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The Development and Use of Chicken Inbreds
T h e Development and Use of Chicken Inbreds C H A R L E S W.
KNOX
Agricultural Research Administration, Bureau of Animal Industry, Agricultural Research Center, Beltsiille, Maryland (Received for publication December 8, 1945)
A S a general thesis continued inbreeding •£*• increases the homozygosity of the gene complement. This was adequately demonstrated statistically by Wright (1922 and 1923). The reason for inbreeding is to produce purity (homozygousness) in a large number of factors with little or no knowledge of their particular mode of inheritance. The purpose of inbreeding is to obtain successful lines of inbreds and to demonstrate their superior economic possibilities in respect to incrossing, incrossbreeding and topincrossing. A committee of the American Society of Animal Production (1941) defines these terms as follows. Incrossing is considered as the crossing of two different inbred lines of the same breed and variety, incrossbreds the result of crossing two lines of inbreds of different breed and variety and topincross the crossing of inbred males on outbred or relatively slightly inbred females of the same breed and variety. The first part of this paper deals with the development of inbreds and the second part with the use of inbred lines in crosses, in an attempt to produce progeny of superior value to the commercial poultry farmer. INBREEDING In corn breeding, because self-fertilization can be practiced and data obtained on thousands of lines and crosses, much 262
can be left in finding the better lines by chance. In livestock and poultry breeding, "selfing," is impossible and the number of lines which may be developed is limited by cost in animals, in time, and in facilities. Although persistent inbreeding produces homozygousness of genes, it is also true, at least in chickens, that some detrimental factors, rather than good ones, may accumulate. This seems to be especially true of such traits as fertility and hatchability. However, it should be possible, by maintaining rigid standard selection, to guide the inbreeding so that only the factors wanted are retained. Matings with a coefficient of inbreeding of less than 25% can be maintained until most of the deleterious genes are eliminated or their effect minimized. Successive mating of brother and sister, with care to maintain selection for the factors desired, can follow these preliminary matings. Such a procedure was followed in the present experiment. In previous investigations involving inbreeding, Cole and Halpin (1916, 1922), Goodale (1927), Dunn (1923, 1928), Jull (1929, 1930, and 1933), Dumon (1930), Dunderly (1930), Warren (1934), British Ministry of Agriculture and Fisheries (1934), Byerly, Knox and Jull (1934), Hays (1929, 1934), and others found that hatchability decreased rapidly with successive brother-sister matings or their equivalent. In many instances hatchability decreased to such an extent that the
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INTRODUCTION
263
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
TABLE 1.—1945 incubation data for present generation of one inbred family? sire 13625, dam 236 Percent
Male
Mated to
Coefficients of inbreeding* in percent
No. of eggs set
3701
Unselected 3 sister 1307
60.4
Hatch of fertile eggs
Hatch of total eggs
28
89.3
•89.3
12.5 to 28.0
19
100.0
31.6
Unselected sisters 989 1069* 11144
60.4 60.4 60.4
38 32 36
,81.1 84.4 74.3
Inbred White Leghorns
12.5 to 28.0
46
Unselected sisters 10654 1112 1115* 1217*
60.4 60.4 60.4 60.4
Inbred White Leghorns
12.5 to 28.0
Inbred White Leghorns .414
78.9 84.4 • 72.2
80.4
80.4
30 27 30 33
58.6 76.9 82.8 97.0
56.7 74.1 80.0 97.0
48,
91.7
91.7
-
1 Family, as used here, refers to the sons and daughters of a single sire mated to a single dam, in this case 13625 mated to 236. 2 Coefficients of inbreeding refer to the females in the mating. 3 Unselected refers to fertility and hatchability only. * Progeny from these lines will be used to produce the future inbred families. The others will be dropped. Line, as used here, indicates the progeny of a single female.
results obtained forced the termination of the project. Recently several papers have used Wright's coefficients of inbreeding involving a small gene complex and with greater or lesser degree of selection and interpreted results from such coefficients. • Wright's coefficients of inbreeding serve a useful purpose as a statistical measure of the homozygosity of a large gene complex, and is formulated without selection being operative and includes equal transmission of genes by both sexes. Care should be taken in this regard whenever these coefficients are used as a measure or in the interpretation of the results of inbreeding. These papers have established a precedent that this author is following; however, it is doubtful if this procedure can be followed with impunity. For instance, it is possible by proper selection to produce homozygous plumage colors with a 0.0 percent coefficient of inbreeding. On the other hand, the coefficient of inbreeding can be developed to a high degree and
have the plumage color genes remain heterozygous primarily due to selection. It is possible where single quantitative characters are concerned in inbreeding that the results obtained may be a better measure of the success and degree of homozygosity than a particular statistic— at least care must be taken in the interpretation of both. In Table 1 are placed the 1945 incubation data and coefficients of inbreeding for the individuals in a single line of inbred Rhode Island Reds and their crosses with slightly inbred White Leghorn females. In Table 2, the incubation data and coefficients of inbreeding are given for several successive years for an inbred family (data on individuals in Table 1) for incrossbreds, outbred Rhode Island Reds and White Leghorns. The White Leghorns varied from 3 % to 28% coefficient of inbreeding with more than 95% of them being between 20 and 25%. The present generation of inbred Rhode Island Red chickens has a coefficient of inbreeding of
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416
4
264
CHARLES W. KNOX
tion must be applied in the selection of the foundation parental stock at the very beginning of the project and maintained at all times until the specific line of inbreds is breeding true for the particular characteristics desired. Secondly, its success depends upon the relationship that exists between the phenotypic and genotypic complement. Thirdly, environment plays a very important part as in most cases it will cause such great variation of the quantitative trait under observation that it is impossible to make perfect selection. Hence selection, while not perfect, is maintained until inbreeding has progressed sufficiently to produce homozygosity or perfection for the trait desired. On the other hand, if inbreeding makes any deleterious factor homozygous, selection is not effective, in which case it is necessary to discard the entire inbred line and start over again. In this and other experiments many lines were discarded because of this impasse. The data in Table 2 express the results of this conflict between inbreeding (toward homozygosity) and selection (for controlled direction). The average of the inbred family, chosen for illustration, for the first three years is downward, 79.8 percent hatch of fertile eggs to 53.3 percent and 79.5 percent of total eggs set to 43.9 percent. This indicates that both fertility and hatchability were decreasing with each generation of inbreeding, brother-sister mating for the first three years. Selection was maintained successfully for these factors during this time as expressed by the data for the individuals used as breeders, since only slight loss of hatchability is noticeable with practically no loss in fertility during the same three years. For the last two years, selection has asserted itself and the inbreeding has "fixed" to a considerable degree high fertility and hatchability. Only one dam,
