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A Comparison of Combination and Family Selection in Chickens1
BREEDING AND GENETICS A Comparison of Combination and Family Selection in Chickens1 V.A. GARWOOD and P. C. LOWE North Central Regional Poultry Breeding Laboratory, US Department of Agriculture, Science and Education Administration, Agricultural Research, West Lafayette, Indiana 47907 (Received for publication January 28, 1980) ABSTRACT Sire family selection was compared with index selection in six replicates of a singlegeneration experiment. The index comprised individual and sire family performance for rate of lay from 20 to 40 weeks of age in a synthetic White Leghorn type layer strain. Response was slightly greater to combination selection than to sire family selection, but the difference (.2%) was not statistically significant. On the basis of estimated relative efficiency value, a difference of 2.14% between selection criteria would be necessary for statistical significance, and the probability of detecting this difference in an experiment of the given magnitude would be quite low. (Key words: index selection, combination, sire family, rate of lay) 1981 Poultry Science 60:285-288 INTRODUCTION
Details of the theory of combination selection based on information including individual and family records were developed and discussed by Lush (1947). Lerner( 1950) referred particularly tq the use of combination selection in poultry breeding. Osborne (1957) gave detailed developments of theory with respect to gains expected from: a) combination selection in which weights were assigned to full-sib and individual records, and b) combination selection in which weights were assigned to full sib and half-sib family averages and to individual records. In general, the relative efficiency of combination to family selection is about 1.0 when the phenotypic correlation (t) between members of a family is small. Consequently, the advantage of combination over family selection is greatest with large family sizes and traits of low heritability. Combination and family selection, along with mass selection, were compared experimentally with chickens by Kinney et al. (1970) for survivor's rate of lay from first egg to 40 weeks of age. Accumulated response over generations was greater in the combination than in the family system. When responses were standardized for selection intensity and pheno-
1 This investigation was conducted as a part of the cooperative research of the NC-89 Regional Poultry Breeding Project entitled "Nature and Utilization of Genetic Variation in Poultry Improvement."
typic standard deviation, that response to combination selection was not significantly different from that to family selection. In other work, reported by Wilson (1974), Tribolium castaneum was the experimental organism. Responses to combination and family selection were not significantly different when either larval weight or pupal weight was the selected trait in both a replicated, multiple generation selection experiment and a replicated, single generation selection experiment. Similarly, Campo and Tagarro (1977) found no significant difference in response between the two systems when pupal weight was the selected trait in two unreplicated, multiple-generation experiments differing by family size (10 vs. 4). The objective of the present experiment was to compare the results of a replicated, single generation of selection based on sire family averages with results based on an index combining sire family and individual performances for survivor's rate of lay from 20 to 40 weeks of age in chickens. MATERIALS AND METHODS The initial generation was derived from a synthetic population, North Central Randombred (NCR), based upon six commercial White Leghorn type strains. Description of the establishment of the population has been given by Garwood et al. (1980). In brief, the line is maintained through random selection and random mating of 60 males and 360 females each generation. 285
GARWOOD AND LOWE
286
To produce the initial generation for the present experiment, the population was increased in 1976 by mating each of 72 males with 6 females, each of which produced 5 daughters. The sire families produced were randomized equally to the two selection criteria and within each criterion to six replicates. All sire families were produced in a single hatch. The pullets were reared together in floor pens until 18 weeks old when they were randomized individually to 72 floor pens with 30 individuals each. Individual egg production was measured by a trapnest period of 3 days per week from the 20th through the 3 9th week of age. Rate of lay for each surviving pullet was derived by dividing her egg production by 60 and was transformed to degrees for purposes of analyses and selection. Selection under the family criterion was based upon sire family averages, but combination selection was practiced in the second criterion for individual index values derived by use of the index (I):
