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Improvement of Reproductive Traits and Body Measurements of Turkeys1
Improvement of Reproductive Traits and Body Measurements of Turkeys 1 R. E. COOK,2 W. L. BLOW, C. C. COCKERHAM AND E. W. GLAZENER Poultry Department, North Carolina Agricultural Experiment Station, Raleigh, North Carolina (Received for publication July 11, 1961)
relatively heavy emphasis on increased body weight and fleshing characteristics. This objective is understandable since turkeys are raised entirely for meat. With this trend toward heavy, broad breasted birds the accompanying changes in reproductive ability, if not negative, have been minimal increases. Although the interests of growers and hatching egg producers may be somewhat different, their interests so overlap that the genetic-economic value of turkeys is a function of the various characters which affect their different enterprises. The breeder, in this case, is interested in developing superior meat birds with a high reproductive rate. This may be desirable economically, but the genetic relationship between traits may prevent attaining the maximum improvement that would be possible if each character were independent of other characters. The problem becomes one of placing the correct relative emphasis on different characters so as to maximize the improvement of breeding value. In this study an attempt has been made to develop a selection index involving certain reproductive traits and physical measurements of turkeys. The traits used in this study were (1) total egg production during the first hundred days of production, (2) percent fertility of all eggs set, 1 Published with the approval of the Director of Research as Paper No. 1329 of the Journal Series. 2 Present address: Department of Poultry Husbandry, University of Florida, Gainesville, Florida.
(3) percent hatchability of fertile eggs, (4) body weight and (5) breast width at six months of age. The various genetic and environmental parameters influencing these characters were estimated in this study and from this information a selection index for turkeys was developed. Several investigators have studied the relationship between physical measurements of turkeys and reproductive ability. Funk (1950) and Berg (1953) have found some evidence of negative correlations between the two kinds of traits. Funk reported phenotypic correlations of — .08 between body weight and eight weeks' production and —.21 between percent hatch of all eggs set and body weight. These were in close agreement with Berg (1953) who also found that there were negative correlations between width of breast and egg production and fertility. Berg (1953) also reported very high phenotypic correlations between width of breast and body weight at 24 weeks of age. There seems little doubt on the basis of this report and others that width of breast and body weight are positively correlated. There is little information available on the correlations between various factors affecting reproductive performance in turkeys. Maw and McCartney (1956) reported a phenotypic correlation of .06 between fertility and hatchability of fertile eggs. Funk (1950) reported phenotypic correlations of .18 and —.04 between hatchability of all eggs set and egg production in two varieties of turkeys.
556
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programs for the imSELECTION provement of turkeys have placed
557
IMPROVEMENT OF TURKEYS
MATERIALS
The data used in this study were obtained from the North Carolina Agricultural Experiment Station flock of Broad Breasted Bronze turkeys over a period of years from 1944 to 1955. The management of the birds has been described previously by Blow and Glazener (1954), and Blow, Stewart and Glazener (1958). The data were based on 3,043 birds from 509 dams and 67 sires. The emphasis in the selection of both sexes in the past had been primarily on body weight and breast fleshing. Since matings were made in the flock prior to
TABLE 1.—The traits invoiced in this study Trait
Description of the trait
Xi
Total egg production from the date of first egg in the pullet year to 100 days later Percent fertility of all eggs set from an individual hen Percent hatchability of fertile eggs Body weight in pounds at six months of age Breast width in inches at six months of age
Xi Xz Xt Xi
the onset of production, selection for egg production consisted mainly of eliminating those individuals which produced very small families. Only natural matings were used during the course of the experiment. The hatching season for flock replacements generally extended over a six or eight week's period with biweekly hatches starting approximately the middle of March. The poults in each hatch were distributed at random and maintained in units of 100 poults in the brooder house until they were approximately nine weeks of age. After this period they were maintained in a single flock on range until they were weighed and measured at six months of age. The symbols assigned each trait and a description of the traits involved in this study are presented in Table 1. All females in this flock which laid at least one egg were involved in the data. The average values of the five characters and the number of observations by years are presented in Table 2. The experimental unit in the case of fertility and hatchability was the average percent of an individual bird's eggs. Percent fertility was determined by candling. Observations on fertility and hatchability were made on all birds in this flock prior to 1953. From this date these two characters were recorded only for those birds which went into single male matings to propagate the next generation. Unfortunately, some of the data on body weight and
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The several workers who have investigated the heritability of body weight and breast width have generally reported rather high heritabilities for these characters. These include Kondra and Shoffner (1955) who obtained estimates averaging .4 to .5 for body weight and slightly smaller estimates for breast width. The estimates from this study were slightly higher than similar estimates presented by Abplanap and Kosin (1952). Studies of the heritability of fertility and hatchability have been limited. The estimates available have varied widely with a range from .12 (Abplanap and Kosin, 1953) to 1.98 (Kondra and Shoffner, 1955) for hatchability. These high estimates are in sharp contrast with the generally low estimates published for similar characters in chickens. Wilson and Johnson (1946) estimated the heritability of egg numbers in turkeys from birds surviving their pullet year of production at .02. Several previously published estimates of the heritability of egg production in the flocks used in the present study have been approximately .15 which is considerably lower than the estimate of .40 found by Shaklee, Knox and Marsden (1952) in a flock of Beltsville Whites.
558
R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. GLAZENER TABLE 2.-—Average value
of i'.he traits and the number of observations inthe years recorded, Br. width
Body wt.
Hatch.
Fert.
Egg prod. Year No. 124 196 147 220 206 172 233 271 181 390 434 469 3,043
58.80 54.38 60.45 60.00 60.76 64.82 56.27 58.88 55.23 59.83 60.73 51.57 58.05
No. 124 196 147 220 206 172 231 267 177 65 75 90 1,970
No.