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approximately 68%. The data in Table 1 are for the individuals chosen for further breeding work, whereas the data in Table 2 are for the entire family, some of which are not being used for continuance of the project. The family averages are included for outbred Rhode Island Reds, outbred Single Comb White, Leghorns and incrossbreds from Single Comb White Leghorn females as a measure of the value of the inbred family. Waters (1932), publishing results on a project for the inbreeding of White Leghorn fowls, which was initiated by and under the leadership of the present author from 1926 to 1931 inclusive, stated that the success of this inbreeding study is due in no small measure to the rigid selection practiced, for genetic advance and careful selection travel the same highway. Waters and Lambert (1936) published data on ten years of inbreeding on this same project. They showed a gradual decrease in average hatchability of inbreds but practically no decrease for those inbreds chosen as parents. In Tables 1 and 2 the data indicate that successive generations of brother and sister matings or their equivalent can be maintained without loss of fertility or hatchability. This is contrary to the findings of previous investigators; however there is a possible explanation. Mating brothers and sisters or their equivalent for successive generations develops a high coefficient of inbreeding. Careful selection for good fertility and hatchability factors, seems necessary in order to avoid very p'oor fertility and hatchability which usually resulted from inbreeding. It is evident therefore that much depends on the early selection for high fertility and hatchability and maintenance of selection for these characteristics. The success of selection depends mainly upon three factors. The first is that selec-
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
265
TABLE 2.—Incubation data on successive generations of a single family of inbred R. I. Reds, incrossbreds, outbred1 R. I. Reds, and outbred? White Leghorns ', Year
Male or males
1945 3 Inbred R. I. Reds 3 Inbred R. I. Reds 4 Outbred R. I. Reds
Percent Mated to
8 unselected sisters" 6 Inbred W. Leghorns 4 63 Outbred R. I. Reds
4 Outbred W. Leghorns 69 Outbred W. Leghorns
60.4
254
80.5
79.8
12.5 to 25.0
113
90.7
67.9
0.0
1195
85.9
72.7
0.0
1574
83.1
75.7
51.3
35
94.3
94.3
1 Inbred R. I. Red
2 Inbred W. Leghorns 3 unselected sisters
12.5 to 25.0 51.3
40 104
100.0 73.5
100.0 73.4
1 Inbred R. I. Red
2 Inbred W. Leghorns
12.5 to 25.0
40
100.0
100.0
0.0
1377
83.5
70.4
0.0 38.9
1311 16
80.3 81.3
74.1 81.3
93
76.3
31.2
4 Outbred R. I. Reds
Selected sister 236
Hatch Hatch of fertile of total eggs eggs
70 Outbred R. I. Reds
4 Outbred W. Leghorns 66 Outbred W. Leghorns 1943 1131 Selected sister 6730 3 Inbred W. Leghorns 2 Inbred R. I. Reds
5 unselected sisters
2 Inbred R. I. Reds
7 Inbred W. Leghorns
4 Outbred R. I. Reds
6 Outbred R. I. Reds
4 Outbred W. Leghorns 73 Outbred W. Leghorns 1942 2374
Selected sister 4801 3 Inbred W. Leghorns
2 Inbred R. I. Reds
5 unselected sisters
2 Inbred R. I. Reds
6 Inbred W. Leghorns
4 Outbred R. I. Reds
75 Outbred R. I. Reds
3.0 to 25.0 38.9
81
53.3
43.9
158
74.0
50.1
0.0
1481
91.8
78.1
0.0
1237
77.7
66.4
27 54
84.6 64.7
81.5 61.1
3.0 to 25.0
27.6 3.0 to 25.0 27.6
118
83.3
72.6
3.0 to 25.0
106
73.9
71.9
0.0
1550
85.4
62.1
4 Outbred W. Leghorns
67 Outbred W. Leghorns
0.0
1280
80.6
69.2
2843
Selected sister 695
0.0
28
89.3
89.3
No matings made with White Leghorns 6 unselected sisters
0.0
106
79.8
79.5
4 Outbred R. I. Reds
68 Outbred R. I. Reds
0.0
1373
86.8
81.0
4 Outbred W. Leghorns
65 Outbred W. Leghorns
0.0
1290
80.2
69.4
1 Inbred R. I. Red
1 Hatching eggs purchased periodically and the best male obtained from these eggs used in one of the four2 outbred matings. No outside eggs or stock purchased since 1935. 8 Coefficients of inbreeding refer to the females in the mating. 4 Inbred and incrossbred progeny from these pullets are half sisters as are similar groups for preceding years.
i
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1944 13625
Coefficients No. of of inbreeding3 eggs in percent set
266
CHARLES W. KNOX TABLE 3.—Production and body weight data on a single inbred family and their incrossbreds1 Inbred 2 pullets
969 989
Average egg weight in grams
Days to maturity
48 206 206 206 186 233 219 185 148 32
54.2 63.4 58.6 60.8 59.6 56.0 53.4 54.2 59.5 50.7 57.0 57.5
344 242 175 248 179 228 169 165 260 335
166.9 196.1
234.5 202.1
Mature body weight in grams 1970 3150 1985 2312 2614 2380 2709 2580 2520 2162 2438 2016
1 Progeny of an inbred R. I. Red male of this family by inbred white Leghorn females. The resulting incrossbreds, therefore, are half-sisters to the inbreds. 2 These inbred pullets are sisters and have a coefficient of inbreeding of 60.4 percent.
1065, among the 8 (Table 1) gave less than excellent hatching results. This may have been due to poorer environmental conditions during the growing period (note in Table 3 the difference in mature body weight when compared with that of the others) or the "pull" of inbreeding toward poor fertility and hatchability. In any event, this would indicate that selection cannot be diminished until some future generation shows perfect "fixation" by inbreeding. The measure of success in maintaining the fertility and hatchability of the inbreds is reflected in the results obtained when the Rhode Island Red inbreds are crossed with the inbred White Leghorn females, the resulting inbreds being halfsisters to the inbred Rhode Island Red progeny. Another measure is the comparison of hatchability of total eggs set for outbred Rhode Island Reds, which was approximately 62 to 81 percent and for the outbred White Leghorns which was 66 to 75 percent for the same years, as expressed in Table 2. The hatchability of fertile eggs was about 84 to 92 percent and 78 to 83 percent respectively. During the same period the Rhode Island Red inbreds varied from a low of 44 percent
hatch of total eggs set to 80 percent and from 53 percent to 83 percent of fertile eggs. The incrossbreds varied from a low average hatchability of total eggs set of 50 percent to a high of 100 percent and of fertile eggs set from 74 percent to a high of 100 percent. By comparing the data for the various groups, as given in Table 2, it appears that successive generations of brother-sister matings can be made without materially affecting the fertility or hatchability of the inbreds when compared to the incrossbreds or outbred Rhode Island Reds and White Leghorns. As shown in Table 3 selection and inbreeding are involved in a similar manner for most of the quantitative traits of interest to the poultry farmer, namely annual egg production, average annual egg weight, days to maturity, and mature body weight. Breeding is a matter of compromise whether it is some form of outbreeding or inbreeding and selection for a group of characteristics is also a matter of compromise so that in this experiment the best selection may not be used for a single characteristic but rather the best selection for the group. According to Hazel and Lush (1942) group selection appears to be much more efficient than selection for
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1065 1069 1112 1114 1115 1217 1307 1311 Averages (10) Incrossbreds (11)
Average number of eggs
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
when used in incrossbreeding. The variation in individual averages for egg production, egg weight, days to maturity, and body weight indicate that selection must be continuous and the excellent possibilities for success in selecting an inbred line of high egg production, good egg weight, early maturity and excellent body weight. In this conclusion can be included excellent fertility and hatchability as indicated by the data in Table 1. The data for average egg production, egg weight, days to maturity and mature body weight in Table 4 are for the results of brother-sister matings for successive years and for outbred Rhode
TABLE 4.—Production and mature body weight data for a single inbred R. I. Red family generations and outbred1 R. I. Reds
Year
Pullets
Selected Inbred 1115 1944 to 1945
1943 to 1944
1942 to 1943
1941 to 1942
1940 to-1941
1
Av. Coefficients Annual egg of inbreeding production annual egg wt. in percent in numbers in grams 60.4
in:
Days to maturity
Mature body weight in grams
219
53.4
169
2709
All Inbreds (10)
60.4
167
57.0
234
2438
11 Incrossbreds 2
12.5 to 25.0
196
57.5
198
1966
193 Outbred R. I. Reds
00.0
155
55.1
209
2224
Selected Inbred 236
51.3
200'
57.6 3
182
2853
All Inbreds (3)
51.3
115
56.9
268
2347
36 Incrossbreds 2
12.5 to 25.0
219
61.3
207
2236
196 Outbred R. I. Reds
00.0
195
57.4
212
2628
Selected Inbred 6730
38.9
225
57.8
213
2646
All Inbreds (7)
38.9
145
57.6
240
2572
16 Incrossbreds 2
12.5 to 25.0
172
61.5
215
2369
219 Outbred R. I. Reds
00.0
155
57.4
224
2491
Selected Inbred 4801
27.6
221
58.0
196
2398
All Inbreds (9)
27.6
189
56.3
195
2493
170 Outbred R. I. Reds
00.0
186
54.9
187
2650
185
2630
Selected Inbred 695
00.0
271
57.7
All Inbreds (5)
00.0
238
59.3
179
2632
173 Outbred R. I. Reds
00.0
208
58.4
198
2527
Use for reference points. Half-sisters to the inbred pullets (inbred R. I. Red male mated with inbred White Leghorns). * Estimated for period after death.
