- Email: [email protected]
Combining Ability of Inbred Lines of Leghorns1,2
400
A. J. WYATT
to pellets as compared to their controls. Evidence indicates that experimental groups of goslings should be reared out of sight of one another to prevent their follow-the-leader tendencies from biasing the reults. ACKNOWLEDGMENT
REFERENCES Pepper, W. F., 1952. The influence of aureomycin on the interrelationship between manganese and
Combining Ability of Inbred Lines of Leghorns1,5 A. J. WYATT Department of Poultry Husbandry, Iowa State College, Ames (Received for publication September 15, 1952)
T
HE use of the topcross test for evaluating the combining ability of inbred lines was originally proposed by corn breeders (Lindstrom, 1931; Jenkins and Brunson, 1932). The procedure has subsequently been adopted for use with poultry. Information in the literature concerning the combining ability of inbred lines of chickens is meager, although some use of the topcross has been made. Waters (1938) topcrossed inbred White Leghorn sires on outbred White Leghorn females. He observed that the topcross progeny were superior to both parents in hatchability and in viability at eight and 24 weeks of age. Waters (1941)
1 Part of a thesis submitted to the faculty of Iowa State College in partial fulfillment of the requirements for the M.S. degree. 2 Journal paper No. J-2159 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 1040.
also studied the egg weight of similar topcrosses. He found no significant difference between egg weight of the topcross progeny and that of the inbreds or the randombred Leghorns. Maw (1942) also crossed inbred Leghorn males with noninbred Leghorn females and observed the topcross progeny to be superior to non-inbred Leghorns in viability and production. He observed little difference in body weight, but egg weight of the topcrosses was significantly lower than that of the non-inbred controls. Glazener and Blow (1951) made topcrosses involving sires from eight inbred lines. Females from five breeds and varieties were used as testers. The performance of the topcrosses was compared with that of the inbreds for body weight, feathering, mortality, and hatchability. They found the regression of topcross performance on inbred performance to be positive and
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
We wish to thank Merck and Co. Ltd., Montreal, Canada, for the vitamin B12 supplement, vitamin Bj2 and penicillin supplement, riboflavin and niacin used in this work.
salt. M.S.A. Thesis, University of Toronto. Scott, M. L., 1951. Unidentified vitamins in turkey nutrition. Proc. Cornell Nutrition Conference, pp. 73-77. Scott, H. M., and W. R. Glista, 1950. The effect of aureomycin and arsonic acid on chick growth. Poultry Sci. 29: 921-923. Scott, M. L., and G. F. Heuser, 1952. Studies in duck nutrition 4. Bowed legs in ducks caused by niacin deficiency. Poultry Sci. 31: 752-754. Slinger, S. J., W. F. Pepper and D. C. Hill, 1952a. Interaction between penicillin and grass juice concentrate in turkeys. Poultry Sci. 31: 187-188. Slinger, S. J., W. F. Pepper and D. C. Hill, 1952b. Effect of penicillin on the tolerance of turkeys to fat. Arch. Biochem. Biophys. 37: 266-269. Snedecor, G. W., 1946. Statistical Methods: The Iowa State College Press, Fourth Edition, pp. 81-82.
COMBINING ABILITY OF INBRED LINES
METHODS AND MATERIALS
It seems desirable to define the following terms as used in this study. Topcross: A mating of inbred sires with non-inbred females (the females were single crosses in this case.) Tester: A non-inbred population used to evaluate an inbred line. Single Cross: A cross of two inbred lines. The data used came from 13 inbred lines which were currently available at the Iowa Experiment Station. Information concerning these lines is summarized in Table 1. Lines 8, 9, 10, and 15 all originated at the Iowa Experiment Station TABLE I.—Summary of lines
Line
Approx. inbreeding coefficient
White Leghorn
8 9 10 IS 21
.70 .85 .55 .62 .25
New Hampshire
C D 13 500
.18 .18 .55 .30
B
.60
Barred Rock
20 R
.40 .18
White Rock
550
.30
Breed
R.I. Red
and were somewhat related. Lines C, D, R, 13, 20, and 21 originated from hatching eggs purchased from breeders throughout the -tJountry. Lines 500 and 550 were developed at the Minnesota Experiment Station and were brought to Iowa as mature males. Line B, developed at the U.S.D.A. Experiment Station at Beltsville, Maryland, was brought to Iowa as hatching eggs. Topcross matings were made in 1949 using sires from the five Leghorn inbred lines. The testers used were the female progeny of five single crosses. The compsoition of the tester females was as follows: Tester A B 4 5R 5B
Composition of Tester New Hamp. DXNew Hamp. C R.I. Red BXNew Hamp. D W. Rock 550XBarred Rock 20 New Hamp. 500XNew Hamp. 13 New Hamp. 500XBarred Rock R.
