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A Genetic Analysis of Growth and Egg Production in Meat-Type Chickens
A Genetic Analysis of Growth and Egg Production in Meat-Type Chickens R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
Ohio Agricultural Experiment Station, Columbus 10, Ohio (Received for publication February 5, 1962)
1. GENETIC PARAMETERS IN THE RANDOMBRED WHITE GOLD POPULATION
The composition and methods used for maintenance of the Ohio Randombred White Gold chickens were described by Goodman and Jaap (1960b). Genetic parameters have been calculated from progenies of diallel matings as explained by Goodman and Jaap (1961). Data from growth measured by body weight at 8, 16 and 24 weeks of age have been collected and analyzed in a similar fashion. The genetic, enviromental and phenotypic correlations between these body weight data and egg production characters have been calculated from the differences between and within full sister families. The only previous report using a randombred population of chickens for a genetic analysis of body weight and egg production traits is that of
King (1961) using the egg-type Regional Cornell Randombred White Leghorn. (a) Growth Rate. Sixty-three lots of 120 to 200 female chickens were started at three-week intervals between August 25, 1955 and March 19, 1959. Body weights at 8, 16 and 24 weeks of age are summarized by yearly (54-week) groups in Table 1. Each group of 18 hatches includes data from 2500 to 3500 pullets. Eight-week body weight of these females remained remarkably constant during this period of over 3^ years. Environmental conditions as feeding and management were fairly uniform. Therefore, little change in growth was expected from environmental sources. These data in Table 1 indicate that there was no genetic change in growth rate to eight weeks of age and that the large number of sires and dams was fairly effective in preventing genetic drift. In contrast, it appears that body weights at 16 and 24 weeks of age may have declined, during the last two years summarized in Table 1. We are not certain whether this depression in weight of the two latter groups could have resulted from chronic respiratory disease having a greater effect after eight weeks of age toward the latter part of this J>\ year period. There was much variation in body weights between hatched groups. Therefore, all body weight variances were calculated using deviations in hundredths of a pound from the mean of their tri-weekly hatch group. Also, only the first two pullets hatched from each paired mating were used in the analyses of the diallel matings. The genetic variance was subdivided into three
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HAT economically important characters might be affected by rapidly increasing rate of growth from selection for increased body weight at eight weeks of age? The results may be predicted from the genetic parameters in a randombred population. The validity of these parameters may be checked by measuring the response as well as the correlated responses obtianed from a selection experiment. The first part of this report is concerned with the genetic parameters in the Ohio Randombred White Gold meat-type chickens. The second part reports experiments which measure the increase in eight-week body weight and the correlated egg production characters after growth rate has been increased by selection.
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
TABLE 1.—Average body weights (lbs.) of randombred females in successive 54-week groups of IS hatches from 1955 into 1959 Body Weights Hatches 8 Weeks 1-18 19-36 37-54 55-63
1.92 1.88 1.90 1.89
16 Weeks 24 Weeks 4.27 4.20 4.07 4.12
5.83 5.86 5.77 5.66
TABLE 2.—Phenotypic variances (P) and heritaabilities Qfi) for body weight of females in pounds Age in . Weeks 8 16 24
P
h„2
W
hi 2
.0637 .2050 .4262
.47 .71 .61
.85 .53 .20
.26 .21 .20
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components and expressed as heritable fractions of the total. Heritability could be estimated from the following sources: Between sire families (h s 2 ), between dam families (ha2) and sire X dam interaction (hi 2 ). The phenotypic variances (P) together with these three heritability estimates are given in Table 2. Phenotypic variance (P) for body weight of females at eight weeks of age (.0637) was relatively large in this randombred population. Unpublished values of P for females of strains A and B used by Goodman and Jaap (1960a) were .0480 and .0394, respectively, at this age. It is probable that the phenotypic variance in this population may have been higher than that of most meat-type strains under selection and closed for more generations. Maternal effects on body weight were relatively high at eight weeks of age, ha2 being .85 as compared with .47 for hs2. Part of these maternal effects may be attributable to size of egg from which the chick hatched (unpublished data). By 16 and 24 weeks of age the sire component of the variance exceeded that from the dam demonstrating that sex-linked genetic effects were more important than maternal effects at these ages. The method used in calculating h s 2 (four times the measurable fraction from sires) should overestimate additive effects of sex-linked genes because the sire component of the variance contains one-half rather than one quarter of the total when the data pertain
to female progeny. Assuming no maternal effects, sex-linked genes accounted for approximately 10 percent of the phenotypic variance at 16 weeks and 20 percent at 24 weeks of age. These estimates are derived by substracting hd2 from hs2 and dividing by two. The fraction of the phenotypic variance (P) estimated to have arisen from gene interaction (hi2, Table 2) was .26, .21 and .20 at 8, 16 and 24 weeks, respectively. These amount to one-fourth to one-fifth of the total genetic effects. It is possible that this relatively large amount of genetic variance from non-additive genetic effects may have arisen from the recent crossbred ancestry. All data accumulating from the time the population was closed were used in the calculations. Goodman and Jaap (1960a) found very little, if any, non-additive effects in two populations which had been closed and under selection for growth rate over 20 generations. It is possible that much of this non-additive genetic variance may have arisen from epistatic combinations of gene blocks brought into the population from its diverse ancestry. (b) Egg Characters. Heritabilities for egg production, egg weight and egg shape index have been reported by Goodman and Jaap (1961). Table 3 gives the means (X) and phenotypic variances (P) which were not given in that report together with heritabilities estimated from the sire components of the variance (h s 2 ). The latter should provide the best estimate of the
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ANALYSIS OF GROWTH AND PRODUCTION TABLE 3.—Means (x), phenotypic variance (P) and heritabilities (hs2) from the sire component for egg characters in the Randombred White Gold chicken Character No. of eggs laid in 69 trap days from 23rd to 46th week 30th week egg weight (gm.) 30th week egg shape (100W/L) 30th week egg albumen height (mm.)