2
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one trait at a time. This type of selection, therefore, would be a cause for some irregularity of progress for some single trait and would also take a longer time to reach the same goal than if selection is made for a single trait. Taken as a whole, however, a better line of inbreds is produced, one that can be continued and also will not be poorer than the standard breds used as foundation stock. The data in Table 3 are for the same family used to illustrate inbreeding and hatchability (Table 1). The data in this table show how well the quantitative traits are being maintained with inbreeding and also how well this family is doing
267
268
CHARLES W. KNOX
Thus far only a single family of Rhode Island Red inbreds have been used for illustration and discussion. However, many lines have been used in the experiment most have been discarded and a few have been continued and their number increased. HYBRIDS
In previous publications, Knox and 01sen (1938) and Knox (1939), attention was brought to the fact that the term hybrid, through usage, could mean the progeny of any one of several different types of crossing:
1. Crossing individuals of different genera, species, or sub-species. 2. Crossing individuals of different breeds or varieties. 3. Top crossing inbred males on outbred females of the same breed and variety. 4. Crossing inbred individuals of different breeds or varieties. 5. Crossing inbred individuals of the same breed and variety from different inbred lines. In previous work Smith and HaigThomas (1913), Phillips (1913, 1915, 1916, 1921), Cutler (1918), Serebrovsky (1929), Tiniakoff (1933, 1934), Warren and Scott (1935), Quinn, Burrows and Byerly (1937), and others have reported work with true hybrids, hybrids of the first type. These hybrids although of academic interest appear to be of no immediate economic value. Crossing different breeds and varieties, the second type of hybrid production, has received considerable attention as shown by Pearl and Surface (1910), Warren (1927, and 1930), Horlacher and Smith (1938), Knox and Olsen (1938), Jeffrey (1939), Knox (1939), Knox, Quinn and Godfrey (1943), Hoffman (1943), and Horlacher, Smith and Wiley (1941). The consensus of opinion seems to be that crossing different breeds and varieties produces progeny that grow faster to 12 weeks of age than the standardbred parental stock used in the cross and also that the crossbred stock has better viability. If the parental stock differed markedly in body weight, the adult body weights of the crossbreds were intermediate. The effect of crossbreeding on average annual egg production is variable with slightly more evidence of less production by the crossbreds than by purebred progeny.
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Island Reds. The annual average results for the outbred Reds which acted as a control, are given each year for comparison, as the quantitative traits measured appeared to vary considerably from year to year. No culling or selection was made on any of the females of the various groups from the time they were hatched until the end of their laying year. During a five year period, 1940-1945, the average annual egg production of the outbred Rhode Island Red pullets varied from 208 eggs in 1940-1941 to 155 in 1942-1943. During the same five year period the inbred Rhode Island Reds varied from 238 in 1940-1941 to 115 in 1943-1944. Considering the 1944-1945 year also, it appears that the inbred Rhode Island Reds compare favorably with the outbred Reds. As a matter of fact they compare favorably with the outbreds in all traits with the exception of the average days to maturity. This trait seems to be later with such successive year of inbreeding. Referring to Table 3, however, it will be noted that there are several possibilities for selecting early maturing individuals in the inbred line. Four of the inbred pullets matured in less than 180 days. This inbred line appears to offer excellent opportunity for selection and further study of all four of these traits.
T H E DEVELOPMENT AND USE or CHICKEN INBREDS
successful commercial egg production. Therefore this part of the experiment was designed to cross inbreds of high coefficients of inbreeding, 1. with inbreds of the same breed and variety (incrossing with Rhode Island Reds), 2. with inbreds of a different breed and variety (incrossbreeding with Single Comb White Leghorns), and 3. with outbred stock of the same breed and variety (topincrossing with Rhode Island Reds). Comparisons of results were made also with outbred stock of the same breed and variety (Rhode Island Reds) and with outbred stock of a different breed and variety (Single Comb White Leghorns). For these purposes the Single Comb Rhode Island Red and Single Comb White Leghorn were used and the experiment started in its present form in 1940 although some preliminary work was started in 1935. During the years 1935-1936 and 19361937 only two single male ma tings were involved. Eight were used in 1940 and this number was gradually increased until 20 matings were used in 1943 and 1944. Most matings had an inbred Rhode Island Red male mated to at least three of his sisters (inbred line), three inbred Rhode Island Reds from a different line (to produce incross progeny), three outbred Rhode Island Reds (to produce topincross progeny), and three slightly inbred White Leghorns (to produce incrossbred progeny). Approximately 175 outbred Rhode Island Red and White Leghorn pullets each year completed a period of 365 days from the date of first egg and the number of eggs that they laid during this period was used in computing the average annual egg production. In 1935 and 1936 about 50 incrossbred and crossbred Rhode Island Red by White Leghorns, respectively, were used in calculating the average an-
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There may be several reasons for such a difference of results; first, the amount of variation encountered in studies of egg production; second, crossbreds are sometimes broodier than either parental strain so that egg production may be reduced because of broodiness; third, many reports were made with too few numbers of individuals, and lastly, even fewer investigators attempted to repeat the crosses in subsequent years, which in view of the variation encountered in egg production strains and between years must be repeated for several years, before adequate interpretation of results can be made. The statement that crossing breeds with a hatchability of about 80 percent lowered hatchability about as often as it increased it among the Beltsville flocks, was made by Byerly, Knox and Jull (1934). A similar statement may be made in regard to egg production, to wit, the crossing of breeds or varieties with an average annual egg production of 200 eggs or more produced progeny with a lower annual egg production than that of the parental standardbreds about as often as it increased it among the Beltsville flocks. The last three types of hybrids depend upon the use of inbreds. The use of inbreds in crossing is really attempting to simulate the crossing of inbred strains of corn for the production of successful hybrids. The value of inbreds, when used in crosses, is that heterosis in the hybrids is so controlled that similar sets of favorable genes are produced in subsequent generations of progeny. Consequently inbreeding reduces variability to a minimum and subquent crosses of inbreds give relatively similar and predictable results from year to year from the same inbred lines. It would appear that the use of inbreds in crosses offers considerable possibilities for producing hybrids, as herein defined, for
269
270
CHARLES W. KNOX
was practiced and the coefficient of inbreeding would be negligible. Not all inbred lines behave alike when crossed and although averages are given for all lines, in Table 5, it is probable that from now on inbreds can be selected that will prove to be even better for crossing than reported here. Hence the average annual egg production given here for the incrossbreds is a minimum figure and probably can be increased. The superiority in egg production of the progeny from the inbred Rhode Island
TABLE 5.—Annual egg production data of all inbred R. I. Reds, incrossbreds, and outbred R. I. Reds1 and White leghorns2 Outbred White Outbred Leghorn males Reds cf mated to mated to Outbred OutInbred Reds Inbred Out9 9 bred bred White W. Leg. Leg. Reds Reds
Inbred R. I. Red males mated to Year UnreInbred Inbred lated Reds Reds Inbred > 2 5 % < 2 5 % Reds 1935-36 1936-37 1940-41 1941-42 1942-43 1943-44 Average
183 155 139 159 159.0
195 165 165 202 181.8
Outbred R.I. Reds
231 236 234 187 190 216 197.7
180 185 199 191.3
196 225 224.4
201 188 208 186 155 195 189.1
199 196 204 193 196 184 195.3
219 213
2093
Differences in average annual egg production between Incrosses & Outbred Reds
Outbred W. Leg.
Inbred Reds
30 48 26
32 40 30
51
41 30 35.0
0 41 28.6
57 66 58.0
1 Hatching eggs purchased periodically and the best male obtained from these eggs used in one of the four outbred matings. 2 No outside eggs or stock purchased since 1935. 3 5 pullets only.
The data given in Table 5 give the average results for the inbreds, crosses, outbred Rhode Island Reds and Single Comb White Leghorns for the various years that data were obtained. The outbred Reds were maintained as outbreds by periodically purchasing 100 hatching eggs from one of the nation's foremost breeders about once every three years and the best male hatched from these eggs, used in one of the four current breeding pens. In the outbred White Leghorns no foundation stock was purchased since 1935; however practically no inbreeding
Red males mated to the inbred Leghorn females is in evidence each year. They average to lay 26 to 48 eggs more than outbred Rhode Island Red pullets and with the exception of one year out of 5 laid 30 to 41 eggs more than the outbred White Leghorns. The one year in question 1942-1943, the incrossbreds were kept in the same .house, cared for by the same attendant and otherwise were similarly managed and subjected to similar environment as the outbred Rhode Island Red pullets. Both the Rhode Island Red and incrossbred pullets suffered laryngo-
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nual egg production for the incrossbred and crossbred pullets. For each of the last three years, there were about 175 incrossbred pullets. In 1940 there were 246 Rhode Island Red inbred pullets with an average annual egg production of 183 eggs. This number of inbreds was increased until in the last two years they involved approximately 350 pullets each year with an average annual egg production of 139 and 159 eggs, respectively. There were approximately 50 incross and topincross pullets each year.
