- Email: [email protected]
Topcross Testing for Broiler Production*
Topcross Testing for Broiler Production E. W. GLAZENER AND W. L. BLOW
North Carolina Agricultural Experiment Station, Raleigh (Received for publication February 19,1951)
follows: New Hampshire, Barred Plymouth Rock, White Plymouth Rock, Rhode Island Red, and S. C. White Leghorn. The New Hampshire and Barred Plymouth Rock females were broiler-type, the White Plymouth Rock females utility, and the Rhode Island Red and S. C. White Leghorn females egg-type. Sixteen topcross matings, two pens for each inbred line, were used in this study. Three males from each of the eight inbred lines were mated with the five sets of tester females in each breeding pen, with approximately 18 to 20 females per pen.
METHODS AND MATERIALS
Four hatches of chicks, primarily during the month of March, were made for the inbreds and their topcrosses. All chicks were brooded together in starting batteries for approximately 10 days, then transferred to conventional floor brooding. A standard starting and broiler mash was fed. At 6 and 10 weeks of age the chickens were classified as either well-feathered or poorly-feathered and weighed to the nearest ounce. In figuring mortality to 10 weeks of age, birds that were not accountable were classified as dead.
The North Carolina Experiment Station has eight inbred lines of chickens, one S. C. White Leghorn, one Rhode Island Red, three Barred Plymouth Rock, and three New Hampshire, which are inbred from 35 to 55 percent. The lines vary according to their type in origin. NCI, S. C. White Leghorn, NC6, Rhode Island Red, NCI2, New Hampshire, and NC18, Barred Plymouth Rock, are eggtype lines; NC11, New Hampshire, NC16 and NC17, Barred Plymouth Rock, are broiler-type lines; and NC13, New Hampshire, is a utility line. In addition to maintaining the inbred lines, topcross matings for the eight lines were made in 1950. Cockerels from each line were mated to females from five breeds and varieties secured from private breeders. The tester females were as * Contribution from the Department of Poultry Science, North Carolina Agricultural Experiment Station as Journal Paper 379.
RESULTS AND DISCUSSION
The performance of the inbreds and their topcrosses for 10-week body weight, feathering, mortality, fertility, and hatchability of fertile eggs is given in Table 1. The top line of each section in the table gives the performance of the inbred line. For example, at the top of Table 1 is NCI (WL). The data in this line are the performance for the inbred line of S. C. White Leghorns. Listed directly under the
870
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
T I T T L E information is available in the •*-*' literature concerning the combining ability of inbred lines and the performance of their topcrosses for broilers. Waters (1938) observed that inbred White Leghorn sires topcrossed on White Leghorn females produced offspring that were superior in hatchability and viability to random-bred Leghorns. Growth rates were not reported. The following investigation was conducted in an attempt to learn something of the combining quality and performance of inbred lines for broilers.
TOPCROSS TESTING FOR BROILER PRODUCTION
871
TABLE 1.—Weights, feathering, mortality, fertility and hatchability of inbreds and their topcross progeny
Line and crosses
Mean 10-wk. wts.
Percent poorlyfeathered at 6 wks.
No. at 10 wks.
Male
Males
Female
oz.
Percent Females mortality to 10 wks. oz.
