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Performance of Layer-Type Chickens as Related to Body Conformation and Composition. 1. A Static Analysis of Shank Length and Body Weight at 20 Weeks of Age1
BREEDING AND GENETICS Performance of Layer-Type Chickens as Related to Body Conformation and Composition. 1. A Static Analysis of Shank Length and Body Weight at 20 Weeks of Age1 J. F. TIERCE Indian River International, Nacogdoches, Texas 75961 A. W. NORDSKOG Department of Animal Science, Iowa State University, Ames, Iowa 50010
ABSTRACT The growth of an animal part (y) relative to the whole (x), is expressed in terms of an exponential equation, y=axfi, where a and /3 are growth constants. For the present study x is body weight and y is shank length determined for 10 lines of chickens at 20 weeks of age. The derived values of a and (3, assumed to be limiting relative growth constants for bone, were estimated by least squares of the log-transformed (linear) exponential equation. From the means of a and (3, calculated for each of seven Leghorn and three Fayoumi lines, individual values of a; and /3; were calculated for more than 24,000 pedigreed birds included in 9 years of records. The individual values (i) were calculated as: a; = antilog (log y; - 0 log x;) and: ft = log a - log y; logx; If the initial growth constant, a, is the primary hereditary growth parameter governing conformation, then the parameter, 0, should not be as highly heritable as a. To test this hypothesis, the heritability (h 2 ) of both a and /3 were calculated for each of the 10 lines. For a sire component h 2 values ranged from .12 ± .05 to .45 + .10 and for (3 from .05 to .41. The pooled h 2 estimate of a over lines (.31 ± .02) was not significantly different from that for (3 (.31 ± .02). Compared with the original data on body weight and shank length, the transformation significantly reduced the heritability estimated. We conclude that the parameters, a and /3, are useful only in describing phenotypic and not genetic differences in relative growth. (Key words: body size, body weight, chickens) 1985 Poultry Science 6 4 : 6 0 5 - 6 0 9 INTRODUCTION During t h e past t h r e e decades a least-cost profit-oriented approach t o commercial egg p r o d u c t i o n has created a d e m a n d for a genetically m o r e efficient egg-laying hen. T o m e e t this d e m a n d , breeders have strived to develop smaller or i n t e r m e d i a t e weight hens capable of laying large eggs at a high rate. This endeaver has featured use of t h e dwarf gene to reduce b o d y size, or otherwise, b y simple selection
'Journal Paper No. J. 11321 of the Iowa Agriculture and Home Economics Experiment Station, Project 2576, a contributing project to the North Central Regional Project, Biotechnology in Poultry, NC168. 605
schemes, t o develop " m i n i " chickens. Usually t h e criterion used t o evaluate b o d y size has been t h a t of b o d y weight at some specific age. B o d y weight is a function o f skeletal size ( b o n e ) , fleshing (muscle), and c o n d i t i o n (fat). Dickerson and Hughes ( 1 9 6 4 ) and N o r d s k o g and Briggs ( 1 9 6 8 ) showed t h a t fleshing o r c o n d i t i o n is an economically i m p o r t a n t comp o n e n t in t e r m s of laying-house p e r f o r m a n c e . If variation in fleshing condition is due m o s t l y t o variable h u s b a n d r y practices, t h e n selecting for small or intermediate hens o n t h e basis of b o d y weight alone might n o t always exclude birds with p o o r viability and egg p r o d u c t i o n . This r e p o r t will deal w i t h t h e relationship of shank length ( b o n e ) t o b o d y weight ( t o t a l size), as d e t e r m i n e d o n m o r e t h a n 2 4 , 0 0 0 Leghorn a n d F a y o u m i pullets a t 2 0 w e e k s of
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
(Received for publication January 16, 1984)
TIERCE AND NORDSKOG
606
age. This population is defined as "static", because the data are restricted to a single age group. The objectives of the present study were to calculate heritability (h 2 ) estimates of the "allometric" growth constants a and j3, and to consider their uniqueness when contrasted with h estimates of body weight (x) and shank length (y) all derived from the same population of 20-week old pullets.
