THE IMPORTANCE OF INTERACTIONS DETECTED BETWEEN …Workers with poultry differ in their assessment of the importance of the phenomenon in the various circumstances studied. ABPLANALP

THE IMPORTANCE OF INTERACTIONS DETECTED BETWEEN GENOTYPE AND ENVIRONMENTAL FACTORS FOR

CHARACTERS OF ECONOMIC SIGNIFICANCE IN POULTRY1

P. HULL2 A N D R. s. GOWE

Canada Department of Agriculture, Research Branch, Ottawa, Canada

Received July 3, 1961

T is usual to find that animals from any particular domesticated stock are I reared in a variety of environmental conditions, and that these conditions in their turn often differ to some degree from those in which the stock was originally developed. A stock of animals selected in the past for high performance in some trait cannot be expected to do uniformly well in all environments in which it can be maintained. It is reasonable to assume further that, in some cases at least, optimum performance can only be obtaincci by selecting distinct strains under the conditions in which they are to be reared. The objectives of the present investigation were to determine the degree to which genotype-environment interactions were of importance in one case of economic interest, and to see if it were possible to indicate, from the outcome of this and other tests previously reported, under what general conditions genotype-environment interactions would be expected to be of practical significance.

When previous results were considered it was found that genotype-environment interactions varied in their importance in different situations. BONNIER and HANSON (1 948), studying the lactation of identical twin cattle reared with two intensities of feeding, concluded on indirect evidence that the genotype- environment interaction was likely to be very large. A study by MORLEY (1956) of the performance of half-sib groups of Australian Merino sheep, separated and reared on high and low planes of nutrition, showed that genotype-environment interactions were unimportant for fleece characteristics, but significant for body weight. KING and YOUNG (1955), comparing three different breeds of sheep overwintered in warm or cold environments and on high or low planes of nutrition, concluded that no interactions occurred either in the case of fleece characteristics or growth rate.

FALCONER and LATYSZEWSKI (1952) compared the body weights of two strains of mice. both of which were selected for high body weight but under two different intensities of feeding. When samples of these two strains were raised in both the high-plane and low-plane environments, it. was found that the interaction between strain and feeding intensity was marked. YOUNG (1953), studying both body weight and litter size in three inbred lines of mice reared under four

1 Contribution No. 73, Animal Research Institute, Research Branch, Ottawa, Canada. 2 National Research Council of Canada Postdoctorate Fellow.

Genetics 47: 143-159 February 1962

144 P. HULL A N D R. S. GOWE

different temperatures and feeding regimes, has also emphasized the importance of genotype-environment interaction.

Workers with poultry differ in their assessment of the importance of the phenomenon in the various circumstances studied. ABPLANALP (1956) found no evidence of interaction between sire group and hatch for egg weight, winter egg production, total egg production or age a t sexual maturity. OSBORNE ( 1952), however. had previously reported an interaction between sib group and early and late hatches. considering only age at sexual maturity. LOWRY, LERNER and TAYLOR (1956) found no interaction between sire or dam progeny groups (within a selected strain) and cage uersus floor management for age at sexual maturity, production index and egg weight. GOWE (1 956), comparing seven strains of Leghorns housed in pens or balieries, found an interaction between strain and environment for egg production, body weight and egg weight, but not for age at sexual maturity. The work of GUTTERIDGE and O'NEIL (1942) showed that there was no interaction between three strains of poultry and three farms for egg production, egg weight or date of sexual maturity, but indicated that there was a genotype-environment interaction for maximum body weight. GOWE and WAKELY (1 954) similarly found no evidence of any interaction of sire progeny groups (within strains) with six widely separated farms for egg production traits or mortality. NORDSKOG and KEMPTHORNE (1 960), considering the performance (measured as age at sexual maturity, egg production, mortality and egg weight) of several commercial strains, decided that there was highly significant interaction of genotype with location. Unfortunately strain-location interactions appeared to be confounded with hatch-location interactions. It was only by assum- ing the latter were of no importance that these authors could conclude that genotype-location interaction effects were significant. HILL and NORDSKOG ( 1956) had previously found a significant variety (strain) location interaction for mortality and a highly significant variety (strain) year interaction for mortality and hen- day egg production. DICKERSON (1956) suggested that one possible reason why a strain of Leghorns did not respond to selection was that there was an interaction between genotype and year.