in each criterion, approximately 17% were saved for breeding. In procedures to produce the test generation, the selected females of a criterion-replicate subclass were inseminated with pooled semen from 3 random males of the nonpedigreed NCR population. About 120 random female chicks were hatched per subclass to average approximately 4 daughters per selected dam. All test pullets were hatched in a single hatch, reared together, and housed in individual cages when 18 weeks old. Traits measured on each pullet were: age at first egg, rate of lay from 20 to 40 weeks of age (transformed to degrees for analysis of variance), and egg weight at 40 weeks of age. Egg weight was the average weight of eggs collected for 3 consecutive days immediately before pullets were 40 weeks old. Because of disproportionate subclass numbers produced by differential mortality rates, the difference in response to the two criteria of selection was tested by analysis of variance of unweighted criterion-replicate subclass means. The model was:
I = wtf + w2p Yj; = n + S; + Rj + SRj: + Ejj where, f is the mean transformed rate of lay of the respective sire family for the individual with a transformed rate of lay of p (with p contributing to f ) . The values for w were derived by Lush (1947):
where, Y;.- is the mean in the i t n selection criterion (S) and the j 1 " replicate (R). RESULTS AND DISCUSSION
nd • r-t 1-r w i = 1 + (nd-l)t 1-t • W 2 - 1-t where d is the number (6) of full-sib families in the sire family each with n progeny (5), and r and t, respectively, are the average genetic and average phenotypic correlation between members of the sire family; t was derived as h 2 r; h 2 was the heritability for transformed rate of lay on the basis of individual observations. Analysis of data from the previous generation, consisting of 104 sire families, yielded a value of 28.6% for h 2 . The value for r was .284 and that for t was .084, and the index became: I = 2.43f+ p An effort was made to keep selection intensity constant across criteria. The superior sire family (30 daughters) of the 6 within each replicate was selected. Similarly, within each replicate of die combination criterion, the 30 individuals with the best index values were selected. Thus,
The expected standardized selection differentials and the associated standard deviations in the initial generation are given in Table 1 for each criterion-replicate subclass. Slightly greater selection intensity (i) occurred for combination selection than for sire family selection (1.32 vs. 1.13), but the difference was not significant (P>.05). Also presented in Table 1 are the subclass mean performances of individual progeny in the test generation. Although progeny of combination selected females laid at a slightly greater rate than those of sire family selection (57.1 vs. 56.9%), the difference was not statistically significant (P>.05). Likewise, no significant differences were found between criteria for age at first egg and egg weight. Wilson (1974) and Campo and Tagarro (1977) also reported that combination selection exceeded sire family selection but not significantly. The lack of significance was interpreted in the two reports as being contrary to the theoretical expectations of Lush (1947). Unequal selection intensities were recognized
COMBINATION VS. FAMILY SELECTION
287
TABLE 1. Expected selection intensities (i) and a ociated standard deviations (a) for sire family (F) and combination (C) selection together wii mean performances of the test generation. Test generation performance Rate i F
a
at first egg (d)
of lay (%)
Egg VSt.
(g)
C
F
C
F
C
F
C
F
C
1.23 1.19 1.50 1.29
1.29 1.26 1.18 1.43 1.29 1.41
3.04 6.20 4.19 4.04 4.78 2.87
24.81 20.22 17.20 21.34 15.59 15.28
54.6 46.0 60.5 64.4 57.6 58.4
61.3 57.8 60.0 57.5 57.5 48.5
184 209 175 175 175 180
176 181 171 183 178 193
56 59 59 62 58 60
60 59 63 58 59 60
1.13
1.32
4.18
19.07
56.9
57.1
183
180
59
60
.62 .96
by Campo and Tagarro (1977) as a source of discrepancy between realized results and theoretical expectations, but the experiments were also unreplicated, and the effects of genetic drift were not considered. The combination index used by Wilson (1974) is not reported in detail. Only the weight applied to the deviation of family mean from the population mean is presented; none is given for the individual deviation, the implication being that the weight is unity. If the implication is true, the weight presented for family deviation is not optimum for maximum gain. Use of the weight as presented would effect a shift of emphasis toward individual selection and a reduction in the expected genetic gain, particularly for lowly or moderately heritable traits. Several factors in the present work are possible sources of uncontrolled error. An obvious source is sampling error, some of which arises from the use of different random tester males for each subclass. No measure was possible for determining the adequacy of the replication used for eliminating this particular source. A second source concerns the realized selection differentials for the two criteria. If the criteria had, for any reason, different rates of reproduction and mortality, these effects would have been reflected directly in the responses through the realized selection differentials. No measure of the differentials was possible as the test generation was not pedigreed. A larger source of error concerns the methods of management during selection and testing and the index used. During the selection phase,