The data for the analysis came from a selection experiment involving seven White Leghorn lines and three Fayoumi lines (Nordskog et al., 1974). The Leghorn base population was developed from 12 diallel single crosses between four commercial lines. The Fayoumi lines were derived from a flock maintained at Iowa State University over more than 20 years. The number of records analyzed, selection criteria, and number of male and female breeders per line are summarized in Table 1. Leghorn line A and Fayoumi line J were selected for high rate of egg production to 32 weeks of age. Leghorn lines B and C were selected, respectively, for high and low body weight; and Leghorn lines D and E were selected, respectively, for high and low egg weight. Fayoumi lines K and L were selected, respectively, for high body weight and high egg weight. Leghorn F was selected antagonistically for high body weight
log (y- jk ) = logaj + 0j log (x i j k and solving for the unknowns aj and (3; for line i, where yj; k ,X" k = shank length (y) and body weight (x) of the kth bird in the jth year of the ith i m e . Values of a-^ and | 3 - k were calculated for each individual by substituting y - k and x » k and either &j or & into the preceding linear equation, where aijk
TABLE 1. Lines, selection criteria, and number of records
Line Leghorn A B C D E F
Selection criteria1
Number of individual records
H-EP H-BW L-BW H-EW L-EW H-BW L-EW L-BW, H-EW
6007 1763 2118 2264 2245
H-EP H-BW H-EW
3555 1244 2165
2164 2228
Fayoumi
J K L 1
L=Low, H=high, BW=body weight, EW=egg :gg weight, weight,EP= EP=i =egg production.
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
MATERIALS AND METHODS
and low egg weight and Leghorn G was selected antagonistically for low body weight and high egg weight (Nordskog et al., 1974). Two or three hatches, 2 weeks apart, were produced each year. All chicks were pedigree wing-banded and brooded, intermingled to 8 weeks of age, and then placed on a summer pasture. At 20 weeks, body weight and shank length were recorded on each pullet. Body weight was measured to the nearest .045 kg (.1 lb) and shank length, measured with parallel jaw calipers, to the nearest millimeter. Because all measurements were taken on the same pullets at the same age, essentially at maturity, the data were regarded as a sample from a static population. For each of the 10 lines, least squares estimates of the initial growth constant, a, and the "coefficient of static allometry", 0, were first calculated using a simple least squares analysis of the log-transformed linear equation:
PERFORMANCE OF LAYERS AND BODY COMPOSITION
initial growth constant for the k t n bird in the j t " year of the ith line, henceforth referred to as a, and ft-^ = coefficient of static
Estimates for ft ranged from .1340 ± .0074 to .2006 ± .0070 in the Leghorn lines and from .1622 ± .0099 to .2261 ± .0125 in the Fayoumi lines. The largest was for the Fayoumi line K, selected for high body weight and the smallest for Leghorn line C, selected for low body weight. Lines with higher estimates of ft were selected for either high body weight or egg weight whereas lines with lower estimates were selected for low egg weight or body weight. Using the line estimates of a and B and the individual bird measurements of 20-week shank length and body weight, individual estimates of
the initial growth constant (oq) and the allometry coefficients (ft) were calculated for over 24,000 pedigreed individuals. The heritability estimates of a and B are presented in Table 3. For the sire component estimates (h 2 s) the pooled averages are .31 ± .02 for a and .27 + .02 for B. For the dam component (h 2 d) the pooled estimates are slightly larger but not significantly so. Also, the estimates for a are slightly, but not significantly, larger than for B. The magnitude of the h 2 s estimates for a, presented in rank order, seem not to be related to the prior selection applied to the different individual lines (Table 1), but the rank order correlation (R) between the estimates of a and 8 was equal to 1 based on the sire components (P<.01). For the dam components, the R=.25 was not significant. Because the individual bird estimates of