| dc.description | INTRODUCTION
Low genetic potential for production traits and frequent
outbreak of diseases specifically Newcastle disease has
been noted in a significant number of studies to be
among the major factors limiting productivity of the local
chickens in the Tropics (Katule, 1991; Alexander, 2001;
Pedersen, 2002). Their performance can partly be
improved substantially by crossbreeding with improved/
specialized egg type or meat type chickens as it has
been shown in previous studies (Ali et al., 2000;
Pedersen, 2002; Theerachai et al., 2003; Segura-Correa
et al., 2004). However, these crossbreeding programs
are threatened by current global move on conservation of
indigenous genetic resources (Msoffe, 2003; Kosgey,
*Corresponding author. E-mail: jlwelamira@yahoo.com.
2004). Therefore, there is a need for looking for alterna tive approaches for genetic improvement. Genetic
improvement through selection within local chickens
could be a good option. A recent study by Lwelamira
(2007) in two Tanzania local chicken ecotypes viz. Kuchi
and Tanzania Medium (Medium) have shown existence
of significant additive genetic variation in various traits in
these ecotypes and hence expecting adequate response
to selection. Based on their performances, the study
further revealed Kuchi ecotype to be a good starting
material for improving meat production and Medium
ecotype for egg production. Since relative economic
weights for various traits in these ecotypes are lacking as
it has been in most other local/unimproved chicken
strains/groups (Menge et al., 2005), this study was
therefore carried out to evaluate various desired-gain
selection indices for improving Kuchi ecotype for meat
134 Afr. J. Agric. Res.
Table 1. Desired-gain selection indices for Kuchi and Medium ecotypes
Ecotype Index Ab Bwt16 AFE EN-90 EW
Kuchi
I1k + + - 0 -
I2k - + - 0 -
I3k + + - - -
Medium
I1m + 0 + + +
I2m - 0 + + +
I3m - - + + +
I4m + - + + +
I5m + 0 + + 0
I6m - 0 + + 0
I7m - - + + 0
I8m + - + + 0
Ab= Antibody response against Newcastle disease virus (NDV) vaccine
(primary humoral immune response at 4 weeks of age assessed two weeks
post vaccination); Bwt16 = Body weight at 16 weeks of age; EN-90 = Egg
number in the first 90 days; EW = Egg weight between 33 to 36 weeks of
age, AFE= age at first egg; + = A trait is improved, 0 = A trait is held
constant, - = A trait is dropped from the index.
production and Medium ecotype for egg production.
MATERIALS AND METHODS
Various desired gain selection indices specific for each ecotype
were constructed and compared. Previous study by Lwelamira
(2007) showed that Kuchi ecotype had good prospects for further
improvement in terms of meat production, and Medium ecotype for
egg production. Therefore, selection indices were constructed with
the intention of improving meat production for Kuchi and egg
production for Medium ecotype (Table 1).
Selection indices were constructed according to Yamada et al.
(1975) (that is desired-gain indices) as applied in subsequent
studies (Gill and Verma, 1983; Hazary et al., 1998; Nishida et al.,
2001; Noda et al., 2002; Kaushik and Khanna, 2003). The indices
were evaluated in terms of number of generations required to
achieve the pre-defined/ desired gains and correlated responses.
Use of index I in selection usually involves calculation of
weighting factors b for traits to be used as selection criteria
(equation 1).
I = b X 1
Where: I= Selection index; b = n × 1 vector of weighting factors; X =
n × 1 vector of source of information, usually phenotypic
measurements on the candidate for selection or its relatives.
In the present study, information source (X) was individual own
performance for antibody response and body weight, and fullsib
averages for egg production and related traits.
Based on the Yamada index, b in the present study was
calculated as:
b = (G R)
–1Q 2
Where:
G = n × m genetic variance-covariance matrix of the traits used as
selection criteria and traits in the breeding objectives;
Q = m × 1 vector of intended genetic changes for m traits assigned
by breeder;
R = n × n matrix of Wright’s coefficients of relationship.
Genetic variance-covariances were derived from genetic
parameters estimated for these ecotypes in a study by Lwelamira
(2007). These are presented in Table 2.
Desired genetic changes for various traits (that is Q) were
calculated as the difference between desired and observed means.
Intended performances /improvement and hence desired gains/
genetic changes for traits highlighted for improvement in different
selection indices in Tables 1 are presented in Table 3.