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
were obtained from outbred progeny. The crosses between the inbred lines were not any better than for the outbred or top incross progeny, possibly because enough relationship remained between the lines to make the gene complements similar. In any event best results were obtained from the incrossbreds between Rhode Island Reds and White Leghorns in respect to most of the commercial traits considered. SUMMARY
The data presented indicate: 1. That lines of inbreds may be produced without a decrease of fertility, hatchability, mature body weight, or annual egg production. 2. That selection must be initiated and maintained during inbreeding in order to control the direction that homozygosity takes during inbreeding. 3. That incrossbreds produced from inbred Rhode Island Reds mated to Single Comb White Leghorns gave superior results in annual egg production to the standardbred Rhode Island Reds and White Leghorns in this experiment. 4. That incrossing and topincrossing were not as favorable for annual egg production as incrossbreeding, but were improving with each successive year that the inbreds improved, namely the last three years. LITERATURE CITED
British Ministry of Agriculture and Fisheries, 1934. Experiments on inbreeding poultry. Bull. 83. Byerly, Theodore C, C. W. Knox, and Morely A. Jull. 1934. Some genetic aspects of hatchability. Poultry Sci: 13: 230-238. Cole, L. J. and J. G. Halpin, 1916. Preliminary report of an experiment on close inbreeding. Jour. Amer. Assn. Instr. Invest, in Poul. Husb. 3: 7-8. •, 1922. Results of eight years of inbreeding Rhode Island Reds. Anat. Rec. 23: 97. Cutler, D. W., 1918. On the sterility of hybrids between the pheasant and the Gold Campine fowl. Jour. Gen. 7:155-165. Dumon, A. G., 1930. The effect of inbreeding. Proc. Fourth World's Poult. Cong. 48-71.
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tracheitis and recurrent outbreaks of infectious coryza of epizootic proportions. In spite of this the average annual egg production of the incrossbreds was 196 eggs, the same as for the Single Comb White Leghorn pullets, but 41 eggs more than were laid by the Rhode Island Reds which were subjected to similar conditions. The Single Comb White Leghorn pullets were fed a similar diet to that of the others but were kept in another house on a different part of the experimental farm and in charge of a different poultryman. These were not subjected to epizootic disease conditions. However they did not average to lay any more eggs than the incrossbreds, the average production of the White Leghorns for the same year being 196 eggs. It will be noted that as inbreeding continued from year to year there appears to be a marked increase in the egg production of the incross progeny (standardbred Rhode Island Reds). It may be possible to increase this number of eggs further until it at least equals that of the incrossbred and topincross progeny. It should be kept in mind that the data are for all inbreds and their crosses. One of the inbred lines produced incrossbreds that had an average annual egg production of 254 eggs. Hence it may be possible to increase proportionately the average production of the incross, topincross and incrossbred progeny. At the present time, however, the incross and topincross progeny have lower average egg production than the incrossbred progeny but at least as favorable annual production as the outbred Reds and outbred White Leghorns. Maw (1942) found top-crossing (topincrossing) gives better results than crosses between inbred lines where some relationship exists between the lines. The results obtained from topincrosses of the present paper were not any better than
271
272
CHARLES W. KNOX Pearl, Raymond and F . M. Surface, 1910. Studies on hybrid poultry. Me. Agr. Exp. Sta. Bull. 179. Phillips, J. C. 1913. Reciprocal crosses between Reeve's pheasant and the common ring-neck pheasant producing unlike hybrids. Amer. Nat. 47: 701-704. , 1915. Experimental studies of hybridization among ducks and pheasants. Jour. Exp. Zool. 18:69-112. , 1916. Two pheasant crosses. Jour. Hered. 7: 7-12. , 1921. A further report on specie crosses in birds. Gen. 6: 366-383. Quinn, J. P., W. H. Burrows, and T. C. Byerly, 1937. Turkey-chicken hybridization. Jour. Hered. 28:169-173. Serebrovsky, A. S., 1929. Untersuchungen ttber Artbastarde bei Hiihnern. Jour. Gen. 21:327-340. Smith, G., and R. Haig-Thomas, 1913. On sterile and hybrid pheasants. Jour. Gen. 3:39-52. The American Society of Animal Production, 1941. Report of committee on investigations. Proceedings of the 1940 annual meeting of the American Society of Animal Production, p. 378. Tiniakoff, G. G., 1933. Hybrid of peacock and hen. (Russian with English summary.) Verhandl. d. landwirtsch. Inst. f. Bastardierung u. Acclimatisation in Ascania Nova. 1: 85-98. , 1934. Peacock and hen hybrids and a comparative analysis of their parents. Jour. Biol. 3:41-63. Warren, D. C , 1927. Hybrid vigor in poultry. Poultry Sci. 7:1-8. , 1930. Crossbred poultry. Kan. Agr. Exp. Sta. Bull. 252. -, 1934. The influence of some factors on the hatchability of the hen's egg. Kan. Agr. Exp. Sta. Bull. 37. Warren, D. C , and H. M. Scott, 1935. An attempt to produce turkey-chicken hybrids. Jour. Hered. 26: 105-107. Waters, N. F., 1932. Inbreeding White Leghorns. Proc. 6th Inter. Cong. Genet. 2: 205-206. Waters, N . F., and W. V. Lambert, 1936. Inbreeding in the White Leghorn fowl. Iowa Res. Bull. 202, pp. 1-55. Wright, Sewall, 1922. Coefficients of inbreeding and relationship. Amer. Nat. 56: 330-338. — , 1923. The measurement of inbreeding and relationship. Jour. Hered. 14: 339-348.
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Dunn, L. C , 1928. Verhandlungen des V. Internationalen Kongresses fur Vererbungswissenchaft, 1:609. Goodale H. D., 1927. Six consecutive generations of brother to sister matings in White Leghorns. Poultry Sci. 6: 274-276. Hays, F. A., 1929. Inbreeding in relation to egg production. Mass Agr. Exp. Sta. Bull. 258. , 1934. Effect of inbreeding on fecundity in Rhode Island Reds. Mass. Agr. Exp. Sta. Bull. 312. Hazel, L. N . and Jay L. Lush, 1942. The efficiency of three methods of selection. Jour, of Hered. 33: 393-399. Hoffman, Edmund, 1943. White Cornish White Rock cross for Superior broiler stock. U. S. Egg and Poultry 368-369 and 381. Horlacher, W. R., and Robert M. Smith, 1938. Preliminary report of crossbreeding for broiler production. Ark. Agr. Ext. Sta. Bull. 354. Horlacher, W. R., Robert M. Smith and Williams H. Wiley, 1941. Crossbreeding for broiler production. Arkansas Agr. Exp. Sta. Bull. 411. Jeffrey, Fred P., 1939., Crossbreeding for egg production. Hints to poultry men, New Jersey Agr. Exp. Sta. 26: 5. Jull, Morley A., 1929. Studies in hatchability. H I . Hatchability in relation to coefficients of inbreeding. Poultry Sci. 8: 361-368. , 1930. Studies in hatchability. IV. The effect of intercrossing inbred strains of chickens on fertility and hatchability. Poultry Sci. 9:149-156. —, 1933. The effects on various characters of inbred lines of White Leghorns. Jour. Hered. 24: 93-101. Knox, Charles W., 1939. Crossbreeding in the domestic fowl. Proc. 7th World's Poultry Cong. 58-61. Knox, Charles W., and Marlow W. Olsen, 1938. A test of crossbred chickens S. C. White Leghorns, and Rhode Island Reds. Poultry Sci. 17: 193-199. Knox, Charles W., Joseph P. Quinn and Albert B. Godfrey, 1943. Comparison of Rhode Island Reds, White Wyandottes, Light Sussex, and crosses among them to produce Fi, and threeway cross progeny. Poultry Sci. 22: 83-87. Maw, A. J. G., 1942. Crosses between inbred lines of the domestic fowl. Poultry Sci. 21: 548-553.