a and B were, in fact, only transformations of metric observations on body weight and shank length, it seemed obligatory that these be compared with the h 2 estimates of the original metrical measurements of body weight and shank length; these are presented in rank order by lines in Table 4. The body weight sire component estimates ranged from .21 ± .07 (Line C selected for low body weight) to .59 ± .12 (Line D selected for large egg weight). Pooled across lines, the estimates were .43 ± .03 and .63 ± .03 for the sire and dam components, respectively. For shank length they ranged from .22 ± .08 (Leghorn B selected for high body weight) to .56 ± .11 (Fayoumi L selected for high egg weight). The pooled values of .41 ± .03 and .53 ± .02 based on the sire and dam components, respectively, are significantly larger than those
TABLE 2. Estimates of static allometry constants by lines based on the relationship shank length — a(body weight)? Line A B C D E F G
J K L
ft .8017 .8455 .7968 .8110 .7892 .8142 .7965 .7776 .7925 .7969
±.0022 ±.0053 +.0053 ± .0038 ± .0034 ± .0044 ± .0033 ± .0029 ± .0070 ± .0044
.1829 ± .0047 .1507 ±.0081 .1340 ± .0074 .2006 ± .0070 .1754 ±.0082 .1731 ±.0075 .1625 ± .0073 .2043 ± .0070 .2261 ± .0125 .1622 ± .0099
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allometry for the k t n bird in the j t n year of the i r h line, henceforth referred to as B. For m n each of P lines, there were : 2 _ i j.2i n jk = N individuals, giving a total of PN derived values of a and p for the analysis. Year-generation estimates of population parameters were pooled to obtain single averages of heritabilities and genetic correlations by lines. These were tested for statistical significance using approximate standard errors as described by Dickerson (1969). Estimates of the allometry constants are given in Table 2. Values of aj ranged from .7892 ± .0033 to .8455 ± .0053 for the seven Leghorn lines and from .7760 + .0029 to .7969 ± .0044 for the three Fayoumi lines. Leghorn lines, B, D, and F, selected for high body weight or for high egg weight, had the largest values for aj. The smallest were those of Fayoumi line J, selected for high egg production and Leghorn line E, selected for low egg weight.
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TABLE 3. Heritability (h2) estimates of static allometry constants1 based on (sire and dam components) of variance Sire component (h 2 s)
Dam component (h 2 d)
(3
a
F A B C E K G
.45 .41 .38 .37 .29 .26 .26 .26 .18 .12
± .10 ± .09 ± .08 ± .09 ± .05 ± .01 ± .01 ± .07 ± .09 ± .05
.41 ± .09 .40 ± .09 .34 ± .07 .32 ± .08 .27 ± .05 .25 ± .08 .25 ± .07 .21 ± .06 .15 + .08 .05 ± .04
.18 ± .31 ± .39 ± .36 ± .46 ± .31 ± .37 + .20 ± .62 ± .27 ±
Pooled average
.31 + .02
.27 ± .02
.36 ± .02
Line
a
L D
J
1
1.
.01 .08 .06 .08 .05 .09 .08 .07 .13 .08
.12 .35 .26 .37 .48 .20 .24 .04 .55 .28
± .07 ± .08 ± .06 ± .08 ± .01 ± .10 ± .08 ± .07 ± .07 ± .08
.26 ± .02
.25
Shank length = a(body weight)&.
of the transformation-derived allometry constants (Table 3). The rank order correlation between body weight and shank length was .48 for h 2 s but negative for h 2 d (R=-.32). However, to reach statistical significance an R=.64 is required for a sample of only 10 lines. Finally, the rank order correlation between the heritability estimates of BW with the h 2 s estimates of a and 0 were positive, R(BW,a)=.20 and R(BW,/3)=.35, but not statistically significant.
Table 5 contrasts the phenotypic and genetic correlations between body weight and shank length with those between the transformed values a and )3. For the latter, the phenotypic and genetic correlations were high with pooled estimates of .95 and .96, respectively. These results demonstrate that genetic parameters estimated as simple log-transformation functions of body weight and shank length are essentially equivalent to those using the original
TABLE 4. Heritability estimates (h2) of 20-week body weight (BW) and shank length (SL) based on sire and dam components of variance Dam component (h 2 d) 2
Sire component (h 2 s) 1 Linie
BW
L D A F E
.31 .59 .53 .44 .31 .33 .55 .21 .27 .31
J
G C K B Rank correlation
1
Paternal half-sib analysis.
2
Full-sib analysis.
