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Selection of Egg-Laying Chickens as Juveniles on the Average Genetic Merit of Their Parents
BREEDING AND GENETICS Selection of Egg-Laying Chickens as Juveniles on the Average Genetic Merit of Their Parents CHARLES SMITH Centre for the Genetic Improvement of Livestock, Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario NIG 2W1 (Received for publication October 22, 1987)
1988 Poultry Science 67:1655-1659 INTRODUCTION
In egg-laying chickens selection is often practiced on part record, rather than on the whole egg-laying record, so as to reduce the generation interval and increase the annual rate of genetic response (Bohren et al., 1970). The generation interval can be reduced further by breeding all the test laying birds early in their laying periods, hatching and rearing all the progeny, and then selecting among the progeny before lay on the average genetic merit of their parents. Such selection on pedigree records here is called juvenile (J) selection after Nicholas and Smith (1983) and Smith (1986). The objectives of this study were to examine the logistics and theoretical response rates with J selection schemes for egg-laying chickens, and to compare them with adult (A) selection schemes where parents are selected after the laying period. The breeding objective was taken as net economic merit, a function of several traits with different economic values, but was modeled on hen-housed production. MATERIALS AND METHODS
Simple versions of the logistics and theory were used to describe and compare the different selection systems, and to make the main points. Thus only additive genetic variation and equilib-
rium rates of genetic change were considered, and the same mating ratios and proportions selected were used in the two schemes. In practice, many variations could be made in logistics, in considering nonadditive genetic variance, and in the selection times and methods. An attempt could be made to optimize the schemes, as is done in practice with current schemes. Time Schedules. Likely time periods involved in testing egg-laying chickens are given in Table 1. After a full record (or part record) is obtained, time is needed for evaluation of candidates genetically (14 days), for setting up of pens (7 days), and, if mated, for fertile eggs to be produced (7 days). As detailed, the length of the period for collecting eggs for hatching depends on the number of progeny required, on the rate of lay, on fertility and hatchability, and on the survival rate of the progeny. To fit into one hatch, a collection period of 24 days should produce about seven female progeny/dam. For collections late in the laying period, however, the rate of lay, fertility, and hatchability will all be lower; a longer period (about 36 days) and two hatches may be needed to get seven female progeny. With one hatch the generation interval will be the same for all progeny. With two hatches the generation interval used is that for the last-hatched individuals, as selection has to await their records.
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ABSTRACT Egg-laying chickens are usually selected for breeding on a selection index of individual and family records (or part records) of laying performance. The generation interval is 1 yr or longer. An alternative would be to mate all potential parents early in the laying period and to select the next generation as juveniles before lay, based on the average genetic merit of their parents. Selection of individuals from the best indexing matings, rather than selection of the best individuals, reduces the estimated annual genetic response by a factor of approximately V T 5 . But the generation interval can be reduced by a larger proportion and so the annual genetic response can be increased. For selection on the same length of record, and with the same selection intensity, there could be an increase in the rate of genetic response in the juvenile scheme by a factor of about at least 1.14 to 1.34 compared with response rate in the adult scheme for the same number tested per generation. However, the juvenile scheme needs much larger numbers hatched and reared, and its higher costs may limit its use in practice. (Key words: layer, juvenile selection, adult selection, selection response)
SMITH
1656 TABLE 1. Time schedules for selection systems Time periods
140 476 14 7 7 24 > 21 42 119
'Days needed to provide seven female offspring (7 = 24 • rate of lay • fertility • hatchability • survival • sex-ratio = 24 • .85 • .93 • .83 • .90 • .5).
A pedigree diagram with the information available in the two schemes is shown in Figure 1. In the A scheme individual sires (S) and dams (D) are selected on a selection index of individual (for dams), full sib, and half-sib records. Suppose there are 20 breeding males and 200 breeding females/generation, with 7 female progeny and 2 male progeny retained per mating for selection and breeding the next generation. At the end of the test period, the best 20 males (.05 = 20/400) and 200 females (.14 = 200/ 1,400) are selected as sires and dams of the next generation with selection accuracies of r s and rD, respectively. In the J scheme there are also 1,400 females on laying test; these are bred to give (7 x 1,400) female progeny and (2 x 1,400) retained male progeny. These become candidates (C) for selection at the end of their parents' laying test. The J candidates are selected on the average breeding merit of their parents' S and D; the accuracy of evaluation is expressed as r s and rD, as above. To maintain the same selection intensities, the best 1,400 (9,800 X .14) candidate females are selected for testing and the best 140 (2,800 X .05) candidate males are selected as mates. The remaining candidate males and females are discarded, or used for other purposes. In the J scheme, to obtain progeny that are both candidates for selection and ready for lay and for their own testing period when their parents are evaluated, progeny should be 119 days of age (140 - 1 4 - 7 ) when their parents' full records (or part records) are available. The hatching eggs will have been collected 164 days earlier (119 + 24 + 21). The simplest case to consider is where the
Sire's half sibs
Sire's full sibs
Sire \
Dam /
Dam's full sibs
Dam's half sibs
Candidate Male or female
FIGURE 1. Pedigree diagram with information for selection of parents (sire and dam) in the adult scheme and of candidates in the juvenile scheme. No records are available for sires or candidates.
