EFFICIENCY OF SELECTION METHODS

Laly John C “Statistical methodology for selection procedures in poultry breeding” Thesis. Department of Statistics, University of Calicut, 2005

CHAPTER 7

EFFICIENCY OF SELECTION METHODS

7.1 Introduction

Selection is the means with which all improvement of domesticated animals

and plants has been made. The simplest form of selection is to choose individuals on

the basis of their own phenotypic values. An outline of various methods of selection

used in animal breeding and a comparison amongst them is given below:

The net value of an individual depends on several traits. Hence, it is necessary

to apply selection simultaneously to all traits of economic importance. In poultry,

genetic economic value of a layer bird depends on many traits like age at sexual

maturity, body weight, number of eggs produced and weight of egg. The traits to be

considered for simultaneous selection obviously depend to a large extent, upon their

genetic significance and economic values. Genetic significance of a character means

its response to selection and response to selection depends on (1) the magnitude of

heritability of the character and (2) the genetic association of the character with other

characters.

Once the decision is taken upon the traits to be improved, the next problem is

how the selection should be applied on these traits in order to achieve the maximum

improvement of overall economic value.

Common selection methods are

(1) Tandem selection

(2) Independent Culling levels and

(3) Selection Index

Tandem selection:

Selection is practiced for one trait at a time until satisfactory improvement has

been achieved in this trait. Then, selection efforts for this trait are relaxed and efforts

are directed towards the improvement of a second, then for a third and so on, until

finally each trait has been improved to the desired level. This method is the least

efficient of all the three methods, from the stand point of the amount of genetic

progress made for the time and effort e d by the breeder. Efficiency of tandem

selection depends a great deal upon the genetic correlation between the traits selected

for. If the genetic correlation is positive and desirable, improvement in one trait by

selection would automatically result in improvement in the other trait not selected for,

then the method could be quite efficient. Otherwise, if there were little or no genetic

correlation between the traits, which means that they are inherited independently, the

efficiency would be less. When the two traits are negatively correlated, improvement

in one trait is nullified or neutralized by the regression in the other. For example, in

poultry, selection for egg number will result in reduction in egg weight, as they are

negatively correlated.

Independent Culling Levels:

Selection is made simultaneously for all the characters but, independently,

rejecting all individuals that fail to meet the minimum standard for any one trait.

This method is more efficient than tandem method and has an important

advantage over the latter in that, selection is practised for more than one trait at a

time. It is never more efficient than the selection index method. But, in cases where,

the traits under consideration manifest at different ages, it offers the practical

advantage of disposing a proportion of inferior individuals earlier.

When independent culling level is applied for more than one trait, it has the

following disadvantages:

1) It does not permit superiority in some traits to compensate for deficiencies in

the other.

2) Characters measured early in life are likely to be overemphasized at the

expense of selection for important traits measured later.

3) Determination of optimum culling levels when the traits are correlated.

Selection index or total score method

Selection is made for all traits simultaneously by using some kind of a total

score or index of the net merit of an individual, constructed by combining together the

scores for component characters. The individuals with highest score are kept for

breeding purposes.

Since the traits to be considered in selection may not be equally important

economically, some kind of weighing is required. Unless appropriate weighing is

adopted some traits will receive too much and others too little attention. The amount

of weight given to each trait depends on its relative economic values, its heritability

and the genetic and phenotypic correlations between the different traits.

Selection index method is more efficient than tandem and independent culling

level methods, because it results in more genetic improvement for the time and effort

expended in its use.

Index selection in poultry breeding

An individual's own phenotypic value is not the only source of informationto

its breeding value, additional information is provided by the phenotypic values of its

relatives, particularly those of full and half sibs. The use of information from relatives

is of great importance in the application of selection to poultry breeding, as the

primary trait of selection, egg number, is low heritable. When the heritability of the

character to be selected is low, the mean values of close relatives often provide with a

more reliable guide to the breeding value than the individual's own phenotypic value.

The correlation between the breeding value and the phenotypic value can be increased

by combining the information from one or more relatives of the individual for the

given trait resulting in greater response to selection.

