighted averages. DISCUSSION
Intrapopulation selection based on part-year records was effective in increasing rate of egg production in the CCids and RRids strains and their crosses over a 7-generation interval. Neither response curves nor regressions of sire components of variance on generation suggested depletion of additive genetic variance. Egg production records from pullets tested as candidates for selection and from those used in comparisons of breeding systems yielded heritability estimates of 0.10 + 0.05 and 0.10+ 0.16, respectively, for partyear rate. These estimates are in essential agreement with previous estimates of 0.06 and 0.18 for the randombred CCc,
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rate of lay was measured from first egg to 385 or 500 days of age. Age at sexual maturity was largely excluded in selecting for rate of lay as the number of eggs and number of trap days used in calculating rate were counted beginning with the first egg recorded for each hen. However, birds with unusually late sexual maturity were excluded from consideration, as production of at least 10 eggs on trap days prior to 260 days of age was required before rate was considered as estimated reliably enough for selection purposes. Changes in age at first egg over the 7 generations of study occurred in some strain combinations (Table 3). Significant positive regressions were found for the RRr, RRids and RrCr strain combinations. In contrast no trend was apparent for the selected strain derived from the CCc foundation. Graphic representation of changes in age at first egg (Figure 1) suggests a reversal of trend from earlier to later sexual maturity for the first 3 and the last 4 generations, respectively. Body and egg weights at both 32 and 55 weeks of age tended to decrease with advancing generations. The downward trend was more apparent under the ids than under the r breeding system. Albumen quality and specific gravity scores tended to decrease under the r breeding system. These changes indicate better albumen quality, but thinner egg shells. Changes under the ids breeding system were generally of opposite sign but nonsignificant for the same characteristics. Genetic correlations based on estimates of heritabilities, phenotypic variances and correlated responses obtained in this study are presented in Table 4A. Such estimates were not calculated for the CCr, RRr and RCids populations, as statistical evidence of response of the primary trait
1293
P A R T - Y E A R E G G PRODUCTION
AGE AT FIRST EGG, WK.
32-WK. BODY WT, KG.
55-WK. BODY WT, KG.
32-WK. EGG WT, GM.
55-WK. EGG WT, GM.
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RATE OF LAY TO 2 6 0 DAYS, %
o en O O
o en to
z O
>
^
^
LU Q
SPECIFIC GRAVITY, SCORE
ALBUMEN QUALITY, SCORE
-.8
0
1
2
3
4
6
7
GENERATIONS
6 OF
1
2
3
4
6
7
SELECTION
FIG. 1. Direct and correlated responses to selection shown graphically on a generation-by-generation basis. Responses were estimated by deviations of selected strains from control strams and by deviations of crosses of selected strains from crosses of control strains.
1294
J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
TABLE 4.—Genetic correlations of rate of lay from first egg to 260 days of age with other productivity trails Selected strains and their crosses
Age at first egg 0.73 0.00
RrCr CrRr
Body ' Wt. 32-week
55-week
Egg wt. 32-week
55-week
Albumen quality
Specific gravity
A. Estimated from correlated responses1 0.35 0.45 -0.39 -0.66 -0.51 -1.57 -1.31 -2.17
-1.17 -2.15
-0.23 0.23
-0.17 2.54
-0.66 -2.29
-1.18 -3.92
-0.96 -1.60
-1.59 -2.22
0.29 2.06
0.05 0.41
RidsCids CidsRids
0.23 -0.30
-0.66 -0.66
-1.18 -1.18
-0.14 -0.31
-0.28 -1.03
0.10 0.20
-0.29 0.00
0.51
-1.04
-1.57
-0.51
-1.16
-0.11
0.03
Unwtd. Av.
2
CCr RRr
B. Estimated from sire components. of covariance and variances -0.13 -0.29 0.01 1.10 1.16 -0.08 0.20 -0.54 -0.26 -0.68 -0.71 0.65
RrCr CrRr
0.96 -0.56
0.50 0.68
0.39 0.58
0.28 -0.55
0.20 -0.21
0.31 0.25
0.78 0.04
CCids RRids
-0.32 -0.34
-0.24 0.20
-0.42 0.03
-0.69 -0.21
-1.27 -0.06
-0.20
-0.01 -0.54
RidsCids CidsRids
-0.08 0.21
0.58 0.02
-0.19 -0.47
-0.30 -0.42
-0.58 -0.59
0.20 -0.14
-1.06 0.29
0.25
-0.42
-0.50
-0.18
-1.16
0.06
-0.58
0.04 + 0.55
0.44 + 0.66
-.17 + 0.37
-.42 + 0.80
-.41 + 2.14
0.15 + 0.42
0.10 + 0.43
RCids Wtd. Av.