selection in the J and A schemes is based on the same length of test record and where the same mating ratios and selection intensities are used. In this case accuracy of selection of S and of D, as breeders in the A scheme and as parents of existing juvenile C, will be the same. If a part record is used, the same co-heritability (rGh!h2) will apply to both schemes, where h1 and h2 are the square roots of the heritabilities for the whole and part records, respectively, and rG is the genetic correlation between whole and part record. Comparisons made with the same record length are thus independent of the estimates of these parameters, and so are quite general. If good estimates of the parameters are available, then there are options for varying the length of the test record in both schemes, so as to maximize the genetic response rate in each. Predicted Genetic Response. In the A selection scheme the predicted annual genetic response (R) by selection of adults, the S and D of the next generation, is: RA = [(i s r s + iDrD)/(Ls + LD)]hcr
[1]
where i is the selection intensity, h is the square root of the heritability, a is the standard deviation, and L is the age of parents when their offspring are born (Van Vleck et al., 1987). The accuracy (r) of selection is the correlation of the breeding value (G) with the estimated breeding value (G), estimated by a selection index of the performance of the individual (for D), the full sibs, and the half sibs. In a conventional selection index (by definition), CovGG/VarG = 1. Then: r2 = (CovGG)2/(VarG.VarG) = CovGG/VarG.
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Average age at first egg Age at end of normal laying period Genetic evaluation Set up pens Get fertile eggs Egg collection (one hatch) Incubation Molt Age of progeny at end of parental record in the juvenile system (140—14—7)
Days
JUVENILE SELECTION FOR EGG PRODUCTION
For J selection C: G c = (Gs + GD)/2 G c = (G s + GD)/2. The correlation (r c ) between G c and G c (for both sexes) = CovG c G c /VVarG c VarG c
= -5 \ A ! + r2D
as VarGc = VarG, CovG s G s , and CovGsGs/ VarG = r|. With random mating, CovGsGD = 0. The predicted annual genetic response to selection of J male (m) and female (f) candidates as parents for the next generation is then: RT
= (im + if) h a V r | + rf) 2(L m + Lf)
With the same selection intensities used in the A and J schemes, i s = i m andi D = if, so: Rj _(is + iD)Vrs + rD A 2 (i s r s + i D r D )
R
(L s +L D ) (Lm + Lf)
For the special case of r s = rD (Smith, 1986): R
'
=
, A
( L S
+ L D )
[3]
RA (Lm + Lf) Thus, selecting C in the J scheme from the best indexing matings rather than selection of the best individuals reduces the genetic response by approximately VT~5. But the generation interval can be reduced by a greater proportion, and so the net genetic response is increased. Thus the response from the J scheme will be greater than that from the A scheme if the generation interval for J is less than V " - 5 = .707 of that for A. In egg-laying chickens, males are selected more intensely than females, so i s is greater than iD. But males are less accurately selected than females (which have their own record, as well as records on full sibs and half sibs), so r s is less than rD. With such a combination of values of i and r (is>iD> r s < r D) f° r m e sexes, it can be shown that Rj will always be greater than RA when the ratio of the generation intervals for J and A is less than v ^ 5 . For example, selecting 1 out of 20 males (.05) and 1 out of
7 females (. 14), the selection intensities are 2.06 and 1.58, respectively. If the accuracy of selection was .45 in males and .5 in females, then Equation 3 becomes: Rj/RA = .713 (L s + LD)/ (Lm + Lf) and similarly for other cases. So .707 can be taken as a lower limit for equivalence of the two schemes. Thus the response in the J scheme will be greater than that in the A scheme by at least .707 times the reciprocal of the respective generation intervals. This result can be used generally for comparison of J and A schemes for the same mating ratios, selection intensities, record lengths, heritability, and accuracies of selection. RESULTS
Selection on Full Laying Record. With selection on a full laying record of (476 - 140) = 336 days, the generation interval in the A selection scheme is 577 days (476 + 42 + 7 + 7 + 24 + 21), to allow for molting (and genetic evaluation), setting up mating pens, getting fertile eggs, and collecting and hatching the eggs. In the J scheme the progeny need to be hatched 119 days before the end of the parental record, so the generation interval is 357 days (476 119 days). The ratio of generation intervals for J and A is .619 (357/577). This is less than .707, and so an increased response rate of at least 1.14 (.707/.619) can be expected with the juvenile scheme. Long Part Record. The above A scheme includes the time spent molting. An alternative would be to collect the hatching eggs before the end of lay, and select on a part record (in both schemes). If it takes 52 days (14 + 7 + 7 + 24) to evaluate, set up mating pens, get fertile eggs and collect hatching eggs, the record must be terminated at 424 (476 - 52) days. The generation interval will be 497 (476 + 21) days. At the end of the parental record the progeny must be 119 days, so the generation interval in the J scheme is 305 days (424 - 119 days). The ratio of generation intervals is then .614 (305/ 497), so the response in the J scheme should be at least 1.15 (.707/.614) of that in the A scheme. At the end of the laying period the rate of lay, fertility, and hatchability may all be lower than earlier. Therefore, a longer collection period will be needed to get the same number of female progeny. If the collection period were extended to 36 days with two hatches, the respective generation intervals would be 293 and 497 days (ratio: .589). The response with the J scheme would then be 1.20 times that for the A scheme.