Av. 74.30 78.23 78.36 75.89 78.49 74.89 75.58 65.86 64.51 82.28 77.95 71.37 74.06
124 195 147 219 206 171 230 264 177 65 72 90 1,960
breast width was lost when the department made a move to new quarters in 1952. These losses are reflected in the years without any observations for these two characters. The year to year variation in the reproductive characters was quite normal. The decreased values for breast width in 1948 and 1949 were due in part to the use of an instrument developed by Bird (1945) which measured breast width at a point below the anterior end of the keel which was one-fifth the body depth. In other years the breast width was measured with calipers at If inches depth approximately \\ inches back from the anterior end of the keel. Changes in body weight and breast width during the latter part of the study were due to some extent to the introduction of several males from another strain of turkeys into this previously closed flock.
T H E PARAMETERS
The following statistics, in addition to the net worth of each character, were needed for the construction of the index: (1) phenotypic variances of the traits (2) phenotypic covariances for all pairs of traits (3) genetic variances of the traits
Av.
No.
79.27 80.62 80.99 82.19 75.11 79.55 66.89 83.13 79.58 71.74 80.88 74.32 78.12
— 190 147
—
201 170
—
42 35 390 433 469 2,077
No.
Av.
Av.
— 190 147
11.56 12.91
—
—
2.27 2.51
—
12.76 14.16
201 170
1.82 1.69
—
— —
— —
12.33 13.27 12.48 14.12 16.27 13.80
35 390 433 469 2,035
2.09 2.79 3.06 3.39 2.72
(4) genetic covariances for all pairs of traits. Analysis of the data was based on the assumption that performance of any of the characters can be expressed as indicated in the following equation: Xnk i = M+y
i+sn+dak+e,,jkl
where X,-,-*; represents the 1th progeny of the k th dam mated to the j t h sire in the i th year. King and Henderson (1954) have discussed the estimation of variance components in poultry with "nested" data having unequal subclass numbers. The variances and covariances in these data were analyzed as shown in Table 3. TABLE 3.—The analysis of variance and covariance showing the theoretical composition of the mean squares and mean cross-products
Source of variation
Total Years Between sires within years Between dams within sires and years Between full-sibs
Expected composiMean tion of the mean squares squares or the mean cross-products*
Mi
o-e2+ko'<7d2+h
M2 Mz
* The composition for a mean cross-product is the same as for the corresponding mean square except that components of covariance are substituted for components of variance.
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1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 Means
Av.
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IMPROVEMENT OF T U R K E Y S
T h e components of variance and their standard errors for all traits from the analyses of variance are shown in Table 4. These standard errors are indicative of the large sampling error associated with the estimates of the genetic variance. When the sire or d a m component of variance is multiplied by four to estimate the genetic variance, the variance of this estimate is increased by sixteen times. As is so often the case in this type of mating, the dam component of variance is appreciably larger t h a n the sire component for each of the five traits except breast width. The heritability estimates derived from these variance components are presented in Table 5. Sampling effects may play a
TABLE 5.—Estimates of heritability from the analysis of variance for each trait Heritability estimates from Traits Egg production Fertility Hatchability Body weight Breast width
4
4a/
0.19 0.11 0.12 0.35
0.29 0.25 0.31 0.55 0.16
0.31
large part in the excess of the estimates of heritability from the dam component over that computed from the sire component. However, the excess of the estimates from the d a m component presents a case for the possible involvement of extraneous effects such as maternal influences and dominance deviations. Although these results indicate t h a t all of the characters are amenable to selection, body weight is the character which would show the greatest response. The components of covariance and their standard errors from the analyses of covariance and estimates of the genetic correlations between different pairs of the traits are presented in Tables 6 and 7, respectively. T h e most striking feature of these tables is the inconsistency of the estimates of the same covariance and correlation from the sire and dam components. I n only three of the ten cases is there agreement in sign of the values presented. Estimates of the genetic correlations from the dam components indicate t h a t for the traits considered in this study the character associations are generally in favor of the breeder. Very minor exceptions are the small negative genetic correlations between egg produc-
TABLE 4.—The components of variance and their standard errors from the analysis of variance for each trail Source of variation
Xt
11.04 + 3.33 16.94 + 3.79 206.62 + 5.89
Xi,
11.96+ 5.97 26.69+ 9.61 392.37 + 14.24
6.59 + 3.26 17.27+4.97 198.80 + 7.22
0.112 + 0.033 0.177 + 0.027 0.996 + 0.035
0.0082 + 0.00077 0.0044 + 0.0018 0.0920 + 0.0032
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Genetic interpretation of variance components arising from progeny differences has been discussed b y Comstock and Robinson (1948). The sire and d a m components each contain one-fourth of the additive genetic variance with the remainder being contained in the variance between full sibs. Using the parental variances to estimate the genetic variance may introduce bias because of certain nonadditive genetic and environmental contributions. The source of bias in these estimates has been discussed so frequently in previous literature on herkabilities t h a t further discussion a t this point hardly seems necessary. The additive genetic covariance between traits is distributed in the components of covariance in a manner analogous to t h a t of the additive genetic variance; and when estimated from the parental components of covariance, is subject to the same kind of bias.