It will be seen that in some of the cases reported. genotype-environment interactions are very marked, and in others they are apparently unimportant. On the basis of limited evidence some investigators have concluded that genotype- environment interactions are of no general significance in animal breeding and others that the phenomenon is so widespread that present breeding practices must be revised. It is more reasonable to assume that neither of these general conclusions is justifiable and that each case of practical importance to the breeder must be considered separately. However, the large amount of data available for analysis in the present investigation made it easier to make an accurate assessment of the importance of genotype-environment interactions in a number of comparable cases, and the results of this investigation together with cases already reported do suggest that it is possible to distinguish some conditions where genotype-environment interactions are likely to be important from others where they can be expected to be unimportant.

GENOTYPE-ENVIRONMENT INTERACTION 145

EXPERIMENTAL METHODS

Two strains of White Leghorns, the Ottawa Strain and the New Strain (re- ferred to as Strains 1 and 2, respectively, in the rest of this paper) were used throughout the experiment. An account OI the origin of these strains will be found in GOWE et al. (1959). Both strains were closed flocks with different origins that were being selected for high egg production. The breeding popula- tions of each strain were maintained at Ottawa. Data from three generations on test in the years, 1956-57, 1957-58 and 1958-59 (years 1, 2 and 3 ) were used for the study.

Each year 21 to 26 selected males of each strain were individually mated to random samples of 12 to 18 selected females of the same strain. The breeders were trapnested and the eggs marked so that all progeny were identified as to sire and dam. Hatching eggs from both strains were saved for two weeks and incubated in the same machines for each hatch. Since there were three hatches each year, eggs were saved over a total period of six weeks and there were four weeks between the first and the third hatch. Each of the three hatches was assigned to a separate test farm so that hatches and farms (two environmental effects) were confounded.

The sex of each chick was determined at hatching and the six to eight full-sib families with the largest number of daughters were used from each half-sib family. If there were more than six full sisters, the family was randomly reduced to six females. For the first two years the plan was to try and house from two to three daughters per dam per treatment at each of the three locations. The two treatments given at each location were (1) a program where feed was restricted during the rearing period to about 70 percent of what the full-fed group ate: this had been found by GOWE et al. (1960) to result in a more efficient production of eggs for a given quantity of feed, or (2) ad libitum feeding throughout the life of the bird. Six daughters per hatch would allow for one daughter dying in each treatment during the rearing period and still permit housing two full-sibs per treatment at each location. The design of the experiment was changed for the third year to allow for one daughter per dam per treatment per location with about twice the number of dams per sire with progeny on test.

The female chicks of both strains assigned to each location were distributed to the chick boxes by a restricted randomization procedure to equalize the ship- ping environment for the progeny of each sire.

Four branch farms of the Research Branch, Canada Department of Agriculture cooperated in this experiment. The farms were located at Agassiz (British Columbia), Morden in year 1 and Brandon in years 2 and 3 (both in Manitoba), and Harrow (Ontario). The two farms in Manitoba were considered as one location for the purposes of these analyses since the climate, latitude and build- ings at the two locations were roughly equivalent.

At the three test locations all the chicks were intermingled for the brooding periods. In year 1 the pullets were randomly divided within dam, sire and strain into two groups and transferred to range at ages varying from 50 to 57 days at the three locations. After one week on range, one group was restricted in the


feed it received to 70 percent of what the lull-fed birds ate the previous week. This continued until the pullets were housed at 147 days of age. For years 2 and 3 the groups were separated at 21 days of age in the brooder house and the restricted feeding program was started at this time with the restricted group receiving 90 percent of the amount the full-fed birds ate the previous week for the period 21 to 28 days of age, 80 percent of the amount the full-fed birds ate for the next week and 70 percent of the amount the full-fed birds ate for the period 36 days to 147 days of age.

The same all-mash ration was fed to both groups. All birds had free access to poultry pastures from eight weeks to 147 days. Both groups of birds were housed at 147 days of age and put on an all-mash laying ration fed ad libitum. Birds from the two groups were housed in replicated pens randomly allotted in the laying houses at each location. Progeny of both strains were randomly distributed throughout the pens within treatment. The laying house management program was standardized as far as possible between farms and very carefully between pens within farms. Details of the management program and rations can be found in the report by GOWE et al. (1 960).

Individual body weights were taken to the nearest ten grams at 147 days and at 350 days of age. All pullets were trapnested five days a week from 147 days of age until the end of the test at 500 days of age. Estimates of the age in days at first egg (sexual maturity) and individud egg production came from these trap records. The mean hen-housed egg production (LERNER’S (1950) Production Index) was the mean production of the total number of birds housed. Survivor egg production was the mean production of only those birds that survived until the end of the test period (500 days). All eggs laid over a ten-day period, when the birds were approximately 365 days of age, were individually weighed to the nearest gram. The mean egg weight for each bird was obtained and this mean used in subsequent analyses.