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= .25 (CovG s G s + CovG D G D )/>/(.25)VarG
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SMITH
1658
DISCUSSION
Useful gains of at least 14 to 34% in estimated rates of genetic response and economic merit have been shown to be possible with the J selection system, in contrast to various A selection systems, using a pedigree index. But a large increase (some sevenfold if there are seven female offspring per dam) in the total number hatched and reared is required in the nucleus breeding unit. This would add substantial costs to the enterprise. Some of these costs might be offset if individuals not selected for breeding were used in nucleus multiplication or in female parent lines for crossing to produce commercial crossbred progeny. These would replace others normally needed for these purposes, and indeed might reduce the genetic lag from nucleus to commercial level. In a J scheme all the facilities would be used more intensively. With a shorter generation interval the total number hatched per year would be higher, by the reciprocal of the proportional reduction in generation interval by the J scheme. The same testing space would be used but it would be used almost continuously, rather than periodically, as parents need to be dehoused to make way for their selected progeny. However, there would be savings in the costs of keeping males for breeding until the end of their sisters' records. Males in the J scheme could be culled after mating, some 140 days earlier than in an A scheme. The approach described here is simple, but
general. The results theoretically hold for all mating ratios, selection rules, and intensities and for any heritability and co-heritability factors, by assuming the same structures and parameters and the same length of record (or part record) in both the J and A schemes. It has been shown that the advantage of J selection is higher with a shorter part record. Also, selection on part record should yield higher annual rates of genetic response in both schemes than selection on whole record. This is because the part-whole genetic correlation is usually higher than the ratio of corresponding part record length to whole record length (Flock, 1977; Gowe and Fairfull, 1985). With reliable estimates of the genetic correlations for the different part record periods, the lengths of part record that would maximize responses in each system could be derived, and the responses compared. However, the trend in commercial breeding is to use extended records, as the part-whole genetic correlation decreases after a period of selection on part record, and there is concern about persistency of lay (R. S. Gowe and R. W. Fairfull, Animal Research Centre, Ottawa, Ontario, Canada, personal communication). The rate of inbreeding per year would probably be increased in the J scheme. More males may be represented in the next generation in the J scheme, as the selection is on matings, not individual parents. However, this is offset by the lower generation interval; the increase in inbreeding rate will be proportional to the square of the ratio of A to J generation length. One way to maintain selection pressure and reduce the increase in inbreeding rate would be to use more males from selected sibships, with a correspondingly lower mating ratio of females per male. This would lead to fewer half sibs per sibship, giving a lower accuracy of selection; the response rate would also fall slightly. Estimation of rates of inbreeding with selection and use of close relatives is difficult (Burrows, 1984). Computer simulation of the selected populations with empirical calculation of response rates and inbreeding levels in simulated selection stocks may be required. Selection index methods have been used to estimate the accuracy of selection using records from the full and half sibships. In practice estimated breeding values would be derived by improved statistical methods using a Best Linear Unbiased Prediction (BLUP) animal model (Henderson, 1984; Kennedy, 1988). This takes account of information on all relatives (ancestors
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Short Part Record. At the other extreme, consider the shortest part record possible for the parents. Such a part record allows parents to have progeny ready for selection but before lay, at the end of the parental record. Fertility is low at the start of lay (140 days), but should be high after 4 wk of lay, at 168 days. After egg collection (24 days) and hatching (21 days), the minimum parental age at the hatch of progeny is 213 days, the generation interval for the J scheme. The age of the parents at the end of their part record is 332 days (213 + 119 days), giving the parents a record length of 192 days (332 - 140). In the A selection scheme the generation interval is 405 days (332 + 14 + 7 + 7 + 24 + 21), and the generation ratio is .526. Thus the response with the J scheme would be 1.34 times that for the A scheme. With the shorter part record, the advantage of the J scheme is greater.