3
-0.49 0.53
1 Based on the Falconer (1954) method of estimation, i.e. genetic correlation=AG'hP/AGh'p'; where AG, h and P are the regressions of selection response on generation number, square root of heritability estimates and phenotypic standard deviations, respectively. The prime indicates the statistic for the trait not under direct selection. 2 Estimates were obtained within generation and replication and pooled by weighting the estimates with their invariance. 3 Invariances were used as the weighting factors.
0.18 for the RRc and the average of 0.18 from a literature survey (see Kinney et ak, 1968; and Kinney and Lowe, 1968). Additive genetic variance for other productivity traits was also indicated from estimates of this study and those of Kinney et al. (1968) and Kinney and Lowe (1968). The smaller heritability estimates for egg weight presented here may be due to the restriction that only one egg per pullet was weighed at each age. Correlated responses in age at first egg, albumen quality and shell thickness as indicated by specific gravity were in some cases significant. They were not, however,
consistent over the various selected strains and crosses and in the case of age at first egg an apparent reversal of response occurred midway through the experiment. There was a general tendency for the selected strains and their crosses to decrease in body and egg weight at 32 and 55 weeks of age. The effect was greater at 55 weeks for both traits. These correlated responses were larger and more often significant under the ids breeding system where clear-cut changes in part-year rate of lay were obtained by selection. Other studies in which strains were se-
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CCids RRids
1295
PART-YEAR EGG PRODUCTION
ity and persistence of egg laying. Some selection pressure was used to increase egg size in advanced generations of both studies and to improve fertility, hatchability and livability by Gowe and Strain. Morris (1963) did not report any adjustments of his data for increasing levels of inbreeding, although deleterious effects on productivity were clearly evident in his strains (Morris, 1962) and are therefore confounded with selection effects. Conclusions from the foregoing considerations are: (a) usable genetic variance was available for major productivity traits within each of two egg production populations of chickens based on crosses of commercial strains, (b) exhaustion of additive genetic variation did not accompany 7 generations of selection for the lowly heritable trait of part-year rate of egg production and (c) negative genetic correlations exist between rate of lay and body and egg weight. It appears likely that undesirable genetic associations between components of total productivity are more likely responsible for lack of improvement in egg production strains at this time than is exhaustion of genetic variation. SUMMARY
Data from strains of chickens which responded to selection for part-year rate of egg production were analyzed for evidence of change in additive genetic variance and for correlated responses in traits not under direct selection. There was no suggestion of depletion of additive genetic variance resulting from 7 generations of selection. No consistent changes in age at first egg, albumen quality and egg shell thickness were detected. However, body and egg weights at 32 and 55 weeks of age were generally reduced and the response was more striking in those strains (and their crosses) showing greater response to se-
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lected primarily for high egg production early in the year indicated decreases in body and egg weights similar to those we observed. Thus, Nordskog et al. (1967), though unable to detect increased rate of lay due to selection for that trait, found significant negative changes in these secondary characteristics. We suggested (Saadeh et al., 1968) that such changes in traits not under direct selection might be interpreted as indicative of a cryptic gain in the primary trait. Morris (1963) observed downward trends in body and egg weight and Gowe's results (cited by Clayton, 1968) concur in terms of egg weight response. The absence of correlated responses for age at first egg in the White Leghorn strains of this study are in contrast with the downward trend in 2 other selection experiments with this breed (Morris, 1963; Gowe and Strain, 1963; and Gowe's later results cited by Clayton, 1968). Such a response obtained in the other experiments is not surprising, however, since age at