Ten topcross matings were made, two pens for each Leghorn line. Two males from each of the 5 inbred lines of Leghorns were mated with the five sets of tester females in each breeding pen, with 18 to 20 females per pen. Chicks were produced in five hatches; four hatches being in March and one in April. The chicks were brooded in a multiple-pen permanent brooder house until eight weeks of age. The birds were then weighed to the nearest 10 grams and moved to the range. At about 24 weeks of age, the birds were housed. All birds were trapnested three days per week. Egg production was computed as the ratio of the number of eggs laid on trapnest days to the number of trapnest days during the period from housing to June 3. Egg weights were measured to the nearest gram during a two-week period in March. Mortality percentages to housing time time were calculated after excluding birds recorded as missing. Housing weights were corrected to 165 days. For the analysis
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
significant for 10-week weight and for feathering. A regression approaching zero was reported for hatchability and for mortality to 10 weeks. Line X tester interactions were found to be significant for broiler weight and hatchability. These workers concluded that the topcross test appears limited in broiler production. The purpose of the present investigation was to obtain additional information concerning combining ability of inbred lines of chickens.
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COMBINING ABILITY OF INBRED LINES TABLE 3.—Analyses of variance Mean squares Source of variation
Body wt. at 8 weeks
B
d.f. d.f.
Mean Sq.
4 Between lines 4 83,691 Between testers 4 4 251,479 Between pens within lines 5 5 8,208,375t Lines X Testers 93,757f 16 16 Residual 11,075 20 2,184
n°t
d
^T' Hatcha-
Mortality
Egg prod.
Egg wt.
628.25 137.75
26.5 27.0
4.13 23.58f
250.80 228.88 137.85
111.8 83.4 59.9
12.22f 2.40 4.22
lays
«**
Brooder Range
Laying
.2012 .0632
20.0 37.5
188.01 177.52 47.27 144.87
.0408 .0388 .0243
30.0* 19.4* 8.0
67.71 84.46 52.28 120.25 49.31 73.56
between the various lines and testers are of primary importance. Differences between testers were significant for egg weight, and the lineX tester interaction was significant for body weight at eight weeks and for hatchability. The poultry breeder is frequently interested in predicting topcross performance from the phenotypic performance of the inbred line. A measure of such relationship could be obtained by computing the regression of topcross performance on inbred performance as was done by Glazener and Blow (1951). For these data, however, an examination of Table 2 seems sufficient for this purpose. Consider, for example, eight week weight of males. The best performing inbred is line 21 with a mean weight of 451 grams at eight weeks, and the line with the lowest performance is line 15 with a mean weight of 338 grams. Examination of the mean of the progeny of the topcrosses involving these lines shows that line 15, the lowest performing inbred, yielded the highest performing topcross progeny. The topcross progeny of line 21, the highest performing inbred, were of intermediate rank. Similarly the topcross progeny of the other inbreds did not rank with the performance of the inbreds themselves. This indicates little relationship between topcross performance and inbred performance for eight
week weight of males in these data. Similar examination of the other traits studied reveals a situation quite similar to that for weight of males at eight weeks; consequently, in these data, there seems to be little relation between the performance of the inbred and the performance of the topcross progeny. Since the relationship of topcross performance to inbred performance is a function of heritability, this result indicates that genes acting in an additive fashion contribute little to the variance between lines. In view of the available estimates of heritability, such a result is somewhat conflicting, especially as regards body weight and egg weight. Glazener and Blow (1951) found the regression of topcross performance on inbred performance to be .29 for weight at 10 weeks. Heritability of broiler weight has been reported as approximately 50 percent by Lerner et al. (1947), and Glazener et al. (1951) reported estimates ranging from 51 to 79 percent. Estimates of heritability of egg weight andd)ody weight are reviewed by Shoffner and Sloan (1948) and by Lerner and Cruden (1951). The estimates range from .46 to .84 for the former and from .20 to .75 for the latter. These estimates are not all comparable, however. Heritability of egg production seems to be in the intermediate to low range (Shoffner and Sloan,
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
* Significant at .05 level, t Significant at .01 level.