These intended performances were chosen in such a way that
they are within the capacity of the population as shown by
performance of some individuals in an environment under
consideration. Furthermore, the chosen intended performances
were close to the performance of the crosses between local and
exotic birds (Ali et al., 2000; Theerachai et al., 2003). In addition,
the chosen intended performance for body weight also depended
on weight at which a chicken can be marketed (that is at least 1 kg)
(Pedersen, 2002; Theerachai et al., 2003; Acamovic et al., 2005).
Based on the capacity of Kuchi population, body weight at 16
weeks of age (Bwt16) was chosen as the target body weight in
various selection indices as it is possible to target much higher
weights in a breeding objective (that is above 1.5 kg under intensive
conditions, and hence chickens would be fetching good prices)
compared to body weights at 8 and 12 weeks of age, and at earlier
age than at 20 weeks of age (Lwelamira, 2007).
Furthermore, based on Yamada index, expected genetic gains
per generation in all k traits under study (that is direct and
correlated responses) including m traits in the breeding objectives
was calculated using equation 3.
G*' .b' RG *
I
I
i
=
3
Lwelamira and Kifaro 135
Table 2. Genetic variance-covariances for Kuchi and Medium ecotypes.
Ecotype Ab Bwt16 AFE EN-90 EW
Kuchi
Ab 0.304 -4.000 - -0.100 -
Bwt16 -4.000 5268.000 - -58.680 -
EN-90 -0.100 -58.680 - 20.170 -
Medium
Ab 0.307 -3.84 -0.23 -0.11 -0.07
Bwt16 -3.84 3963.2 -137.85 -51.36 35.75
AFE -0.23 -137.85 108.73 -7.98 5.66
EN-90 -0.11 -51.36 -7.98 26 -1.76
EW -0.07 35.73 5.66 -1.76 6.09
Ab = Antibody response against Newcastle disease virus (NDV) vaccine
(primary humoral immune response at 4weeks of age assessed two weeks post
vaccination) (HIlog2); Bwt16 = body weight (g) at 16 weeks of age; EN-90 = egg
number in the first 90 days; EW = egg weight (g) between 33 to 36 weeks of
age; AFE = age at first egg (days).
Table 3. Observed and desired mean and desired gain for Kuchi and Medium ecotypes under intensive management system.
Ecotype Trait Unit Observed mean Desired mean Desired change Percentage change (%)
Kuchi
Ab HI (log2) 5.0 6 1 20
Bwt16 gram 1394 1800 406 29
Medium
Ab HI (log2) 5.1 6 0.9 18
EN-90 No. 49 68 19 39
EW Gram 42 50 8 19
AFE Days 168 154 -14 8
Ab = Antibody response against Newcastle disease virus (NDV) vaccine (primary humoral immune response at 4weeks of age
assessed two weeks post vaccination); Bwt16 = body weight at 16 weeks of age, EN-90 = egg number in the first 90 days, EW =
egg weight between 33 to 36 weeks of age, AFE = age at first egg.
Where:
iI = intensity of selection based on the index;
I = standard deviation of the index calculated as shown in equation
4;
G* = k x 1 vector of genetic gains per generation in k traits;
G* = n x k genetic variance-covariance matrix.
= (b'Pb) σI 4
Where P is an n x n phenotypic variance-covariance matrix.
Again as with genetic variance-covariances, P and G* were also
derived from a study by Lwelamira (2007). Phenotypic variance covariances are presented in Table 4. Genetic variance covariances including all traits recorded (that is traits included in the
indices and those not included but their correlated responses were
examined) for the purpose of determining G* are presented in Table
5.
The number of generations q required to attain the pre-defined
breeding objectives was calculated using equation 5 (Yamada et
al., 1975; Nishida et al., 2001). All matrix equations were solved
using Interactive Matrix Language (IML) procedures of SAS (2000).