ANALYTICAL METHODS

The objective of the analyses was to determine the importance of the interaction between different genotypes and two sets of environmental factors. It was possible at the same time to estimate the extent to which certain environmental factors interacted with one another. This computation was performed in turn for each of the six traits already described (housing and mature body weight, age at sexual maturity, egg weight at maturity and egg production of all the hens housed and of the survivors only to 500 days) within each of the two strains. Since this required 12 separate analyses each involving 5000 progeny, the computation was programmed and carried out by means of an electronic computer.

The genotype-environment interactions studied were those of sire breeding value with (1) the environmental differences between the three farms (including hatch effects) and (2) the two rearing programs-restricted and full-feeding. Further, it was possible to examine the second order interaction of farm with


rearing treatment with sire breeding value. Since essentially the same rearing treatments were repeated at the three farms over all three years, it was also possible to estimate the interaction of the rearing treatment with years, of farm effects with years and also of farm effects with treatments with years .

There were small differences between the numbers of progeny from each sire in any farm-treatment group due mainly to differences in mortality between the sire groups. The analyses were carried out using sire group means. The variance within sire groups for a particular trait was estimated by a separate analysis for each strain. Mean squares derived from the analysis of sire group means were adjusted by multiplication by the harmonic mean of the number of offspring within the subgroups, for comparison with the within sire group variance.

In two cases all of the progeny of a particular sire given one particular treatment at one location were lost. In these cases the missing mean was estimated and the computation completed by the method described by YATES (1933). In this case if Zi’ijkz is the estimate of the missing mean in year i and si is the number of sires in that year, then

- sizij.. + 3Yi.k. + 2zi.J - 22i. ..

5si - 3 XijkZ =

The appropriate correction was made to the mean squares in the analyses based on data including such estimated values.

It was assumed that within each strain the performance ( X i j k z m ) measured for trait X , of the n t h progeny of the jth sire within the ith year at the kth farm, given the Zth treatment, was made up of the following contributions:

X i j k Z n l = /A + yi + Sj(i) + f k + y f i k + Sffk(i) + tz + ytiz + Stjl(i) + f t k l + Y f t i k l + Sftjkl(i) + ~ ( i j k ~ )

where p is the overall mean for trait X in that strain.

y i is the effect of year i measured over all sires in all environments; s j ( i ) is the effect of sire j within year i averaged over all environments; f k is the effect of farm k averaged over all sires in both treatments; yfilc is the interaction of farm k with year i; sfjk(i) is the interaction of sire i within year i with farm k; t l is the effect of treatment Z measured over all sires at all farms; yti 1 is the interaction of treatment Z with year i; stj l( i) is the interaction of sire j within year i with treatment 1; f t k l is the interaction of farm k with treatment 2; yftikz is the interaction of farm k, treatment Z and year i; s f t j k l ( . i ) is the interaction of sire i, farm k and treatment Z within year i; W ( i j k l ) is a contribution due to genetic differences within sire groups plus

The sires tested were regarded as a random sample from each strain. Since -they were, in fact, chosen on the basis of high egg production of their sibs, estimates of genetic variance based on this sample were biased downwards. The year effects, the farm effects and the two levels of nutrition were not chosen at random; it is

that due to environmental influences peculiar to each bird.

148 P. HULL A N D R. S . GOWE

assumed that the observed variability is peculiar to the set of circumstances which were examined. It was not considered justifiable to regard the year effects in this experiment (1956-57, 1957-58 and 1958-59) as a random sample of years, since consecutive years might be either more or less similar than years chosen at random. The three farms were deliberately picked to cover as wide a range of climatic conditions as was possible and were not to be thought of as a random sample from any particular population of farms. Nor could the two levels of nutrition be regarded as anything but two arbitrarily fixed treatments. The expected composition of the mean squares, after multiplying by the harmonic mean of the subgroup number ( h ) would then be as shown in Table 1 . Appro- priate tests of significance of the various mean squares were made, based on the expectations shown in Table 1.