SL ± .10 ± .12 ± .08 ± .10 ± .09 ± .07 ± .12 ± .07 + .10 ± .09
.56 .54 .48 .39 .38 .37 .35 .29 .28 .33
(BW, SL) (BW,a) (BW,(3)
.48 .20 .35
± .11 ±.11 ± .07 ± .10 + .09 ± .08 ± .09 ± .09 + .11 ± .08
BW
SL
.62 ± .09 .63 ± .08 .81 ± .06 .5 5 + .09 .75 ± .09 .33 ± .06 .49 ± .08 .54 ± .09 .37 + .13 .50 ± .10
.28 .49 .58 .64 .43 .59 .51 .64 .62 .34
-.32
+ .07 ± .08 ± .05 ± .09 ± .08 ± .07 ± .08 ± .09 ± .13 ± .10
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Rank correlations (a,|3)
(3
609
PERFORMANCE OF LAYERS AND BODY COMPOSITION
TABLE 5. Phenotypic and genetic correlations between 20-week body measurements (body weight and shank length) and between derived static allometry constants (a and (3) Phenotypic Line
J
K L Pooled 1
Body measurements
Allometry constants
Body measurements
Allometry constants
.45 .37 .35 .52 .45 .43 .43 .45 .46 .37 .43
.98 .98 .93 .98 .88 .98 .98 .96 .99 .99 .95
.77 .19 .49 .69 .74 .47 .93 .44 .70 .73 .64
.99 .98 .97 .99 .95 .99 .95 1.00 1.00 1.00 .96
Estimates based on paternal half-sib analysis.
set of metric observations. The lower heritabilities of the log-transformed values are probably only a consequence of the transformed scale. The phenotypic and genetic correlations between the metric measures of 20-week shank length and body weight were considerably lower than the transformed values with a pooled estimate of .43. For the genetic correlations, the pooled estimate was .64. DISCUSSION The heritabilities and correlations (both phenotypic and genetic) of the original measurements on body weight and shank length agree with those given by Kinney (1969). Pontecorvo (1938) suggested that a, the initial growth constant, is the primary hereditary factor governing conformation; this implies that j3 would be less highly heritable. In our study, the pooled estimates of the heritability of a and j3 were not significantly different (.27 vs. .31), but they were lower than the estimate using the original metric measurements. This can be attributed to a scale effect on the transformed population distribution. The assumption for proceeding with this analysis, however, was that a and /3 are distinct entities genetically. The parameter tx was derived as a linear difference in the log values of y and x; p was derived as the ratio of the difference, log y — log ot to log x. This leads to a "common element effect". That is, the high correlation between a and ]3 is caused by the correlated errors of their estimates. Also this
probably accounts for the substantial rank order correlation between h2a and h 2 0 among the 10-line groups. In a study on growth of Hereford cattle, Kidwell et al. (1952) concluded that most of the variation associated with a. was simply the inverse of the variation associated with |3. We conclude that the parameters a and /3 are useful in describing only phenotypic differences in body weight-shank length relationships between populations and that essentially they reflect only the variation of the untransformed metric measures of body weight and shank length plus a scale effect of the transformed distribution. REFERENCES Dickerson, G. E., 1969. Techniques for research in quantitative animal genetics. Pages 36—79 in Techniques and Procedures in Animal Science Research. Am. Soc. Anim. Sci., Albany, NY. Dickerson, G. E., and W. F. Hughes, 1964. Body weight: A critical factor in laying performance. The Kimberchik News (Aug.): 1-8. Kidwell, J. F., P. W. Gregory, and H. R. Gilbert, 1952. A genetic investigation of allometric growth in Hereford cattle. Genetics 37:158—174. Kinney, T. B., 1969. A summary of reported estimates of heritabilities and of genetic and phenotypic correlations for traits of chickens. US Dept. Agric, Agric. Res. Serv., Agric. Handbook No. 363. Nordskog, A. W„ and D. M. Briggs, 1968. The body weight egg production paradox. Poultry Sci. 48:498-504. Nordskog, A. W., H. S. Tolman, D. W. Casey, and C. Y. Lin, 1974. Selection in small populations of chickens. Poultry Sci. 53:1188-1219. Pontecorvo, G., 1938. Allometric growth of the forelimb in cattle. J. Agric. Sci. 29:111—114.