I I
I
i i
q
(b'Pb)
= =
σ
5
Population structure and derivation of selection intensities
In this study it was assumed that in each generation 40 best males
are randomly mated to 240 best females with a mating ratio of one
male to six females. Due to fertility and hatchability problems, the
average number of progeny per dam obtained in the study by
Lwelamira (2007) under intensive management was 4 and 5 for
Kuchi and Medium ecotypes, respectively. However, the average
number of progeny per dam could be increased by increasing the
number of hatches, by improving hatchability (that is hatchability of
fertile eggs for the parent stock was 63% for Kuchi and 66% for
Medium ecotypes) and by improving egg storage conditions before
hatching (Abdou et al., 1990; Ruiz and Lunam, 2002). Since the
parent stock in the study by Lwelamira (2007) was collected from
field and consisted of mature birds, in which some of them were
likely to be too old for good laying performance, the average laying
rate for the parent stock was low (around 35%) compared to that of
136 Afr. J. Agric. Res.
Table 4. Phenotypic variance-covariances for Kuchi and Medium ecotypes.
Ecotype Ab Bwt16 AFE EN-90 EW
Kuchi
Ab 1.126 -4.64 - -0.17 -
Bwt16 -4.64 11972.74 - -112.94 -
EN-90 -0.17 -112.94 - 63.04 -
Medium
Ab 1.06 -7.996 -0.45 -0.38 -0.215
Bwt16 -7.996 9436.18 -182.6 -90.44 67.8
AFE -0.45 -182.6 209.09 -18.85 7.57
EN-90 -0.38 -90.44 -18.85 86.68 -3.57
EW -0.215 67.8 7.57 -3.57 12.18
Ab = Antibody response against Newcastle disease virus (NDV) vaccine (primary humoral
immune response at 4weeks of age assessed two weeks post vaccination) (HIlog2); Bwt16
= Body weight (g) at 16 weeks of age; EN-90 = egg number in the first 90 days; EW = egg
weight (g) between 33 to 36 weeks of age; AFE = age at first egg (days).
around 50% for their offspring, in which all of them were young (that
is in their early period of laying). Hence, by involving young mature
breeding females in the breeding program, as would be the case in
the current simulation work, it would more likely result into
increased number of progeny per dam. Moreover, since hatchability
under artificial incubation is known to be negatively affected by the
age of the dam (Tona et al., 2001), therefore by involving relatively
young mature hens as breeding females, would result into
increased hatchability and hence the number of offspring per dam.
Consequently, in the current study it was assumed that on average
each female produces 10 progeny, with 5 chicks of each sex (that is
in total there would be 2400 chicks). Taking into account
mortality/loss of about 10% by the time birds are selected and
mated, then the total number of birds available for selection would
be 2400 - (2400 x 0.10) = 2160 (that is 1080 birds for each sex).
Therefore, by selecting around 40 best males and 240 best females
in each generation would lead to the proportion selected of about
3.7 and 22.2% for males and females, respectively giving an
average selection intensity of 1.78. This average selection intensity
is within the range of that of around 1.5 to 2 mostly used in
commercial chicken breeding (Ameli et al., 1991; Su et al., 1997).
RESULTS
Desired-gain selection indices/ breeding scenarios
for Kuchi ecotype
Responses per generation in various traits resulting from
various desired-gain selection indices for Kuchi ecotype
under intensive management are shown in Table 6.
Selection index coefficients are presented in Table 7.
Results show that predicted direct response to selection
in 16 weeks body weight under intensive management
ranged from 70 to 81 g per generation. Results from
Table 6 also reveal that all selection indices considered
for Kuchi predicted to result into favourable correlated
responses to other body weight measurements (24 - 81 g
per generation), egg weight (0.5 to approximately 1 g per
generation), and age at sexual maturity (- 2 to -3 days per
generation) with little change in eggshell thickness and
egg shape index. Dropping antibody response (humoral
immune response) from the index (I2k) resulted into a
slight deterioration of this trait. On the other hand, in
situation where egg number was dropped from the index
(that is control of change in egg number not considered)
(I3k), this was predicted to result into substantial drop in
egg number per generation (that is approximately one
egg per generation).
Results from present study further show that the
number of generations required achieving the desired
gains ranged from 5 to 6 generations (Table 6).
Assuming that the activities of mating, collection of eggs,
incubation and hatching; rearing of birds from day old to
sexual maturity; and recording of egg production for a
period of 3 months after sexual maturity for birds from all
hatches/batches lasting for about 2, 6, and 5 (that is 2 +
3) months, respectively, as it has been shown in a study
by Lwelamira (2007), this would lead to a generation
interval of 13 months (that is 2 + 6 + 5) for selection index
I1k and I2k (where apart from improving body weight, egg
number is also held constant), and 8 months (that is 2 +
6) for selection index I3k (where control of correlated
response in egg number is ignored). Hence, to attain the
desired gain in body weight it would require 6.3, 5.4 and
3.6 years of selection for index I1k, I2k and I3k,
respectively.