In poultry, the family classification is usually a hierarchical one, viz., each sire

is mated to a number of dams and each dam produces one or more offspring. In such

cases, the relative merits of individual selection and family selection in breeding for

low heritable traits like egg number, were first discussed by Lush (1947) and Lerner

(1950). For poultry, selection of individuals on the basis of index with appropriate

weights attached to its own performance and the average performance of sire and dam

families (combined selection) was developed by Osbome (1957 a, b).

In IWN and IWP strains of White Leghom, the selection index of Osborne

(1957 a,b) was applied with criterion of selection as part year egg production from S1

to SI4 generations. When selection criterion was part year egg production, a decline

was noticed in the late production. Hence, from Sls generation, selection criterion was

shifted to annual egg production.

Effectiveness of selection is assessed with respect to the direct and correlated

responses. Direct response to selection is the change observed in the trait under

selection, that is primary trait. The selection for a primary trait brings about

simultaneous changes in correlated traits. Correlated response is defined as the

concomitant change observed in an unselected trait when selection is directly applied

for one trait. It may result either from genetic effects or environmental influences or a

combination of both (Falconer and McKay, 1996).

The direct and correlated responses in IWN and IWP strains, after 1 4 1 ' l \

generations of selection for EN40, were reported by (2000). In this chapter, jS?"i/c

an evaluation of the response after six generations of selection for annual egg

production is presented. On the basis of the results obtained from this and the

conclusion drawn from analyses in the previous chapters, the possibility of alternate

selection plans is discussed.

7.2 Review of Literature

Selection is the most important tool in improving egg production in poultry.

Various researchers have proposed several selection indices from time to time. The

genetic parameters, heritability and genetic correlation play key roles for the decision

of selection criterion. Hence, any report on selection experiments will be

accompanied by estimates of genetic parameters of the corresponding population. A

brief review of the various selection indices is given.

Osborne (1957 a, b) proposed selection index for improving egg production

and this index is still used widely. The index uses the individual's own performance,

along with its dam and sire family (IDS) information.

Hogsett and Nordskog (1 958) discussed the construction of an index based on

egg production rate, body weight and egg weight. Method for determining the

economic weights for the component traits was outlined. From a review of genetic

parameter estimates, Hogsett and Nordskog (1958) found reports of both positive and

negative correlations between body weight and egg weight. Thus within lines, very

small birds tend to lay less eggs than larger birds, but extremely larger fat birds also

lay fewer eggs. The economic weights depend not only on market conditions

fluctuating by season and year, but also on the mean level of performance of the

breeding stock itself.

Morris (1 963) selected for 72 week egg production based on a partial record of

production. Annual production increased initially but plateaued subsequently. Morris

(1963), citing other studies with similar results concluded that the evidence is very

strong that continued selection for partial record egg production will provide steady

gain in the character selected, but will eventually be achieved at the expense of the

production period. Gowe and Strain (1963) also concluded that gains could be made

in the partial record , but after a few generation, no further gains in annual egg W-

production were obtainable.

In India, for the two decades from mid seventies to early nineties, poultry

breeding research was focused on selection based on part year egg production with

the ultimate objective of improving annual egg production. Combined selection index

of Osborne (1 957 a,b) was adopted widely.

During 1980's, different researchers proposed several selection indices, with

part year egg production as the primary trait of selection.

Narain et al. (1983) proposed a selection index involving egg production upto

240 days of age as the primary trait and age at first egg and egg weight as the

auxiliary traits and compared its efficiency with Osborne's index using data from a

poultry population maintained at the Bhopal centre of A.I.C.R.P. on Poultry

Improvement. It was found that efficiency increased when age at first egg and egg

weight were used as auxiliary traits. The gain in efficiency was claimed to be high for

high values of heritability of the selected trait and auxiliary traits are to be

uncorrelated. The index was not used further and Bhopal centre itself was terminated

by ICAR for want of any genetic improvement of the stock.

Das et al. (1983) tried multi-trait index selection with egg number upto 40

weeks of age, 35 week egg weight and 20 week body weight in two generations of

egg type chicken and reported economic gains of Rs.5.66 and Rs.9.07 respectively.