TABLE 1

Composition of mean squares (after multiplication by h) within strains

Mean square Degiccs of freedom Expectation of mean square __________-- _ _ _ _ ~ ~ _ _ _ Source of variation

Between years

Between sires within years

Between farms

Years x farms interaction

Between treatments

Years x treatments interaction

Sires (within years) x farms interaction

Sires (within years) x treatments interaction

Farms x treatments interaction

Farms x treatments x years interaction

Sires (within years) x farms x treatments interaction

Within subgroups

where g = total number of progeny si = number of sires in year i h = harmonic mean of number of progeny in group S" = '/2 p s i - Z(Si)2]

ZSi

GENOTYPE-ER VIRONMENT INTERACTION 149

RESULTS

The mean performance of birds of strains 1 and 2 is given in Table 2, first for each year (the average for all farms and both treatments), then separately for each farm (the average for both treatments in all years), and finally for the full and the restricted groups (the average for all farms in all years). The overall mean performance for each strain for each trait is also shown.

The 12 separate analyses of variance and tests of significance are summarized in Tables 3a,b,c. The components of variance derived from the mean squares in Table 3 are listed in Tables 4a,b,c. The 12 components estimated are sym- bolized as:

U; = effect of years

U: = effect of farms

a2, = effect of treatments

ai(,) = variance due to sires within years

U:, = variance due to interaction of farms and years

ai, = variance due to interaction of treatments and years aiF = variance due to interaction of sires and farms aiT = variance due to interaction of sires and treatments ai,, = variance due to interaction of farms and treatments

aiTy = variance due to interaction of farms, treatments and years aiFT = variance due to interaction of sires, farms and treatments

U; = within sire group variance

TABLE 2 Mean performance of strains I and 2 in difjerent environmental conditions

Year Farm Treatment Strain Character Strain 1 2 3 1 2 3 F R mean

Mean body wt. (in kg) at housing

Mean body wt. (in kg) at matunty

Mean age at first egg (days)

Mean March egg weight (gm)

Mean number of eggs produced in 500 days per hen housed

Mean number of eggs produced per hen surviving to 500 days

1 1.47 1.45

2 1.51 1.47

1 2.01 2.03

2 2.07 2.09

1 168 174

2 170 175

1 58.3 56.9

2 59.4 59.2

1 152 161

2 160 162

1 169 174

2 173 173

1.39

1.39

1.98

1.99

171

171

57.6

57.5

167

166

179

176

1.52 1.38

1.54 1.38

2.05 1.99

2.11 2.02

168 175

169 176

58.7 57.3

59.1 57.6

155 156

160 158

175 169

176 169

1.41 1.63

1.42 1.63

1.98 2.04

2.00 2.08

169 162

171 164

58.7 58.2

59.2 58.5

I69 161

171 162

178 174

178 174

1.24

1.27

1.97

2.01

180

179

58.2

58.7

160

164

174

174

1.44

1.46

2.01

2.05

171

172

58.2

58.6

160

163

174

174


TABLE 3

Analyses of uariance

Body weight at housing I3iid.v weight at maturity Strain 2

(a) Strain 1 Strain 2 Strain 1

Source df M.S.s l00 df h l . S . t 1 0 0 [if R r s . t ioo df M.S.+lOO

Y 2 28,5751 S ( Y ) 70 2,1901 F 2 91,7461 F x Y 4 15,7021 T 1 2,058,1451 T x Y 2 4,912+ S X F 140 3091 S X T 70 664.1 F X T 2 2,0621: F X T X Y 4 4,0241: S X F X T 138 232 W 521 5 22 1

2 71,4771 68 2,5453: 2 127.3431 4 14.8061: 1 1,853,9891: 2 6,613t

68 1.0271: 2 8,6411 4 6,6451

136 25 8

134 218 5644 234

2 7,869 70 5,7651 2 27,017f 4 3,2251: 1 51,6651: 2 1,025"

1 40 524' 70 320 2 2.9901 4 147

138 408 4844 40 1

2 52,9003 68 5,6862 2 57,9042 4 8,8331 1 60,8062 2 415

136 578* 68 41 5 2 3,9501 4 535

134 347 5314 444

-4ge a t first egg Strain 1 Strain 2

(b )

Source df hl S. df 14 S

hlarch egg weight Strain I Strain 2

df RI.S. df Y1.S

Y 2 S!Y) 70 F 2 F X Y 4 T 1 T X Y 2 S X F 140 S X T 70 T x F 2 T X F X Y 4 S X F X T 138 W 5093

13,7301 1,1571:

25,3961: 4.0613:

W6,5 161 6,8951:

289 4601

6,8741: 2,6021

222 265

2 68 2 4 1 2

136 68 2 4

134 5556

11,714-f 2.1211:

21.6411: 2,2241

320,2801 2.2081

203 315+

8.4941: 975t 219 232

2 744.781 70 122.513 2 1,015.571: 4 104.938 1 2.08 2 85.981

140 15.78t 70 9.66 2 22.40 4 31.63

138 9.12 4466 11.03

2 68 2 4 1 2

136 68 2 4

134 5006

1,900.14$ 225.363:

1,324.231 45.50* 21.67

107.841 13.38 8.48

26.62 27.42

9.90 11.96

Egg production per hen housed Egg production: survivors to 500 days Strain 2 Strain 1 Strain 2

(C) Strain 1

Source df M.S. df hr.s. df L1.S. clf 1r.s Y 2 S / Y ) 70 F 2 F X Y 4 T 1 T X Y 2 S X F 140 S X T 70 T X F 2 T X F X Y 4 S X F X T 138 H' 5215

109,199p 9.8521:

11 1,8081: 27,9861:

1,103 1,522 3,130 3,162 1,096 1,186 2,610 2,718

2 68 2 4 1 2

136 68 2 4

134 5644

17,3221: 7,6791

89.1 461 17,1081 7,102

999 2.854 2,693

480 3.489 2,209 2,333

2 70 2 4 1 2

140 70 2 4

138 44.66

40,3971 4,2821

33.4191 5,6451

524 4,001 1,266 1,605

131 709

1,346 1,297

2 68 2 4 1 2

136 68 2 4

134 4988

4,807 4,7161

36,3283: 2,927

638 72

3,220 1,318

824 596 960

1,128

* P < .05 :- P < .01 : P < .a01

GENOTYPE-ENVIRONMENT INTERACTION

TABLE 4

Components of variance

151

Body weight at housing-100 Body weight at maturityilO0 Strain 1 Strain 2 Strain 1 Strain 2

14.75

26.71

51.13

25.82

767.01

4.75

3.59

12.02

2.05

12.72

0.88

221.37

36.44

28.75

67.12

23.05

652.44

5.90

0.92

19.74

8.90

20.37

- 1.17

233.81

1.29

79.66

16.22

4.96

20.95

0.86

5.50

- 2.40

3.16

- 0.96

0.62

44M.00

26.64.

69.65

32.35

14.01

22.72

0.00

4.55

- 0.76

4.07

0.63

- 0.77

443.67

(b) Age at first egg March egg weight Strain I Strain 2 Strain 1 Strain 2

0 2 7.20 5.16 0.41 1 1.006

U2 12.44 23.93 1.787 3.019

U2 14.37 11.53 0.661 0.788

0 2 F Y 6.48 3.26 0.177 0.058

0 2 175.60 114.72 -0.004 0.005

S ( Y )

U2 T Y 7.37 2.04. 0.101 0.119

0 2 ST 5.42 2.13 0.044 -0.099

0 2 1 .oo - 1.10 0.229 0.062 SF

0 2 7.62 8.90 0.018 0.0m

0 2 F T Y 8.17 2.44 0.089 0.063 FT

0 2 - 3.53 - 0.91 -0.008 -0.1 76 SFT

W 0 2 264.78 231.78 1 I .036 11.965

Egg production per hen housed Egg production survwors to 500 days Strain 1 Strain 2 Strain 1 Strain 2

(C)

0 2 55.55 5.08 2.3.94. .05

0 2 96.78 66.36 48.02 51.18 S(Y)

a2 60 77 45.49 21.32 21.26

0 2 F Y 44.70 22.54 8.71 3.10

0 2 - 0.77 1.55 - 0.48 - 0.27


TABLE &Continued

Components of uariance

rJ2 - 1.83 - 1.79 3.18 - 1.51

0 2 16.74 19.39 1.47 3.94.

U 2 14.97 8.93 9.92 5.43

0 2 - 1.69 - 1.82 - 1.61 - 0.17

0 2 - 4.78 4.05 - 2.53 - 1.32

0 2 - 8.84 - 9.24 4.72 -14.39

0 2 2718.32 2333.38 1296.81 1128.28

TI.

SP

ST

FT

FTI-

EFT

w-

DISCUSSION

Effects of enuironmental differences on performance

The environmental differences considered were those associated with (1 ) differences among the three years, (2) differences among the three farms, and ( 3 ) differences between the two rearing programs. The two strains were treated separately throughout.