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R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. TABLE 6.—The components of covariance and their standard errors from the analysis of covariance for each trait Source of covariation
Traits
°d{Xi, Xj)
-1.879+3.86 8.574±5.06 3.721 + 2.93 1.582 + 3.67 -0.065 + 0.079 0.273 + 0.24 0.005 + 0.069 -0.017+0.063 -2.219+3.12 9.242 + 4.96 0.489±0.16 -0.075±0.51 0.051±0.22 0.249+0.15 -0.255 + 0.36 0.204+.0.38 -0.008+0.11 0.120+0.35 0.016+0.0073 0.020±0.0055
ire(Xi, Xj) 19.547±7.33 16.830±5.24 0.095±0.35 0.033±0.11 52.500+7.31 -1.480+0.73 -0.480 + 0.22 -1.201 + 0.52 -0.270+0.16 0.089 + 0.0077
tion and breast width and between fertility and body weight. Estimates of the genetic correlation between characters derived from the sire components of covariance indicate there may be a negative genetic correlation between egg production and fertility, fertility and hatchability, and hatchability and body weight. These data also indicate that there is a fairly strong positive association between fertility and body weight. Previously published estimates of correlations between reproductive traits have not indicated that there are negative correlations between the components which affect reproductive performance. In addition, it is generally accepted that poor fertility is primarily a problem associated with large-type turkeys. Valid application of the principle of index selection assumes that the statistics TABLE 7.—Estimates of genetic correlations between traits from the analyses of variance and covariance Genetic correlations from Traits correlated 4*AXi, XiX-^2 X1XX3 A1X.X4 XiX-^5 X^XXz X2XX4 Xi^Xf, A3X.A4 XzXX$ XJX-X'B
-0.16 0.44 -0.06 0.02 -0.25 0.42 0.16 -0.30 -0.03 0.S3
Xj)
4*d(Xi, 0.40 0.09 0.16 -0.06 0.43 -0.03 0.73 0.12 0.40 0.72
Xj)
used in constructing the indexes are reasonable approximations to the biological parameters of the populations. By using estimates of the genetic variances and genetic covariances derived from the sire components of variance and covariance, the bias resulting from dominance deviations and maternal effects is circumvented. There is no necessity for estimates of the same parameter to agree if one is aware of the sources of bias in these estimates. The troublesome inconsistency arises from the very large sampling errors associated with the estimates of these parameters. Unfortunately, previously published estimates of the genetic parameters presented in this paper are very limited. THE INDEX
The construction and evaluation of the index was patterned on those methods developed by Smith (1936) and Hazel (1943). The approach is essentially one of multiple regression in which the regression coefficients are derived from a set of n simultaneous equations, where n is the number of independent variables used in arriving at the total score of the index. Although the two workers used different statistics in constructing the indexes, the techniques are equivalent and lead to the same results. In constructing selection indexes, estimates of certain genetic and phenotypic parameters are required for each trait. In addition to these statistics a definition of net worth or relative economic value is needed for each trait. The aggregate genotype (H) of a bird may be designated as: H = a1Gi+a2G2+ • • • + «/?* where G; is the genetic or breeding value of the i t h character and a» is the economic worth of the same character. The selection index is defined as:
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X1XX2 XiXXz XiXXi XiXXi XiXXi XiXXt XiXXr, XzXXi XiXXt XiXXi
"s(Xi, Xj)
GLAZENER
IMPROVEMENT OF TURKEYS
I = b1X1+b,X2+
• • • +biXi,
In computing the relative economic weights for the various characters a laying season of 100 days was assumed with an average production of 58 percent as observed in this study. A feed cost of $.0455 per egg was computed using the average feed consumption of breeder hens observed by Berry and Frazier (1945) and
the current prices of feed. The current price of hatching eggs is approximately $.26 each. Assuming that all costs of producing eggs except cost of feed remain fixed and that there is a linear relationship between feed consumption and production, a one percent increase in production would be worth $.2145. The economic values of fertility and hatchability can be computed by using a price of hatching eggs of $.26 based on either 75 percent fertility or 65 percent hatchability. Assuming that an increase in these two characters does not add any additional production costs, then a one percent increase in fertility would result in an increase of $.003467 in the value of each egg and a one percent increase in hatchability would add $.0040 to the value of each egg. With a flock production of 58 eggs per bird, a one percent increase in fertility and hatchability for the breeding season would be worth $.2011 and $.2320 per hen, respectively. If body weight is increased in the same growing period by one pound, the net profit for the extra pound of gain would equal the selling price minus the feed cost since the other costs remain fixed to a large extent. The difference between selling price and feed cost for a pound of gain was estimated using current market conditions at $.076. Of the approximately 80,000,000 turkeys grown in 1959, only one in twenty of these individuals had an opportunity to express a phenotype for characters affecting both reproduction and meat production. It would appear that any index which fails to consider this point would give excessive emphasis to the various characters affecting reproductive performance. An alternative would be to develop an index for the various characters in which body weight is given additional economic value because of the
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where the b's are regression coefficients or relative weights for the phenotypic values, X's, of the characters in the index. The b's, which maximize the correlation between I and H are solutions to a set of simultaneous linear equations involving the genetic and phenotypic variances and covariances and the economic worths. Yamada (1958) and Hogsett and Nordskog (1958) have discussed the representation in matrix notation and subsequent solution of these equations to obtain the value of the b's. Increasing number of traits included in indexes will affect the amount of selection which can be applied to any one trait. In attempting to improve poultry there are any number of characters which contribute to the net worth of the fowl. If an index is to have any great utility, however, the number of traits included must be limited so that the index will be moderately simple. In improving the performance of turkeys there is no doubt that the five characters in this study make important contributions to the economic worth of turkeys, though it may be difficult to assess the contribution of breast width. Since it is difficult to assess the economic worth of this trait, an index was constructed to select for the three reproductive traits and body weight using breast width as an indicator of the other traits. Economic weights were computed for each character with an economic weight of zero assigned to breast width.
561
562
R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. GLAZENER
small fraction of turkeys which ever express the phenotype for reproductive performance. This was accomplished by
„
of selection with the index then the expected genetic change in egg production would be
ii
AGi =
—
•
— >
ci
ai=2.82,
a2=2.65,
a4 = 19.00,
a3 = 3.05,
a5 = zero.