404
A. J. WYATT
The significant line X tester interaction
for weight at eight weeks, and for hatchability (Table 3) found in these data is evidence that dominance and non-linear gene interactions may be important in determining the differences between lines. This finding is in agreement with that of Glazener and Blow (1951). Although the apparent differences between lines in these data appear to be due to non-additive genetic variance, they may also be due to environmental effects. However, the inbreeding decline observed in the case of hatchability (Shoffner, 1948; and Wilson, 1948b) is evidence for the existence of some kind of genetic variance. This suggests that non-additive genetic variance is important in these lines. It thus appears that hybridization may be an important aspect of poultry breeding. Highly heterozygous testers such as the single crosses used in this study would undoubtedly possess many genes for general combining ability, but they might also contain genes with strong dominance and epistatic effects. To the extent that this is true, the results would tend to be more influenced by specific combining ability. However, the use of several testers would greatly reduce the probability of such an occurrence. For traits in which the differences between lines are primarily the result of factors other than genes with additive effects, topcross testing for general combining ability would seem to have only limited use. Test crosses might have considerable utility, but the problem would then seem to be one of selecting lines to be used in testing for specific combining ability. These data indicate that the differences between lines result from effects other than those due to additive genes; hence, the use of the topcross test for evaluating combining ability of inbred lines seems to be of limited value.
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
1948; Wilson, 1948a; Lerner and Hazel, 1947; and Lerner and Cruden, 1947) while that of viability (Shoffner and Sloan, 1948; and Robertson and Lerner, 1949) and hatchability (Shoffner and Sloan, 1948; and Wilson, 1948b) is low. Further inspection of Table 2 shows that the different testers failed to rank the inbred lines in the same order as measured by the mean performance of the topcross progeny. This is further evidence that genes with additive effects contribute little to the variance between lines. In spite of the rather marked differences observed in the performance of the inbred lines (Table 2) for all the traits studied, the analyses of variance in Table 3 show that the lines exhibited no differences in general combining ability for any of these traits. This result further strengthens the evidence that the differences among these lines are due primarily to factors other than additive genes. In the case of all-or-none characters such as mortality and hatchability, this apparent lack of line differences might be explained by the existence of a threshold. If, for example, resistance to death is dependent on one or several more or less continuously variable factors, it seems reasonable that the phenotype, death in this case, would appear only when the level of resistance falls below a certain threshold. It is also possible that hatchability may be influenced by maternal effects which might account for the lack of additive genetic genetic differences between lines. If maternal effects were of primary importance in these data, however, a significant difference between testers would be expected. The line X tester interaction would also be expected to be non-significant. Neither of these conditions was met by these data.
FORMALIZED VACCINE FOR NEWCASTLE SUMMARY
ACKNOWLEDGMENT
The author is indebted to Arne W. Nordskog for suggestions concerning certain phases of the analysis. REFERENCES Glazener, E. W., and W. L. Blow, 1951. Topcross testing for broiler production. Poultry Sci. 30: 870-874. Jenkins, M. T., and A. M. Brunson, 1932. Methods
of testing inbred lines of maize in crossbred combination. J. Am. Soc. Agron. 24: 523-530. Lerner, I. M., V. S. Asmundson and D. M. Cruden, 1947. The improvement of New Hampshire fryers. Poultry Sci. 26: 515-524. Lerner, I. M., and D. M. Cruden, 1948. The heritability of accumulative monthly and annual egg production. Poultry Sci. 27: 67-78. Lerner, I. M., and L. N. Hazel, 1947. Population genetics of a poultry flock under artificial selection. Genetics, 32: 325-339. Lindstrom, E. W., 1931. Prepotency of inbred sires on commercial varieties of maize. J. Am. Soc. Agron. 23: 652-661. Maw, A. J. G., 1942. Crosses between inbred lines of the domestic fowl. Poultry Sci. 21: 548-553. Robertson, A., and I. M. Lerner, 1949. The heritability of all-or-none traits: viability of poultry. Genetics, 34: 395-411. Shoffner, R. N., 1948. The reaction of the fowl to inbreeding. Poultry Sci. 27: 448-452. Shoffner, R. N., and H. J. Sloan, 1948. Heritability studies in the domestic fowl. Official Report Eighth World's Poultry Congress: 269-281. Waters, N. F., 1938. The influence of inbred sires topcrossed on White Leghorn fowl. Poultry Sci. 17:490-497. Waters, N. F., 1941. Genetic aspects of egg weight observed during inbreeding experiments. Poultry Sci. 20: 14-27. Wilson, W. O., 1948a. Egg production rate and fertility in inbred chickens. Poultry Sci. 27: 719726. Wilson, W. O., 1948b. Viability of embryos and of chicks in inbred chickens. Poultry Sci. 27: 727735.