KNOX
Agricultural Research Administration, Bureau of Animal Industry, Agricultural Research Center, Beltsiille, Maryland (Received for publication December 8, 1945)
A S a general thesis continued inbreeding •£*• increases the homozygosity of the gene complement. This was adequately demonstrated statistically by Wright (1922 and 1923). The reason for inbreeding is to produce purity (homozygousness) in a large number of factors with little or no knowledge of their particular mode of inheritance. The purpose of inbreeding is to obtain successful lines of inbreds and to demonstrate their superior economic possibilities in respect to incrossing, incrossbreeding and topincrossing. A committee of the American Society of Animal Production (1941) defines these terms as follows. Incrossing is considered as the crossing of two different inbred lines of the same breed and variety, incrossbreds the result of crossing two lines of inbreds of different breed and variety and topincross the crossing of inbred males on outbred or relatively slightly inbred females of the same breed and variety. The first part of this paper deals with the development of inbreds and the second part with the use of inbred lines in crosses, in an attempt to produce progeny of superior value to the commercial poultry farmer. INBREEDING In corn breeding, because self-fertilization can be practiced and data obtained on thousands of lines and crosses, much 262
can be left in finding the better lines by chance. In livestock and poultry breeding, "selfing," is impossible and the number of lines which may be developed is limited by cost in animals, in time, and in facilities. Although persistent inbreeding produces homozygousness of genes, it is also true, at least in chickens, that some detrimental factors, rather than good ones, may accumulate. This seems to be especially true of such traits as fertility and hatchability. However, it should be possible, by maintaining rigid standard selection, to guide the inbreeding so that only the factors wanted are retained. Matings with a coefficient of inbreeding of less than 25% can be maintained until most of the deleterious genes are eliminated or their effect minimized. Successive mating of brother and sister, with care to maintain selection for the factors desired, can follow these preliminary matings. Such a procedure was followed in the present experiment. In previous investigations involving inbreeding, Cole and Halpin (1916, 1922), Goodale (1927), Dunn (1923, 1928), Jull (1929, 1930, and 1933), Dumon (1930), Dunderly (1930), Warren (1934), British Ministry of Agriculture and Fisheries (1934), Byerly, Knox and Jull (1934), Hays (1929, 1934), and others found that hatchability decreased rapidly with successive brother-sister matings or their equivalent. In many instances hatchability decreased to such an extent that the
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INTRODUCTION
263
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
TABLE 1.—1945 incubation data for present generation of one inbred family? sire 13625, dam 236 Percent
Male
Mated to
Coefficients of inbreeding* in percent
No. of eggs set
3701
Unselected 3 sister 1307
60.4
Hatch of fertile eggs
Hatch of total eggs
28
89.3
•89.3
12.5 to 28.0
19
100.0
31.6
Unselected sisters 989 1069* 11144
60.4 60.4 60.4
38 32 36
,81.1 84.4 74.3
Inbred White Leghorns
12.5 to 28.0
46
Unselected sisters 10654 1112 1115* 1217*
60.4 60.4 60.4 60.4
Inbred White Leghorns
12.5 to 28.0
Inbred White Leghorns .414
78.9 84.4 • 72.2
80.4
80.4
30 27 30 33
58.6 76.9 82.8 97.0
56.7 74.1 80.0 97.0
48,
91.7
91.7
-
1 Family, as used here, refers to the sons and daughters of a single sire mated to a single dam, in this case 13625 mated to 236. 2 Coefficients of inbreeding refer to the females in the mating. 3 Unselected refers to fertility and hatchability only. * Progeny from these lines will be used to produce the future inbred families. The others will be dropped. Line, as used here, indicates the progeny of a single female.
results obtained forced the termination of the project. Recently several papers have used Wright's coefficients of inbreeding involving a small gene complex and with greater or lesser degree of selection and interpreted results from such coefficients. • Wright's coefficients of inbreeding serve a useful purpose as a statistical measure of the homozygosity of a large gene complex, and is formulated without selection being operative and includes equal transmission of genes by both sexes. Care should be taken in this regard whenever these coefficients are used as a measure or in the interpretation of the results of inbreeding. These papers have established a precedent that this author is following; however, it is doubtful if this procedure can be followed with impunity. For instance, it is possible by proper selection to produce homozygous plumage colors with a 0.0 percent coefficient of inbreeding. On the other hand, the coefficient of inbreeding can be developed to a high degree and
have the plumage color genes remain heterozygous primarily due to selection. It is possible where single quantitative characters are concerned in inbreeding that the results obtained may be a better measure of the success and degree of homozygosity than a particular statistic— at least care must be taken in the interpretation of both. In Table 1 are placed the 1945 incubation data and coefficients of inbreeding for the individuals in a single line of inbred Rhode Island Reds and their crosses with slightly inbred White Leghorn females. In Table 2, the incubation data and coefficients of inbreeding are given for several successive years for an inbred family (data on individuals in Table 1) for incrossbreds, outbred Rhode Island Reds and White Leghorns. The White Leghorns varied from 3 % to 28% coefficient of inbreeding with more than 95% of them being between 20 and 25%. The present generation of inbred Rhode Island Red chickens has a coefficient of inbreeding of
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416
4
264
CHARLES W. KNOX
tion must be applied in the selection of the foundation parental stock at the very beginning of the project and maintained at all times until the specific line of inbreds is breeding true for the particular characteristics desired. Secondly, its success depends upon the relationship that exists between the phenotypic and genotypic complement. Thirdly, environment plays a very important part as in most cases it will cause such great variation of the quantitative trait under observation that it is impossible to make perfect selection. Hence selection, while not perfect, is maintained until inbreeding has progressed sufficiently to produce homozygosity or perfection for the trait desired. On the other hand, if inbreeding makes any deleterious factor homozygous, selection is not effective, in which case it is necessary to discard the entire inbred line and start over again. In this and other experiments many lines were discarded because of this impasse. The data in Table 2 express the results of this conflict between inbreeding (toward homozygosity) and selection (for controlled direction). The average of the inbred family, chosen for illustration, for the first three years is downward, 79.8 percent hatch of fertile eggs to 53.3 percent and 79.5 percent of total eggs set to 43.9 percent. This indicates that both fertility and hatchability were decreasing with each generation of inbreeding, brother-sister mating for the first three years. Selection was maintained successfully for these factors during this time as expressed by the data for the individuals used as breeders, since only slight loss of hatchability is noticeable with practically no loss in fertility during the same three years. For the last two years, selection has asserted itself and the inbreeding has "fixed" to a considerable degree high fertility and hatchability. Only one dam,