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
A B C D E F G
Genetic 1
Phenotypic
ABSTRACT The growth of an animal part (y) relative to the whole (x), is expressed in terms of an exponential equation, y=axfi, where a and /3 are growth constants. For the present study x is body weight and y is shank length determined for 10 lines of chickens at 20 weeks of age. The derived values of a and (3, assumed to be limiting relative growth constants for bone, were estimated by least squares of the log-transformed (linear) exponential equation. From the means of a and (3, calculated for each of seven Leghorn and three Fayoumi lines, individual values of a; and /3; were calculated for more than 24,000 pedigreed birds included in 9 years of records. The individual values (i) were calculated as: a; = antilog (log y; - 0 log x;) and: ft = log a - log y; logx; If the initial growth constant, a, is the primary hereditary growth parameter governing conformation, then the parameter, 0, should not be as highly heritable as a. To test this hypothesis, the heritability (h 2 ) of both a and /3 were calculated for each of the 10 lines. For a sire component h 2 values ranged from .12 ± .05 to .45 + .10 and for (3 from .05 to .41. The pooled h 2 estimate of a over lines (.31 ± .02) was not significantly different from that for (3 (.31 ± .02). Compared with the original data on body weight and shank length, the transformation significantly reduced the heritability estimated. We conclude that the parameters, a and /3, are useful only in describing phenotypic and not genetic differences in relative growth. (Key words: body size, body weight, chickens) 1985 Poultry Science 6 4 : 6 0 5 - 6 0 9 INTRODUCTION During t h e past t h r e e decades a least-cost profit-oriented approach t o commercial egg p r o d u c t i o n has created a d e m a n d for a genetically m o r e efficient egg-laying hen. T o m e e t this d e m a n d , breeders have strived to develop smaller or i n t e r m e d i a t e weight hens capable of laying large eggs at a high rate. This endeaver has featured use of t h e dwarf gene to reduce b o d y size, or otherwise, b y simple selection
'Journal Paper No. J. 11321 of the Iowa Agriculture and Home Economics Experiment Station, Project 2576, a contributing project to the North Central Regional Project, Biotechnology in Poultry, NC168. 605
schemes, t o develop " m i n i " chickens. Usually t h e criterion used t o evaluate b o d y size has been t h a t of b o d y weight at some specific age. B o d y weight is a function o f skeletal size ( b o n e ) , fleshing (muscle), and c o n d i t i o n (fat). Dickerson and Hughes ( 1 9 6 4 ) and N o r d s k o g and Briggs ( 1 9 6 8 ) showed t h a t fleshing o r c o n d i t i o n is an economically i m p o r t a n t comp o n e n t in t e r m s of laying-house p e r f o r m a n c e . If variation in fleshing condition is due m o s t l y t o variable h u s b a n d r y practices, t h e n selecting for small or intermediate hens o n t h e basis of b o d y weight alone might n o t always exclude birds with p o o r viability and egg p r o d u c t i o n . This r e p o r t will deal w i t h t h e relationship of shank length ( b o n e ) t o b o d y weight ( t o t a l size), as d e t e r m i n e d o n m o r e t h a n 2 4 , 0 0 0 Leghorn a n d F a y o u m i pullets a t 2 0 w e e k s of
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
(Received for publication January 16, 1984)
TIERCE AND NORDSKOG
606
age. This population is defined as "static", because the data are restricted to a single age group. The objectives of the present study were to calculate heritability (h 2 ) estimates of the "allometric" growth constants a and j3, and to consider their uniqueness when contrasted with h estimates of body weight (x) and shank length (y) all derived from the same population of 20-week old pullets.