Newcastle Disease: Response to Formalized Vaccine E. F. WALLER AND M. R. GARDINER Department of Animal and Poultry Industry, University of Delaware, Newark, Delaware (Received for publication September 15, 1952)
N
EWCASTLE disease has been a serious problem among the Delaware broiler producers since 1946. The literature dealing with the various aspects of this disease is becoming voluminous Misc. Publication Series #153. Published as Miscellaneous Paper No. 153 with the approval of the Director of the Delaware Agricultural Experiment Station.
and a review of it is beyond the scope of this paper. Approximately 95 percent of the chickens started in this area are vaccinated at least once with some type of Newcastle vaccine. As a general rule, if the vaccination is done too early the immunity is of short duration, and if the vaccination is withheld for three or more weeks, natural outbreaks may decimate
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
Topcrosses were made using sires from five inbred lines of Leghorns. Five female testers were used. Each was a single cross of inbred lines of heavy breeds. The performance of the topcross progeny was compared with that of the Leghorn inbreds, and data were analyzed for body weight, hatchability, mortality, egg production, and egg weight. Significant line X tester interactions were obtained for body weight at eight weeks and for hatchability. Evidence is presented which indi-. cates that differences between the lines in this study are primarily the result of effects other than those due to additive genes. These data indicate only a limited value of the topcross test for evaluating combining ability of inbred lines.
405
A. J. WYATT
to pellets as compared to their controls. Evidence indicates that experimental groups of goslings should be reared out of sight of one another to prevent their follow-the-leader tendencies from biasing the reults. ACKNOWLEDGMENT
REFERENCES Pepper, W. F., 1952. The influence of aureomycin on the interrelationship between manganese and
Combining Ability of Inbred Lines of Leghorns1,5 A. J. WYATT Department of Poultry Husbandry, Iowa State College, Ames (Received for publication September 15, 1952)
T
HE use of the topcross test for evaluating the combining ability of inbred lines was originally proposed by corn breeders (Lindstrom, 1931; Jenkins and Brunson, 1932). The procedure has subsequently been adopted for use with poultry. Information in the literature concerning the combining ability of inbred lines of chickens is meager, although some use of the topcross has been made. Waters (1938) topcrossed inbred White Leghorn sires on outbred White Leghorn females. He observed that the topcross progeny were superior to both parents in hatchability and in viability at eight and 24 weeks of age. Waters (1941)
1 Part of a thesis submitted to the faculty of Iowa State College in partial fulfillment of the requirements for the M.S. degree. 2 Journal paper No. J-2159 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 1040.
also studied the egg weight of similar topcrosses. He found no significant difference between egg weight of the topcross progeny and that of the inbreds or the randombred Leghorns. Maw (1942) also crossed inbred Leghorn males with noninbred Leghorn females and observed the topcross progeny to be superior to non-inbred Leghorns in viability and production. He observed little difference in body weight, but egg weight of the topcrosses was significantly lower than that of the non-inbred controls. Glazener and Blow (1951) made topcrosses involving sires from eight inbred lines. Females from five breeds and varieties were used as testers. The performance of the topcrosses was compared with that of the inbreds for body weight, feathering, mortality, and hatchability. They found the regression of topcross performance on inbred performance to be positive and
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
We wish to thank Merck and Co. Ltd., Montreal, Canada, for the vitamin B12 supplement, vitamin Bj2 and penicillin supplement, riboflavin and niacin used in this work.