Desired-gain selection indices/breeding scenarios for
Medium ecotype
Results from Table 8 show that the number of
generations required for achieving the desired gains in
various selection indices for Medium ecotype in which
their coefficients are presented in Table 9 ranged from 8
to 11 generations, which are higher than those obtained
in various selection indices for achieving the desired
gains in Kuchi under intensive management. Again by
assuming a generation interval of 13 months as in some
breeding scenarios/selection indices for Kuchi under
Lwelamira and Kifaro 137
Table 5. Genetic variance-covariances including all traits for determination of G* for Kuchi and Medium ecotypes.
Ecotype Ab Bwt8 Bwt12 Bwt16 Bwt20 AFE EN-90 EW STH ESI
Kuchi
Ab 0.304 -2.086 -3.2 -4 -3.04 -0.10 -0.10 -0.015 -0.05 0.17
Bwt16 -4 1797.55 3654.74 5268 5315.49 -143.84 -58.68 43.35 -9.85 17.38
EN-90 -0.10 -9.27 -37.7 -58.68 -81.35 -7.7 20.17 -2.32 -1.32 0.134
Medium
Ab 0.307 -1.76 -3.2 -3.84 -3.04 -0.23 -0.11 -0.07 0.017 0.18
Bwt16 -3.84 1508.21 3202.66 3963.2 4238.96 -137.85 -51.36 35.73 -7.63 -5.1
AFE -0.23 -9.03 -72.34 -137.85 -195.94 108.73 -7.98 5.66 0.63 2.54
EN-90 -0.11 -16.19 -38.32 -51.36 -83.84 -7.98 26 -1.76 -0.46 2.07
EW -0.07 9.26 35.67 35.73 57.97 5.66 -1.76 6.09 -0.15 0.60
Ab = Antibody response against Newcastle disease virus (NDV) vaccine (primary humoral immune response at 4weeks of age assessed two weeks post
vaccination) (HIlog2); Bwt8, Bwt12, Bwt16 and Bwt20 = body weight (g) at 8, 12, 16 and 20 weeks of age, respectively; AFE = age at first egg (days); EN-90
= egg number in the first 90 days after sexual maturity, EW, STH and ESI = egg weight (g), shell thickness (µ) and egg shape index (%), respectively,
between 33 to 36 weeks of age.
Table 6. Direct and correlated responses per generation resulting from various desired gain selection indices for Kuchi
ecotype.
Index Ab Bwt8 Bwt12 Bwt16 Bwt20 AFE EN-90 EW STH ESI Gen
I1k (0.17) 23.80 48.31 (69.92) 70.31 -2.50 (0.00) 0.50 -0.24 0.39 5.80
I2k -0.07 28.18 56.42 (81.12) 80.83 -2.65 (0.00) 0.58 -0.22 0.28 5.00
I3k (0.18) 24.96 51.58 (74.83) 76.31 -2.21 -0.95 0.63 -0.19 0.39 5.43
Gen = number of generations required to achieve the desired gains; Ab = Antibody response against Newcastle disease virus
(NDV) vaccine (primary humoral immune response at 4weeks of age assessed two weeks post vaccination) (HIlog2); Bwt8, Bwt12,
Bwt16 and Bwt20 = body weight (g) at 8, 12, 16 and 20 weeks of age, respectively; AFE = age at first egg (days); EN-90 = egg
number in the first 90 days after sexual maturity, EW, STH and ESI = egg weight (g), shell thickness (µ) and egg shape index (%),
respectively, between 33 to 36 weeks of age. Figures in parentheses are direct response to selection (that is traits included in the
index), while those out of parentheses are correlated responses (that is traits not included in the index).
Table 7. Selection index coefficients for Kuchi ecotype.