Percent eggs fertile
Percent fertile eggs hatched
564 104 52 119 76 82
1 0 0 13 28 0
0 0 0 0 0 0
31.6 39.6 39.9 38.9 33.8 36.4
27.0 33.6 31.0 32.1 29.9 29.4
12 9 12 6 16 9
90 96 87 95 93 85
77 83 79 92 88 88
NC6 (RIR) XNH XBPR XWPR XRIR XWL
316 84 77 103 73 73
76 10 56 44 39 0
53 11 0 23 24 0
33.2 42.4 41.1 40.3 38.9 40.1
29.3 34.6 34.3 34.0 31.8 31.5
36 10 5 9 8 4
86 92 88 89 76 61
71 79 89 90 77 88
NC11 (NH) XNH XBPR XWPR XRIR XWL
509 97 43 101 88 52
6 0 0 23 24 0
0 0 0 0 0 0
43.0 44.8 46.3 42.3 37.2 40.0
36.1 36.7 35.6 34.4 32.5 33.5
15 5 8 12 5 10
84 97 97 90 80 51
62 83 79 92 88 88
NC12 (NH) XNH XBPR XWPR XRIR XWL
259 85 80 73 65 22
16 0 25 33 24 0
3 0 0 9 12 0
32.3 41.1 37.2 39.2 37.3 32.4
27.5 36.7 33.0 34.0 32.2 31.8
19 4 7 13 4
71 89 92 77 97 25
66 81 83 82 83 78
NC13 (NH) XNH XBPR XWPR XRIR XWL
148 76 69 65 60 18
6 0 13 31 30 0
1 0 0 0 14 0
36.8 44.8 39.0 39.7 39.5 40.0
31.1 35.4 33.0 32.9 31.7 28.6
11 7 9 13 9 5
77 80 81 74 67 29
67 79 78 84 85 64
NC16 (BPR) XNH XBPR XWPR XRIR XWL
123 114 89 74 96 63
55 0 50 46 41 17
16 0 17 28 15 0
40.8 48.8 43.3 42.1 42.5 40.9
35.2 38.7 34.5 36.4 34.4 33.0
14 2 11 10 9 10
84 89 95 96 89 70
51 84 90 85 88 84
NC17 (BPR) XNH XBPR XUPR XRIR XWL
136 82 68 68 42 29
23 0 31 47 32 17
11 0 29 23 14 0
37.3 45.6 38.3 39.5 40.9 40.0
35.0 38.6 35.0 36.1 31.9 32.5
13 11 7 13 2 15
72 77 98 84 56 41
63 80 86 86 63 88
NC18 (BPR) XNH XBPR XWPR XRIR XWL
85 104 79 82 97 43
94 13 44 34 29 0
67 9 15 24 23 0
27.6 45.2 36.8 40.4 40.6 36.5
25.5 36.3 31.8 31.8 32.4 30.2
42 11 11 6 6 4
85 93 94 88 91 43
54 86 90 87 92 87
8
•
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
NCI (WL) XNH XBPR XWPR XRIR XWL
872
E. W. GLAZENER AND W. L. BLOW
inbred line is the performance of its topcrosses, NCI inbred malesXnon-inbred NH, BPR, WPR, RIR, and WL females. The same scheme follows for the other inbred lines. Estimates of the association between the performance of the inbred lines and their topcrosses were computed for weight, feathering, mortality, and hatchability by the regression of topcross performance on that of the inbred. The topcross perform-
failure of the topcross progeny of a line to vary the same no matter which tester parent was used are primary points of interest. The difference between lines and between testers for broiler weight and feathering, and the interaction of lineX tester for broiler weight and hatchability of fertile eggs are significant at the .01 level. As is observed in Table 1, the males from NC11 had a weight of 43.0 ounces
TABLE 2.—Analysis of variance for topcross performance
Feathering
D.F.
Mean sq.
D.F.
Mean sq.
Total Between sexes Between hatches Between lines Between tester LineX tester Error
316 1 3 7 4 28 273
4,218.00 292.67 108.14f 275.75t 11.68t 4.57
209 1 2* 7 4 28 167
15,286.00 1,385.00 l,817.00t 3,961.50f 338.89 317.04
Mortality D.F.
Mean sq.
D.F.
Mean sq.
159
147 3 7 4 28 105
Hatchability
190.70 29.60 47.55 66.17 81.15
3 7 4 28 117
286.53 176.97 197.92 176.24f 94.91
* Only three hatches reported, as feather picking was prevalent in the fourth hatch, f Line, tester, and line X tester mean squares significant at .01 level.