Year effects: From Tables 3a, b. c, it can be seen that there were highly significant year effects on all traits except two (mature body weight in strain 1 and survivor egg production in strain 2). The selection procedure during the period of the experiment was assumed to have made no important contribution to year effects. There was only a slight upward trend in egg production apparent at this stage of the selection program. Body weight at housing and at maturity was greater in years 1 and 2 than in year 3 (see Table 2). The decline in mature body weight was more pronounced in strain 2. There was also a decline in egg weight in both strains in year 3. The total number of eggs produced by both strains was, however, higher in year 3 than in the first two years. There is no doubt that the year effects in these three traits are related.

Farm eflects: These were very highly significant in all cases (see Tables 3a, b. c) . Birds reared at farm 2 became sexually mature later and produced smaller eggs than did those at the other two farms. At farm 2 birds also produced the lowest total number of eggs. Body weights were greatest at farm 1 and the birds reached sexually maturity earliest here. The number of eggs produced by hens surviving to 500 days was highest at farm 3. Farm 2 provided the least favorable environment for the expression of high body weight, early maturity and high egg production. It is worth noting that the extremes of temperature found at farm 2 are not encountered at either of the other farms. This may provide an indication of the type of environmental differences between the farms which were responsible for the very highly significant effects of farm on performance for all characters studied. It was found that there was a very highly significant interaction of location effects with years. The overall fluctuations in climate from year to year would be expected to affect each location to a different

GENOTYPE-EN VIRONMENT INTERACTION 153

extent. Each farm, however, provided an environment which was sufficiently distinctive to influence production in a characteristic way over all 3 years.

EfJects of nutritional treatment: Restricted feeding caused a marked decrease in body weight at housing: the restricted-fed birds were then only 75 percent of the weight of the full-fed birds. The body weight of the group whose food intake was restricted during the rearing period was only slightly less than that of the full-fed groups at maturity. In fact it was found that after they had been returned to ad libitum feeding for approximately 27 weeks, the weight of these birds was 95 percent of those given full feeding throughout their lives. Sexual maturity was delayed by from 15 to 18 days on the average in the birds reared on the lower plane of nutrition. The effects of the restricted feeding program on egg weight at maturity and total number of eggs produced per bird during the fixed test period were small. The variance component associated with the rearing treatment differences made up approximately 60 percent of the total in the case of body weight at housing and 30 percent of the total in the case of age to sexual maturity, but only about four percent of the total for the trait body weight at maturity. For the other three characters the mean squares associated with the treatment differences were insignificant.

The interaction of the effect of nutritional treatment with years was highly significant.for body weight at housing, age at first egg and egg weight. This suggested that the final result of feed restriction depends in part on the environmental circumstances peculiar to the years in which this treatment was given. For example, it was possible that unfavorable climatic conditions caused a more pronounced reduction in body weight in some groups on restricted feeding. An alternative explanation is that varying exposure to diseases such as coccidiosis, which are in turn partly dependent on climatic conditions, made it necessary to return the restricted groups to full-feeding for treatment to the disease for longer periods in one year than another. There was also a highly significant interaction between nutritional treatment and farm effect for body weight at housing and at maturity, and also for date of sexual maturity, which could be understood on the basis of similar reasoning. Finally, the second order interaction of treatment x farm x year was found to be highly significant also for the traits, body weight at housing and age at sexual maturity. Since the climatic factors with which the treatment effects were presumed to interact vary both with location and year, this was not unexpected.

Interaction of genotype and environment It was possible to estimate the proportion of the total variance due to the

interaction of sire breeding value within years with farms and also with nutritional treatment as well as the second order interaction of sires with farms with treatments. The design of this experiment was such that, within years, farm effects would be confounded with hatch effects, if the latter exist. It is reasonable to assume, however, that any interaction of genotype with hatch would be unimportant in this experiment since the conditions of hatching were constant and the times of hatching differed only by a matter of two or three weeks.

154 E’. HULL AND R. S. GOWE

A significant interaction between sire genotype and farm was found in four cases (see Tables 3a,b). For strain 1 there was a highly significant interaction (P < .OOl) between genotype and farm both for body weight at housing and for egg weight at maturity, and an interaction (P < .OS) for body weight at maturity. When the corresponding mean squares for strain 2 were examined, it was seen that the only significant interaction between genotype and location was for body weight at maturity (P < .05). There was no indication of a significant interaction between genotype and farm for age of sexual maturity, egg production per hen housed or survivor egg production for either strain (Tables 3b,c).