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approximately 68%. The data in Table 1 are for the individuals chosen for further breeding work, whereas the data in Table 2 are for the entire family, some of which are not being used for continuance of the project. The family averages are included for outbred Rhode Island Reds, outbred Single Comb White, Leghorns and incrossbreds from Single Comb White Leghorn females as a measure of the value of the inbred family. Waters (1932), publishing results on a project for the inbreeding of White Leghorn fowls, which was initiated by and under the leadership of the present author from 1926 to 1931 inclusive, stated that the success of this inbreeding study is due in no small measure to the rigid selection practiced, for genetic advance and careful selection travel the same highway. Waters and Lambert (1936) published data on ten years of inbreeding on this same project. They showed a gradual decrease in average hatchability of inbreds but practically no decrease for those inbreds chosen as parents. In Tables 1 and 2 the data indicate that successive generations of brother and sister matings or their equivalent can be maintained without loss of fertility or hatchability. This is contrary to the findings of previous investigators; however there is a possible explanation. Mating brothers and sisters or their equivalent for successive generations develops a high coefficient of inbreeding. Careful selection for good fertility and hatchability factors, seems necessary in order to avoid very p'oor fertility and hatchability which usually resulted from inbreeding. It is evident therefore that much depends on the early selection for high fertility and hatchability and maintenance of selection for these characteristics. The success of selection depends mainly upon three factors. The first is that selec-
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
265
TABLE 2.—Incubation data on successive generations of a single family of inbred R. I. Reds, incrossbreds, outbred1 R. I. Reds, and outbred? White Leghorns ', Year
Male or males
1945 3 Inbred R. I. Reds 3 Inbred R. I. Reds 4 Outbred R. I. Reds
Percent Mated to
8 unselected sisters" 6 Inbred W. Leghorns 4 63 Outbred R. I. Reds
4 Outbred W. Leghorns 69 Outbred W. Leghorns
60.4
254
80.5
79.8
12.5 to 25.0
113
90.7
67.9
0.0
1195
85.9
72.7
0.0
1574
83.1
75.7
51.3
35
94.3
94.3
1 Inbred R. I. Red
2 Inbred W. Leghorns 3 unselected sisters
12.5 to 25.0 51.3
40 104
100.0 73.5
100.0 73.4
1 Inbred R. I. Red
2 Inbred W. Leghorns
12.5 to 25.0
40
100.0
100.0
0.0
1377
83.5
70.4
0.0 38.9
1311 16
80.3 81.3
74.1 81.3
93
76.3
31.2
4 Outbred R. I. Reds
Selected sister 236
Hatch Hatch of fertile of total eggs eggs
70 Outbred R. I. Reds
4 Outbred W. Leghorns 66 Outbred W. Leghorns 1943 1131 Selected sister 6730 3 Inbred W. Leghorns 2 Inbred R. I. Reds
5 unselected sisters
2 Inbred R. I. Reds
7 Inbred W. Leghorns
4 Outbred R. I. Reds
6 Outbred R. I. Reds
4 Outbred W. Leghorns 73 Outbred W. Leghorns 1942 2374
Selected sister 4801 3 Inbred W. Leghorns
2 Inbred R. I. Reds
5 unselected sisters
2 Inbred R. I. Reds
6 Inbred W. Leghorns
4 Outbred R. I. Reds
75 Outbred R. I. Reds
3.0 to 25.0 38.9
81
53.3
43.9
158
74.0
50.1
0.0
1481
91.8
78.1
0.0
1237
77.7
66.4
27 54
84.6 64.7
81.5 61.1
3.0 to 25.0
27.6 3.0 to 25.0 27.6
118
83.3
72.6
3.0 to 25.0
106
73.9
71.9
0.0
1550
85.4
62.1
4 Outbred W. Leghorns
67 Outbred W. Leghorns
0.0
1280
80.6
69.2
2843
Selected sister 695
0.0
28
89.3
89.3
No matings made with White Leghorns 6 unselected sisters
0.0
106
79.8
79.5
4 Outbred R. I. Reds
68 Outbred R. I. Reds
0.0
1373
86.8
81.0
4 Outbred W. Leghorns
65 Outbred W. Leghorns
0.0
1290
80.2
69.4
1 Inbred R. I. Red
1 Hatching eggs purchased periodically and the best male obtained from these eggs used in one of the four2 outbred matings. No outside eggs or stock purchased since 1935. 8 Coefficients of inbreeding refer to the females in the mating. 4 Inbred and incrossbred progeny from these pullets are half sisters as are similar groups for preceding years.
i
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1944 13625
Coefficients No. of of inbreeding3 eggs in percent set
266
CHARLES W. KNOX TABLE 3.—Production and body weight data on a single inbred family and their incrossbreds1 Inbred 2 pullets
969 989
Average egg weight in grams
Days to maturity
48 206 206 206 186 233 219 185 148 32
54.2 63.4 58.6 60.8 59.6 56.0 53.4 54.2 59.5 50.7 57.0 57.5
344 242 175 248 179 228 169 165 260 335
166.9 196.1
234.5 202.1
Mature body weight in grams 1970 3150 1985 2312 2614 2380 2709 2580 2520 2162 2438 2016
1 Progeny of an inbred R. I. Red male of this family by inbred white Leghorn females. The resulting incrossbreds, therefore, are half-sisters to the inbreds. 2 These inbred pullets are sisters and have a coefficient of inbreeding of 60.4 percent.
1065, among the 8 (Table 1) gave less than excellent hatching results. This may have been due to poorer environmental conditions during the growing period (note in Table 3 the difference in mature body weight when compared with that of the others) or the "pull" of inbreeding toward poor fertility and hatchability. In any event, this would indicate that selection cannot be diminished until some future generation shows perfect "fixation" by inbreeding. The measure of success in maintaining the fertility and hatchability of the inbreds is reflected in the results obtained when the Rhode Island Red inbreds are crossed with the inbred White Leghorn females, the resulting inbreds being halfsisters to the inbred Rhode Island Red progeny. Another measure is the comparison of hatchability of total eggs set for outbred Rhode Island Reds, which was approximately 62 to 81 percent and for the outbred White Leghorns which was 66 to 75 percent for the same years, as expressed in Table 2. The hatchability of fertile eggs was about 84 to 92 percent and 78 to 83 percent respectively. During the same period the Rhode Island Red inbreds varied from a low of 44 percent
hatch of total eggs set to 80 percent and from 53 percent to 83 percent of fertile eggs. The incrossbreds varied from a low average hatchability of total eggs set of 50 percent to a high of 100 percent and of fertile eggs set from 74 percent to a high of 100 percent. By comparing the data for the various groups, as given in Table 2, it appears that successive generations of brother-sister matings can be made without materially affecting the fertility or hatchability of the inbreds when compared to the incrossbreds or outbred Rhode Island Reds and White Leghorns. As shown in Table 3 selection and inbreeding are involved in a similar manner for most of the quantitative traits of interest to the poultry farmer, namely annual egg production, average annual egg weight, days to maturity, and mature body weight. Breeding is a matter of compromise whether it is some form of outbreeding or inbreeding and selection for a group of characteristics is also a matter of compromise so that in this experiment the best selection may not be used for a single characteristic but rather the best selection for the group. According to Hazel and Lush (1942) group selection appears to be much more efficient than selection for
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1065 1069 1112 1114 1115 1217 1307 1311 Averages (10) Incrossbreds (11)
Average number of eggs
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
when used in incrossbreeding. The variation in individual averages for egg production, egg weight, days to maturity, and body weight indicate that selection must be continuous and the excellent possibilities for success in selecting an inbred line of high egg production, good egg weight, early maturity and excellent body weight. In this conclusion can be included excellent fertility and hatchability as indicated by the data in Table 1. The data for average egg production, egg weight, days to maturity and mature body weight in Table 4 are for the results of brother-sister matings for successive years and for outbred Rhode
TABLE 4.—Production and mature body weight data for a single inbred R. I. Red family generations and outbred1 R. I. Reds
Year
Pullets
Selected Inbred 1115 1944 to 1945
1943 to 1944
1942 to 1943
1941 to 1942
1940 to-1941
1
Av. Coefficients Annual egg of inbreeding production annual egg wt. in percent in numbers in grams 60.4
in:
Days to maturity
Mature body weight in grams
219
53.4
169
2709
All Inbreds (10)
60.4
167
57.0
234
2438
11 Incrossbreds 2
12.5 to 25.0
196
57.5
198
1966
193 Outbred R. I. Reds
00.0
155
55.1
209
2224
Selected Inbred 236
51.3
200'
57.6 3
182
2853
All Inbreds (3)
51.3
115
56.9
268
2347
36 Incrossbreds 2
12.5 to 25.0
219
61.3
207
2236
196 Outbred R. I. Reds
00.0
195
57.4
212
2628
Selected Inbred 6730
38.9
225
57.8
213
2646
All Inbreds (7)
38.9
145
57.6
240
2572
16 Incrossbreds 2
12.5 to 25.0
172
61.5
215
2369
219 Outbred R. I. Reds
00.0
155
57.4
224
2491
Selected Inbred 4801
27.6
221
58.0
196
2398
All Inbreds (9)
27.6
189
56.3
195
2493
170 Outbred R. I. Reds
00.0
186
54.9
187
2650
185
2630
Selected Inbred 695
00.0
271
57.7
All Inbreds (5)
00.0
238
59.3
179
2632
173 Outbred R. I. Reds
00.0
208
58.4
198
2527
Use for reference points. Half-sisters to the inbred pullets (inbred R. I. Red male mated with inbred White Leghorns). * Estimated for period after death.