ance for each line was taken as the unweighted mean of the five topcross means for each line. These regressions, with their standard errors were as follows: Weight Feathering Mortality Hatchability
.29+.08 . 18 ± . 06 -.06+.03 - . 1 3 +.16
The positive values for weight and feathering are significant at the .05 level, whereas those for mortality and hatchability are non-significant. For these four characters, an analysis of variance was computed (Table 2). The total sum of squares was divided into the sums of squares associated with sex, hatch, line, and tester, the line X tester interaction, and the unaccounted variation designated as the error. In this analysis, estimates of the mean squares which are associated with differences in topcross performance between the various lines and tester breeds, and the success or
as compared to that of 32.3 ounces for NC12 at 10 weeks. Furthermore, the topcrosses of these two New Hampshire lines were in the same relative rank. The NC11 and NC16 lines were the most rapid-growing of the eight lines and also produced the topcrosses with the superior 10-week body weights. The linear regression values for the four characters are indicative of the relative variation between line means which is due to additive genes. If the heritability of individual differences were 100 percent, the regression of topcross performance on that of the inbred would be .5. The deviation between .5 and the regression value for weight, .3, may be due to interline non-additive genetic variance and intra-line environmental and genetic variance. If the number of chicks involved in each line is sufficiently large, then the deviation probably reflects mainly nonadditive inter-line genetic differences. The
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
Body weight
Source of variation
TOPCROSS TESTING FOR BROILER PRODUCTION
the inheritance of rate of feathering at broiler age. Mortality to 10 weeks of age and hatchability of fertile eggs reacted very similarly in the topcross test in that neither character showed a significant regression or a significant difference between lines or testers in the analysis of variance. These characters are generally accepted as having heritabilities of a low magnitude. Lush, Lamoreux and Hazel estimated the heritability of livability as 14 percent and Shoffner and Sloan (1948) hatchability as 18 percent. In spite of the rather marked differences observed in the performance of the inbred lines, the inbred lines exhibited no difference in combining ability for these two traits. This result suggests that the differences among the lines are the effect of factors primarily other than additive genes. The apparent lack of any line differences in the topcrosses for mortality may well be the result of the operation of threshold effects for this character, where only a minimum level of vitality is necessary to insure survival. That hatchability is influenced by maternal effects is quite possible. Marsden and Olsen (1950) in a study of fertility and hatchability in turkeys observed that the dam might have an important nongenetic effect on hatchability. If maternal effects were of primary importance in this particular case, however, a significant difference should exist between testers for hatchability. Also, the line and the line X tester interaction should be nonsignificant. These conditions are not satisfied. According to the positive interaction obtained for hatchability, factors of a non-additive nature are paramount in this trait. Sprague and Tatum (1942) assumed that in previously untested material, additive genes which affect general combining ability are of most importance;
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
regression value obtained for weight suggests that a large portion of the variance between line means is the result of genes which act in an additive manner. Lerner, Asmundson and Cruden (1947) secured a value of approximately 50 percent and Glazener, Blow, Dearstyne and Bostian (1951) a value ranging from 51 to 79 percent for heritability of broiler weight on the basis of individual differences. In traits with high heritabilities, such as weight, however, genetic effects which are not additive in nature may be important. The role of dominance and gene interactions cannot be overlooked in the inheritance of 10-week weight as the interaction of the inbred line X tester indicate (Table 2). Thus, hybridization may have its place in a broiler program, but the possibilities are probably limited as compared with hybrid poultry for egg production. Feathering at six weeks behaved very similar to 10-week body weight. NC6 and NCI8, two of the lines with poor feathering, produced topcrosses with relatively few well-feathered birds. On the basis of the regression value, having an extra 10 percent of the birds characterized by a line well-feathered meant that the topcross progeny of the line would have an additional increment of 1.9 percent well-feathered over the average of all topcross progeny. Significant line differences for this trait in the topcross and the significant regression are evidence that general combining ability is conditioned to a large extent by additive genes. The non-significant interaction for feathering denotes that additive genes or non-genetic factors played a relatively more important role in the topcrosses than dominance or gene interactions. This result is not in complete accordance with the present theories concerning the inheritance of feathering. The observation seems to reveal the need of more research concerning
873
874
E. W. GLAZENEE AND W. L. BLOW