x
29 52.6 74 7.28
P
h82
122.91 23.31
.28 .60
12.76
.11
1.72
.05
(c) Correlations. Complete data for body weight at the three ages, as well as the number, weight, shape and albumen height
TABLE 4.—Genetic (rg), environmental (re) and phenotypic (r) correlations plus a calculated penotypic correlation (rp) body weight and egg production characters 8 Wk: body wt. and 16-wk. body wt. 24-wk. Body wt. Egg prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height a t 30 wks.
r
s
re
r
.77 .36 .15 .25 .17 .13
.33 .33 -.07 -.15 -.22 .02
64 33 03 08 - 01 08
.60 .09 .28 .05 .00
.53 -.02 -.25 -.10 .11
57 03 09 - 01 05
.10 .20 .06 -.09
-.13 -.23 -.15 .30
02 09 00 04
-.16 .12 .00
.12 .04 -.01
01 07 - 03
.22 .12
-.10 .19
.05 .16
.20
.20
.20
16-wk. body wt. and 24 wk. body wt. Egg. prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. 24 wk. body wt. and Egg prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. Egg prod, to 46 wks. and Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. Egg wt. at 30 wks. and Egg shape a t 30 wks. Albumen height a t 30 wks. Egg shape a t 30 wks. and Albumen height at 30 wks.
r
P
.57 .34 .04 .12 .01
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total additive genetic effects of both autosomal and sex-linked genes. The phenotypic variance (P) for egg production of survivors at 46 weeks of age is in terms of number of eggs laid in 69 trapnesting days, three per week during the laying period from 23 to 46 weeks of age. Egg production during the years these data were collected was low as indicated by the mean production of 42 percent. A rate of 62 percent production was obtained for a similar age period from the same population in a later test. Much of the environmental depression of egg production in this earlier test appeared to have resulted from chronic respiratory disease. Egg shape was measured as the width divided by length and multiplied by 100. Egg weight was recorded in grams and albumen height in millimeters. The parameters (P) and (hs2) will be used later for prediction of the amount of change expected from the genetic correlations with growth rate. It was pointed out in the previous report (Goodman and Jaap, 1961) that sex-linked genetic effects were indicated for egg production from the 23rd to the 46th week as well as egg weight when the pullets were 30 weeks of age. For this reason the hs2 values given in Table 3 are apt to be greater than the true genetic value of all additive effects in females.
of eggs, were available for 1176 females produced by 397 paired matings involving 217 dams and 211 sires. The mean number of daughters per full sister family was 2.96. The genetic, environmental and phenotypic correlations calculated from these data are presented in Table 4. The model used for these calculations was that given by Lerner (1950, page 235) using twice the covariance between full sister families as the numerator of the fraction. Correlations between weights at the three ages were all positive (Table 4). The genetic correlation (rg) between 8 and 24 week body weights was relatively low (.36) in comparison to those between 8 and 16 weeks (.77), and 16 and 24 weeks (.60). This may indicate some difference in genetic effects on the latter part of this growth period from those acting prior to eight weeks of age. It should be emphasized that part of the
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
In this formula rp is the expected phenotypic correlation for traits x and y when h is the square root of their respective heritabilities, ra is the additive genetic correlation, and re the environmental correlation when e2 is 1-h2. In our calculations we used the values of h s 2 from Tables 2 and 3 as the preferred measure for h in the above formula. Likewise, rg from Table 4 was used as the only available estimate of ra. These expected phenotypic correlations are included in Table 4 for ease of comparison. The differences between r and rp (Table 4) were relatively small. Apparently nonadditive effects included in the estimation of the genetic correlation, rg, did not cause much bias in prediction of the phenotypic correlation, rp. This information has provided additional confidence in the value of these heritabilities and correlations for use in calculating the response expected from selection experiments. (d) Correlated Responses Expected. Since all of the genetic correlations with eight-week body weight were positive, increases in egg production, egg weight, broadness of eggs (egg shape) and height of albumen would be expected if rate of growth were increased by selection for in-
TABLE 5.—Changes expected and realized from an eight-week body weight selection differential of four standard deviations Changes Expected
Realized
+3.9 +2.8 +0.6 +0.1
-5.0 +2.8 + 1.9 +0.3
Egg production (%) Egg weight (gm.) Egg broadness (index) Albumen height (mm.)
creased weight at eight weeks of age. The amount of expected change (CRy) in a correlated character (y) which should accompany genetic changes in eight-week body weight may be calculated from the data in the foregoing tables by means of the following formula (Falconer, 1960): CRy = i hx hy rg apy In this formula i represents the selection differential in terms of units of standard deviation for weight at eight weeks, rg is the genetic correlation from Table 4, and cpy is the phenotypic standard deviation for the correlated trait y. One standard deviation for female body weight is 0.25 lb. at eight weeks of age when the phenotypic variance is 0.0637 (P, Table 2). The phenotypic variance for males is greater than that for females. Although it fluctuates considerably among different broods, a variance of 0.11 would appear to be comparable to 0.06 for females. Using these estimates, the selection differential (i) per generation was approximately 0.7 of a standard deviation for dams TABLE 6.—8-week body weights of 4th generation selected and Randombred While Gold chickens Number Line
Body Weight (lbs.)