An interaction between genotype and treatment was found for two characters, body weight at housing (P < .OO1 for both strains) and age at sexual maturity (P < .OO1 for both strains),

In no case was the second order interaction genotype x farm x treatment significant.

The preceding analyses of variance indicated that, for some of the characters considered in this investigation, there was a statistically significant interaction between genotype and environment, and that the two strains may differ in the degree to which within strain genotypes interact with the range of locations considered. Any significant interaction could be due to (1) differences in the variances among sires in the several environments, or (2) changes in the rank of sire progeny groups in these environments. The latter would be of great importance when deciding on an appropriate selection procedure. It is possible to test the homogeneity of the sire variances within years firstly for full-fed and restricted-fed treatments, and secondly for each of the three farms by the method described by BARTLETT (1937). The tests listed in Table 5 indicated that the crude ~2 value for heterogeneity of variance among farms (df = 2) and between treatments (df = 1 ) were significant in only one instance at the five percent level.

was expected to be between 0 and 1. Values

close to one would indicate that genotype-environment interactions were unimportant in the particular conditions of the test. Decreasing values would indicate

6 The value of Or

a; + U& ffLt +

TABLE 5

Test of homogeneity of between sire uariance in different enuironments ~~ ~ ~

Strain I Strain 2 Heterogeneity x 2 Heterogeneity x 2 Heterogeneity x2 Heterogeneity x 2

between treatments between farms between treatments between farms - ____~ Character

Body weight at housing 1.31 2.52 4.37 3.12 Body weight at maturity 0.52 5.10 0.69 3.46

March egg weight 0.80 0.12 0.45 0.65 Hen housed egg production 0.01 3.73 0.01 4.70 Survivor egg production 0.34 1.44 0.05 1.58

Age to first egg 3.59 1.45 0.15 0.40

xz with 1 df at P=O.05 is 3.84 (treatments). x2 with 2 df a t PzO.05 is 5.99 (farms),


that the interaction of genotype and environment was more likely to influence the genetic behavior of the stock under these environmental conditions. These ratios are listed for the six characters of the two stocks in Table 6.

The proportion of the total phenotypic variance within farms and treatments which was due to genetic variance between sire groups was also estimated. This

was given by the ratio . (If the genetic variance for the trait in question

was low it would be unlikely, but not impossible, to find an important interaction of genotype with environmental treatment.) Similarly the proportion of environ-

a i

U; + U;

mental variance due to the treatment effect was expressed as , and a;+ a;

the proportion of environmental variance due to farm effects was expressed as

where U: is the variance due to uncontrolled environmental effects a:+ U:'

within sire progeny groups. These ratios are shown in Table 6. The environmental variance within sire progeny group within farms within treatments, u2

,?' was obtained from the relationship U;= U;- 30: since it is known that U: estimates % a2, and oLestimates % U: + U; where a; is the component of variance due to additive genetic effects.

The interaction between genotype and farm environment was unimportant for all traits, the additive genetic variance formed a high percentage of the total genetic variance in all cases (Table 6 ) . Since the total genetic variance for hen-

TABLE 6

Interrelation of variance components

Body weight at housing

1 2

0.11 0.11

0.26 0.3 1

0.88 0.97

0.84 0.82

0.69 0.59

Body weight at maturity

Age to first egg

1 2

0.17 0.14

0.09 0.12

0.W 0.94

0.1 1 0.09

1.03 1.01

0.80 0.92

1 2

0.04 0.09

0.06 0.06

0.93 1.05

0.43 0.41

March egg weight

Egg production per hen housed

1 2

0.14 0.20

0.10 0.21

0.89 0.98

0.00 0.00

0.98 1.03

0.03 0.03

0.02 0.02

0.85 0.77

0.00 0.00

0.87 0.88

1 2

Egg production survivors to 500 days

1 2

0.04 0.04

0.02 0.02

0.97 0.93

0.00 0.00

0.83 0.90


housed and survivor egg production was small, response to selection among sires in these characters was expected to be small. At the same time there was no evidence to indicate that the response would differ according to the location where the selection was performed.

The interactions between genotype and nutritional treatment for body weight at housing (both strains) and for age at sexual maturity (strain l ) , appeared from Table 6 to be of considerable importance. Additive genetic variance made up only 60-70 percent of the total genetic variance in these cases. These results suggest that if a strain of birds was required to have large body size or early maturity under conditions of restricted feeding, it would be better to consider selecting among birds reared on a restricted feeding program rather than among full-fed birds within these strains.