But the genetic response in egg v"

Singh and Kurnar selection viz., individual, --P

sire and dam family averages and their combinations and found that combined

selection (Osborne's index) was superior to dam family, sire family and individual

selection at all levels of heritability and family size. Efficiency is increased with

increase in family size especially at low level of heritability.

Ayyagari et al. (1984) fitted multi-trait, multi-source selection indices based

on age at maturity, body weight at 20 and 40 weeks, EN40 and EW40, to evaluate the

usefulness of additional sources of information in multi-trait selection indices. The

sources of information used were individual's phenotypic performance, its full and

half sisters averages for all the component traits. The results revealed that the

component traits in the index did not gain alike from the additional sources of

information and based on the responses obtained from the component traits, suggested

that in a multi-trait index, supplementing additional sources of information only to the

primary trait (egg number) appeared adequate.

Singh et al. (1986) constructed 26 selection indices of different combinations

of five traits BW20, ASM, EN40, EW40 and BW40 in a White Leghorn population

and found that the index involving the two traits EN40 and EW40 was the best. It was

concluded that aggregate genetic worth of the bird would depend on some important

characters and not all the characters.

Singh and Kumar (1992 a) proposed a selection index based on BW20, ASM,

EW and EN40 in one generation and selected females were retained for recording

their annual egg production. This information on dams was used in the selection of

succeeding generation. The index was fitted so as to get an improvement of 2 g in

EW40 and 25 eggs in 72 week egg number, without any change in ASM. The index

was fitted in an assumed population and the genetic parameter estimates were taken

from the literature.

Singh and Kumar (1992 b) proposed multi-source, multi trait indices in single,

two and three stages to improve genetically antagonistic traits- annual egg production

and egg weight. Selection of males was based on part year egg production of full and

half sisters and dam's annual egg production and females based on indices combining

part year egg production (IDS), individual egg weight and dam's annual egg

production. A two-stage selection index with selection for EW in the initial stage and

index based on part year egg production (IDS) and dam's annual egg production was

suggested.

c, j\- r] h+- *

/ " " 4 -- All the above indices were based on part year egg production/ad some used

k dam's annual egg production in addition. Reports on selection index based on annual

egg production is scanty.

A brief review of the responses realized after continuous selection for part

year egg production is given below. Reports are not found on the responses realized

from selection based on annual egg production .

Johari et al. (1995) estimated the average realized genetic gains after 17

generations of selection for part period egg production up to 280 days of age, as 2.35,

1.92, 1.18 and 1.82 eggs in IWG, IWH, IWI and IWJ respectively and were highly

significant (P<0.01). The corresponding genetic gains in various correlated traits were

4.05, 1.85, 7.23 and 6.91g for BW20, -9.53,-12.76,-2.18 and-5.08g for BW40, -1.90,-

1.63,-0.87 and -1.55 days for ASM and -0.46, -0.30, -0.26 and -0.26 for EW40.

After six generations (1 988-94) of family index selection (IDS) for part period

egg production up to 280 days of age, Bais et al. (2000) got the average realized

genetic gains as 1.18 and 2.18 eggs per generation in IWH and IWI lines respectively.

The corresponding genetic gains in different correlated traits were -0.17 and -0.60 g

for 40th week egg weight; 0.87 and -0.13 days for age at sexual maturity; 9.43 and

3.13 g for 20-week-body weight and 19.24 and 5.03 g for 40-week-body weight.

Except for 20-week-body weight in the IWH line, the responses in all the traits of

both the selected lines were non-significant.

Adeyinka et al. (2001) evaluated the direct and correlated responses to

selection over 5 years (1991-1995) in a population of layer-type chickens. The

genotypic response was 3.14 eggs per year while the phenotypic response was 1.61

eggs. Correlated response for age at first egg, body weight at 40 weeks and egg

weight were -3.92 days, 6.12 g and 0.89 g respectively. Ravikumar et al. (2003)

reported that the genetic and phenotypic responses realized in a White Leghorn line

were 1.93k0.47 and -1.86k0.67 respectively, after six generations of combined

selection for egg production to 280 days of age and were non significant.