Males Females Growth Selected Randombred Genetic gain
240 208
231 237
Males
Females
3.02 2.49
2.54 2.12
0.53
0.42
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environmental correlations is due to correlations between non-additive genetic effects demonstrated to be present at all three ages on body weight as well as egg weight and egg shape. The relative importance of non-additive genetic effects on the correlations may be indicated by the discrepancy between the actual phenotypic correlation (r) and that expected (rp) from the genetic and environmental correlations. Therefore, the expected phenotypic correlations with eight-week body weight were calculated using the following formula from Falconer (1960): rp = hx hy ra + ex ey re
ANALYSIS OF GROWTH AND PRODUCTION
lb. and females were 0.17 lb. above their hatch group. This would be a selection differential (i) of about 0.7 of the standard deviation for females and 1.4 for males. The average selection intensity for both sexes was, therefore, slightly in excess of one standard deviation per generation. The expected gain in eight-week weight per generation calculated by multiplying h s 2 by the selection differential would be about 0.14 lb. average for both sexes. The amount of gain in eight-week weight in the selected line, after four generations of selection, was measured by the increase in body weight when brooded intermingled with chicks of the Randombred line. The results of this test are given in Table 6. The gain in weight of the selected line was 0.53 lb. for males and 0.42 lb. for females 2. GROWTH AND CORRELATED RESPONSES or an average of 0.48 for both sexes. In A growth selected line was started from view of the fact that the selection differenthe first closed generation of the Random- tial could not be calculated accurately, we bred White Gold population. Selection was consider the expected gain of 0.56 lb. was based solely upon the amount each sex ex- a reasonable approximation of that obtained ceeded the average weight for that sex from the selection experiment. within each hatch group at eight weeks of age. Two hatches two weeks apart produced 500 to 700 chickens in each generation. About half of the needed breeders were selected from each hatch group. The growth-selected parents were randomly mated in flock matings involving 32 sires in first, second and fourth generation. Only 21 sires were used as parents of the third selected generation. The number of dams varied from 120 to 160. The number of sires was greater than that commonly used in commercial practice in order to minimize changes in genetic constitution which might arise from gene drift or inbreeding depression. (a) The Growth Response. Since flock Age [months] matings were used the actual selection difFIG. 1. Growth of White Gold pullets from Seferential could not be measured accurately. lected (S X S), Randombred (R X R ) , Selected At eight weeks of age males selected for sires by Randombred dams (S X R ) , and Ranbreeders exceeded their flock mates by 0.45 dombred sires by Selected dams (R X S).
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and 1.4 for sires, or an average of 1.1 for both sexes. Table 5 gives the changes in egg production, egg weight, egg broadness and albumen height expected when the accumulated selection differential is four standard deviations of body weight. Four standard deviations was the approximate accumulated selection differential used to produce the females of a growth-selected line for measurement of the correlated changes in these traits (Fig. 1, and Tables 8 and 9). The changes realized in these test females are included in Table 5 for direct comparison with those expected. The design of the experiment in which these realized responses were measured is reported in the following section.
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
TABLE 7.—8-week body weight of White Gold and commercial broiler crossbreds in 1959 Body Weight
SX S SX R
Males Females < White Gold Growth Selected Line 2.97 Commercial Crossbred A 3.01 Commercial CrossbredB 2.88
2.51 2.46 2.37
number of female progeny of each type surviving to 12 months of age were:
2.74 2.74 2.62
R X S R X R
162 172
These females were brooded and reared intermingled. At 22 weeks of age, one-eighth of each of the four types were pro-rated at radom to eight pens for egg production records. Body weights were recorded when the females were 9, 16 and 24 weeks of age and monthly thereafter through the eleventh month after hatching. These growth data are presented graphically in Figure 1. At nine weeks of age the S X S females exceeded the weight of the R X R females by 0.55 lb. The maximum weight difference between the S and R lines was reached by the fifth month after hatching. From the fifth to the eleventh month both lines gained about the same amount, 1.34 and 1.35 lb. Therefore, the increased growth rate resulting from selection for large size at eight weeks of age ceased after the fifth month of the growing period. The gain of 0.55 lb. at nine weeks of age resulted in an increase of 0.84 lb. in adult body weight. These post selection age-responses jvere predicted from the genetic correlations between 8, 16 and 24 week body weights in the Randombred line. The S X R cross (Figure 1) grew more rapidly than the R X S. Since the S sires producing the S X R pullets were highly selected in contrast to unselected S pullets used to produce the R X S line, S X R would have been expected to grow more rapidly than R X S. The difference in weight of these reciprocals at 9 weeks was .11 lb. which is even less than might be expected with such a high selection differential in S sires. Apparently most of the difference between the S and R lines, therefore, was due to additive gene effects. Since dominance of sex-linked genes is impossi-
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Since the growth rate of the Randombred White Gold was equal to that of the Lancaster X Nichols 12 in the first closed generation (19SS), the Growth-selected line was compared with two popular broiler crosses in 1959. Six hundred chicks, 300 of each sex, of each kind were brooded intermingled within sex. The results of these comparisons are presented in Table 7. After four generations of selection the Growthselected White Gold line grew at a rate equal to that of the superior commercial crossbred A and better than that of B. Therefore, commercial poultry breeders were estimated to have increased the eightweek weight of broilers by slightly less than one-half pound between 195S and 1959. (b) Correlated Responses. Tests for responses correlated with the change in growth rate of the Selected line were made with females. Their sires were highly selected individuals, a selection differential of over £ lb. from the fourth selected generation, and the dams were unselected females from the third generation. The test pullets were produced both as pure lines as well as reciprocal crosses between the Growth-selected (S) and the Randombred (R) lines. The S X S and S X R progeny were from the same 16 S sires used to artificially inseminate three to four hens per sire in each of the groups of S and R females, respectively. Likewise R X R and R X S were produced from 16 randomly chosen R sires used in inseminate 50 to 60 hens in each of the R and S lines. The
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ANALYSIS OF GROWTH AND PRODUCTION TABLE 8.—Survivors' egg production between the 25th and 48th week of age Type of Females
Egg Production (%)
SXS SXR RXS RXR
57 59 60 62
( R X R , Table 8) was high, 62%. A second test of the egg production of the R and S lines was made using the fourth-generation S pullets whose eightweek body weight was given in Table 6. Triplicate pens of each line were used and pen records of egg production were recorded for six months following the 25th week after hatching. The average monthly hen-day production during this period was 47% for the 226 pullets housed from the S line and 56% for 214 pullets housed from the R line. The spread between the two lines was greater (9%) than the 5% recorded in the first test. There is no question that the genetic correlation between eight-week weight and egg production is negative. A genetic correlation of —0.19 would be necessary to produce a correlated decrease (CRy) of 5% in egg production. The correlated responses measured for egg weight, egg shape and albumen height were in the same direction as those expected (Table 5). The data for eggs are presented in Table 9. Eggs from the S X S pullets at 32 weeks of age exceeded those from the R X R line by 2.8 gm. in weight, they were 1.9 units broader and albumen of the the broken-out egg was 0.3 mm. higher. A selection differential of one standard deviation in 8-week body weight had been used for four generations. The S X S would have been expected to exceed R X R (Table 5) by 2.8 gm. in egg weight, 0.6 units in broadness (egg shape) and about 0.1 mm. in albumen height. On this basis the change TABLE 9.—Egg weight, egg shape and albumen height for females recorded the 32nd week after hatching Egg Weight (gm.)