facilities were limited to floor pens with a 3-day-per-week trapnest record, and in the derivation of the index only an estimate of heritability of rate of lay for a 7-day-per-week trapnest record was available. The net effect would be to decrease the accuracy of estimation of part record rate of lay and to diminish the observed differences between the two criteria of selection. The condition has been pointed out previously by Nordskog and Crump (1948) wherein they reported that with trapnesting on a half-time basis, the resulting loss in accuracy could be recovered by increasing the population size by 7%. Nordskog (1948) reported that with individual selection, a 3% loss in genetic progress could be expected with trapnesting on a halftime basis. Wheat and Lush (1961) also reported that genetic losses of 2, 4, and 15% could be expected for trapnest records of 4, 3, and 1 day per week, respectively, when compared with a trapnest record of 7 days per week. With regard to the index used, Harris (1964), in agreement with the conclusion of Williams (1962), has reported that when insufficient data are available for index construction, a base index, sajXi, rather than an estimated index, zbiXj (where ai coefficients reflect the relative economic values of the component traits X; and bj are the estimated coefficients) is preferable. But here the relative economic values (aj) were not known, hence, they were set to 1.0 and the b; based upon heritability for a seven day trapnest record were used. Because a control was not necessary for the primary objective it was not included in the experiment. Consequently, neither an accurate
GARWOOD AND LOWE
288
estimate of the ratio of gains by the two criteria of selection nor a comparison of the ratio with the theoretical counterpart is possible. The expected relative effectiveness (E) of combination to sire family selection, in terms of individual performance and equal selection intensities, can be estimated as: h 2 [n(3d-l)-21 : 2 2 •-[•,{4(nd-l)-h [n(d+l)-2]} 4h (nd-l)
+
h2[n(d+l)+2]: {4+h 2 [n(d+l)-2]}4h 2
—Y2 !
nd J
[4nd{4+h [n(d+l)-2]}] n(d+l)+2
^
where d, h 2 , and n are as defined for equation [1]. With the family structure (6 dams/sire, 5 offspring/dam) and heritability (^30%) of the present experiment, the expected relative efficiency of combination to sire family selection is 1.3. By the phenotypic standard deviation estimated from an analysis of the untransformed criterion-replicate subclass means of rate of lay in Table 1, the genetic gain by sire family selection ( A F ) , with a standardized selection differential of 1.0, is estimated as 1.57% for the selected trait rate of lay. Thus, the expected genetic gain by combination selection becomes 1.3AF=2.04%. By the method outlined by Cochran and Cox (1950), it can be shown that a difference between criteria of 2.14% would be needed for significance. The probability of detecting this difference by an experiment of the present size is less than 50%. Consequently, a conclusion mat the results obtained are
contradictory to theoretical expectations does not appear to be plausible. REFERENCES Campo, J. L., and P. Tagarro, 1977. Comparison of three selection methods for pupal weight in Tribolium castaneum. Ann. Genet. Sel. Anim. 9:250-268. Cochran, W. G., and G. M. Cox, 1950. Experimental designs. J. Wiley and Sons, Inc., New York, NY. Garwood, V. A., P. C. Lowe, and B. B. Bohren, 1980. An experimental test of the efficiency of family selection in chickens. Theor. Appl. Genet. 56:5-9. Harris, D. L., 1964. Expected and predicted progress from index selection involving estimates of population parameters. Biometrics 20:46—72. Kinney, T. B., B. B. Bohren, J. V. Craig, and P. C. Lowe, 1970. Responses to individual, family and index selection for short term rate of egg production in chickens. Poultry Sci. 49:1052-1064. Lerner, I. M., 1950. Population genetics and animal improvement. University Press, Cambridge, London, England. Lush, J. L., 1947. Family merit and individual merit as bases for selection. Part I. Amer. Nat. 81:241-261. Nordskog, A. W., 1948. Periodical trapnesting and family selection for egg production. Poultry Sci. 27:713-718. Nordskog, A. W., and L. Crump, 1948. Systematic and random sampling for estimating egg production in poultry. Biometrics 4:223-235. Osborne, R., 1957. The use of sire and dam family averages in increasing the efficiency of selective breeding under a hierarchical mating system. Heredity 11:93-116. Wheat, J. D., and J. L. Lush, 1961. Accuracy of partial trapnest records. 1. Repeatability of daily egg records. Poultry Sci. 40:399-402. Williams, J. S., 1962. The evaluation of a selection index. Biometrics 18:375-393. Wilson, S. P., 1974. An experimental comparison of individual, family and combination selection. Genetics 76:823-836.