After four generations of selection, Rath (1986) obtained 8.30k1.60 eggs in

IWN and 6.37k1.60 eggs in IWP as direct response in EN40. The correlated responses

in ASM and EW40 were -4.26k2.03 days, -0.20k0.11 g in IWN and -4.45k2.76 days

and -0.02k0.20 g in IWP. After 14 generations of selection for 40-week egg number

in IWN and IWP strains, Laly et al. (2000) got a direct response of 1.34k0.54 eggs in

EN40 and correlated responses in ASM, BW20 and EN40 as -0.57k0.40 days,

5.20*4.50 g and 0.13*0.05 g in IWN and 1.3k0.43 eggs, -0.73k0.41 days, 6.54k4.74

g and 0.13k0.05 g in IWP.

7.3 Materials and Methods

The data fromSl to S20 generations (1994-2001) on ASM, BW16, BW40,

EN40, EN60, EW28 and EW40 were used in the analysis. The details of number of

birds in each generation are given in chapter 1 (Table 1.1).

In the six generations from Sl j to S20, of both IWN and IWP strains, selection

criterion was annual egg production (EN60) and independent culling level was

applied for egg weight, for 32-week egg weight in S l j and for 28-week egg weight

from S16 to generations. The selection method is given below.

7.3.1 Selection method followed

Combined selection based on Individual, Dam family and Sire family averages

(IDS selection) as per Osborne(1957 a,b) was followed . The index for selection of

males and females were obtained as follows:

I (Female) = (P-P) + b z ( ~ & P ) + b 3 ( ~ , - P )

Where

P = Annual egg production of individual.

P = Population average of annual egg production

Fd = Dam family average

F, = Sire family average

b2 and b3 are weights attached for dam and sire families

n = number of progenies per dam

d = number of dams per sire

h2 = heritability of annual egg production (EN60)

From the individuals selected based on the index, those with egg weight at 28

weeks below population average were culled, so that finally 300 females were

selected. Thus a two stage selection was adopted, with first stage selection for egg

numbers and in the second stage, independent culling for egg weight. Based on the

index for males, 50 males were selected. In each generation, the selected females and

males were kept for breeding with six females per male. The selection was carried out

in the Mannuthy Centre of A.I.C.R.P. on Poultry Improvement.

For fitting selection index, FORTRAN program was developed.

The effectiveness of selection for six generations was assessed by estimating

the response to selection, both genetic and phenotypic in the selected and unselected

traits.

7.3.2 Response to selection

Phenotypic and genetic responses realized in the selected and unselected traits

due to selection for annual egg production for six generations (Sls to SZ0) were

estimated in both IWN and IWP strains. The phenotypic responses per strain are not

free of environmental trend. Hence, a control population was simultaneously

maintained and tested, to critically assess the genetic response in selected populations.

Direct response to selection

Phenotypic response per generation for the selected trait (Direct response) was

estimated by regressing generation mean on generation number.

Similarly, the genetic response was estimated as the regression of the

difference between the means of the trait in the selected and control populations on

the generation number.

The regression coefficient was tested for statistical significance using t- test.

Correlated Response

The correlated responses (phenotypic) realized per generation in the

unselected traits ASM, BW16, BW40, EN40, EN60, EW28 and EW40 were

dobtained by regressing respective generation mean on generation number.

Realized genetic response was estimated by regressing the deviations from

control on respective generation number.

The direct and correlated responses were estimated using MS-EXCEL package.

7.4 Results and Discussion

The means and standard errors of the various traits in the six generations and

genetic and phenotypic responses realized are provided in Tables 7.1 and 7.2

respectively in IWN and IWP. The corresponding means in the control population are

given in Table 7.3. The results are discussed in the following section.

7.4.1 Response to selection

The phenotypic and genetic responses after selection for annual egg

production for six generations in IWN and IWP are presented in Tables 7.1 and 7.2.

Phenotypic responses in control population (Table 7.3) were all non-significant

indicating that it was adequate in measuring the environmental trend.

Direct Response in egg number (EN60)

In all generations, EN60 was greater for IWP than IWN. Significant

phenotypic response of 5.9W0.78 eggs (p<0.01) was realized in IWN and 6.1*0.95

eggs (p<0.05) in IWP for 60-week egg number, per generation. The realized genetic

gains were 3.48k1.78 eggs and 6.21*1.37 eggs in IWN and IWP strains respectively.