Using these relative economic weights and the variances and covariances estimated from the sire components of the analyses, this index becomes /=.56Xi+.22X 2 +.29Xs+7.13X 4 -|-8.68X 6 . The first impression may be that greater emphasis is being placed on body weight and breast width in arriving at a total score or index value. However, the standard deviations of reproductive performance are so much larger than that of body weight, there may be a tendency for superiority in reproductive performance to outweigh the contribution of superiority in body weight to index score. The effects of selection when using the index on the direction and magnitude of change for each character is of major interest. Even though the use of the index may point the way to maximizing the overall rate of improvement, the use of the index may lead to undesirable consequences as far as one or more of the characters is concerned. An example of this would be a negative change in direction for one of the characters during the course of selection. If one standard deviation is the amount
AGi =1.76 eggs,
AG, = 1.34%,
AG3 =.44%,
AG 4 =.261bs.,
AG5= .05 in. Although there are slight negative correlations between some of the characters, these are not large enough to prevent some genetic progress for each of the characters included in the index. In terms of economic gain most of the benefits accruing from the use of this index result from changes in fecundity. When the genetic change for each character is weighted by the relative economic value for that character, the total relative gain from the three characters affecting reproduction is 9.85 compared with 4.94 for the change in body weight. In terms of economic reality the index derived here may have very little application. In order to approach a competitive edge if may be necessary for a turkey breeder to concentrate entirely on improving the body weight at a particular age if increases in reproductive performance do not permit him to sell more poults. Likewise, increased body weight may be of no advantage if a particular strain has established merit for this character, but is comparatively deficient for reproductive performance. In these cases the relative weight to be placed on various characters is a function of the level of performance at the time selection begins.
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multiplying the net worth of an additional pound of gain by nineteen. The relative economic values of the five characters then become:
where a^ is the variance of the index. The expected genetic change for each of the five characters in this study would be:
IMPROVEMENT OF TURKEYS SUMMARY
REFERENCES Abplanalp, H., and I. J. Kosin, 1952. Heritability of body measurements in turkeys. Poultry Sci. 31: 781-791. Berg, R. W., 1953. Some relationships of certain body measurements to meat yield and reproductive performance in turkeys. Unpublished Ph.D. thesis, University of Minnesota Library, March, 1953. Berry, C. N., and F. W. Frazier, 1945. "Egg-onomics," A study of factors in producing hatching
eggs and poults. Turkey World, 20: 20, 64-68. Bird, S., 1945. Measuring roundness of breast in live turkeys. U. S. Egg Poultry Magazine, 51: 206, 235. Blow, W. L., and E. W. Glazener, 1954. Heritability of egg production and egg weight in turkeys. Poultry Sci. 33: 417-421. Blow, W. L., H. A. Stewart and E. W. Glazener, 1958. Genetic variation and covariation of partial and complete records in turkeys. Poultry Sci. 37: 193-200. Comstock, R. E., and H. F. Robinson, 1948. The components of variance in populations of biparental progenies and their use in estimating the average degree of dominance. Biometrics, 4: 254266. Funk, E. M., 1950. The relation of body weight of turkey hens to egg production, egg weight, percentage of hatch and viability of poults. Poultry Sci. 29: 64-66. Hazel, L. N., 1943. Genetic basis for constructing selection indexes. Genetics, 28: 476-490. Hogsett, M. L., and A. W. Nordskog, 1948. Geneticeconomic value in selecting for egg production rate, body weight and egg weight. Poultry Sci. 37: 1404-1419. King, S. C , and C. R. Henderson, 1954. Heritability studies of egg production in the domestic fowl. Poultry Sci. 33: 155-169. Kondra, P. A., and R. N. Shoffner, 1955. Heritability of some body measurements and reproductive characters in turkeys. Poultry Sci. 34: 1262-1267. Maw, A. J. G., and M. G. McCartney, 1956. Variation in fertility of inbred chickens and of fertility and hatchability in White Holland turkeys. Poultry Sci. 35: 1185-1190. Shaklee, W. E., C. W. Knox and S. J. Marsden, 1952. Heritability of egg production in Beltsville Small White turkeys. Poultry Sci. 31: 935. Smith, H. F., 1936. A discriminant function for plant selection. Annals Eugenics, 7: 240-250. Wilson, W. O., and J. E. Johnson, 1946. The inheritance of egg production and hatchability in turkeys. Poultry Sci. 25: 278-284. Yamada, Y., 1958. Heritability and genetic correlations in economic characters in chickens. Japanese J. Genetics, 33: 13-22.
OCTOBER 30-NOVEMBER 1. \* ;STERN POULTRY CONGRESS, NATIONAL ORANGE SHOW GRO NTDS, SAN BERNARDINO, CALIF. OCTOBER 2-4. NEPPCO POULTRY EXPOSITION, FARM SHOW BUILDING, HARRISBURG, PA.
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Data from the records of 3,043 turkeys were used to estimate various genetic and phenotypic parameters. Genetic correlations between egg production, percent fertile eggs, hatchability of fertile eggs, body weight and breast width varied considerably as estimated from the sire and dam components of variance and covariance. The magnitude of the genetic correlation was rather small with more positive estimates being computed from the dam than from the sire components. Heritability estimates were considerably higher for the physical measurements than for the characters affecting reproduction. Genetic correlations estimated from the sire components were negative for egg production and fertility, egg production and body weight, fertility and hatchability, hatchability and body weight, and hatchability and breast width. There was a rather high positive correlation between body weight and fertility. A selection index for turkeys was derived which involved the characters in this study. This index provides an opportunity for a positive change in each of the characters for which selection is being practiced.