For those traits in which line differences are primarily the function of non-additive genes, the situation is somewhat different. Test crosses may have more utility. Yet, testing for general combining ability in lines that are not differentiated appreciably by additive genes appears to have limited application. The problem is one of culling lines before testing for specific desirable combinations, a problem that becomes more complicated as the number of available inbred lines for testing increases. Although the authors have no alternative to offer, culling on the basis of the performance of the line does not seem to have particular merit for traits with low heritabilities. The use of the topcross test for evaluating the general combining ability of inbred lines in poultry has been adopted from the corn breeder. In corn, the depressing effects of inbreeding appear greater than that of poultry with the slower rate of inbreeding. Beneficial results of additive genes, then, in corn may be largely neutralized by the more rapid approach to homozygosity; therefore, a test designed
for general combining ability would have more application in corn than in poultry. SUMMARY
Topcross matings were made by sires of 8 inbred lines. These topcrosses were compared with inbreds for 10-week broiler weight, 6-week feathering, mortality to 10 weeks, and hatchability. Positive significant regressions were obtained for topcrosses on inbreds for broiler weight and feathering; a regression value approaching zero for mortality and hatchability. These results are probably explained in part on the basis of the heritability of the characters considered. Interactions of the line X tester were calculated and found significant for broiler weights and hatchability. The value of the topcrossing test appears limited in broiler production. ACKNOWLEDGMENT The authors wish to thank Dr. R. E. Comstock, Department of Experimental Statistics, for his consultation in the analysis of the data. REFERENCES Glazener, E. W., W. L. Blow, R. S. Dearstyne and C. H. Bostian, 1951. Effects of inbreeding on broiler weights and feathering in the fowl. (In press.) Lerner, I. M., V. S. Asmundson and D. M. Cruden, 1947. The improvement of New Hampshire fryers. Poultry Sci. 26: 515-524. Lush, J. L., W. F. Lamoreux and L. N. Hazel, 1948. The heritability of resistance to death in the fowl. Poultry Sci. 27: 375-388. Marsden, S. J., and M. W. Olsen, 1950. Fertility and hatchability of eggs from two varieties of turkeys and their reciprocal crosses. Poultry Sci. 29: 548553. Shoffner, R. N., and H. J. Sloan, 1948. Heritability studies in the domestic fowl. Eighth World's Poultry Congress 1: 266-280. Sprague, G. F., and L. A. Tatum, 1942. General vs. specific combining ability in single crosses of corn. J. Amer. Soc. Agron. 34: 923-932. Waters, N. F., 1938. The influence of inbred sires topcrossed on White Leghorn fowl. Poultry Sci. 17: 490497.
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
whereas, in selected material that is being tested in single crosses, genes conditioning specific combining ability have the most effect in determining yield differences. If such is true, top or line cross testing would have very little application in determining combining ability or inbred lines that vary for traits affected largely by additive genes. In those cases in which the heritability of a character is high, the appraisal of the general combining ability of a line is possible on the basis of the performance of the inbred line. Therefore, if broiler weight or feathering were the only traits considered in the establishment of a line, the lines could be culled on the basis of their performance and no testing for general combining qualities would be necessary.
North Carolina Agricultural Experiment Station, Raleigh (Received for publication February 19,1951)
follows: New Hampshire, Barred Plymouth Rock, White Plymouth Rock, Rhode Island Red, and S. C. White Leghorn. The New Hampshire and Barred Plymouth Rock females were broiler-type, the White Plymouth Rock females utility, and the Rhode Island Red and S. C. White Leghorn females egg-type. Sixteen topcross matings, two pens for each inbred line, were used in this study. Three males from each of the eight inbred lines were mated with the five sets of tester females in each breeding pen, with approximately 18 to 20 females per pen.
METHODS AND MATERIALS
Four hatches of chicks, primarily during the month of March, were made for the inbreds and their topcrosses. All chicks were brooded together in starting batteries for approximately 10 days, then transferred to conventional floor brooding. A standard starting and broiler mash was fed. At 6 and 10 weeks of age the chickens were classified as either well-feathered or poorly-feathered and weighed to the nearest ounce. In figuring mortality to 10 weeks of age, birds that were not accountable were classified as dead.
The North Carolina Experiment Station has eight inbred lines of chickens, one S. C. White Leghorn, one Rhode Island Red, three Barred Plymouth Rock, and three New Hampshire, which are inbred from 35 to 55 percent. The lines vary according to their type in origin. NCI, S. C. White Leghorn, NC6, Rhode Island Red, NCI2, New Hampshire, and NC18, Barred Plymouth Rock, are eggtype lines; NC11, New Hampshire, NC16 and NC17, Barred Plymouth Rock, are broiler-type lines; and NC13, New Hampshire, is a utility line. In addition to maintaining the inbred lines, topcross matings for the eight lines were made in 1950. Cockerels from each line were mated to females from five breeds and varieties secured from private breeders. The tester females were as * Contribution from the Department of Poultry Science, North Carolina Agricultural Experiment Station as Journal Paper 379.