Significance (p<0.05) was noticed for IWP only.

Correlated responses

Age at sexual maturity

In all generations from Sls to SZ0, IWP strain had low age at sexual maturity

compared to IWN strain as noted from Table 7.1 and Table 7.2. Similar observation

was made by Prabhakaran et al. (2001). The phenotypic response in this trait was

significant in both strains, with an average reduction of 3.5*0.92 days per generation

in IWN and 3.37*1.06 days in IWP strain, consequent to selection for 60-week egg

production.

Table 7.1 Gener vario

I I I Genera- INO. of

Phenotypic

response

Genetic

response

tion wise S traits in

ASM (days)

* - Significant at 5% level ** - Significant at 1% level

# - EW28 measured only from S 16 generation

## - BW 16 measured only from SI1 generation

neans and standard errors, phenotypic and genetic responses of IWN strain

BW40 EN40 EN60 ~ ~ 2 8 ' (g) EW40 (g) ~ ~ 1 6 "

Table 7.2 Generation wise means and standard errors, phenotypic and genetic responses of

# - EW28 measured only from SI6 generation

## - B W 16 measured only from S 18 generation

various traits in IWP strain

* - Significant at 5% level ** - Significant at 1% level

B W I ~ ~ ( ~ )

1006.1 + 0.6

1135.0 + 0.4

1131.4 + 0.8

62.60 + 38.20 70.65f 29.59

The corresponding realized genetic responses were -2.12*1.02 days in IWN and

-2.5 1=t0.60 days in IWP strain. Significance (pC0.05) was noted for IWP strain only.

BW40 (g)

1739.1 + 7.0

1756.6 + 1.3

1773.5 + 2.2

1820.3 + 1.3

1639.0 + 1 .O

1651.0 + 1 .O

-21.33 f 15.80 -3.53 f 31.70

Genera- tion

S~~

S16

S17

S18

S19

S20

Table 7.3 Means and standard errors of various traits in Control Population

Reports in, literature on the response, due to selection for annual egg

production is scanty. But several reports of negative response in ASM at phenotypic

EW40 (g)

52.60 + 0.09

52.49 + 0.02

50.97 + 0.04

53.10 + 0.01

54.14 + 0.02

54.01 + 0.02 0.40 f 0.24 0.20 f 0.32

No. of obs-

1328

1429

1460

2508

2150

2112

EN40

97.48 + 0.49

105.31 + 0.07

100.71 + 0.31

107.13 + 0.05

107.31 + 0.19

121.21 + 0.02 3.74" f 0.12

4.40** f 0.21

ASM (days)

159.22 k 0.39

145.18 * 0.10

150.86 k 0.01

143.26 + 0.07

144.53 * 0.06

137.55 + 0.04

Phenotypic response

Genetic response

BWI~"(~)

1038.2 + 8.0

1045.0 + 8.0

1022.0 + 6.0

Genera- tion

S16

S17

S ~ 8

S19

-3.37* f 1.06 -2.51* f

0.60

EN60

202.60 + 0.91

195.30 + 0.21

178.90 + 0.46

205.96 + 0.11

212.58+ 0.99

224.48+ 0.78

6.1*f 1.95 6.21* + 1.37

BW40 (g)

1639.2 + 11.2

1650.0 + 2.5

1522.0 + 1.3

1566.0 + 1.01

1526.0 + 2.1

3.86 f 5.93

EN40

96.53 + 1 .O

57.18

94.85 + 1.68

90.09 + 1.79

99.37 + 2.37 3.86 f 5.93

Ew28' (9)

48.24 + 0.03

47.88 + 0.002

48.76 + 0.02

50.20 + 0.01

50.42 + 0.01 0.67*f 0.17 0.31 f 0.32

No. of obs-

68

200

200

loo

EN60

177.35 + 1.50

108.53 i 1.60

182.68 + 3.54

185.54 + 3.07

186.62 + 4.89 9.55 f

10.92

ASM (days)

148.84 + 0.56

157.94* 0.11

149.53 i 0.62

153.70 + 0.66

146.96 i 0.94

Phenotypic response

-0.80 f 1.54

~ ~ 2 8 ' (g)