salt. M.S.A. Thesis, University of Toronto. Scott, M. L., 1951. Unidentified vitamins in turkey nutrition. Proc. Cornell Nutrition Conference, pp. 73-77. Scott, H. M., and W. R. Glista, 1950. The effect of aureomycin and arsonic acid on chick growth. Poultry Sci. 29: 921-923. Scott, M. L., and G. F. Heuser, 1952. Studies in duck nutrition 4. Bowed legs in ducks caused by niacin deficiency. Poultry Sci. 31: 752-754. Slinger, S. J., W. F. Pepper and D. C. Hill, 1952a. Interaction between penicillin and grass juice concentrate in turkeys. Poultry Sci. 31: 187-188. Slinger, S. J., W. F. Pepper and D. C. Hill, 1952b. Effect of penicillin on the tolerance of turkeys to fat. Arch. Biochem. Biophys. 37: 266-269. Snedecor, G. W., 1946. Statistical Methods: The Iowa State College Press, Fourth Edition, pp. 81-82.
COMBINING ABILITY OF INBRED LINES
METHODS AND MATERIALS
It seems desirable to define the following terms as used in this study. Topcross: A mating of inbred sires with non-inbred females (the females were single crosses in this case.) Tester: A non-inbred population used to evaluate an inbred line. Single Cross: A cross of two inbred lines. The data used came from 13 inbred lines which were currently available at the Iowa Experiment Station. Information concerning these lines is summarized in Table 1. Lines 8, 9, 10, and 15 all originated at the Iowa Experiment Station TABLE I.—Summary of lines
Line
Approx. inbreeding coefficient
White Leghorn
8 9 10 IS 21
.70 .85 .55 .62 .25
New Hampshire
C D 13 500
.18 .18 .55 .30
B
.60
Barred Rock
20 R
.40 .18
White Rock
550
.30
Breed
R.I. Red
and were somewhat related. Lines C, D, R, 13, 20, and 21 originated from hatching eggs purchased from breeders throughout the -tJountry. Lines 500 and 550 were developed at the Minnesota Experiment Station and were brought to Iowa as mature males. Line B, developed at the U.S.D.A. Experiment Station at Beltsville, Maryland, was brought to Iowa as hatching eggs. Topcross matings were made in 1949 using sires from the five Leghorn inbred lines. The testers used were the female progeny of five single crosses. The compsoition of the tester females was as follows: Tester A B 4 5R 5B
Composition of Tester New Hamp. DXNew Hamp. C R.I. Red BXNew Hamp. D W. Rock 550XBarred Rock 20 New Hamp. 500XNew Hamp. 13 New Hamp. 500XBarred Rock R.
Ten topcross matings were made, two pens for each Leghorn line. Two males from each of the 5 inbred lines of Leghorns were mated with the five sets of tester females in each breeding pen, with 18 to 20 females per pen. Chicks were produced in five hatches; four hatches being in March and one in April. The chicks were brooded in a multiple-pen permanent brooder house until eight weeks of age. The birds were then weighed to the nearest 10 grams and moved to the range. At about 24 weeks of age, the birds were housed. All birds were trapnested three days per week. Egg production was computed as the ratio of the number of eggs laid on trapnest days to the number of trapnest days during the period from housing to June 3. Egg weights were measured to the nearest gram during a two-week period in March. Mortality percentages to housing time time were calculated after excluding birds recorded as missing. Housing weights were corrected to 165 days. For the analysis
Downloaded from http://ps.oxfordjournals.org/ at Florida International University on June 10, 2015
significant for 10-week weight and for feathering. A regression approaching zero was reported for hatchability and for mortality to 10 weeks. Line X tester interactions were found to be significant for broiler weight and hatchability. These workers concluded that the topcross test appears limited in broiler production. The purpose of the present investigation was to obtain additional information concerning combining ability of inbred lines of chickens.
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COMBINING ABILITY OF INBRED LINES TABLE 3.—Analyses of variance Mean squares Source of variation
Body wt. at 8 weeks
B
d.f. d.f.
Mean Sq.
4 Between lines 4 83,691 Between testers 4 4 251,479 Between pens within lines 5 5 8,208,375t Lines X Testers 93,757f 16 16 Residual 11,075 20 2,184
n°t
d
^T' Hatcha-
Mortality
Egg prod.
Egg wt.