Egg Shape (ratio)
Albumen Height (mm.)
58.9 59.5 56.8 56.1
75.9 75.4 74.5 74.0
7.70 7.42 7.70 7.40
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ble because the female has only one sex chromosome, it is possible that some of these additive effects could be attributable to sex-linked genes received from the father. The egg production of the two lines and their reciprocal crosses is given in Table 8 as a percentage of eggs out of the 69 days of trapnesting (three times per week from the 23rd through the 46th week of age). An increase in egg production of the S X S line was predicted from the genetic correlation (Table 4). The realized response, however, was lowered egg production (57%) as compared with the R X R control (62%). Further confirmation of this negative response is provided by the intermediate egg production of the reciprocal crosses. Since the genetic correlation between eight-week body weight and egg production was low (.15) and the confidence interval for such correlations are large, this reversal from a positive estimate to a negative response is explained as a chance deviation in the crude estimation. However, an interesting observation by Hicks (1958) is analagous to this positive estimate and negative response. Hicks obtained a positive genetic correlation between mature body weight and egg production when egg production was poor. In contrast, in years when egg production was superior (normal?) he obtained negative correlations. In our experiments, a positive correlation was obtained when egg production was low (42%) and a negative response occurred when egg production of the Randombred
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
crease in egg production was slightly more than one percent per one-tenth pound gain in eight-week body weight. The increase in egg weight was identical with that predicted from the genetic parameters calculated for the randombred parental line. The realized responses in broadness of egg and increase in height of albumen were greater than those predicted. Poultry breeders were estimated to have made a genetic gain of almost \ lb. in eight-week weight of broilers produced commercially between 1955 and 1959. REFERENCES
SUMMARY
Genetic parameters were calculated for growth and egg production characteristics of the randombred population of White Gold meat-type chickens. These were compared with the responses obtained in a line selected to grow much more rapidly than the randombred line from which it originated. Characters considered were body weight at 8, 16 and 24 weeks of age; egg production between the 23rd and 46th week; and egg weight, shape and height of the albumen of eggs laid during the 30th week after hatching. Genetic effects on all characters were estimated to be positively correlated with eight-week body weight in the randombred population. However, egg production proved to be negatively correlated with the genetic change in body weight. The de-
Falconer, D. S., 1960. Introduction to Quantitative Genetics. The Ronald Press Company, New York 10, N.Y. Goodman, B. L., and R. G. Jaap, 1960a. Improving accuracy of heritability estimates from diallel and triallel matings in poultry. 1. Poultry Sci. 39: 938-944. Goodman, B. L., and R. G. Jaap, 1960b. Improving accuracy of heritability estimates from diallel and triallel matings in poultry. 2. Poultry Sci. 39: 944-949. Goodman, B. L., and R. G. Jaap, 1961. Non-additive and sex-linked genetic effects on egg production in a randombred population. Poultry Sci. 40: 662-668. Hicks, A. F., Jr., 19S8. The interrelationships of mature body weight with egg number, egg weight and egg shape in chickens. Poultry Sci. 37: 1211. King, S. C , 1961. Inheritance of economic traits in the Regional Cornell control population. Poultry Sci. 40: 97S-986.
NEWS AND NOTES (Continued from page 1438) the Agricultural Research Service. In 1955 he became Assistant Director of Livestock Research, and in 1957 Deputy Administrator for Farm Research. While continuing as a member of the Animal Husbandry Division staff, he served from 1944 to 1946 as Chief of the Livestock Branch of the United Nations Relief and Rehabilitation Administration.
In 1943 Dr. Byerly received the Borden Award, administered by the Poultry Science Association, for his contributions to poultry science, especially in the field of avian physiology, including incubation, endocrinology, and feed utilization. He is a Fellow of the Poultry Science Association and of the American Association for the Advancement of Science, and a member of the American Society of
(Continued on page 1473)
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in egg weight of the S line was the same as that expected while the changes in egg shape and albumen height were greater than predicted. The higher values for egg weight and shape in S X R (Table 9) than that of R X S would have been expected from their relative growth rates (Figure 1) and tend to add confirmation to the validity of the genetic correlations. Albumen height in eggs from these reciprocals did not conform to expectation. R X S resembled the S parent and S X R the R parental line.