From these results it is possible to make a hypothesis concerning the conditions in which genotype-environment interactions might be of biological importance. The variation due to the environmental treatments must be large in comparison to the nongenetic variation within the environments. This situation occurred in the present investigation for body weight at housing and age at first egg. In the experiment of FALCONER and LATYSZEWSKI (1 952) already mentioned, in which the relative performance of two strains of mice was found to be the reverse on the high plane of nutrition from what it was on the low plane, the diets them- selves had a very large effect on average performance. Similar situations have been encountered elsewhere. For example there are numerous reports of wide differences in relative merits of different strains of animals when exposed to certain diseases, particularly where genetic resistance to the particular diseases has been found in some of the strains but not in others. Cases of this nature have been described by HUTT (1958). A wide variation in average performance has been found between the environments in which the disease was present and those free from disease. Another case where environmental differences are large is the wide differences between temperate and tropical regions in temperature, relative humidity, presence or absence of disease organisms and levels of nutrition. These in turn may cause great differences in the production of meat, eggs and milk between these two environments. BONSMA (1 955) discusses the factors influenc- ing performance in one such case when comparing British beef cattle with Africander cattle in a subtropical environment. It is usual to find when comparing strains of animals selected in two environments as dissimilar as these that each outperforms the other in its own environment.

The cases where a large genotype-environment component of variance occurs in this experiment have a second characteristic besides a large environmental variance. The between sires variance (additive genetic variance), is also quite large. It is suggested that favorable circumstances for the occurrence of an important genotype-environment interaction generally will be found if the genetic wriance between the groups which are divided and reared in the various environments is large when compared to the total phenotypic within-environment variance. The most extreme instance possible is that which occurs when identical twin pairs are split between two environments. Here, all genetic variance is be-


tween pairs of twins and none within pairs. An important interaction was found under these circumstances in the study reported by BONNIER and HANSON (1948). The ratio of genetic variance between groups to total variance could also be high where the groups used are different inbred lines, and it is perhaps significant that YOUNG ( 1953) detected a large genotype-environment interaction under these circumstances. Smaller between-group variances would be expected if full-sib groups were used and smaller still if half-sib groups were employed. In comparisons involving different strains of animals the ratio of genetic variance (between strain variance) to total within-environment variance would depend on the degree of genetic similarly between the particular strains used.

SUMMARY

A study was undertaken of the magnitude of genotype-environment interaction involving two strains of poultry, three widespread locations, three years, and two rearing programs within each location. Differences among progeny groups of sires within the two strains within years were used for the genetic differences. All breeding stock was kept at one location and progeny were shipped as day-old chicks to the three test locations, each of three hatches going to a separate farm. One half of the birds at each location were reared under a feed restriction program and the other half were full-fed.

The method of analysis involved estimation of components of variation associated with each genetic and environmental source of variation for any character. The components were then expressed as ratios which indicated first the importance of a single fixable environmental factor in relation to total environmental variance, and secondly the importance of variance among the genetically different groups in relation to total variance within the main environments. The importance of genotype-environment interaction was measured by the ratio of between sire genetic variance to total genetic variance (between sire plus interaction variance).

The rearing treatment had a very large effect on traits such as body weight and age at sexual maturity but little effect on the number of eggs laid over a standard test period, since the longer period to sexual maturity cancelled for the most part the higher rate of egg production of the restricted-fed groups.

The year and farm effects were large for most traits but quite variable from year to year.

Large and important interactions were found only when the environmental effects were very large and the genetic differences were wide. For the three traits, body weight at 21 weeks (end of restriction period), and egg weight and body weight at maturity, the interaction between sire genotype and farm were significant in one strain. For the other strain the only significant interaction was for body weight at 21 weeks. Significant interactions for genotype X rearing treatment were found for the two traits, days to sexual maturity and body weight at 21 weeks. In no case was the second order interaction significant for genotype x farm x treatment.


The results of this experiment indicate the conditions under which genotype- environment interactions are likely to be of importance in a practical breeding program.

ACKNOWLEDGMENTS

The senior author is grateful to the National Research Council, Canada, for awarding him a Post-Doctorate Fellowship to work in the Canada Department of Agriculture. The authors would also like to acknowledge the assistance of MR. G. P. KAVANAGH.

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THE IMPORTANCE OF INTERACTIONS DETECTED BETWEEN …Workers with poultry differ in their assessment of the importance of the phenomenon in the various circumstances studied. ABPLANALP