JUVENILE SELECTION FOR EGG PRODUCTION
ACKNOWLEDGMENTS
The suggestion to use the J scheme for genetic improvement of egg laying chickens was made
by R. W. Polkinghorne, Adelaide, S. Australia. Constructive comments by R. W. Fairfull and R. S. Gowe, Ottawa, Ontario and by several industrial poultry geneticists are gratefully acknowledged. The research was supported by the Natural Sciences and Engineering Research Council and Semex of Canada.
REFERENCES Bohren, B. B., T. B. Kinney, S. P. Wilson, and P. C. Lowe, 1970. Genetic gains in annual egg production from selection on part-record parent production in the fowl. Genetics 65:655-667. Bulmer, M. G., 1971. The effect of selection on genetic variability. Am. Nat. 105:201-211. Burrows, P. M., 1984. Inbreeding under selection from related families. Biometrics 40:895-906. Flock, D. K., 1977. Genetic analysis of part-period egg production in a population of White Leghorns under long-termRRS. Z. Tierz. Zuchtungsbiol. 94:89-103. Gowe, R. S., andR. W. Fairfull, 1985. The direct response to long term selection for multiple traits in egg stocks, and changes in the genetic parameters with selection. Pages 125-146 in: Poultry Genetics and Breeding. W. G. Hill, J. M. Manson, and D. Hewitt, ed. Br. Poult. Sci. Ltd., Harlow, UK. Henderson, C. R., 1984. Application of Linear Models in Animal Breeding. Univ. Guelph Press, Guelph, Ontario, Canada. Hudson, G.F.S., and L. R. Schaeffer, 1984. Monte Carlo comparison of sire evaluation models in populations subject to selection and nonrandom mating. J. Dairy Sci. 67:1264-1272. Kennedy, B. W., 1988. Use of mixed model methodology in analysis of designed experiments. Inter. Symp. on Advances in Statistical Methods for Genetic Improvement of Livestock. Gianola and K. Hammond, ed. (in press). Nicholas, F. W., and C. Smith, 1983. Increased rates of genetic change in dairy cattle by embryo transfer and splitting. Anim. Prod. 36:341-353. Smith, C., 1986. Use of embryo transfer in genetic improvement of sheep. Anim. Prod. 42:81-88. Van Vleck, L. D., E. J. Pollak, andE.A.B. Oltenacu, 1987. Genetics for the Animal Sciences. Freeman and Co., New York, NY. Woolliams, J. A., and C. Smith, 1988. The value of indicator traits in the genetic improvement of dairy cattle. Anim. Prod. 46:333-345.
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and collaterals) and allows for the reduced genetic variation in the next generation due to the selection of parents. The systems need to be studied by computer simulation, to simulate and assess the benefits of the BLUP methods (Hudson and Schaeffer, 1984), to take account of gametic disequilibrium effects due to continuous selection (Bulmer, 1971), and to derive optimal breeding systems. Many variations in both the A and J systems could be studied and compared: for length of record, number of full and half sibs, and for selection and mating ratios. For example, more intense selection-selecting breeding males from fewer sibships-could be used. Breeding systems to take account of dominance and epistatic variation, as well as additive genetic variation, might be considered. Though these methods would affect the absolute rates of genetic response, they would probably have little effect on the relative rates of genetic response in the A and J systems. The advantage of J selection schemes would be increased if any other useful information on individuals were available before selection, such as genetic markers or physiological indicators genetically correlated with economic merit (Woolliams and Smith, 1988). This would allow selection among males within full sibships and would increase the accuracy of selection of all candidates, both contributing to extra responses. In summary, the J selection schemes offer higher rates of genetic response than A schemes. However they are likely to be more expensive to run and they incur higher rates of inbreeding. The value of extra genetic response on a national scale usually far exceeds the extra costs of selection in nucleus breeding stocks; consumer benefitxost ratios are large. The J scheme may also be useful to breeders in improving their competitive position in the sale of breeding stock.