563
relatively heavy emphasis on increased body weight and fleshing characteristics. This objective is understandable since turkeys are raised entirely for meat. With this trend toward heavy, broad breasted birds the accompanying changes in reproductive ability, if not negative, have been minimal increases. Although the interests of growers and hatching egg producers may be somewhat different, their interests so overlap that the genetic-economic value of turkeys is a function of the various characters which affect their different enterprises. The breeder, in this case, is interested in developing superior meat birds with a high reproductive rate. This may be desirable economically, but the genetic relationship between traits may prevent attaining the maximum improvement that would be possible if each character were independent of other characters. The problem becomes one of placing the correct relative emphasis on different characters so as to maximize the improvement of breeding value. In this study an attempt has been made to develop a selection index involving certain reproductive traits and physical measurements of turkeys. The traits used in this study were (1) total egg production during the first hundred days of production, (2) percent fertility of all eggs set, 1 Published with the approval of the Director of Research as Paper No. 1329 of the Journal Series. 2 Present address: Department of Poultry Husbandry, University of Florida, Gainesville, Florida.
(3) percent hatchability of fertile eggs, (4) body weight and (5) breast width at six months of age. The various genetic and environmental parameters influencing these characters were estimated in this study and from this information a selection index for turkeys was developed. Several investigators have studied the relationship between physical measurements of turkeys and reproductive ability. Funk (1950) and Berg (1953) have found some evidence of negative correlations between the two kinds of traits. Funk reported phenotypic correlations of — .08 between body weight and eight weeks' production and —.21 between percent hatch of all eggs set and body weight. These were in close agreement with Berg (1953) who also found that there were negative correlations between width of breast and egg production and fertility. Berg (1953) also reported very high phenotypic correlations between width of breast and body weight at 24 weeks of age. There seems little doubt on the basis of this report and others that width of breast and body weight are positively correlated. There is little information available on the correlations between various factors affecting reproductive performance in turkeys. Maw and McCartney (1956) reported a phenotypic correlation of .06 between fertility and hatchability of fertile eggs. Funk (1950) reported phenotypic correlations of .18 and —.04 between hatchability of all eggs set and egg production in two varieties of turkeys.
556
Downloaded from http://ps.oxfordjournals.org/ at Rutgers University Libraries/Technical Services on April 26, 2015
programs for the imSELECTION provement of turkeys have placed
557
IMPROVEMENT OF TURKEYS
MATERIALS
The data used in this study were obtained from the North Carolina Agricultural Experiment Station flock of Broad Breasted Bronze turkeys over a period of years from 1944 to 1955. The management of the birds has been described previously by Blow and Glazener (1954), and Blow, Stewart and Glazener (1958). The data were based on 3,043 birds from 509 dams and 67 sires. The emphasis in the selection of both sexes in the past had been primarily on body weight and breast fleshing. Since matings were made in the flock prior to
TABLE 1.—The traits invoiced in this study Trait
Description of the trait
Xi
Total egg production from the date of first egg in the pullet year to 100 days later Percent fertility of all eggs set from an individual hen Percent hatchability of fertile eggs Body weight in pounds at six months of age Breast width in inches at six months of age
Xi Xz Xt Xi
the onset of production, selection for egg production consisted mainly of eliminating those individuals which produced very small families. Only natural matings were used during the course of the experiment. The hatching season for flock replacements generally extended over a six or eight week's period with biweekly hatches starting approximately the middle of March. The poults in each hatch were distributed at random and maintained in units of 100 poults in the brooder house until they were approximately nine weeks of age. After this period they were maintained in a single flock on range until they were weighed and measured at six months of age. The symbols assigned each trait and a description of the traits involved in this study are presented in Table 1. All females in this flock which laid at least one egg were involved in the data. The average values of the five characters and the number of observations by years are presented in Table 2. The experimental unit in the case of fertility and hatchability was the average percent of an individual bird's eggs. Percent fertility was determined by candling. Observations on fertility and hatchability were made on all birds in this flock prior to 1953. From this date these two characters were recorded only for those birds which went into single male matings to propagate the next generation. Unfortunately, some of the data on body weight and
Downloaded from http://ps.oxfordjournals.org/ at Rutgers University Libraries/Technical Services on April 26, 2015
The several workers who have investigated the heritability of body weight and breast width have generally reported rather high heritabilities for these characters. These include Kondra and Shoffner (1955) who obtained estimates averaging .4 to .5 for body weight and slightly smaller estimates for breast width. The estimates from this study were slightly higher than similar estimates presented by Abplanap and Kosin (1952). Studies of the heritability of fertility and hatchability have been limited. The estimates available have varied widely with a range from .12 (Abplanap and Kosin, 1953) to 1.98 (Kondra and Shoffner, 1955) for hatchability. These high estimates are in sharp contrast with the generally low estimates published for similar characters in chickens. Wilson and Johnson (1946) estimated the heritability of egg numbers in turkeys from birds surviving their pullet year of production at .02. Several previously published estimates of the heritability of egg production in the flocks used in the present study have been approximately .15 which is considerably lower than the estimate of .40 found by Shaklee, Knox and Marsden (1952) in a flock of Beltsville Whites.
558
R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. GLAZENER TABLE 2.-—Average value
of i'.he traits and the number of observations inthe years recorded, Br. width
Body wt.
Hatch.
Fert.
Egg prod. Year No. 124 196 147 220 206 172 233 271 181 390 434 469 3,043
58.80 54.38 60.45 60.00 60.76 64.82 56.27 58.88 55.23 59.83 60.73 51.57 58.05
No. 124 196 147 220 206 172 231 267 177 65 75 90 1,970
No.