2
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one trait at a time. This type of selection, therefore, would be a cause for some irregularity of progress for some single trait and would also take a longer time to reach the same goal than if selection is made for a single trait. Taken as a whole, however, a better line of inbreds is produced, one that can be continued and also will not be poorer than the standard breds used as foundation stock. The data in Table 3 are for the same family used to illustrate inbreeding and hatchability (Table 1). The data in this table show how well the quantitative traits are being maintained with inbreeding and also how well this family is doing
267
268
CHARLES W. KNOX
Thus far only a single family of Rhode Island Red inbreds have been used for illustration and discussion. However, many lines have been used in the experiment most have been discarded and a few have been continued and their number increased. HYBRIDS
In previous publications, Knox and 01sen (1938) and Knox (1939), attention was brought to the fact that the term hybrid, through usage, could mean the progeny of any one of several different types of crossing:
1. Crossing individuals of different genera, species, or sub-species. 2. Crossing individuals of different breeds or varieties. 3. Top crossing inbred males on outbred females of the same breed and variety. 4. Crossing inbred individuals of different breeds or varieties. 5. Crossing inbred individuals of the same breed and variety from different inbred lines. In previous work Smith and HaigThomas (1913), Phillips (1913, 1915, 1916, 1921), Cutler (1918), Serebrovsky (1929), Tiniakoff (1933, 1934), Warren and Scott (1935), Quinn, Burrows and Byerly (1937), and others have reported work with true hybrids, hybrids of the first type. These hybrids although of academic interest appear to be of no immediate economic value. Crossing different breeds and varieties, the second type of hybrid production, has received considerable attention as shown by Pearl and Surface (1910), Warren (1927, and 1930), Horlacher and Smith (1938), Knox and Olsen (1938), Jeffrey (1939), Knox (1939), Knox, Quinn and Godfrey (1943), Hoffman (1943), and Horlacher, Smith and Wiley (1941). The consensus of opinion seems to be that crossing different breeds and varieties produces progeny that grow faster to 12 weeks of age than the standardbred parental stock used in the cross and also that the crossbred stock has better viability. If the parental stock differed markedly in body weight, the adult body weights of the crossbreds were intermediate. The effect of crossbreeding on average annual egg production is variable with slightly more evidence of less production by the crossbreds than by purebred progeny.
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Island Reds. The annual average results for the outbred Reds which acted as a control, are given each year for comparison, as the quantitative traits measured appeared to vary considerably from year to year. No culling or selection was made on any of the females of the various groups from the time they were hatched until the end of their laying year. During a five year period, 1940-1945, the average annual egg production of the outbred Rhode Island Red pullets varied from 208 eggs in 1940-1941 to 155 in 1942-1943. During the same five year period the inbred Rhode Island Reds varied from 238 in 1940-1941 to 115 in 1943-1944. Considering the 1944-1945 year also, it appears that the inbred Rhode Island Reds compare favorably with the outbred Reds. As a matter of fact they compare favorably with the outbreds in all traits with the exception of the average days to maturity. This trait seems to be later with such successive year of inbreeding. Referring to Table 3, however, it will be noted that there are several possibilities for selecting early maturing individuals in the inbred line. Four of the inbred pullets matured in less than 180 days. This inbred line appears to offer excellent opportunity for selection and further study of all four of these traits.
T H E DEVELOPMENT AND USE or CHICKEN INBREDS
successful commercial egg production. Therefore this part of the experiment was designed to cross inbreds of high coefficients of inbreeding, 1. with inbreds of the same breed and variety (incrossing with Rhode Island Reds), 2. with inbreds of a different breed and variety (incrossbreeding with Single Comb White Leghorns), and 3. with outbred stock of the same breed and variety (topincrossing with Rhode Island Reds). Comparisons of results were made also with outbred stock of the same breed and variety (Rhode Island Reds) and with outbred stock of a different breed and variety (Single Comb White Leghorns). For these purposes the Single Comb Rhode Island Red and Single Comb White Leghorn were used and the experiment started in its present form in 1940 although some preliminary work was started in 1935. During the years 1935-1936 and 19361937 only two single male ma tings were involved. Eight were used in 1940 and this number was gradually increased until 20 matings were used in 1943 and 1944. Most matings had an inbred Rhode Island Red male mated to at least three of his sisters (inbred line), three inbred Rhode Island Reds from a different line (to produce incross progeny), three outbred Rhode Island Reds (to produce topincross progeny), and three slightly inbred White Leghorns (to produce incrossbred progeny). Approximately 175 outbred Rhode Island Red and White Leghorn pullets each year completed a period of 365 days from the date of first egg and the number of eggs that they laid during this period was used in computing the average annual egg production. In 1935 and 1936 about 50 incrossbred and crossbred Rhode Island Red by White Leghorns, respectively, were used in calculating the average an-
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There may be several reasons for such a difference of results; first, the amount of variation encountered in studies of egg production; second, crossbreds are sometimes broodier than either parental strain so that egg production may be reduced because of broodiness; third, many reports were made with too few numbers of individuals, and lastly, even fewer investigators attempted to repeat the crosses in subsequent years, which in view of the variation encountered in egg production strains and between years must be repeated for several years, before adequate interpretation of results can be made. The statement that crossing breeds with a hatchability of about 80 percent lowered hatchability about as often as it increased it among the Beltsville flocks, was made by Byerly, Knox and Jull (1934). A similar statement may be made in regard to egg production, to wit, the crossing of breeds or varieties with an average annual egg production of 200 eggs or more produced progeny with a lower annual egg production than that of the parental standardbreds about as often as it increased it among the Beltsville flocks. The last three types of hybrids depend upon the use of inbreds. The use of inbreds in crossing is really attempting to simulate the crossing of inbred strains of corn for the production of successful hybrids. The value of inbreds, when used in crosses, is that heterosis in the hybrids is so controlled that similar sets of favorable genes are produced in subsequent generations of progeny. Consequently inbreeding reduces variability to a minimum and subquent crosses of inbreds give relatively similar and predictable results from year to year from the same inbred lines. It would appear that the use of inbreds in crosses offers considerable possibilities for producing hybrids, as herein defined, for
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CHARLES W. KNOX
was practiced and the coefficient of inbreeding would be negligible. Not all inbred lines behave alike when crossed and although averages are given for all lines, in Table 5, it is probable that from now on inbreds can be selected that will prove to be even better for crossing than reported here. Hence the average annual egg production given here for the incrossbreds is a minimum figure and probably can be increased. The superiority in egg production of the progeny from the inbred Rhode Island
TABLE 5.—Annual egg production data of all inbred R. I. Reds, incrossbreds, and outbred R. I. Reds1 and White leghorns2 Outbred White Outbred Leghorn males Reds cf mated to mated to Outbred OutInbred Reds Inbred Out9 9 bred bred White W. Leg. Leg. Reds Reds
Inbred R. I. Red males mated to Year UnreInbred Inbred lated Reds Reds Inbred > 2 5 % < 2 5 % Reds 1935-36 1936-37 1940-41 1941-42 1942-43 1943-44 Average
183 155 139 159 159.0
195 165 165 202 181.8
Outbred R.I. Reds
231 236 234 187 190 216 197.7
180 185 199 191.3
196 225 224.4
201 188 208 186 155 195 189.1
199 196 204 193 196 184 195.3
219 213
2093
Differences in average annual egg production between Incrosses & Outbred Reds
Outbred W. Leg.
Inbred Reds
30 48 26
32 40 30
51
41 30 35.0
0 41 28.6
57 66 58.0
1 Hatching eggs purchased periodically and the best male obtained from these eggs used in one of the four outbred matings. 2 No outside eggs or stock purchased since 1935. 3 5 pullets only.
The data given in Table 5 give the average results for the inbreds, crosses, outbred Rhode Island Reds and Single Comb White Leghorns for the various years that data were obtained. The outbred Reds were maintained as outbreds by periodically purchasing 100 hatching eggs from one of the nation's foremost breeders about once every three years and the best male hatched from these eggs, used in one of the four current breeding pens. In the outbred White Leghorns no foundation stock was purchased since 1935; however practically no inbreeding
Red males mated to the inbred Leghorn females is in evidence each year. They average to lay 26 to 48 eggs more than outbred Rhode Island Red pullets and with the exception of one year out of 5 laid 30 to 41 eggs more than the outbred White Leghorns. The one year in question 1942-1943, the incrossbreds were kept in the same .house, cared for by the same attendant and otherwise were similarly managed and subjected to similar environment as the outbred Rhode Island Red pullets. Both the Rhode Island Red and incrossbred pullets suffered laryngo-
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nual egg production for the incrossbred and crossbred pullets. For each of the last three years, there were about 175 incrossbred pullets. In 1940 there were 246 Rhode Island Red inbred pullets with an average annual egg production of 183 eggs. This number of inbreds was increased until in the last two years they involved approximately 350 pullets each year with an average annual egg production of 139 and 159 eggs, respectively. There were approximately 50 incross and topincross pullets each year.