The data for the analysis came from a selection experiment involving seven White Leghorn lines and three Fayoumi lines (Nordskog et al., 1974). The Leghorn base population was developed from 12 diallel single crosses between four commercial lines. The Fayoumi lines were derived from a flock maintained at Iowa State University over more than 20 years. The number of records analyzed, selection criteria, and number of male and female breeders per line are summarized in Table 1. Leghorn line A and Fayoumi line J were selected for high rate of egg production to 32 weeks of age. Leghorn lines B and C were selected, respectively, for high and low body weight; and Leghorn lines D and E were selected, respectively, for high and low egg weight. Fayoumi lines K and L were selected, respectively, for high body weight and high egg weight. Leghorn F was selected antagonistically for high body weight
log (y- jk ) = logaj + 0j log (x i j k and solving for the unknowns aj and (3; for line i, where yj; k ,X" k = shank length (y) and body weight (x) of the kth bird in the jth year of the ith i m e . Values of a-^ and | 3 - k were calculated for each individual by substituting y - k and x » k and either &j or & into the preceding linear equation, where aijk
TABLE 1. Lines, selection criteria, and number of records
Line Leghorn A B C D E F
Selection criteria1
Number of individual records
H-EP H-BW L-BW H-EW L-EW H-BW L-EW L-BW, H-EW
6007 1763 2118 2264 2245
H-EP H-BW H-EW
3555 1244 2165
2164 2228
Fayoumi
J K L 1
L=Low, H=high, BW=body weight, EW=egg :gg weight, weight,EP= EP=i =egg production.
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
MATERIALS AND METHODS
and low egg weight and Leghorn G was selected antagonistically for low body weight and high egg weight (Nordskog et al., 1974). Two or three hatches, 2 weeks apart, were produced each year. All chicks were pedigree wing-banded and brooded, intermingled to 8 weeks of age, and then placed on a summer pasture. At 20 weeks, body weight and shank length were recorded on each pullet. Body weight was measured to the nearest .045 kg (.1 lb) and shank length, measured with parallel jaw calipers, to the nearest millimeter. Because all measurements were taken on the same pullets at the same age, essentially at maturity, the data were regarded as a sample from a static population. For each of the 10 lines, least squares estimates of the initial growth constant, a, and the "coefficient of static allometry", 0, were first calculated using a simple least squares analysis of the log-transformed linear equation:
PERFORMANCE OF LAYERS AND BODY COMPOSITION
initial growth constant for the k t n bird in the j t " year of the ith line, henceforth referred to as a, and ft-^ = coefficient of static
Estimates for ft ranged from .1340 ± .0074 to .2006 ± .0070 in the Leghorn lines and from .1622 ± .0099 to .2261 ± .0125 in the Fayoumi lines. The largest was for the Fayoumi line K, selected for high body weight and the smallest for Leghorn line C, selected for low body weight. Lines with higher estimates of ft were selected for either high body weight or egg weight whereas lines with lower estimates were selected for low egg weight or body weight. Using the line estimates of a and B and the individual bird measurements of 20-week shank length and body weight, individual estimates of
the initial growth constant (oq) and the allometry coefficients (ft) were calculated for over 24,000 pedigreed individuals. The heritability estimates of a and B are presented in Table 3. For the sire component estimates (h 2 s) the pooled averages are .31 ± .02 for a and .27 + .02 for B. For the dam component (h 2 d) the pooled estimates are slightly larger but not significantly so. Also, the estimates for a are slightly, but not significantly, larger than for B. The magnitude of the h 2 s estimates for a, presented in rank order, seem not to be related to the prior selection applied to the different individual lines (Table 1), but the rank order correlation (R) between the estimates of a and 8 was equal to 1 based on the sire components (P<.01). For the dam components, the R=.25 was not significant. Because the individual bird estimates of a and B were, in