Av. 74.30 78.23 78.36 75.89 78.49 74.89 75.58 65.86 64.51 82.28 77.95 71.37 74.06
124 195 147 219 206 171 230 264 177 65 72 90 1,960
breast width was lost when the department made a move to new quarters in 1952. These losses are reflected in the years without any observations for these two characters. The year to year variation in the reproductive characters was quite normal. The decreased values for breast width in 1948 and 1949 were due in part to the use of an instrument developed by Bird (1945) which measured breast width at a point below the anterior end of the keel which was one-fifth the body depth. In other years the breast width was measured with calipers at If inches depth approximately \\ inches back from the anterior end of the keel. Changes in body weight and breast width during the latter part of the study were due to some extent to the introduction of several males from another strain of turkeys into this previously closed flock.
T H E PARAMETERS
The following statistics, in addition to the net worth of each character, were needed for the construction of the index: (1) phenotypic variances of the traits (2) phenotypic covariances for all pairs of traits (3) genetic variances of the traits
Av.
No.
79.27 80.62 80.99 82.19 75.11 79.55 66.89 83.13 79.58 71.74 80.88 74.32 78.12
— 190 147
—
201 170
—
42 35 390 433 469 2,077
No.
Av.
Av.
— 190 147
11.56 12.91
—
—
2.27 2.51
—
12.76 14.16
201 170
1.82 1.69
—
— —
— —
12.33 13.27 12.48 14.12 16.27 13.80
35 390 433 469 2,035
2.09 2.79 3.06 3.39 2.72
(4) genetic covariances for all pairs of traits. Analysis of the data was based on the assumption that performance of any of the characters can be expressed as indicated in the following equation: Xnk i = M+y
i+sn+dak+e,,jkl
where X,-,-*; represents the 1th progeny of the k th dam mated to the j t h sire in the i th year. King and Henderson (1954) have discussed the estimation of variance components in poultry with "nested" data having unequal subclass numbers. The variances and covariances in these data were analyzed as shown in Table 3. TABLE 3.—The analysis of variance and covariance showing the theoretical composition of the mean squares and mean cross-products
Source of variation
Total Years Between sires within years Between dams within sires and years Between full-sibs
Expected composiMean tion of the mean squares squares or the mean cross-products*
Mi
o-e2+ko'<7d2+h
M2 Mz
* The composition for a mean cross-product is the same as for the corresponding mean square except that components of covariance are substituted for components of variance.
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1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 Means
Av.
559
IMPROVEMENT OF T U R K E Y S
T h e components of variance and their standard errors for all traits from the analyses of variance are shown in Table 4. These standard errors are indicative of the large sampling error associated with the estimates of the genetic variance. When the sire or d a m component of variance is multiplied by four to estimate the genetic variance, the variance of this estimate is increased by sixteen times. As is so often the case in this type of mating, the dam component of variance is appreciably larger t h a n the sire component for each of the five traits except breast width. The heritability estimates derived from these variance components are presented in Table 5. Sampling effects may play a
TABLE 5.—Estimates of heritability from the analysis of variance for each trait Heritability estimates from Traits Egg production Fertility Hatchability Body weight Breast width
4
4a/
0.19 0.11 0.12 0.35
0.29 0.25 0.31 0.55 0.16
0.31
large part in the excess of the estimates of heritability from the dam component over that computed from the sire component. However, the excess of the estimates from the d a m component presents a case for the possible involvement of extraneous effects such as maternal influences and dominance deviations. Although these results indicate t h a t all of the characters are amenable to selection, body weight is the character which would show the greatest response. The components of covariance and their standard errors from the analyses of covariance and estimates of the genetic correlations between different pairs of the traits are presented in Tables 6 and 7, respectively. T h e most striking feature of these tables is the inconsistency of the estimates of the same covariance and correlation from the sire and dam components. I n only three of the ten cases is there agreement in sign of the values presented. Estimates of the genetic correlations from the dam components indicate t h a t for the traits considered in this study the character associations are generally in favor of the breeder. Very minor exceptions are the small negative genetic correlations between egg produc-
TABLE 4.—The components of variance and their standard errors from the analysis of variance for each trail Source of variation
Xt
11.04 + 3.33 16.94 + 3.79 206.62 + 5.89
Xi,
11.96+ 5.97 26.69+ 9.61 392.37 + 14.24
6.59 + 3.26 17.27+4.97 198.80 + 7.22
0.112 + 0.033 0.177 + 0.027 0.996 + 0.035
0.0082 + 0.00077 0.0044 + 0.0018 0.0920 + 0.0032
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Genetic interpretation of variance components arising from progeny differences has been discussed b y Comstock and Robinson (1948). The sire and d a m components each contain one-fourth of the additive genetic variance with the remainder being contained in the variance between full sibs. Using the parental variances to estimate the genetic variance may introduce bias because of certain nonadditive genetic and environmental contributions. The source of bias in these estimates has been discussed so frequently in previous literature on herkabilities t h a t further discussion a t this point hardly seems necessary. The additive genetic covariance between traits is distributed in the components of covariance in a manner analogous to t h a t of the additive genetic variance; and when estimated from the parental components of covariance, is subject to the same kind of bias.