RESULTS AND DISCUSSION
The performance of the inbreds and their topcrosses for 10-week body weight, feathering, mortality, fertility, and hatchability of fertile eggs is given in Table 1. The top line of each section in the table gives the performance of the inbred line. For example, at the top of Table 1 is NCI (WL). The data in this line are the performance for the inbred line of S. C. White Leghorns. Listed directly under the
870
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
T I T T L E information is available in the •*-*' literature concerning the combining ability of inbred lines and the performance of their topcrosses for broilers. Waters (1938) observed that inbred White Leghorn sires topcrossed on White Leghorn females produced offspring that were superior in hatchability and viability to random-bred Leghorns. Growth rates were not reported. The following investigation was conducted in an attempt to learn something of the combining quality and performance of inbred lines for broilers.
TOPCROSS TESTING FOR BROILER PRODUCTION
871
TABLE 1.—Weights, feathering, mortality, fertility and hatchability of inbreds and their topcross progeny
Line and crosses
Mean 10-wk. wts.
Percent poorlyfeathered at 6 wks.
No. at 10 wks.
Male
Males
Female
oz.
Percent Females mortality to 10 wks. oz.
Percent eggs fertile
Percent fertile eggs hatched
564 104 52 119 76 82
1 0 0 13 28 0
0 0 0 0 0 0
31.6 39.6 39.9 38.9 33.8 36.4
27.0 33.6 31.0 32.1 29.9 29.4
12 9 12 6 16 9
90 96 87 95 93 85
77 83 79 92 88 88
NC6 (RIR) XNH XBPR XWPR XRIR XWL
316 84 77 103 73 73
76 10 56 44 39 0
53 11 0 23 24 0
33.2 42.4 41.1 40.3 38.9 40.1
29.3 34.6 34.3 34.0 31.8 31.5
36 10 5 9 8 4
86 92 88 89 76 61
71 79 89 90 77 88
NC11 (NH) XNH XBPR XWPR XRIR XWL
509 97 43 101 88 52
6 0 0 23 24 0
0 0 0 0 0 0
43.0 44.8 46.3 42.3 37.2 40.0
36.1 36.7 35.6 34.4 32.5 33.5
15 5 8 12 5 10
84 97 97 90 80 51
62 83 79 92 88 88
NC12 (NH) XNH XBPR XWPR XRIR XWL
259 85 80 73 65 22
16 0 25 33 24 0
3 0 0 9 12 0
32.3 41.1 37.2 39.2 37.3 32.4
27.5 36.7 33.0 34.0 32.2 31.8
19 4 7 13 4
71 89 92 77 97 25
66 81 83 82 83 78
NC13 (NH) XNH XBPR XWPR XRIR XWL
148 76 69 65 60 18
6 0 13 31 30 0
1 0 0 0 14 0
36.8 44.8 39.0 39.7 39.5 40.0
31.1 35.4 33.0 32.9 31.7 28.6
11 7 9 13 9 5
77 80 81 74 67 29
67 79 78 84 85 64
NC16 (BPR) XNH XBPR XWPR XRIR XWL
123 114 89 74 96 63
55 0 50 46 41 17
16 0 17 28 15 0
40.8 48.8 43.3 42.1 42.5 40.9
35.2 38.7 34.5 36.4 34.4 33.0
14 2 11 10 9 10
84 89 95 96 89 70
51 84 90 85 88 84
NC17 (BPR) XNH XBPR XUPR XRIR XWL
136 82 68 68 42 29
23 0 31 47 32 17
11 0 29 23 14 0
37.3 45.6 38.3 39.5 40.9 40.0
35.0 38.6 35.0 36.1 31.9 32.5
13 11 7 13 2 15
72 77 98 84 56 41
63 80 86 86 63 88
NC18 (BPR) XNH XBPR XWPR XRIR XWL
85 104 79 82 97 43
94 13 44 34 29 0
67 9 15 24 23 0
27.6 45.2 36.8 40.4 40.6 36.5
25.5 36.3 31.8 31.8 32.4 30.2
42 11 11 6 6 4
85 93 94 88 91 43
54 86 90 87 92 87
8
•
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
NCI (WL) XNH XBPR XWPR XRIR XWL
872
E. W. GLAZENER AND W. L. BLOW
inbred line is the performance of its topcrosses, NCI inbred malesXnon-inbred NH, BPR, WPR, RIR, and WL females. The same scheme follows for the other inbred lines. Estimates of the association between the performance of the inbred lines and their topcrosses were computed for weight, feathering, mortality, and hatchability by the regression of topcross performance on that of the inbred. The topcross perform-
failure of the topcross progeny of a line to vary the same no matter which tester parent was used are primary points of interest. The difference between lines and between testers for broiler weight and feathering, and the interaction of lineX tester for broiler weight and hatchability of fertile eggs are significant at the .01 level. As is observed in Table 1, the males from NC11 had a weight of 43.0 ounces
TABLE 2.—Analysis of variance for topcross performance
Feathering
D.F.