fact, only transformations of metric observations on body weight and shank length, it seemed obligatory that these be compared with the h 2 estimates of the original metrical measurements of body weight and shank length; these are presented in rank order by lines in Table 4. The body weight sire component estimates ranged from .21 ± .07 (Line C selected for low body weight) to .59 ± .12 (Line D selected for large egg weight). Pooled across lines, the estimates were .43 ± .03 and .63 ± .03 for the sire and dam components, respectively. For shank length they ranged from .22 ± .08 (Leghorn B selected for high body weight) to .56 ± .11 (Fayoumi L selected for high egg weight). The pooled values of .41 ± .03 and .53 ± .02 based on the sire and dam components, respectively, are significantly larger than those
TABLE 2. Estimates of static allometry constants by lines based on the relationship shank length — a(body weight)? Line A B C D E F G
J K L
ft .8017 .8455 .7968 .8110 .7892 .8142 .7965 .7776 .7925 .7969
±.0022 ±.0053 +.0053 ± .0038 ± .0034 ± .0044 ± .0033 ± .0029 ± .0070 ± .0044
.1829 ± .0047 .1507 ±.0081 .1340 ± .0074 .2006 ± .0070 .1754 ±.0082 .1731 ±.0075 .1625 ± .0073 .2043 ± .0070 .2261 ± .0125 .1622 ± .0099
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on February 13, 2015
allometry for the k t n bird in the j t n year of the i r h line, henceforth referred to as B. For m n each of P lines, there were : 2 _ i j.2i n jk = N individuals, giving a total of PN derived values of a and p for the analysis. Year-generation estimates of population parameters were pooled to obtain single averages of heritabilities and genetic correlations by lines. These were tested for statistical significance using approximate standard errors as described by Dickerson (1969). Estimates of the allometry constants are given in Table 2. Values of aj ranged from .7892 ± .0033 to .8455 ± .0053 for the seven Leghorn lines and from .7760 + .0029 to .7969 ± .0044 for the three Fayoumi lines. Leghorn lines, B, D, and F, selected for high body weight or for high egg weight, had the largest values for aj. The smallest were those of Fayoumi line J, selected for high egg production and Leghorn line E, selected for low egg weight.
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TIERCE AND NORDSKOG
TABLE 3. Heritability (h2) estimates of static allometry constants1 based on (sire and dam components) of variance Sire component (h 2 s)
Dam component (h 2 d)
(3
a
F A B C E K G
.45 .41 .38 .37 .29 .26 .26 .26 .18 .12
± .10 ± .09 ± .08 ± .09 ± .05 ± .01 ± .01 ± .07 ± .09 ± .05
.41 ± .09 .40 ± .09 .34 ± .07 .32 ± .08 .27 ± .05 .25 ± .08 .25 ± .07 .21 ± .06 .15 + .08 .05 ± .04
.18 ± .31 ± .39 ± .36 ± .46 ± .31 ± .37 + .20 ± .62 ± .27 ±
Pooled average
.31 + .02
.27 ± .02
.36 ± .02
Line
a
L D
J
1
1.
.01 .08 .06 .08 .05 .09 .08 .07 .13 .08
.12 .35 .26 .37 .48 .20 .24 .04 .55 .28
± .07 ± .08 ± .06 ± .08 ± .01 ± .10 ± .08 ± .07 ± .07 ± .08
.26 ± .02
.25
Shank length = a(body weight)&.
of the transformation-derived allometry constants (Table 3). The rank order correlation between body weight and shank length was .48 for h 2 s but negative for h 2 d (R=-.32). However, to reach statistical significance an R=.64 is required for a sample of only 10 lines. Finally, the rank order correlation between the heritability estimates of BW with the h 2 s estimates of a and 0 were positive, R(BW,a)=.20 and R(BW,/3)=.35, but not statistically significant.
Table 5 contrasts the phenotypic and genetic correlations between body weight and shank length with those between the transformed values a and )3. For the latter, the phenotypic and genetic correlations were high with pooled estimates of .95 and .96, respectively. These results demonstrate that genetic parameters estimated as simple log-transformation functions of body weight and shank length are essentially equivalent to those using the original
TABLE 4. Heritability estimates (h2) of 20-week body weight (BW) and shank length (SL) based on sire and dam components of variance Dam component (h 2 d) 2
Sire component (h 2 s) 1 Linie
BW
L D A F E
.31 .59 .53 .44 .31 .33 .55 .21 .27 .31
J
G C K B Rank correlation
1
Paternal half-sib analysis.
2
Full-sib analysis.