Ohio Agricultural Experiment Station, Columbus 10, Ohio (Received for publication February 5, 1962)
1. GENETIC PARAMETERS IN THE RANDOMBRED WHITE GOLD POPULATION
The composition and methods used for maintenance of the Ohio Randombred White Gold chickens were described by Goodman and Jaap (1960b). Genetic parameters have been calculated from progenies of diallel matings as explained by Goodman and Jaap (1961). Data from growth measured by body weight at 8, 16 and 24 weeks of age have been collected and analyzed in a similar fashion. The genetic, enviromental and phenotypic correlations between these body weight data and egg production characters have been calculated from the differences between and within full sister families. The only previous report using a randombred population of chickens for a genetic analysis of body weight and egg production traits is that of
King (1961) using the egg-type Regional Cornell Randombred White Leghorn. (a) Growth Rate. Sixty-three lots of 120 to 200 female chickens were started at three-week intervals between August 25, 1955 and March 19, 1959. Body weights at 8, 16 and 24 weeks of age are summarized by yearly (54-week) groups in Table 1. Each group of 18 hatches includes data from 2500 to 3500 pullets. Eight-week body weight of these females remained remarkably constant during this period of over 3^ years. Environmental conditions as feeding and management were fairly uniform. Therefore, little change in growth was expected from environmental sources. These data in Table 1 indicate that there was no genetic change in growth rate to eight weeks of age and that the large number of sires and dams was fairly effective in preventing genetic drift. In contrast, it appears that body weights at 16 and 24 weeks of age may have declined, during the last two years summarized in Table 1. We are not certain whether this depression in weight of the two latter groups could have resulted from chronic respiratory disease having a greater effect after eight weeks of age toward the latter part of this J>\ year period. There was much variation in body weights between hatched groups. Therefore, all body weight variances were calculated using deviations in hundredths of a pound from the mean of their tri-weekly hatch group. Also, only the first two pullets hatched from each paired mating were used in the analyses of the diallel matings. The genetic variance was subdivided into three
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HAT economically important characters might be affected by rapidly increasing rate of growth from selection for increased body weight at eight weeks of age? The results may be predicted from the genetic parameters in a randombred population. The validity of these parameters may be checked by measuring the response as well as the correlated responses obtianed from a selection experiment. The first part of this report is concerned with the genetic parameters in the Ohio Randombred White Gold meat-type chickens. The second part reports experiments which measure the increase in eight-week body weight and the correlated egg production characters after growth rate has been increased by selection.
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
TABLE 1.—Average body weights (lbs.) of randombred females in successive 54-week groups of IS hatches from 1955 into 1959 Body Weights Hatches 8 Weeks 1-18 19-36 37-54 55-63
1.92 1.88 1.90 1.89
16 Weeks 24 Weeks 4.27 4.20 4.07 4.12
5.83 5.86 5.77 5.66
TABLE 2.—Phenotypic variances (P) and heritaabilities Qfi) for body weight of females in pounds Age in . Weeks 8 16 24
P
h„2
W
hi 2
.0637 .2050 .4262
.47 .71 .61
.85 .53 .20
.26 .21 .20
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components and expressed as heritable fractions of the total. Heritability could be estimated from the following sources: Between sire families (h s 2 ), between dam families (ha2) and sire X dam interaction (hi 2 ). The phenotypic variances (P) together with these three heritability estimates are given in Table 2. Phenotypic variance (P) for body weight of females at eight weeks of age (.0637) was relatively large in this randombred population. Unpublished values of P for females of strains A and B used by Goodman and Jaap (1960a) were .0480 and .0394, respectively, at this age. It is probable that the phenotypic variance in this population may have been higher than that of most meat-type strains under selection and closed for more generations. Maternal effects on body weight were relatively high at eight weeks of age, ha2 being .85 as compared with .47 for hs2. Part of these maternal effects may be attributable to size of egg from which the chick hatched (unpublished data). By 16 and 24 weeks of age the sire component of the variance exceeded that from the dam demonstrating that sex-linked genetic effects were more important than maternal effects at these ages. The method used in calculating h s 2 (four times the measurable fraction from sires) should overestimate additive effects of sex-linked genes because the sire component of the variance contains one-half rather than one quarter of the total when the data pertain
to female progeny. Assuming no maternal effects, sex-linked genes accounted for approximately 10 percent of the phenotypic variance at 16 weeks and 20 percent at 24 weeks of age. These estimates are derived by substracting hd2 from hs2 and dividing by two. The fraction of the phenotypic variance (P) estimated to have arisen from gene interaction (hi2, Table 2) was .26, .21 and .20 at 8, 16 and 24 weeks, respectively. These amount to one-fourth to one-fifth of the total genetic effects. It is possible that this relatively large amount of genetic variance from non-additive genetic effects may have arisen from the recent crossbred ancestry. All data accumulating from the time the population was closed were used in the calculations. Goodman and Jaap (1960a) found very little, if any, non-additive effects in two populations which had been closed and under selection for growth rate over 20 generations. It is possible that much of this non-additive genetic variance may have arisen from epistatic combinations of gene blocks brought into the population from its diverse ancestry. (b) Egg Characters. Heritabilities for egg production, egg weight and egg shape index have been reported by Goodman and Jaap (1961). Table 3 gives the means (X) and phenotypic variances (P) which were not given in that report together with heritabilities estimated from the sire components of the variance (h s 2 ). The latter should provide the best estimate of the
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ANALYSIS OF GROWTH AND PRODUCTION TABLE 3.—Means (x), phenotypic variance (P) and heritabilities (hs2) from the sire component for egg characters in the Randombred White Gold chicken Character No. of eggs laid in 69 trap days from 23rd to 46th week 30th week egg weight (gm.) 30th week egg shape (100W/L) 30th week egg albumen height (mm.)