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1988 Poultry Science 67:1655-1659 INTRODUCTION
In egg-laying chickens selection is often practiced on part record, rather than on the whole egg-laying record, so as to reduce the generation interval and increase the annual rate of genetic response (Bohren et al., 1970). The generation interval can be reduced further by breeding all the test laying birds early in their laying periods, hatching and rearing all the progeny, and then selecting among the progeny before lay on the average genetic merit of their parents. Such selection on pedigree records here is called juvenile (J) selection after Nicholas and Smith (1983) and Smith (1986). The objectives of this study were to examine the logistics and theoretical response rates with J selection schemes for egg-laying chickens, and to compare them with adult (A) selection schemes where parents are selected after the laying period. The breeding objective was taken as net economic merit, a function of several traits with different economic values, but was modeled on hen-housed production. MATERIALS AND METHODS
Simple versions of the logistics and theory were used to describe and compare the different selection systems, and to make the main points. Thus only additive genetic variation and equilib-
rium rates of genetic change were considered, and the same mating ratios and proportions selected were used in the two schemes. In practice, many variations could be made in logistics, in considering nonadditive genetic variance, and in the selection times and methods. An attempt could be made to optimize the schemes, as is done in practice with current schemes. Time Schedules. Likely time periods involved in testing egg-laying chickens are given in Table 1. After a full record (or part record) is obtained, time is needed for evaluation of candidates genetically (14 days), for setting up of pens (7 days), and, if mated, for fertile eggs to be produced (7 days). As detailed, the length of the period for collecting eggs for hatching depends on the number of progeny required, on the rate of lay, on fertility and hatchability, and on the survival rate of the progeny. To fit into one hatch, a collection period of 24 days should produce about seven female progeny/dam. For collections late in the laying period, however, the rate of lay, fertility, and hatchability will all be lower; a longer period (about 36 days) and two hatches may be needed to get seven female progeny. With one hatch the generation interval will be the same for all progeny. With two hatches the generation interval used is that for the last-hatched individuals, as selection has to await their records.
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ABSTRACT Egg-laying chickens are usually selected for breeding on a selection index of individual and family records (or part records) of laying performance. The generation interval is 1 yr or longer. An alternative would be to mate all potential parents early in the laying period and to select the next generation as juveniles before lay, based on the average genetic merit of their parents. Selection of individuals from the best indexing matings, rather than selection of the best individuals, reduces the estimated annual genetic response by a factor of approximately V T 5 . But the generation interval can be reduced by a larger proportion and so the annual genetic response can be increased. For selection on the same length of record, and with the same selection intensity, there could be an increase in the rate of genetic response in the juvenile scheme by a factor of about at least 1.14 to 1.34 compared with response rate in the adult scheme for the same number tested per generation. However, the juvenile scheme needs much larger numbers hatched and reared, and its higher costs may limit its use in practice. (Key words: layer, juvenile selection, adult selection, selection response)
SMITH
1656 TABLE 1. Time schedules for selection systems Time periods
140 476 14 7 7 24 > 21 42 119
'Days needed to provide seven female offspring (7 = 24 • rate of lay • fertility • hatchability • survival • sex-ratio = 24 • .85 • .93 • .83 • .90 • .5).
A pedigree diagram with the information available in the two schemes is shown in Figure 1. In the A scheme individual sires (S) and dams (D) are selected on a selection index of individual (for dams), full sib, and half-sib records. Suppose there are 20 breeding males and 200 breeding females/generation, with 7 female progeny and 2 male progeny retained per mating for selection and breeding the next generation. At the end of the test period, the best 20 males (.05 = 20/400) and 200 females (.14 = 200/ 1,400) are selected as sires and dams of the next generation with selection accuracies of r s and rD, respectively. In the J scheme there are also 1,400 females on laying test; these are bred to give (7 x 1,400) female progeny and (2 x 1,400) retained male progeny. These become candidates (C) for selection at the end of their parents' laying test. The J candidates are selected on the average breeding merit of their parents' S and D; the accuracy of evaluation is expressed as r s and rD, as above. To maintain the same selection intensities, the best 1,400 (9,800 X .14) candidate females are selected for testing and the best 140 (2,800 X .05) candidate males are selected as mates. The remaining candidate males and females are discarded, or used for other purposes. In the J scheme, to obtain progeny that are both candidates for selection and ready for lay and for their own testing period when their parents are evaluated, progeny should be 119 days of age (140 - 1 4 - 7 ) when their parents' full records (or part records) are available. The hatching eggs will have been collected 164 days earlier (119 + 24 + 21). The simplest case to consider is where the
Sire's half sibs
Sire's full sibs
Sire \
Dam /
Dam's full sibs
Dam's half sibs
Candidate Male or female
FIGURE 1. Pedigree diagram with information for selection of parents (sire and dam) in the adult scheme and of candidates in the juvenile scheme. No records are available for sires or candidates.