sexual maturity is considerably influenced by additive genetic variance and is a major component of number of eggs per hen housed for the part-year record, used as their primary selection criterion. A discrepancy exists between the results of this selection experiment (see also Saadeh et al., 1968) and those of Morris (1963) and Gowe and Strain (1963; see also Clayton, 1968). As previously noted, we found that rate of lay over longer periods was increased by selection for part-year rate, while they reported a plateauing of response in annual egg production when selection was for part-year number of eggs per pullet housed. Differences in selection criteria may account for these divergent results. Their lack of improvement for full-year productivity could be associated with a negative genetic correlation between early sexual matur-
1296
J. V. CRAIG, D. K. BISWAS AND H. K. SAADEH
lection for part-year rate of lay. It was concluded that undesirable genetic associations between components of total productivity are more likely responsible for lack of improvement in egg production strains at this time than is exhaustion of genetic variation. ACKNOWLEDGMENT
REFERENCES Clayton, G. A. 1968. Some implications of selection results in poultry. World's Poultry Sci. J. 24:3757. Dickerson, G. E., 1955. Genetic slippage in response to selection for multiple objectives. Cold Spring
NEWS AND
NOTES
(Continued from page 1275) volume. The present supplement is the third to be issued on an annual basis. The bibliography regards farm buildings as part of farm equipment and therefore gives reference to any matter affecting their design, construction, economics, and use. It includes the findings of scientific research, of such systematic but less precise forms of investigations as surveys, and of field trials and similar developmental phases of new techniques. It includes information from any country where conditions are relevant to Great Britian. It gives abstracts of the original material, classified by farm enterprise, and a guide to the contents. The titles and prices of previous reports in the series are given below. All are published by the Agricultural Research Council and obtainable from Her Majesty's Stationery Office.
(a) Bibliography of Farm Buildings Research {194558) Part 1. Buildings for Pigs. (2/6) Part 2. Buildings for Potato Storage. (2/-) Part 3. Buildings for Poultry. (4/6) Part 4. Buildings for Cattle. (4/6) Part 5. Buildings for Drying and Storage of Grain. (3/-) Part 6. Buildings for the Processing and Storage of Fodder. (4/-) Part 7. Miscellaneous Items. (2/-) (b) First Supplement (1958-61) Part 1. Buildings for Pigs. (4/-) Part 3. Buildings for Poultry. (4/-) Part 4. Buildings for Cattle. (6/-) Part 5. Buildings for Drying and Storage of Grain. (2/6)
(Continued on page 1364)
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We thank S. C. King, J. R. Carson, S. P. Wilson, T. B. Kinney, P. C. Lowe and B. T. Weinland of the NC-47 Regional Laboratory Staff for supplying samples of the control populations and for collection of data at the Regional Poultry Breeding Laboratory. We are also grateful to L.T. Smith and S. Wearden for their assistance at the Kansas Experiment Station during the earlier years of the study.
Harbor Symposia on Quantitative Biol. 20: 213— 224. Falconer, D. S., 1954. Validity of the theory of genetic correlation. J. Heredity, 45:42-44. Fisher, R. A., 1949. The Design of Experiments. 5th edition. Oliver and Boyd, Edinburgh. Gowe, R. S., and J. H. Strain, 1963. Effect of selection for increased egg production based on partyear records in two strains of White Leghorns. Canad. J. Genet. Cyt. 5: 99-100. Kinney, T. B., Jr., and P. C. Lowe, 1968. Genetic and phenotypic variation in the Regional Red Controls over nine years. Poultry Sci. 47: 11051110. Kinney, T. B., Jr., P. C. Lowe, B. B. Bohren and S. P. Wilson, 1968. Genetic and phenotypic variation in randombred White Leghorn controls over several generations. Poultry Sci. 47:113-123. Morris, J. A., 1962. The effect of mild inbreeding in two lines of White Leghorn. Australian J. Agr. Res. 13:362-375. Morris, J. A., 1963. Continuous selection for egg production using short-term records. Australian J. Biol. Sci. 14: 909-925. Nordskog, A. W., A. Festing and M. W. Verghese, 1967. Selection for egg production and correlated responses in the fowl. Genetics, 55:179-191. Saadeh, H. K., J. V. Craig, L. T. Smith and S. Wearden, 1968. Effectiveness of alternative breeding systems for increasing rate of egg production in chickens. Poultry Sci. 47: 1057-1072.