Mean sq.
D.F.
Mean sq.
Total Between sexes Between hatches Between lines Between tester LineX tester Error
316 1 3 7 4 28 273
4,218.00 292.67 108.14f 275.75t 11.68t 4.57
209 1 2* 7 4 28 167
15,286.00 1,385.00 l,817.00t 3,961.50f 338.89 317.04
Mortality D.F.
Mean sq.
D.F.
Mean sq.
159
147 3 7 4 28 105
Hatchability
190.70 29.60 47.55 66.17 81.15
3 7 4 28 117
286.53 176.97 197.92 176.24f 94.91
* Only three hatches reported, as feather picking was prevalent in the fourth hatch, f Line, tester, and line X tester mean squares significant at .01 level.
ance for each line was taken as the unweighted mean of the five topcross means for each line. These regressions, with their standard errors were as follows: Weight Feathering Mortality Hatchability
.29+.08 . 18 ± . 06 -.06+.03 - . 1 3 +.16
The positive values for weight and feathering are significant at the .05 level, whereas those for mortality and hatchability are non-significant. For these four characters, an analysis of variance was computed (Table 2). The total sum of squares was divided into the sums of squares associated with sex, hatch, line, and tester, the line X tester interaction, and the unaccounted variation designated as the error. In this analysis, estimates of the mean squares which are associated with differences in topcross performance between the various lines and tester breeds, and the success or
as compared to that of 32.3 ounces for NC12 at 10 weeks. Furthermore, the topcrosses of these two New Hampshire lines were in the same relative rank. The NC11 and NC16 lines were the most rapid-growing of the eight lines and also produced the topcrosses with the superior 10-week body weights. The linear regression values for the four characters are indicative of the relative variation between line means which is due to additive genes. If the heritability of individual differences were 100 percent, the regression of topcross performance on that of the inbred would be .5. The deviation between .5 and the regression value for weight, .3, may be due to interline non-additive genetic variance and intra-line environmental and genetic variance. If the number of chicks involved in each line is sufficiently large, then the deviation probably reflects mainly nonadditive inter-line genetic differences. The
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
Body weight
Source of variation
TOPCROSS TESTING FOR BROILER PRODUCTION
the inheritance of rate of feathering at broiler age. Mortality to 10 weeks of age and hatchability of fertile eggs reacted very similarly in the topcross test in that neither character showed a significant regression or a significant difference between lines or testers in the analysis of variance. These characters are generally accepted as having heritabilities of a low magnitude. Lush, Lamoreux and Hazel estimated the heritability of livability as 14 percent and Shoffner and Sloan (1948) hatchability as 18 percent. In spite of the rather marked differences observed in the performance of the inbred lines, the inbred lines exhibited no difference in combining ability for these two traits. This result suggests that the differences among the lines are the effect of factors primarily other than additive genes. The apparent lack of any line differences in the topcrosses for mortality may well be the result of the operation of threshold effects for this character, where only a minimum level of vitality is necessary to insure survival. That hatchability is influenced by maternal effects is quite possible. Marsden and Olsen (1950) in a study of fertility and hatchability in turkeys observed that the dam might have an important nongenetic effect on hatchability. If maternal effects were of primary importance in this particular case, however, a significant difference should exist between testers for hatchability. Also, the line and the line X tester interaction should be nonsignificant. These conditions are not satisfied. According to the positive interaction obtained for hatchability, factors of a non-additive nature are paramount in this trait. Sprague and Tatum (1942) assumed that in previously untested material, additive genes which affect general combining ability are of most importance;
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