Details of the theory of combination selection based on information including individual and family records were developed and discussed by Lush (1947). Lerner( 1950) referred particularly tq the use of combination selection in poultry breeding. Osborne (1957) gave detailed developments of theory with respect to gains expected from: a) combination selection in which weights were assigned to full-sib and individual records, and b) combination selection in which weights were assigned to full sib and half-sib family averages and to individual records. In general, the relative efficiency of combination to family selection is about 1.0 when the phenotypic correlation (t) between members of a family is small. Consequently, the advantage of combination over family selection is greatest with large family sizes and traits of low heritability. Combination and family selection, along with mass selection, were compared experimentally with chickens by Kinney et al. (1970) for survivor's rate of lay from first egg to 40 weeks of age. Accumulated response over generations was greater in the combination than in the family system. When responses were standardized for selection intensity and pheno-
1 This investigation was conducted as a part of the cooperative research of the NC-89 Regional Poultry Breeding Project entitled "Nature and Utilization of Genetic Variation in Poultry Improvement."
typic standard deviation, that response to combination selection was not significantly different from that to family selection. In other work, reported by Wilson (1974), Tribolium castaneum was the experimental organism. Responses to combination and family selection were not significantly different when either larval weight or pupal weight was the selected trait in both a replicated, multiple generation selection experiment and a replicated, single generation selection experiment. Similarly, Campo and Tagarro (1977) found no significant difference in response between the two systems when pupal weight was the selected trait in two unreplicated, multiple-generation experiments differing by family size (10 vs. 4). The objective of the present experiment was to compare the results of a replicated, single generation of selection based on sire family averages with results based on an index combining sire family and individual performances for survivor's rate of lay from 20 to 40 weeks of age in chickens. MATERIALS AND METHODS The initial generation was derived from a synthetic population, North Central Randombred (NCR), based upon six commercial White Leghorn type strains. Description of the establishment of the population has been given by Garwood et al. (1980). In brief, the line is maintained through random selection and random mating of 60 males and 360 females each generation. 285
GARWOOD AND LOWE
286
To produce the initial generation for the present experiment, the population was increased in 1976 by mating each of 72 males with 6 females, each of which produced 5 daughters. The sire families produced were randomized equally to the two selection criteria and within each criterion to six replicates. All sire families were produced in a single hatch. The pullets were reared together in floor pens until 18 weeks old when they were randomized individually to 72 floor pens with 30 individuals each. Individual egg production was measured by a trapnest period of 3 days per week from the 20th through the 3 9th week of age. Rate of lay for each surviving pullet was derived by dividing her egg production by 60 and was transformed to degrees for purposes of analyses and selection. Selection under the family criterion was based upon sire family averages, but combination selection was practiced in the second criterion for individual index values derived by use of the index (I):
in each criterion, approximately 17% were saved for breeding. In procedures to produce the test generation, the selected females of a criterion-replicate subclass were inseminated with pooled semen from 3 random males of the nonpedigreed NCR population. About 120 random female chicks were hatched per subclass to average approximately 4 daughters per selected dam. All test pullets were hatched in a single hatch, reared together, and housed in individual cages when 18 weeks old. Traits measured on each pullet were: age at first egg, rate of lay from 20 to 40 weeks of age (transformed to degrees for analysis of variance), and egg weight at 40 weeks of age. Egg weight was the average weight of eggs collected for 3 consecutive days immediately before pullets were 40 weeks old. Because of disproportionate subclass numbers produced by differential mortality rates, the difference in response to the two criteria of selection was tested by analysis of variance of unweighted criterion-replicate subclass means. The model was:
I = wtf + w2p Yj; = n + S; + Rj + SRj: + Ejj where, f is the mean transformed rate of lay of the respective sire family for the individual with a transformed rate of lay of p (with p contributing to f ) . The values for w were derived by Lush (1947):
where, Y;.- is the mean in the i t n selection criterion (S) and the j 1 " replicate (R). RESULTS AND DISCUSSION
nd • r-t 1-r w i = 1 + (nd-l)t 1-t • W 2 - 1-t where d is the number (6) of full-sib families in the sire family each with n progeny (5), and r and t, respectively, are the average genetic and average phenotypic correlation between members of the sire family; t was derived as h 2 r; h 2 was the heritability for transformed rate of lay on the basis of individual observations. Analysis of data from the previous generation, consisting of 104 sire families, yielded a value of 28.6% for h 2 . The value for r was .284 and that for t was .084, and the index became: I = 2.43f+ p An effort was made to keep selection intensity constant across criteria. The superior sire family (30 daughters) of the 6 within each replicate was selected. Similarly, within each replicate of die combination criterion, the 30 individuals with the best index values were selected. Thus,
The expected standardized selection differentials and the associated standard deviations in the initial generation are given in Table 1 for each criterion-replicate subclass. Slightly greater selection intensity (i) occurred for combination selection than for sire family selection (1.32 vs. 1.13), but the difference was not significant (P>.05). Also presented in Table 1 are the subclass mean performances of individual progeny in the test generation. Although progeny of combination selected females laid at a slightly greater rate than those of sire family selection (57.1 vs. 56.9%), the difference was not statistically significant (P>.05). Likewise, no significant differences were found between criteria for age at first egg and egg weight. Wilson (1974) and Campo and Tagarro (1977) also reported that combination selection exceeded sire family selection but not significantly. The lack of significance was interpreted in the two reports as being contrary to the theoretical expectations of Lush (1947). Unequal selection intensities were recognized
COMBINATION VS. FAMILY SELECTION
287
TABLE 1. Expected selection intensities (i) and a ociated standard deviations (a) for sire family (F) and combination (C) selection together wii mean performances of the test generation. Test generation performance Rate i F
a
at first egg (d)
of lay (%)
Egg VSt.
(g)
C
F
C
F
C
F
C
F
C
1.23 1.19 1.50 1.29
1.29 1.26 1.18 1.43 1.29 1.41
3.04 6.20 4.19 4.04 4.78 2.87
24.81 20.22 17.20 21.34 15.59 15.28
54.6 46.0 60.5 64.4 57.6 58.4
61.3 57.8 60.0 57.5 57.5 48.5
184 209 175 175 175 180
176 181 171 183 178 193
56 59 59 62 58 60
60 59 63 58 59 60
1.13
1.32
4.18
19.07
56.9
57.1
183
180
59
60
.62 .96
by Campo and Tagarro (1977) as a source of discrepancy between realized results and theoretical expectations, but the experiments were also unreplicated, and the effects of genetic drift were not considered. The combination index used by Wilson (1974) is not reported in detail. Only the weight applied to the deviation of family mean from the population mean is presented; none is given for the individual deviation, the implication being that the weight is unity. If the implication is true, the weight presented for family deviation is not optimum for maximum gain. Use of the weight as presented would effect a shift of emphasis toward individual selection and a reduction in the expected genetic gain, particularly for lowly or moderately heritable traits. Several factors in the present work are possible sources of uncontrolled error. An obvious source is sampling error, some of which arises from the use of different random tester males for each subclass. No measure was possible for determining the adequacy of the replication used for eliminating this particular source. A second source concerns the realized selection differentials for the two criteria. If the criteria had, for any reason, different rates of reproduction and mortality, these effects would have been reflected directly in the responses through the realized selection differentials. No measure of the differentials was possible as the test generation was not pedigreed. A larger source of error concerns the methods of management during selection and testing and the index used. During the selection phase,