SL ± .10 ± .12 ± .08 ± .10 ± .09 ± .07 ± .12 ± .07 + .10 ± .09
.56 .54 .48 .39 .38 .37 .35 .29 .28 .33
(BW, SL) (BW,a) (BW,(3)
.48 .20 .35
± .11 ±.11 ± .07 ± .10 + .09 ± .08 ± .09 ± .09 + .11 ± .08
BW
SL
.62 ± .09 .63 ± .08 .81 ± .06 .5 5 + .09 .75 ± .09 .33 ± .06 .49 ± .08 .54 ± .09 .37 + .13 .50 ± .10
.28 .49 .58 .64 .43 .59 .51 .64 .62 .34
-.32
+ .07 ± .08 ± .05 ± .09 ± .08 ± .07 ± .08 ± .09 ± .13 ± .10
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Rank correlations (a,|3)
(3
609
PERFORMANCE OF LAYERS AND BODY COMPOSITION
TABLE 5. Phenotypic and genetic correlations between 20-week body measurements (body weight and shank length) and between derived static allometry constants (a and (3) Phenotypic Line
J
K L Pooled 1
Body measurements
Allometry constants
Body measurements
Allometry constants
.45 .37 .35 .52 .45 .43 .43 .45 .46 .37 .43
.98 .98 .93 .98 .88 .98 .98 .96 .99 .99 .95
.77 .19 .49 .69 .74 .47 .93 .44 .70 .73 .64
.99 .98 .97 .99 .95 .99 .95 1.00 1.00 1.00 .96
Estimates based on paternal half-sib analysis.
set of metric observations. The lower heritabilities of the log-transformed values are probably only a consequence of the transformed scale. The phenotypic and genetic correlations between the metric measures of 20-week shank length and body weight were considerably lower than the transformed values with a pooled estimate of .43. For the genetic correlations, the pooled estimate was .64. DISCUSSION The heritabilities and correlations (both phenotypic and genetic) of the original measurements on body weight and shank length agree with those given by Kinney (1969). Pontecorvo (1938) suggested that a, the initial growth constant, is the primary hereditary factor governing conformation; this implies that j3 would be less highly heritable. In our study, the pooled estimates of the heritability of a and j3 were not significantly different (.27 vs. .31), but they were lower than the estimate using the original metric measurements. This can be attributed to a scale effect on the transformed population distribution. The assumption for proceeding with this analysis, however, was that a and /3 are distinct entities genetically. The parameter tx was derived as a linear difference in the log values of y and x; p was derived as the ratio of the difference, log y — log ot to log x. This leads to a "common element effect". That is, the high correlation between a and ]3 is caused by the correlated errors of their estimates. Also this
probably accounts for the substantial rank order correlation between h2a and h 2 0 among the 10-line groups. In a study on growth of Hereford cattle, Kidwell et al. (1952) concluded that most of the variation associated with a. was simply the inverse of the variation associated with |3. We conclude that the parameters a and /3 are useful in describing only phenotypic differences in body weight-shank length relationships between populations and that essentially they reflect only the variation of the untransformed metric measures of body weight and shank length plus a scale effect of the transformed distribution. REFERENCES Dickerson, G. E., 1969. Techniques for research in quantitative animal genetics. Pages 36—79 in Techniques and Procedures in Animal Science Research. Am. Soc. Anim. Sci., Albany, NY. Dickerson, G. E., and W. F. Hughes, 1964. Body weight: A critical factor in laying performance. The Kimberchik News (Aug.): 1-8. Kidwell, J. F., P. W. Gregory, and H. R. Gilbert, 1952. A genetic investigation of allometric growth in Hereford cattle. Genetics 37:158—174. Kinney, T. B., 1969. A summary of reported estimates of heritabilities and of genetic and phenotypic correlations for traits of chickens. US Dept. Agric, Agric. Res. Serv., Agric. Handbook No. 363. Nordskog, A. W„ and D. M. Briggs, 1968. The body weight egg production paradox. Poultry Sci. 48:498-504. Nordskog, A. W., H. S. Tolman, D. W. Casey, and C. Y. Lin, 1974. Selection in small populations of chickens. Poultry Sci. 53:1188-1219. Pontecorvo, G., 1938. Allometric growth of the forelimb in cattle. J. Agric. Sci. 29:111—114.
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A B C D E F G
Genetic 1
Phenotypic