selection in the J and A schemes is based on the same length of test record and where the same mating ratios and selection intensities are used. In this case accuracy of selection of S and of D, as breeders in the A scheme and as parents of existing juvenile C, will be the same. If a part record is used, the same co-heritability (rGh!h2) will apply to both schemes, where h1 and h2 are the square roots of the heritabilities for the whole and part records, respectively, and rG is the genetic correlation between whole and part record. Comparisons made with the same record length are thus independent of the estimates of these parameters, and so are quite general. If good estimates of the parameters are available, then there are options for varying the length of the test record in both schemes, so as to maximize the genetic response rate in each. Predicted Genetic Response. In the A selection scheme the predicted annual genetic response (R) by selection of adults, the S and D of the next generation, is: RA = [(i s r s + iDrD)/(Ls + LD)]hcr
[1]
where i is the selection intensity, h is the square root of the heritability, a is the standard deviation, and L is the age of parents when their offspring are born (Van Vleck et al., 1987). The accuracy (r) of selection is the correlation of the breeding value (G) with the estimated breeding value (G), estimated by a selection index of the performance of the individual (for D), the full sibs, and the half sibs. In a conventional selection index (by definition), CovGG/VarG = 1. Then: r2 = (CovGG)2/(VarG.VarG) = CovGG/VarG.
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Average age at first egg Age at end of normal laying period Genetic evaluation Set up pens Get fertile eggs Egg collection (one hatch) Incubation Molt Age of progeny at end of parental record in the juvenile system (140—14—7)
Days
JUVENILE SELECTION FOR EGG PRODUCTION
For J selection C: G c = (Gs + GD)/2 G c = (G s + GD)/2. The correlation (r c ) between G c and G c (for both sexes) = CovG c G c /VVarG c VarG c
= -5 \ A ! + r2D
as VarGc = VarG, CovG s G s , and CovGsGs/ VarG = r|. With random mating, CovGsGD = 0. The predicted annual genetic response to selection of J male (m) and female (f) candidates as parents for the next generation is then: RT
= (im + if) h a V r | + rf) 2(L m + Lf)
With the same selection intensities used in the A and J schemes, i s = i m andi D = if, so: Rj _(is + iD)Vrs + rD A 2 (i s r s + i D r D )
R
(L s +L D ) (Lm + Lf)
For the special case of r s = rD (Smith, 1986): R
'
=
, A
( L S
+ L D )
[3]
RA (Lm + Lf) Thus, selecting C in the J scheme from the best indexing matings rather than selection of the best individuals reduces the genetic response by approximately VT~5. But the generation interval can be reduced by a greater proportion, and so the net genetic response is increased. Thus the response from the J scheme will be greater than that from the A scheme if the generation interval for J is less than V " - 5 = .707 of that for A. In egg-laying chickens, males are selected more intensely than females, so i s is greater than iD. But males are less accurately selected than females (which have their own record, as well as records on full sibs and half sibs), so r s is less than rD. With such a combination of values of i and r (is>iD> r s < r D) f° r m e sexes, it can be shown that Rj will always be greater than RA when the ratio of the generation intervals for J and A is less than v ^ 5 . For example, selecting 1 out of 20 males (.05) and 1 out of
7 females (. 14), the selection intensities are 2.06 and 1.58, respectively. If the accuracy of selection was .45 in males and .5 in females, then Equation 3 becomes: Rj/RA = .713 (L s + LD)/ (Lm + Lf) and similarly for other cases. So .707 can be taken as a lower limit for equivalence of the two schemes. Thus the response in the J scheme will be greater than that in the A scheme by at least .707 times the reciprocal of the respective generation intervals. This result can be used generally for comparison of J and A schemes for the same mating ratios, selection intensities, record lengths, heritability, and accuracies of selection. RESULTS
Selection on Full Laying Record. With selection on a full laying record of (476 - 140) = 336 days, the generation interval in the A selection scheme is 577 days (476 + 42 + 7 + 7 + 24 + 21), to allow for molting (and genetic evaluation), setting up mating pens, getting fertile eggs, and collecting and hatching the eggs. In the J scheme the progeny need to be hatched 119 days before the end of the parental record, so the generation interval is 357 days (476 119 days). The ratio of generation intervals for J and A is .619 (357/577). This is less than .707, and so an increased response rate of at least 1.14 (.707/.619) can be expected with the juvenile scheme. Long Part Record. The above A scheme includes the time spent molting. An alternative would be to collect the hatching eggs before the end of lay, and select on a part record (in both schemes). If it takes 52 days (14 + 7 + 7 + 24) to evaluate, set up mating pens, get fertile eggs and collect hatching eggs, the record must be terminated at 424 (476 - 52) days. The generation interval will be 497 (476 + 21) days. At the end of the parental record the progeny must be 119 days, so the generation interval in the J scheme is 305 days (424 - 119 days). The ratio of generation intervals is then .614 (305/ 497), so the response in the J scheme should be at least 1.15 (.707/.614) of that in the A scheme. At the end of the laying period the rate of lay, fertility, and hatchability may all be lower than earlier. Therefore, a longer collection period will be needed to get the same number of female progeny. If the collection period were extended to 36 days with two hatches, the respective generation intervals would be 293 and 497 days (ratio: .589). The response with the J scheme would then be 1.20 times that for the A scheme.