628.25 137.75
26.5 27.0
4.13 23.58f
250.80 228.88 137.85
111.8 83.4 59.9
12.22f 2.40 4.22
lays
«**
Brooder Range
Laying
.2012 .0632
20.0 37.5
188.01 177.52 47.27 144.87
.0408 .0388 .0243
30.0* 19.4* 8.0
67.71 84.46 52.28 120.25 49.31 73.56
between the various lines and testers are of primary importance. Differences between testers were significant for egg weight, and the lineX tester interaction was significant for body weight at eight weeks and for hatchability. The poultry breeder is frequently interested in predicting topcross performance from the phenotypic performance of the inbred line. A measure of such relationship could be obtained by computing the regression of topcross performance on inbred performance as was done by Glazener and Blow (1951). For these data, however, an examination of Table 2 seems sufficient for this purpose. Consider, for example, eight week weight of males. The best performing inbred is line 21 with a mean weight of 451 grams at eight weeks, and the line with the lowest performance is line 15 with a mean weight of 338 grams. Examination of the mean of the progeny of the topcrosses involving these lines shows that line 15, the lowest performing inbred, yielded the highest performing topcross progeny. The topcross progeny of line 21, the highest performing inbred, were of intermediate rank. Similarly the topcross progeny of the other inbreds did not rank with the performance of the inbreds themselves. This indicates little relationship between topcross performance and inbred performance for eight
week weight of males in these data. Similar examination of the other traits studied reveals a situation quite similar to that for weight of males at eight weeks; consequently, in these data, there seems to be little relation between the performance of the inbred and the performance of the topcross progeny. Since the relationship of topcross performance to inbred performance is a function of heritability, this result indicates that genes acting in an additive fashion contribute little to the variance between lines. In view of the available estimates of heritability, such a result is somewhat conflicting, especially as regards body weight and egg weight. Glazener and Blow (1951) found the regression of topcross performance on inbred performance to be .29 for weight at 10 weeks. Heritability of broiler weight has been reported as approximately 50 percent by Lerner et al. (1947), and Glazener et al. (1951) reported estimates ranging from 51 to 79 percent. Estimates of heritability of egg weight andd)ody weight are reviewed by Shoffner and Sloan (1948) and by Lerner and Cruden (1951). The estimates range from .46 to .84 for the former and from .20 to .75 for the latter. These estimates are not all comparable, however. Heritability of egg production seems to be in the intermediate to low range (Shoffner and Sloan,
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* Significant at .05 level, t Significant at .01 level.
404
A. J. WYATT
The significant line X tester interaction
for weight at eight weeks, and for hatchability (Table 3) found in these data is evidence that dominance and non-linear gene interactions may be important in determining the differences between lines. This finding is in agreement with that of Glazener and Blow (1951). Although the apparent differences between lines in these data appear to be due to non-additive genetic variance, they may also be due to environmental effects. However, the inbreeding decline observed in the case of hatchability (Shoffner, 1948; and Wilson, 1948b) is evidence for the existence of some kind of genetic variance. This suggests that non-additive genetic variance is important in these lines. It thus appears that hybridization may be an important aspect of poultry breeding. Highly heterozygous testers such as the single crosses used in this study would undoubtedly possess many genes for general combining ability, but they might also contain genes with strong dominance and epistatic effects. To the extent that this is true, the results would tend to be more influenced by specific combining ability. However, the use of several testers would greatly reduce the probability of such an occurrence. For traits in which the differences between lines are primarily the result of factors other than genes with additive effects, topcross testing for general combining ability would seem to have only limited use. Test crosses might have considerable utility, but the problem would then seem to be one of selecting lines to be used in testing for specific combining ability. These data indicate that the differences between lines result from effects other than those due to additive genes; hence, the use of the topcross test for evaluating combining ability of inbred lines seems to be of limited value.