x
29 52.6 74 7.28
P
h82
122.91 23.31
.28 .60
12.76
.11
1.72
.05
(c) Correlations. Complete data for body weight at the three ages, as well as the number, weight, shape and albumen height
TABLE 4.—Genetic (rg), environmental (re) and phenotypic (r) correlations plus a calculated penotypic correlation (rp) body weight and egg production characters 8 Wk: body wt. and 16-wk. body wt. 24-wk. Body wt. Egg prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height a t 30 wks.
r
s
re
r
.77 .36 .15 .25 .17 .13
.33 .33 -.07 -.15 -.22 .02
64 33 03 08 - 01 08
.60 .09 .28 .05 .00
.53 -.02 -.25 -.10 .11
57 03 09 - 01 05
.10 .20 .06 -.09
-.13 -.23 -.15 .30
02 09 00 04
-.16 .12 .00
.12 .04 -.01
01 07 - 03
.22 .12
-.10 .19
.05 .16
.20
.20
.20
16-wk. body wt. and 24 wk. body wt. Egg. prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. 24 wk. body wt. and Egg prod, to 46 wks. Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. Egg prod, to 46 wks. and Egg wt. at 30 wks. Egg shape at 30 wks. Albumen height at 30 wks. Egg wt. at 30 wks. and Egg shape a t 30 wks. Albumen height a t 30 wks. Egg shape a t 30 wks. and Albumen height at 30 wks.
r
P
.57 .34 .04 .12 .01
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total additive genetic effects of both autosomal and sex-linked genes. The phenotypic variance (P) for egg production of survivors at 46 weeks of age is in terms of number of eggs laid in 69 trapnesting days, three per week during the laying period from 23 to 46 weeks of age. Egg production during the years these data were collected was low as indicated by the mean production of 42 percent. A rate of 62 percent production was obtained for a similar age period from the same population in a later test. Much of the environmental depression of egg production in this earlier test appeared to have resulted from chronic respiratory disease. Egg shape was measured as the width divided by length and multiplied by 100. Egg weight was recorded in grams and albumen height in millimeters. The parameters (P) and (hs2) will be used later for prediction of the amount of change expected from the genetic correlations with growth rate. It was pointed out in the previous report (Goodman and Jaap, 1961) that sex-linked genetic effects were indicated for egg production from the 23rd to the 46th week as well as egg weight when the pullets were 30 weeks of age. For this reason the hs2 values given in Table 3 are apt to be greater than the true genetic value of all additive effects in females.
of eggs, were available for 1176 females produced by 397 paired matings involving 217 dams and 211 sires. The mean number of daughters per full sister family was 2.96. The genetic, environmental and phenotypic correlations calculated from these data are presented in Table 4. The model used for these calculations was that given by Lerner (1950, page 235) using twice the covariance between full sister families as the numerator of the fraction. Correlations between weights at the three ages were all positive (Table 4). The genetic correlation (rg) between 8 and 24 week body weights was relatively low (.36) in comparison to those between 8 and 16 weeks (.77), and 16 and 24 weeks (.60). This may indicate some difference in genetic effects on the latter part of this growth period from those acting prior to eight weeks of age. It should be emphasized that part of the
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
In this formula rp is the expected phenotypic correlation for traits x and y when h is the square root of their respective heritabilities, ra is the additive genetic correlation, and re the environmental correlation when e2 is 1-h2. In our calculations we used the values of h s 2 from Tables 2 and 3 as the preferred measure for h in the above formula. Likewise, rg from Table 4 was used as the only available estimate of ra. These expected phenotypic correlations are included in Table 4 for ease of comparison. The differences between r and rp (Table 4) were relatively small. Apparently nonadditive effects included in the estimation of the genetic correlation, rg, did not cause much bias in prediction of the phenotypic correlation, rp. This information has provided additional confidence in the value of these heritabilities and correlations for use in calculating the response expected from selection experiments. (d) Correlated Responses Expected. Since all of the genetic correlations with eight-week body weight were positive, increases in egg production, egg weight, broadness of eggs (egg shape) and height of albumen would be expected if rate of growth were increased by selection for in-
TABLE 5.—Changes expected and realized from an eight-week body weight selection differential of four standard deviations Changes Expected
Realized
+3.9 +2.8 +0.6 +0.1
-5.0 +2.8 + 1.9 +0.3
Egg production (%) Egg weight (gm.) Egg broadness (index) Albumen height (mm.)
creased weight at eight weeks of age. The amount of expected change (CRy) in a correlated character (y) which should accompany genetic changes in eight-week body weight may be calculated from the data in the foregoing tables by means of the following formula (Falconer, 1960): CRy = i hx hy rg apy In this formula i represents the selection differential in terms of units of standard deviation for weight at eight weeks, rg is the genetic correlation from Table 4, and cpy is the phenotypic standard deviation for the correlated trait y. One standard deviation for female body weight is 0.25 lb. at eight weeks of age when the phenotypic variance is 0.0637 (P, Table 2). The phenotypic variance for males is greater than that for females. Although it fluctuates considerably among different broods, a variance of 0.11 would appear to be comparable to 0.06 for females. Using these estimates, the selection differential (i) per generation was approximately 0.7 of a standard deviation for dams TABLE 6.—8-week body weights of 4th generation selected and Randombred While Gold chickens Number Line
Body Weight (lbs.)