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= .25 (CovG s G s + CovG D G D )/>/(.25)VarG
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SMITH
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DISCUSSION
Useful gains of at least 14 to 34% in estimated rates of genetic response and economic merit have been shown to be possible with the J selection system, in contrast to various A selection systems, using a pedigree index. But a large increase (some sevenfold if there are seven female offspring per dam) in the total number hatched and reared is required in the nucleus breeding unit. This would add substantial costs to the enterprise. Some of these costs might be offset if individuals not selected for breeding were used in nucleus multiplication or in female parent lines for crossing to produce commercial crossbred progeny. These would replace others normally needed for these purposes, and indeed might reduce the genetic lag from nucleus to commercial level. In a J scheme all the facilities would be used more intensively. With a shorter generation interval the total number hatched per year would be higher, by the reciprocal of the proportional reduction in generation interval by the J scheme. The same testing space would be used but it would be used almost continuously, rather than periodically, as parents need to be dehoused to make way for their selected progeny. However, there would be savings in the costs of keeping males for breeding until the end of their sisters' records. Males in the J scheme could be culled after mating, some 140 days earlier than in an A scheme. The approach described here is simple, but
general. The results theoretically hold for all mating ratios, selection rules, and intensities and for any heritability and co-heritability factors, by assuming the same structures and parameters and the same length of record (or part record) in both the J and A schemes. It has been shown that the advantage of J selection is higher with a shorter part record. Also, selection on part record should yield higher annual rates of genetic response in both schemes than selection on whole record. This is because the part-whole genetic correlation is usually higher than the ratio of corresponding part record length to whole record length (Flock, 1977; Gowe and Fairfull, 1985). With reliable estimates of the genetic correlations for the different part record periods, the lengths of part record that would maximize responses in each system could be derived, and the responses compared. However, the trend in commercial breeding is to use extended records, as the part-whole genetic correlation decreases after a period of selection on part record, and there is concern about persistency of lay (R. S. Gowe and R. W. Fairfull, Animal Research Centre, Ottawa, Ontario, Canada, personal communication). The rate of inbreeding per year would probably be increased in the J scheme. More males may be represented in the next generation in the J scheme, as the selection is on matings, not individual parents. However, this is offset by the lower generation interval; the increase in inbreeding rate will be proportional to the square of the ratio of A to J generation length. One way to maintain selection pressure and reduce the increase in inbreeding rate would be to use more males from selected sibships, with a correspondingly lower mating ratio of females per male. This would lead to fewer half sibs per sibship, giving a lower accuracy of selection; the response rate would also fall slightly. Estimation of rates of inbreeding with selection and use of close relatives is difficult (Burrows, 1984). Computer simulation of the selected populations with empirical calculation of response rates and inbreeding levels in simulated selection stocks may be required. Selection index methods have been used to estimate the accuracy of selection using records from the full and half sibships. In practice estimated breeding values would be derived by improved statistical methods using a Best Linear Unbiased Prediction (BLUP) animal model (Henderson, 1984; Kennedy, 1988). This takes account of information on all relatives (ancestors
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Short Part Record. At the other extreme, consider the shortest part record possible for the parents. Such a part record allows parents to have progeny ready for selection but before lay, at the end of the parental record. Fertility is low at the start of lay (140 days), but should be high after 4 wk of lay, at 168 days. After egg collection (24 days) and hatching (21 days), the minimum parental age at the hatch of progeny is 213 days, the generation interval for the J scheme. The age of the parents at the end of their part record is 332 days (213 + 119 days), giving the parents a record length of 192 days (332 - 140). In the A selection scheme the generation interval is 405 days (332 + 14 + 7 + 7 + 24 + 21), and the generation ratio is .526. Thus the response with the J scheme would be 1.34 times that for the A scheme. With the shorter part record, the advantage of the J scheme is greater.