560
R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. TABLE 6.—The components of covariance and their standard errors from the analysis of covariance for each trait Source of covariation
Traits
°d{Xi, Xj)
-1.879+3.86 8.574±5.06 3.721 + 2.93 1.582 + 3.67 -0.065 + 0.079 0.273 + 0.24 0.005 + 0.069 -0.017+0.063 -2.219+3.12 9.242 + 4.96 0.489±0.16 -0.075±0.51 0.051±0.22 0.249+0.15 -0.255 + 0.36 0.204+.0.38 -0.008+0.11 0.120+0.35 0.016+0.0073 0.020±0.0055
ire(Xi, Xj) 19.547±7.33 16.830±5.24 0.095±0.35 0.033±0.11 52.500+7.31 -1.480+0.73 -0.480 + 0.22 -1.201 + 0.52 -0.270+0.16 0.089 + 0.0077
tion and breast width and between fertility and body weight. Estimates of the genetic correlation between characters derived from the sire components of covariance indicate there may be a negative genetic correlation between egg production and fertility, fertility and hatchability, and hatchability and body weight. These data also indicate that there is a fairly strong positive association between fertility and body weight. Previously published estimates of correlations between reproductive traits have not indicated that there are negative correlations between the components which affect reproductive performance. In addition, it is generally accepted that poor fertility is primarily a problem associated with large-type turkeys. Valid application of the principle of index selection assumes that the statistics TABLE 7.—Estimates of genetic correlations between traits from the analyses of variance and covariance Genetic correlations from Traits correlated 4*AXi, XiX-^2 X1XX3 A1X.X4 XiX-^5 X^XXz X2XX4 Xi^Xf, A3X.A4 XzXX$ XJX-X'B
-0.16 0.44 -0.06 0.02 -0.25 0.42 0.16 -0.30 -0.03 0.S3
Xj)
4*d(Xi, 0.40 0.09 0.16 -0.06 0.43 -0.03 0.73 0.12 0.40 0.72
Xj)
used in constructing the indexes are reasonable approximations to the biological parameters of the populations. By using estimates of the genetic variances and genetic covariances derived from the sire components of variance and covariance, the bias resulting from dominance deviations and maternal effects is circumvented. There is no necessity for estimates of the same parameter to agree if one is aware of the sources of bias in these estimates. The troublesome inconsistency arises from the very large sampling errors associated with the estimates of these parameters. Unfortunately, previously published estimates of the genetic parameters presented in this paper are very limited. THE INDEX
The construction and evaluation of the index was patterned on those methods developed by Smith (1936) and Hazel (1943). The approach is essentially one of multiple regression in which the regression coefficients are derived from a set of n simultaneous equations, where n is the number of independent variables used in arriving at the total score of the index. Although the two workers used different statistics in constructing the indexes, the techniques are equivalent and lead to the same results. In constructing selection indexes, estimates of certain genetic and phenotypic parameters are required for each trait. In addition to these statistics a definition of net worth or relative economic value is needed for each trait. The aggregate genotype (H) of a bird may be designated as: H = a1Gi+a2G2+ • • • + «/?* where G; is the genetic or breeding value of the i t h character and a» is the economic worth of the same character. The selection index is defined as:
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X1XX2 XiXXz XiXXi XiXXi XiXXi XiXXt XiXXr, XzXXi XiXXt XiXXi
"s(Xi, Xj)
GLAZENER
IMPROVEMENT OF TURKEYS
I = b1X1+b,X2+
• • • +biXi,
In computing the relative economic weights for the various characters a laying season of 100 days was assumed with an average production of 58 percent as observed in this study. A feed cost of $.0455 per egg was computed using the average feed consumption of breeder hens observed by Berry and Frazier (1945) and
the current prices of feed. The current price of hatching eggs is approximately $.26 each. Assuming that all costs of producing eggs except cost of feed remain fixed and that there is a linear relationship between feed consumption and production, a one percent increase in production would be worth $.2145. The economic values of fertility and hatchability can be computed by using a price of hatching eggs of $.26 based on either 75 percent fertility or 65 percent hatchability. Assuming that an increase in these two characters does not add any additional production costs, then a one percent increase in fertility would result in an increase of $.003467 in the value of each egg and a one percent increase in hatchability would add $.0040 to the value of each egg. With a flock production of 58 eggs per bird, a one percent increase in fertility and hatchability for the breeding season would be worth $.2011 and $.2320 per hen, respectively. If body weight is increased in the same growing period by one pound, the net profit for the extra pound of gain would equal the selling price minus the feed cost since the other costs remain fixed to a large extent. The difference between selling price and feed cost for a pound of gain was estimated using current market conditions at $.076. Of the approximately 80,000,000 turkeys grown in 1959, only one in twenty of these individuals had an opportunity to express a phenotype for characters affecting both reproduction and meat production. It would appear that any index which fails to consider this point would give excessive emphasis to the various characters affecting reproductive performance. An alternative would be to develop an index for the various characters in which body weight is given additional economic value because of the
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where the b's are regression coefficients or relative weights for the phenotypic values, X's, of the characters in the index. The b's, which maximize the correlation between I and H are solutions to a set of simultaneous linear equations involving the genetic and phenotypic variances and covariances and the economic worths. Yamada (1958) and Hogsett and Nordskog (1958) have discussed the representation in matrix notation and subsequent solution of these equations to obtain the value of the b's. Increasing number of traits included in indexes will affect the amount of selection which can be applied to any one trait. In attempting to improve poultry there are any number of characters which contribute to the net worth of the fowl. If an index is to have any great utility, however, the number of traits included must be limited so that the index will be moderately simple. In improving the performance of turkeys there is no doubt that the five characters in this study make important contributions to the economic worth of turkeys, though it may be difficult to assess the contribution of breast width. Since it is difficult to assess the economic worth of this trait, an index was constructed to select for the three reproductive traits and body weight using breast width as an indicator of the other traits. Economic weights were computed for each character with an economic weight of zero assigned to breast width.
561
562
R. E. COOK, W. L. BLOW, C. C. COCKERHAM AND E. W. GLAZENER
small fraction of turkeys which ever express the phenotype for reproductive performance. This was accomplished by
„
of selection with the index then the expected genetic change in egg production would be
ii
AGi =
—
•
— >
ci
ai=2.82,
a2=2.65,
a4 = 19.00,
a3 = 3.05,
a5 = zero.