Males Females Growth Selected Randombred Genetic gain
240 208
231 237
Males
Females
3.02 2.49
2.54 2.12
0.53
0.42
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environmental correlations is due to correlations between non-additive genetic effects demonstrated to be present at all three ages on body weight as well as egg weight and egg shape. The relative importance of non-additive genetic effects on the correlations may be indicated by the discrepancy between the actual phenotypic correlation (r) and that expected (rp) from the genetic and environmental correlations. Therefore, the expected phenotypic correlations with eight-week body weight were calculated using the following formula from Falconer (1960): rp = hx hy ra + ex ey re
ANALYSIS OF GROWTH AND PRODUCTION
lb. and females were 0.17 lb. above their hatch group. This would be a selection differential (i) of about 0.7 of the standard deviation for females and 1.4 for males. The average selection intensity for both sexes was, therefore, slightly in excess of one standard deviation per generation. The expected gain in eight-week weight per generation calculated by multiplying h s 2 by the selection differential would be about 0.14 lb. average for both sexes. The amount of gain in eight-week weight in the selected line, after four generations of selection, was measured by the increase in body weight when brooded intermingled with chicks of the Randombred line. The results of this test are given in Table 6. The gain in weight of the selected line was 0.53 lb. for males and 0.42 lb. for females 2. GROWTH AND CORRELATED RESPONSES or an average of 0.48 for both sexes. In A growth selected line was started from view of the fact that the selection differenthe first closed generation of the Random- tial could not be calculated accurately, we bred White Gold population. Selection was consider the expected gain of 0.56 lb. was based solely upon the amount each sex ex- a reasonable approximation of that obtained ceeded the average weight for that sex from the selection experiment. within each hatch group at eight weeks of age. Two hatches two weeks apart produced 500 to 700 chickens in each generation. About half of the needed breeders were selected from each hatch group. The growth-selected parents were randomly mated in flock matings involving 32 sires in first, second and fourth generation. Only 21 sires were used as parents of the third selected generation. The number of dams varied from 120 to 160. The number of sires was greater than that commonly used in commercial practice in order to minimize changes in genetic constitution which might arise from gene drift or inbreeding depression. (a) The Growth Response. Since flock Age [months] matings were used the actual selection difFIG. 1. Growth of White Gold pullets from Seferential could not be measured accurately. lected (S X S), Randombred (R X R ) , Selected At eight weeks of age males selected for sires by Randombred dams (S X R ) , and Ranbreeders exceeded their flock mates by 0.45 dombred sires by Selected dams (R X S).
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and 1.4 for sires, or an average of 1.1 for both sexes. Table 5 gives the changes in egg production, egg weight, egg broadness and albumen height expected when the accumulated selection differential is four standard deviations of body weight. Four standard deviations was the approximate accumulated selection differential used to produce the females of a growth-selected line for measurement of the correlated changes in these traits (Fig. 1, and Tables 8 and 9). The changes realized in these test females are included in Table 5 for direct comparison with those expected. The design of the experiment in which these realized responses were measured is reported in the following section.
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
TABLE 7.—8-week body weight of White Gold and commercial broiler crossbreds in 1959 Body Weight
SX S SX R
Males Females < White Gold Growth Selected Line 2.97 Commercial Crossbred A 3.01 Commercial CrossbredB 2.88
2.51 2.46 2.37
number of female progeny of each type surviving to 12 months of age were:
2.74 2.74 2.62
R X S R X R
162 172
These females were brooded and reared intermingled. At 22 weeks of age, one-eighth of each of the four types were pro-rated at radom to eight pens for egg production records. Body weights were recorded when the females were 9, 16 and 24 weeks of age and monthly thereafter through the eleventh month after hatching. These growth data are presented graphically in Figure 1. At nine weeks of age the S X S females exceeded the weight of the R X R females by 0.55 lb. The maximum weight difference between the S and R lines was reached by the fifth month after hatching. From the fifth to the eleventh month both lines gained about the same amount, 1.34 and 1.35 lb. Therefore, the increased growth rate resulting from selection for large size at eight weeks of age ceased after the fifth month of the growing period. The gain of 0.55 lb. at nine weeks of age resulted in an increase of 0.84 lb. in adult body weight. These post selection age-responses jvere predicted from the genetic correlations between 8, 16 and 24 week body weights in the Randombred line. The S X R cross (Figure 1) grew more rapidly than the R X S. Since the S sires producing the S X R pullets were highly selected in contrast to unselected S pullets used to produce the R X S line, S X R would have been expected to grow more rapidly than R X S. The difference in weight of these reciprocals at 9 weeks was .11 lb. which is even less than might be expected with such a high selection differential in S sires. Apparently most of the difference between the S and R lines, therefore, was due to additive gene effects. Since dominance of sex-linked genes is impossi-
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Since the growth rate of the Randombred White Gold was equal to that of the Lancaster X Nichols 12 in the first closed generation (19SS), the Growth-selected line was compared with two popular broiler crosses in 1959. Six hundred chicks, 300 of each sex, of each kind were brooded intermingled within sex. The results of these comparisons are presented in Table 7. After four generations of selection the Growthselected White Gold line grew at a rate equal to that of the superior commercial crossbred A and better than that of B. Therefore, commercial poultry breeders were estimated to have increased the eightweek weight of broilers by slightly less than one-half pound between 195S and 1959. (b) Correlated Responses. Tests for responses correlated with the change in growth rate of the Selected line were made with females. Their sires were highly selected individuals, a selection differential of over £ lb. from the fourth selected generation, and the dams were unselected females from the third generation. The test pullets were produced both as pure lines as well as reciprocal crosses between the Growth-selected (S) and the Randombred (R) lines. The S X S and S X R progeny were from the same 16 S sires used to artificially inseminate three to four hens per sire in each of the groups of S and R females, respectively. Likewise R X R and R X S were produced from 16 randomly chosen R sires used in inseminate 50 to 60 hens in each of the R and S lines. The
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ANALYSIS OF GROWTH AND PRODUCTION TABLE 8.—Survivors' egg production between the 25th and 48th week of age Type of Females
Egg Production (%)
SXS SXR RXS RXR
57 59 60 62
( R X R , Table 8) was high, 62%. A second test of the egg production of the R and S lines was made using the fourth-generation S pullets whose eightweek body weight was given in Table 6. Triplicate pens of each line were used and pen records of egg production were recorded for six months following the 25th week after hatching. The average monthly hen-day production during this period was 47% for the 226 pullets housed from the S line and 56% for 214 pullets housed from the R line. The spread between the two lines was greater (9%) than the 5% recorded in the first test. There is no question that the genetic correlation between eight-week weight and egg production is negative. A genetic correlation of —0.19 would be necessary to produce a correlated decrease (CRy) of 5% in egg production. The correlated responses measured for egg weight, egg shape and albumen height were in the same direction as those expected (Table 5). The data for eggs are presented in Table 9. Eggs from the S X S pullets at 32 weeks of age exceeded those from the R X R line by 2.8 gm. in weight, they were 1.9 units broader and albumen of the the broken-out egg was 0.3 mm. higher. A selection differential of one standard deviation in 8-week body weight had been used for four generations. The S X S would have been expected to exceed R X R (Table 5) by 2.8 gm. in egg weight, 0.6 units in broadness (egg shape) and about 0.1 mm. in albumen height. On this basis the change TABLE 9.—Egg weight, egg shape and albumen height for females recorded the 32nd week after hatching Egg Weight (gm.)