JUVENILE SELECTION FOR EGG PRODUCTION
ACKNOWLEDGMENTS
The suggestion to use the J scheme for genetic improvement of egg laying chickens was made
by R. W. Polkinghorne, Adelaide, S. Australia. Constructive comments by R. W. Fairfull and R. S. Gowe, Ottawa, Ontario and by several industrial poultry geneticists are gratefully acknowledged. The research was supported by the Natural Sciences and Engineering Research Council and Semex of Canada.
REFERENCES Bohren, B. B., T. B. Kinney, S. P. Wilson, and P. C. Lowe, 1970. Genetic gains in annual egg production from selection on part-record parent production in the fowl. Genetics 65:655-667. Bulmer, M. G., 1971. The effect of selection on genetic variability. Am. Nat. 105:201-211. Burrows, P. M., 1984. Inbreeding under selection from related families. Biometrics 40:895-906. Flock, D. K., 1977. Genetic analysis of part-period egg production in a population of White Leghorns under long-termRRS. Z. Tierz. Zuchtungsbiol. 94:89-103. Gowe, R. S., andR. W. Fairfull, 1985. The direct response to long term selection for multiple traits in egg stocks, and changes in the genetic parameters with selection. Pages 125-146 in: Poultry Genetics and Breeding. W. G. Hill, J. M. Manson, and D. Hewitt, ed. Br. Poult. Sci. Ltd., Harlow, UK. Henderson, C. R., 1984. Application of Linear Models in Animal Breeding. Univ. Guelph Press, Guelph, Ontario, Canada. Hudson, G.F.S., and L. R. Schaeffer, 1984. Monte Carlo comparison of sire evaluation models in populations subject to selection and nonrandom mating. J. Dairy Sci. 67:1264-1272. Kennedy, B. W., 1988. Use of mixed model methodology in analysis of designed experiments. Inter. Symp. on Advances in Statistical Methods for Genetic Improvement of Livestock. Gianola and K. Hammond, ed. (in press). Nicholas, F. W., and C. Smith, 1983. Increased rates of genetic change in dairy cattle by embryo transfer and splitting. Anim. Prod. 36:341-353. Smith, C., 1986. Use of embryo transfer in genetic improvement of sheep. Anim. Prod. 42:81-88. Van Vleck, L. D., E. J. Pollak, andE.A.B. Oltenacu, 1987. Genetics for the Animal Sciences. Freeman and Co., New York, NY. Woolliams, J. A., and C. Smith, 1988. The value of indicator traits in the genetic improvement of dairy cattle. Anim. Prod. 46:333-345.
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and collaterals) and allows for the reduced genetic variation in the next generation due to the selection of parents. The systems need to be studied by computer simulation, to simulate and assess the benefits of the BLUP methods (Hudson and Schaeffer, 1984), to take account of gametic disequilibrium effects due to continuous selection (Bulmer, 1971), and to derive optimal breeding systems. Many variations in both the A and J systems could be studied and compared: for length of record, number of full and half sibs, and for selection and mating ratios. For example, more intense selection-selecting breeding males from fewer sibships-could be used. Breeding systems to take account of dominance and epistatic variation, as well as additive genetic variation, might be considered. Though these methods would affect the absolute rates of genetic response, they would probably have little effect on the relative rates of genetic response in the A and J systems. The advantage of J selection schemes would be increased if any other useful information on individuals were available before selection, such as genetic markers or physiological indicators genetically correlated with economic merit (Woolliams and Smith, 1988). This would allow selection among males within full sibships and would increase the accuracy of selection of all candidates, both contributing to extra responses. In summary, the J selection schemes offer higher rates of genetic response than A schemes. However they are likely to be more expensive to run and they incur higher rates of inbreeding. The value of extra genetic response on a national scale usually far exceeds the extra costs of selection in nucleus breeding stocks; consumer benefitxost ratios are large. The J scheme may also be useful to breeders in improving their competitive position in the sale of breeding stock.
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