facilities were limited to floor pens with a 3-day-per-week trapnest record, and in the derivation of the index only an estimate of heritability of rate of lay for a 7-day-per-week trapnest record was available. The net effect would be to decrease the accuracy of estimation of part record rate of lay and to diminish the observed differences between the two criteria of selection. The condition has been pointed out previously by Nordskog and Crump (1948) wherein they reported that with trapnesting on a half-time basis, the resulting loss in accuracy could be recovered by increasing the population size by 7%. Nordskog (1948) reported that with individual selection, a 3% loss in genetic progress could be expected with trapnesting on a halftime basis. Wheat and Lush (1961) also reported that genetic losses of 2, 4, and 15% could be expected for trapnest records of 4, 3, and 1 day per week, respectively, when compared with a trapnest record of 7 days per week. With regard to the index used, Harris (1964), in agreement with the conclusion of Williams (1962), has reported that when insufficient data are available for index construction, a base index, sajXi, rather than an estimated index, zbiXj (where ai coefficients reflect the relative economic values of the component traits X; and bj are the estimated coefficients) is preferable. But here the relative economic values (aj) were not known, hence, they were set to 1.0 and the b; based upon heritability for a seven day trapnest record were used. Because a control was not necessary for the primary objective it was not included in the experiment. Consequently, neither an accurate
GARWOOD AND LOWE
288
estimate of the ratio of gains by the two criteria of selection nor a comparison of the ratio with the theoretical counterpart is possible. The expected relative effectiveness (E) of combination to sire family selection, in terms of individual performance and equal selection intensities, can be estimated as: h 2 [n(3d-l)-21 : 2 2 •-[•,{4(nd-l)-h [n(d+l)-2]} 4h (nd-l)
+
h2[n(d+l)+2]: {4+h 2 [n(d+l)-2]}4h 2
—Y2 !
nd J
[4nd{4+h [n(d+l)-2]}] n(d+l)+2
^
where d, h 2 , and n are as defined for equation [1]. With the family structure (6 dams/sire, 5 offspring/dam) and heritability (^30%) of the present experiment, the expected relative efficiency of combination to sire family selection is 1.3. By the phenotypic standard deviation estimated from an analysis of the untransformed criterion-replicate subclass means of rate of lay in Table 1, the genetic gain by sire family selection ( A F ) , with a standardized selection differential of 1.0, is estimated as 1.57% for the selected trait rate of lay. Thus, the expected genetic gain by combination selection becomes 1.3AF=2.04%. By the method outlined by Cochran and Cox (1950), it can be shown that a difference between criteria of 2.14% would be needed for significance. The probability of detecting this difference by an experiment of the present size is less than 50%. Consequently, a conclusion mat the results obtained are
contradictory to theoretical expectations does not appear to be plausible. REFERENCES Campo, J. L., and P. Tagarro, 1977. Comparison of three selection methods for pupal weight in Tribolium castaneum. Ann. Genet. Sel. Anim. 9:250-268. Cochran, W. G., and G. M. Cox, 1950. Experimental designs. J. Wiley and Sons, Inc., New York, NY. Garwood, V. A., P. C. Lowe, and B. B. Bohren, 1980. An experimental test of the efficiency of family selection in chickens. Theor. Appl. Genet. 56:5-9. Harris, D. L., 1964. Expected and predicted progress from index selection involving estimates of population parameters. Biometrics 20:46—72. Kinney, T. B., B. B. Bohren, J. V. Craig, and P. C. Lowe, 1970. Responses to individual, family and index selection for short term rate of egg production in chickens. Poultry Sci. 49:1052-1064. Lerner, I. M., 1950. Population genetics and animal improvement. University Press, Cambridge, London, England. Lush, J. L., 1947. Family merit and individual merit as bases for selection. Part I. Amer. Nat. 81:241-261. Nordskog, A. W., 1948. Periodical trapnesting and family selection for egg production. Poultry Sci. 27:713-718. Nordskog, A. W., and L. Crump, 1948. Systematic and random sampling for estimating egg production in poultry. Biometrics 4:223-235. Osborne, R., 1957. The use of sire and dam family averages in increasing the efficiency of selective breeding under a hierarchical mating system. Heredity 11:93-116. Wheat, J. D., and J. L. Lush, 1961. Accuracy of partial trapnest records. 1. Repeatability of daily egg records. Poultry Sci. 40:399-402. Williams, J. S., 1962. The evaluation of a selection index. Biometrics 18:375-393. Wilson, S. P., 1974. An experimental comparison of individual, family and combination selection. Genetics 76:823-836.