T H E DEVELOPMENT AND USE OF CHICKEN INBREDS
were obtained from outbred progeny. The crosses between the inbred lines were not any better than for the outbred or top incross progeny, possibly because enough relationship remained between the lines to make the gene complements similar. In any event best results were obtained from the incrossbreds between Rhode Island Reds and White Leghorns in respect to most of the commercial traits considered. SUMMARY
The data presented indicate: 1. That lines of inbreds may be produced without a decrease of fertility, hatchability, mature body weight, or annual egg production. 2. That selection must be initiated and maintained during inbreeding in order to control the direction that homozygosity takes during inbreeding. 3. That incrossbreds produced from inbred Rhode Island Reds mated to Single Comb White Leghorns gave superior results in annual egg production to the standardbred Rhode Island Reds and White Leghorns in this experiment. 4. That incrossing and topincrossing were not as favorable for annual egg production as incrossbreeding, but were improving with each successive year that the inbreds improved, namely the last three years. LITERATURE CITED
British Ministry of Agriculture and Fisheries, 1934. Experiments on inbreeding poultry. Bull. 83. Byerly, Theodore C, C. W. Knox, and Morely A. Jull. 1934. Some genetic aspects of hatchability. Poultry Sci: 13: 230-238. Cole, L. J. and J. G. Halpin, 1916. Preliminary report of an experiment on close inbreeding. Jour. Amer. Assn. Instr. Invest, in Poul. Husb. 3: 7-8. •, 1922. Results of eight years of inbreeding Rhode Island Reds. Anat. Rec. 23: 97. Cutler, D. W., 1918. On the sterility of hybrids between the pheasant and the Gold Campine fowl. Jour. Gen. 7:155-165. Dumon, A. G., 1930. The effect of inbreeding. Proc. Fourth World's Poult. Cong. 48-71.
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tracheitis and recurrent outbreaks of infectious coryza of epizootic proportions. In spite of this the average annual egg production of the incrossbreds was 196 eggs, the same as for the Single Comb White Leghorn pullets, but 41 eggs more than were laid by the Rhode Island Reds which were subjected to similar conditions. The Single Comb White Leghorn pullets were fed a similar diet to that of the others but were kept in another house on a different part of the experimental farm and in charge of a different poultryman. These were not subjected to epizootic disease conditions. However they did not average to lay any more eggs than the incrossbreds, the average production of the White Leghorns for the same year being 196 eggs. It will be noted that as inbreeding continued from year to year there appears to be a marked increase in the egg production of the incross progeny (standardbred Rhode Island Reds). It may be possible to increase this number of eggs further until it at least equals that of the incrossbred and topincross progeny. It should be kept in mind that the data are for all inbreds and their crosses. One of the inbred lines produced incrossbreds that had an average annual egg production of 254 eggs. Hence it may be possible to increase proportionately the average production of the incross, topincross and incrossbred progeny. At the present time, however, the incross and topincross progeny have lower average egg production than the incrossbred progeny but at least as favorable annual production as the outbred Reds and outbred White Leghorns. Maw (1942) found top-crossing (topincrossing) gives better results than crosses between inbred lines where some relationship exists between the lines. The results obtained from topincrosses of the present paper were not any better than
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CHARLES W. KNOX Pearl, Raymond and F . M. Surface, 1910. Studies on hybrid poultry. Me. Agr. Exp. Sta. Bull. 179. Phillips, J. C. 1913. Reciprocal crosses between Reeve's pheasant and the common ring-neck pheasant producing unlike hybrids. Amer. Nat. 47: 701-704. , 1915. Experimental studies of hybridization among ducks and pheasants. Jour. Exp. Zool. 18:69-112. , 1916. Two pheasant crosses. Jour. Hered. 7: 7-12. , 1921. A further report on specie crosses in birds. Gen. 6: 366-383. Quinn, J. P., W. H. Burrows, and T. C. Byerly, 1937. Turkey-chicken hybridization. Jour. Hered. 28:169-173. Serebrovsky, A. S., 1929. Untersuchungen ttber Artbastarde bei Hiihnern. Jour. Gen. 21:327-340. Smith, G., and R. Haig-Thomas, 1913. On sterile and hybrid pheasants. Jour. Gen. 3:39-52. The American Society of Animal Production, 1941. Report of committee on investigations. Proceedings of the 1940 annual meeting of the American Society of Animal Production, p. 378. Tiniakoff, G. G., 1933. Hybrid of peacock and hen. (Russian with English summary.) Verhandl. d. landwirtsch. Inst. f. Bastardierung u. Acclimatisation in Ascania Nova. 1: 85-98. , 1934. Peacock and hen hybrids and a comparative analysis of their parents. Jour. Biol. 3:41-63. Warren, D. C , 1927. Hybrid vigor in poultry. Poultry Sci. 7:1-8. , 1930. Crossbred poultry. Kan. Agr. Exp. Sta. Bull. 252. -, 1934. The influence of some factors on the hatchability of the hen's egg. Kan. Agr. Exp. Sta. Bull. 37. Warren, D. C , and H. M. Scott, 1935. An attempt to produce turkey-chicken hybrids. Jour. Hered. 26: 105-107. Waters, N. F., 1932. Inbreeding White Leghorns. Proc. 6th Inter. Cong. Genet. 2: 205-206. Waters, N . F., and W. V. Lambert, 1936. Inbreeding in the White Leghorn fowl. Iowa Res. Bull. 202, pp. 1-55. Wright, Sewall, 1922. Coefficients of inbreeding and relationship. Amer. Nat. 56: 330-338. — , 1923. The measurement of inbreeding and relationship. Jour. Hered. 14: 339-348.
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Dunn, L. C , 1928. Verhandlungen des V. Internationalen Kongresses fur Vererbungswissenchaft, 1:609. Goodale H. D., 1927. Six consecutive generations of brother to sister matings in White Leghorns. Poultry Sci. 6: 274-276. Hays, F. A., 1929. Inbreeding in relation to egg production. Mass Agr. Exp. Sta. Bull. 258. , 1934. Effect of inbreeding on fecundity in Rhode Island Reds. Mass. Agr. Exp. Sta. Bull. 312. Hazel, L. N . and Jay L. Lush, 1942. The efficiency of three methods of selection. Jour, of Hered. 33: 393-399. Hoffman, Edmund, 1943. White Cornish White Rock cross for Superior broiler stock. U. S. Egg and Poultry 368-369 and 381. Horlacher, W. R., and Robert M. Smith, 1938. Preliminary report of crossbreeding for broiler production. Ark. Agr. Ext. Sta. Bull. 354. Horlacher, W. R., Robert M. Smith and Williams H. Wiley, 1941. Crossbreeding for broiler production. Arkansas Agr. Exp. Sta. Bull. 411. Jeffrey, Fred P., 1939., Crossbreeding for egg production. Hints to poultry men, New Jersey Agr. Exp. Sta. 26: 5. Jull, Morley A., 1929. Studies in hatchability. H I . Hatchability in relation to coefficients of inbreeding. Poultry Sci. 8: 361-368. , 1930. Studies in hatchability. IV. The effect of intercrossing inbred strains of chickens on fertility and hatchability. Poultry Sci. 9:149-156. —, 1933. The effects on various characters of inbred lines of White Leghorns. Jour. Hered. 24: 93-101. Knox, Charles W., 1939. Crossbreeding in the domestic fowl. Proc. 7th World's Poultry Cong. 58-61. Knox, Charles W., and Marlow W. Olsen, 1938. A test of crossbred chickens S. C. White Leghorns, and Rhode Island Reds. Poultry Sci. 17: 193-199. Knox, Charles W., Joseph P. Quinn and Albert B. Godfrey, 1943. Comparison of Rhode Island Reds, White Wyandottes, Light Sussex, and crosses among them to produce Fi, and threeway cross progeny. Poultry Sci. 22: 83-87. Maw, A. J. G., 1942. Crosses between inbred lines of the domestic fowl. Poultry Sci. 21: 548-553.