Egg Shape (ratio)
Albumen Height (mm.)
58.9 59.5 56.8 56.1
75.9 75.4 74.5 74.0
7.70 7.42 7.70 7.40
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ble because the female has only one sex chromosome, it is possible that some of these additive effects could be attributable to sex-linked genes received from the father. The egg production of the two lines and their reciprocal crosses is given in Table 8 as a percentage of eggs out of the 69 days of trapnesting (three times per week from the 23rd through the 46th week of age). An increase in egg production of the S X S line was predicted from the genetic correlation (Table 4). The realized response, however, was lowered egg production (57%) as compared with the R X R control (62%). Further confirmation of this negative response is provided by the intermediate egg production of the reciprocal crosses. Since the genetic correlation between eight-week body weight and egg production was low (.15) and the confidence interval for such correlations are large, this reversal from a positive estimate to a negative response is explained as a chance deviation in the crude estimation. However, an interesting observation by Hicks (1958) is analagous to this positive estimate and negative response. Hicks obtained a positive genetic correlation between mature body weight and egg production when egg production was poor. In contrast, in years when egg production was superior (normal?) he obtained negative correlations. In our experiments, a positive correlation was obtained when egg production was low (42%) and a negative response occurred when egg production of the Randombred
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R. G. JAAP, J. H. SMITH AND B. L. GOODMAN
crease in egg production was slightly more than one percent per one-tenth pound gain in eight-week body weight. The increase in egg weight was identical with that predicted from the genetic parameters calculated for the randombred parental line. The realized responses in broadness of egg and increase in height of albumen were greater than those predicted. Poultry breeders were estimated to have made a genetic gain of almost \ lb. in eight-week weight of broilers produced commercially between 1955 and 1959. REFERENCES
SUMMARY
Genetic parameters were calculated for growth and egg production characteristics of the randombred population of White Gold meat-type chickens. These were compared with the responses obtained in a line selected to grow much more rapidly than the randombred line from which it originated. Characters considered were body weight at 8, 16 and 24 weeks of age; egg production between the 23rd and 46th week; and egg weight, shape and height of the albumen of eggs laid during the 30th week after hatching. Genetic effects on all characters were estimated to be positively correlated with eight-week body weight in the randombred population. However, egg production proved to be negatively correlated with the genetic change in body weight. The de-
Falconer, D. S., 1960. Introduction to Quantitative Genetics. The Ronald Press Company, New York 10, N.Y. Goodman, B. L., and R. G. Jaap, 1960a. Improving accuracy of heritability estimates from diallel and triallel matings in poultry. 1. Poultry Sci. 39: 938-944. Goodman, B. L., and R. G. Jaap, 1960b. Improving accuracy of heritability estimates from diallel and triallel matings in poultry. 2. Poultry Sci. 39: 944-949. Goodman, B. L., and R. G. Jaap, 1961. Non-additive and sex-linked genetic effects on egg production in a randombred population. Poultry Sci. 40: 662-668. Hicks, A. F., Jr., 19S8. The interrelationships of mature body weight with egg number, egg weight and egg shape in chickens. Poultry Sci. 37: 1211. King, S. C , 1961. Inheritance of economic traits in the Regional Cornell control population. Poultry Sci. 40: 97S-986.
NEWS AND NOTES (Continued from page 1438) the Agricultural Research Service. In 1955 he became Assistant Director of Livestock Research, and in 1957 Deputy Administrator for Farm Research. While continuing as a member of the Animal Husbandry Division staff, he served from 1944 to 1946 as Chief of the Livestock Branch of the United Nations Relief and Rehabilitation Administration.
In 1943 Dr. Byerly received the Borden Award, administered by the Poultry Science Association, for his contributions to poultry science, especially in the field of avian physiology, including incubation, endocrinology, and feed utilization. He is a Fellow of the Poultry Science Association and of the American Association for the Advancement of Science, and a member of the American Society of
(Continued on page 1473)
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in egg weight of the S line was the same as that expected while the changes in egg shape and albumen height were greater than predicted. The higher values for egg weight and shape in S X R (Table 9) than that of R X S would have been expected from their relative growth rates (Figure 1) and tend to add confirmation to the validity of the genetic correlations. Albumen height in eggs from these reciprocals did not conform to expectation. R X S resembled the S parent and S X R the R parental line.