Using these relative economic weights and the variances and covariances estimated from the sire components of the analyses, this index becomes /=.56Xi+.22X 2 +.29Xs+7.13X 4 -|-8.68X 6 . The first impression may be that greater emphasis is being placed on body weight and breast width in arriving at a total score or index value. However, the standard deviations of reproductive performance are so much larger than that of body weight, there may be a tendency for superiority in reproductive performance to outweigh the contribution of superiority in body weight to index score. The effects of selection when using the index on the direction and magnitude of change for each character is of major interest. Even though the use of the index may point the way to maximizing the overall rate of improvement, the use of the index may lead to undesirable consequences as far as one or more of the characters is concerned. An example of this would be a negative change in direction for one of the characters during the course of selection. If one standard deviation is the amount
AGi =1.76 eggs,
AG, = 1.34%,
AG3 =.44%,
AG 4 =.261bs.,
AG5= .05 in. Although there are slight negative correlations between some of the characters, these are not large enough to prevent some genetic progress for each of the characters included in the index. In terms of economic gain most of the benefits accruing from the use of this index result from changes in fecundity. When the genetic change for each character is weighted by the relative economic value for that character, the total relative gain from the three characters affecting reproduction is 9.85 compared with 4.94 for the change in body weight. In terms of economic reality the index derived here may have very little application. In order to approach a competitive edge if may be necessary for a turkey breeder to concentrate entirely on improving the body weight at a particular age if increases in reproductive performance do not permit him to sell more poults. Likewise, increased body weight may be of no advantage if a particular strain has established merit for this character, but is comparatively deficient for reproductive performance. In these cases the relative weight to be placed on various characters is a function of the level of performance at the time selection begins.
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multiplying the net worth of an additional pound of gain by nineteen. The relative economic values of the five characters then become:
where a^ is the variance of the index. The expected genetic change for each of the five characters in this study would be:
IMPROVEMENT OF TURKEYS SUMMARY
REFERENCES Abplanalp, H., and I. J. Kosin, 1952. Heritability of body measurements in turkeys. Poultry Sci. 31: 781-791. Berg, R. W., 1953. Some relationships of certain body measurements to meat yield and reproductive performance in turkeys. Unpublished Ph.D. thesis, University of Minnesota Library, March, 1953. Berry, C. N., and F. W. Frazier, 1945. "Egg-onomics," A study of factors in producing hatching
eggs and poults. Turkey World, 20: 20, 64-68. Bird, S., 1945. Measuring roundness of breast in live turkeys. U. S. Egg Poultry Magazine, 51: 206, 235. Blow, W. L., and E. W. Glazener, 1954. Heritability of egg production and egg weight in turkeys. Poultry Sci. 33: 417-421. Blow, W. L., H. A. Stewart and E. W. Glazener, 1958. Genetic variation and covariation of partial and complete records in turkeys. Poultry Sci. 37: 193-200. Comstock, R. E., and H. F. Robinson, 1948. The components of variance in populations of biparental progenies and their use in estimating the average degree of dominance. Biometrics, 4: 254266. Funk, E. M., 1950. The relation of body weight of turkey hens to egg production, egg weight, percentage of hatch and viability of poults. Poultry Sci. 29: 64-66. Hazel, L. N., 1943. Genetic basis for constructing selection indexes. Genetics, 28: 476-490. Hogsett, M. L., and A. W. Nordskog, 1948. Geneticeconomic value in selecting for egg production rate, body weight and egg weight. Poultry Sci. 37: 1404-1419. King, S. C , and C. R. Henderson, 1954. Heritability studies of egg production in the domestic fowl. Poultry Sci. 33: 155-169. Kondra, P. A., and R. N. Shoffner, 1955. Heritability of some body measurements and reproductive characters in turkeys. Poultry Sci. 34: 1262-1267. Maw, A. J. G., and M. G. McCartney, 1956. Variation in fertility of inbred chickens and of fertility and hatchability in White Holland turkeys. Poultry Sci. 35: 1185-1190. Shaklee, W. E., C. W. Knox and S. J. Marsden, 1952. Heritability of egg production in Beltsville Small White turkeys. Poultry Sci. 31: 935. Smith, H. F., 1936. A discriminant function for plant selection. Annals Eugenics, 7: 240-250. Wilson, W. O., and J. E. Johnson, 1946. The inheritance of egg production and hatchability in turkeys. Poultry Sci. 25: 278-284. Yamada, Y., 1958. Heritability and genetic correlations in economic characters in chickens. Japanese J. Genetics, 33: 13-22.
OCTOBER 30-NOVEMBER 1. \* ;STERN POULTRY CONGRESS, NATIONAL ORANGE SHOW GRO NTDS, SAN BERNARDINO, CALIF. OCTOBER 2-4. NEPPCO POULTRY EXPOSITION, FARM SHOW BUILDING, HARRISBURG, PA.
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Data from the records of 3,043 turkeys were used to estimate various genetic and phenotypic parameters. Genetic correlations between egg production, percent fertile eggs, hatchability of fertile eggs, body weight and breast width varied considerably as estimated from the sire and dam components of variance and covariance. The magnitude of the genetic correlation was rather small with more positive estimates being computed from the dam than from the sire components. Heritability estimates were considerably higher for the physical measurements than for the characters affecting reproduction. Genetic correlations estimated from the sire components were negative for egg production and fertility, egg production and body weight, fertility and hatchability, hatchability and body weight, and hatchability and breast width. There was a rather high positive correlation between body weight and fertility. A selection index for turkeys was derived which involved the characters in this study. This index provides an opportunity for a positive change in each of the characters for which selection is being practiced.
563