regression value obtained for weight suggests that a large portion of the variance between line means is the result of genes which act in an additive manner. Lerner, Asmundson and Cruden (1947) secured a value of approximately 50 percent and Glazener, Blow, Dearstyne and Bostian (1951) a value ranging from 51 to 79 percent for heritability of broiler weight on the basis of individual differences. In traits with high heritabilities, such as weight, however, genetic effects which are not additive in nature may be important. The role of dominance and gene interactions cannot be overlooked in the inheritance of 10-week weight as the interaction of the inbred line X tester indicate (Table 2). Thus, hybridization may have its place in a broiler program, but the possibilities are probably limited as compared with hybrid poultry for egg production. Feathering at six weeks behaved very similar to 10-week body weight. NC6 and NCI8, two of the lines with poor feathering, produced topcrosses with relatively few well-feathered birds. On the basis of the regression value, having an extra 10 percent of the birds characterized by a line well-feathered meant that the topcross progeny of the line would have an additional increment of 1.9 percent well-feathered over the average of all topcross progeny. Significant line differences for this trait in the topcross and the significant regression are evidence that general combining ability is conditioned to a large extent by additive genes. The non-significant interaction for feathering denotes that additive genes or non-genetic factors played a relatively more important role in the topcrosses than dominance or gene interactions. This result is not in complete accordance with the present theories concerning the inheritance of feathering. The observation seems to reveal the need of more research concerning
873
874
E. W. GLAZENEE AND W. L. BLOW
For those traits in which line differences are primarily the function of non-additive genes, the situation is somewhat different. Test crosses may have more utility. Yet, testing for general combining ability in lines that are not differentiated appreciably by additive genes appears to have limited application. The problem is one of culling lines before testing for specific desirable combinations, a problem that becomes more complicated as the number of available inbred lines for testing increases. Although the authors have no alternative to offer, culling on the basis of the performance of the line does not seem to have particular merit for traits with low heritabilities. The use of the topcross test for evaluating the general combining ability of inbred lines in poultry has been adopted from the corn breeder. In corn, the depressing effects of inbreeding appear greater than that of poultry with the slower rate of inbreeding. Beneficial results of additive genes, then, in corn may be largely neutralized by the more rapid approach to homozygosity; therefore, a test designed
for general combining ability would have more application in corn than in poultry. SUMMARY
Topcross matings were made by sires of 8 inbred lines. These topcrosses were compared with inbreds for 10-week broiler weight, 6-week feathering, mortality to 10 weeks, and hatchability. Positive significant regressions were obtained for topcrosses on inbreds for broiler weight and feathering; a regression value approaching zero for mortality and hatchability. These results are probably explained in part on the basis of the heritability of the characters considered. Interactions of the line X tester were calculated and found significant for broiler weights and hatchability. The value of the topcrossing test appears limited in broiler production. ACKNOWLEDGMENT The authors wish to thank Dr. R. E. Comstock, Department of Experimental Statistics, for his consultation in the analysis of the data. REFERENCES Glazener, E. W., W. L. Blow, R. S. Dearstyne and C. H. Bostian, 1951. Effects of inbreeding on broiler weights and feathering in the fowl. (In press.) Lerner, I. M., V. S. Asmundson and D. M. Cruden, 1947. The improvement of New Hampshire fryers. Poultry Sci. 26: 515-524. Lush, J. L., W. F. Lamoreux and L. N. Hazel, 1948. The heritability of resistance to death in the fowl. Poultry Sci. 27: 375-388. Marsden, S. J., and M. W. Olsen, 1950. Fertility and hatchability of eggs from two varieties of turkeys and their reciprocal crosses. Poultry Sci. 29: 548553. Shoffner, R. N., and H. J. Sloan, 1948. Heritability studies in the domestic fowl. Eighth World's Poultry Congress 1: 266-280. Sprague, G. F., and L. A. Tatum, 1942. General vs. specific combining ability in single crosses of corn. J. Amer. Soc. Agron. 34: 923-932. Waters, N. F., 1938. The influence of inbred sires topcrossed on White Leghorn fowl. Poultry Sci. 17: 490497.
Downloaded from http://ps.oxfordjournals.org/ at UCSF Library on April 25, 2015
whereas, in selected material that is being tested in single crosses, genes conditioning specific combining ability have the most effect in determining yield differences. If such is true, top or line cross testing would have very little application in determining combining ability or inbred lines that vary for traits affected largely by additive genes. In those cases in which the heritability of a character is high, the appraisal of the general combining ability of a line is possible on the basis of the performance of the inbred line. Therefore, if broiler weight or feathering were the only traits considered in the establishment of a line, the lines could be culled on the basis of their performance and no testing for general combining qualities would be necessary.