47.52 + 0.01

46.77 + 0.25

47.03 + 0.41

48.88 + 0.26

-0.42 + 0.55

F 52.38 + 0.10

49.82 + 0.03

51.19 + 0.32

51.78k 0.36

53.49+ 0.28 0.42 + 0.44

and genetic levels as a result of selection for egg production up to 40 weeks are

available. Rath (1986) reported average phenotypic responses of -4.26h2.03 days in

IWN and -4.45h2.76 days in IWP strain after four generations of selection for egg

production up to 40 weeks. After fourteen generations, Laly et al. (2000) obtained

non-significant correlated responses of -0.57~k0.4 days in IWN and -0.73h0.41 days

in IWP strain per generation. Johari et al. (1995), Brah et al. (1999) Bais et al. (2000)

and Adeyinka et al. (2001) reported negative genetic response in ASM as result of

selection for part year egg production.

Part year egg production up to 40 weeks (EN40)

Tables 7.1 and 7.2 dictates the superiority of IWP strain over IWN for egg

production up to 40 weeks in the six generations. This is implied, as IWP had low

average age at sexual maturity compared to IWN, in all the six generations. Selection

for annual egg production resulted in high significant correlated responses at

phenotypic and genetic levels in both strains. The average phenotypic response per

generation was 3.5 h 0.92 eggs in IWN and 3.74 h 0.12 eggs in IWP while the

corresponding figures for genetic response were 4.28h0.09 and 4.4h0.21 eggs. Thus,

selection for annual egg production improved part year egg production also. This

might be due to high genetic correlation between the two traits.

Reports on the correlated response in 40-week egg production resulting from

selection for annual egg production are scanty. This might be due to the fact that

selection criterion was 40 week egg production for last two to three decades.

Many reports on the direct response in EN40 as a result of selection for EN40

are available. Significant phenotypic responses of 1.34*0.54 eggs in IWN and 1.30 k

0.43 eggs in IWP were reported by Laly et al. (2000), when the two strains were

subjected to fourteen generations of IDS method of index selection for EN40. Rath

(1986, Johari et al. (1 995), Brah et a1.(1999), Bais et al. (2000) and Adeyinka et al.

(2001) reported positive response (genetic and phenotypic) in EN40 when selection

criterion was EN40.

Pullet body weight (BW16)

Pullet body weight was recorded at 20 weeks up to S17 generation and from S18

generation, it is recorded at 16 weeks. Since 16-week body weight is of current

interest, the genetic and phenotypic responses in this trait were determined as

correlated response to selection for annual egg production. In both IWN and IWP

strains, the phenotypic as well as genetic responses were positive and non-significant.

The average phenotypic response per generation was 46.55k15.91 g in IWN and

62.6h38.2 g in IWP. The corresponding figures for genetic response were 54.55k7.24

g in IWN and 70.65*29.59 g in IWP.

The average phenotypic response per generation in pullet (20-week) body

weight, after fourteen generations of selection for 40-week egg production was

reported as 5.2k4.5 g in IWN and 6.54k4.74 g in IWP strain by Laly et al. (2000).

These were non significant. Non significant positive gain in 20 week body weight

concomitant to selection on 40 week egg number was reported by Brah et al. (1999)

and Bais et al. (2000).

Mature body weight (BW40)

At 40 weeks, IWP had more average body weight than IWN in all the six

generations (Tables 7.1 and 7.2). In both IWN and IWP strains, negative and non-

significant phenotypic response (-4.42k17.0 and -21.33*15.8) was noticed for

BW40. Genetic responses were also non-significant, positive (2.87h24.94) in IWN

and negative (-3.53k31.70) in IWP. Reports showed positive correlated response in

BW40, due to selection for part year egg production up to 40 weeks (Adeyinka et al.

(2001), Brah et al. (1999) and Bais et al. (2000)).