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1948; Wilson, 1948a; Lerner and Hazel, 1947; and Lerner and Cruden, 1947) while that of viability (Shoffner and Sloan, 1948; and Robertson and Lerner, 1949) and hatchability (Shoffner and Sloan, 1948; and Wilson, 1948b) is low. Further inspection of Table 2 shows that the different testers failed to rank the inbred lines in the same order as measured by the mean performance of the topcross progeny. This is further evidence that genes with additive effects contribute little to the variance between lines. In spite of the rather marked differences observed in the performance of the inbred lines (Table 2) for all the traits studied, the analyses of variance in Table 3 show that the lines exhibited no differences in general combining ability for any of these traits. This result further strengthens the evidence that the differences among these lines are due primarily to factors other than additive genes. In the case of all-or-none characters such as mortality and hatchability, this apparent lack of line differences might be explained by the existence of a threshold. If, for example, resistance to death is dependent on one or several more or less continuously variable factors, it seems reasonable that the phenotype, death in this case, would appear only when the level of resistance falls below a certain threshold. It is also possible that hatchability may be influenced by maternal effects which might account for the lack of additive genetic genetic differences between lines. If maternal effects were of primary importance in these data, however, a significant difference between testers would be expected. The line X tester interaction would also be expected to be non-significant. Neither of these conditions was met by these data.
FORMALIZED VACCINE FOR NEWCASTLE SUMMARY
ACKNOWLEDGMENT
The author is indebted to Arne W. Nordskog for suggestions concerning certain phases of the analysis. REFERENCES Glazener, E. W., and W. L. Blow, 1951. Topcross testing for broiler production. Poultry Sci. 30: 870-874. Jenkins, M. T., and A. M. Brunson, 1932. Methods
of testing inbred lines of maize in crossbred combination. J. Am. Soc. Agron. 24: 523-530. Lerner, I. M., V. S. Asmundson and D. M. Cruden, 1947. The improvement of New Hampshire fryers. Poultry Sci. 26: 515-524. Lerner, I. M., and D. M. Cruden, 1948. The heritability of accumulative monthly and annual egg production. Poultry Sci. 27: 67-78. Lerner, I. M., and L. N. Hazel, 1947. Population genetics of a poultry flock under artificial selection. Genetics, 32: 325-339. Lindstrom, E. W., 1931. Prepotency of inbred sires on commercial varieties of maize. J. Am. Soc. Agron. 23: 652-661. Maw, A. J. G., 1942. Crosses between inbred lines of the domestic fowl. Poultry Sci. 21: 548-553. Robertson, A., and I. M. Lerner, 1949. The heritability of all-or-none traits: viability of poultry. Genetics, 34: 395-411. Shoffner, R. N., 1948. The reaction of the fowl to inbreeding. Poultry Sci. 27: 448-452. Shoffner, R. N., and H. J. Sloan, 1948. Heritability studies in the domestic fowl. Official Report Eighth World's Poultry Congress: 269-281. Waters, N. F., 1938. The influence of inbred sires topcrossed on White Leghorn fowl. Poultry Sci. 17:490-497. Waters, N. F., 1941. Genetic aspects of egg weight observed during inbreeding experiments. Poultry Sci. 20: 14-27. Wilson, W. O., 1948a. Egg production rate and fertility in inbred chickens. Poultry Sci. 27: 719726. Wilson, W. O., 1948b. Viability of embryos and of chicks in inbred chickens. Poultry Sci. 27: 727735.
Newcastle Disease: Response to Formalized Vaccine E. F. WALLER AND M. R. GARDINER Department of Animal and Poultry Industry, University of Delaware, Newark, Delaware (Received for publication September 15, 1952)
N
EWCASTLE disease has been a serious problem among the Delaware broiler producers since 1946. The literature dealing with the various aspects of this disease is becoming voluminous Misc. Publication Series #153. Published as Miscellaneous Paper No. 153 with the approval of the Director of the Delaware Agricultural Experiment Station.
and a review of it is beyond the scope of this paper. Approximately 95 percent of the chickens started in this area are vaccinated at least once with some type of Newcastle vaccine. As a general rule, if the vaccination is done too early the immunity is of short duration, and if the vaccination is withheld for three or more weeks, natural outbreaks may decimate
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Topcrosses were made using sires from five inbred lines of Leghorns. Five female testers were used. Each was a single cross of inbred lines of heavy breeds. The performance of the topcross progeny was compared with that of the Leghorn inbreds, and data were analyzed for body weight, hatchability, mortality, egg production, and egg weight. Significant line X tester interactions were obtained for body weight at eight weeks and for hatchability. Evidence is presented which indi-. cates that differences between the lines in this study are primarily the result of effects other than those due to additive genes. These data indicate only a limited value of the topcross test for evaluating combining ability of inbred lines.
405