Egg weight at 28 weeks (EW28)

With the idea of improving early egg weight, from SI6 generation, egg weight

was recorded at 28 week and independent culling level was applied along with

selection for egg number. Except in SI7, EW28 was high in IWN than IWP. The

phenotypic response per generation was 0.79*0.28 g in IWN and non significant

(Table 7.1). But in IWP, a significant (p<0.05) phenotypic response of 0.67h0.17 g

per generation was noticed (Table 7.2). The genetic response per generation was

0.42k0.46 in IWN and 0.31zk0.32 in IWP. Both the values were non significant. The

genetic correlation between the selected trait EN60 and EW28 are moderately

negative as given in chapter 3. The independent culling for EW28 must have

contributed to the positive response.

Egg weight at 40 weeks (EW40)

EW40 was high for IWN in S16, S18 and S20 and in S17 and S19, high for IWP.

The phenotypic response per generation in EW40 was 0.43h0.28 in IWN (Table 7.1)

and 0.40zk0.24 in IWP (Table 7.2). Genetic responses were 0.23zk0.32 and 0.20&0.32

in IWN and IWP strains respectively. Although, culling level selection was adopted

for EW28, there was positive phenotypic and genetic gains for EW40 in both the

strains. This might be due to the high genetic correlation (0.9 to 1 .O) between EW28

and EW40 in both the strains as seen in Chapter 3+.

In the control line, the regression coefficients for these economic traits were non

significant.

7.5 Conclusion

1. Selection based on annual egg production resulted in significant genetic and

phenotypic gains in both part and annual egg number.

2. ASM and egg number are negatively correlated traits and selection for egg

number lead to significant reduction in ASM in both the strains as expected.

3. In both strains, positive responses were observed in pullet body weight

(B W 16), by selection for annual egg number.

4. Independent culling level selection for egg weight gave positive response in

egg weight at 28 and 40 weeks.

5. The present scheme of selection followed is in the desired direction, as

evidenced from the average genetic and phenotypic responses realized after

six generations of selection for annual egg number and may be continued for a

few more generations with appropriate timely monitoring.

6. IWP strain has most of the desirable traits to be designated as a dam line and

IWN a sire line and the cross (IWN X IWP) will be acceptable to the poultry

industry for commercial exploitation.

7. It is recommended that in future selection programmes, IWP strain may be

further refined as a dam line and IWN as a sire line.

CONCLUDING REMARKS

The salient conclusions drawn from the present investigation and the

recommendations for future selection programmes in IWN and IWP strains of White

Leghorn are given below:.

1. To study the egg production pattern of IWN and IWP strains, fitting of

McNally's and Inverse Polynomial models is easy and of the two, McNally's

model is preferred to Inverse Polynomial. However, when biological

interpretation of parameters is needed, Cornpartmental models are

recommended.

2. We determined the optimum age at sexual maturity and body weight at 16

weeks in IWN and IWP strains. This information is of great significance and

can be used for exploiting the potential of the strains in future selection

programmes.

3. Advancing the age of selection to 56 weeks from 60164 weeks, which will

help the breeder to limit the number of pedigree hatches to be taken is

recommended, since the genetic correlation between EN56 and EN60/EN64 is

found to be very close to 1 and the efficiency of selection based on EN56 is

very high.

4. Even after 20 generations of intense selection, genetic variability still persists

in the two populations IWN and IWP, as seen in chapter 3. Also, the genetic

gains in the various egg production traits from the present scheme of selection

are in the desirable direction. Hence, the present scheme of selection followed

in IWN and IWP strains may be continued for a few more generations with

timely monitoring.

5. Survival analysis showed that the livability of both IWN and IWP strains is

very high.

6. Very focused attention is warranted to further refine IWP strain as a

specialized dam line, since in the present study, the strain has most of the

desirable traits of a dam line to be reckoned.

7. As far as IWN strain is concerned, it desires attention for further refinement to

develop it as a versatile sire line.

8. The cross (IWN X IWP) was a strain cross released from Mannuthy Centre of

All India Coordinated Research Project on Poultry Improvement, under the

patency 'ILM-90'. With the data available for both the strains with respect to

60 week egg number and the connected genetic parameters estimated in the

present study, it is possible to predict that the potential of the cross (IWN X

IWP) is much more than that was reported for ILM-90.

9. During the process of selection of sires, attention should be paid to choose

individuals having desirable body weight from each half sib family and that

too which have had optimum body weight at 16 week and so it is

recommended that the body weight of male progeny is also recorded at 16

weeks of age.