Index Ab Bwt16 EN-90
I1k 4.473 0.083 0.529
I2k - 0.080 0.463
I3k 4.347 0.080 -
Ab = Antibody response against Newcastle disease virus (NDV) vaccine
(primary humoral immune response at 4 weeks of age assessed two
weeks post vaccination) (HIlog2); Bwt16 = body weight (g) at 16 weeks of
age; EN-90 = egg number in the first 90 days.
intensive management (that is 2 months for mating, egg
collection, incubation and hatching; 6 months for rearing
birds to sexual maturity, 5 months for egg recording for
chicks from all hatches), the required number of
generations to achieve the desired gains in selection
index I1m, I2m, I3m, I4m, I5m, I6m, I7m, and I8m would
correspond to 11.5, 11.2, 11.2, 11.5, 8.8, 8.4, 8.4 and 8.7
years of selection, respectively.
It can also be seen in Table 8 that gain per generation
in various desired-gain selection indices for egg number
and age at first egg was around 2 eggs, and -1 to -2
days, respectively, with little change in shell thickness
and egg shape index. In situations where egg weight was
also improved (Selection index I1m, I2m, I3m, and I4m), gain
per generation for this trait ranged from 0.76 to 0.78 g.
Results from Table 8 further show that in selection
indices where either body weight (which is antagonistic to
egg number), or humoral immune responses are dropped
from the index, it was predicted to result into slight
parameters to range from 12 to 15 g per generation. In
138 Afr. J. Agric. Res.
Table 8. Direct and correlated responses per generation resulting from various desired- gain selection indices for Medium
ecotype.
Index Ab Bwt8 Bwt12 Bwt16 Bwt20 AFE EN-90 EW STH ESI Gen
I1m (0.09) -1.26 0.74 (0.00) 2.26 (-1.32) (1.79) (0.76) -0.06 0.28 10.60
I2m -0.01 -1.25 0.72 (0.00) 2.08 (-1.35) (1.84) (0.77) -0.06 0.22 10.36
I3m -0.02 0.41 4.00 4.00 6.04 (-1.36) (1.84) (0.78) -0.07 0.22 10.31
I4m (0.09) -0.19 2.85 2.58 4.81 (-1.32) (1.80) (0.76) -0.06 0.28 10.58
I5m (0.11) -0.33 0.15 (0.00) -1.17 (-1.72) (2.34) (0.00) -0.05 0.26 8.13
I6m -0.012 -0.29 0.11 (0.00) -1.52 (-1.80) (2.44) (0.00) -0.06 0.19 7.79
I7m -0.01 -1.05 -1.37 -1.82 -3.32 (-1.81) (2.45) (0.00) -0.06 0.19 7.75
I8m (0.11) -1.72 -2.58 -3.35 -4.50 (-1.74) (2.37) (0.00) -0.05 0.26 8.04
Gen = number of generations required to achieve the desired gains; Ab = Antibody response against Newcastle disease virus (NDV)
vaccine (primary humoral immune response at 4weeks of age assessed two weeks post vaccination) (HIlog2); Bwt8, Bwt12, Bwt16 and
Bwt20 = body weight (g) at 8, 12, 16 and 20 weeks of age, respectively; AFE = age at first egg (days); EN-90 = egg number in the first
90 days after sexual maturity, EW, STH and ESI = egg weight (g), shell thickness (µ) and egg shape index (%), respectively, between
33 to 36 weeks of age. Figures in parentheses are direct response to selection (that is traits included in the index), while those out of
parentheses are correlated responses (that is traits not included in the index).
Table 9. Selection index coefficients for Medium ecotype.
Index Ab Bwt16 AFE EN-90 EW
I1m 3.397 -0.008 -0.333 1.597 3.573
I2m - -0.012 -0.360 1.543 3.552
I3m - - -0.320 1.593 3.382
I4m 3.505 - -0.303 1.633 3.461
I5m 3.315 0.008 -0.140 1.516 0.549
I6m - 0.004 -0.167 1.463 0.529
I7m - - -0.182 1.445 0.587
I8m 3.209 - -0.170 1.481 0.659
Ab = Antibody response against Newcastle disease virus
(NDV) vaccine (primary humoral immune response at 4
weeks of age assessed two weeks post vaccination)
(HIlog2); Bwt16 = Body weight (g) at 16 weeks of age; EN-90
= Egg number in the first 90 days; EW = Egg weight (g)
between 33 to 36 weeks of age; AFE = age at first egg
(days).
changes in these traits and in number of generations
required to achieve the desired gains (that is efficiency).
DISCUSSION
Desired-gain selection indices/ breeding scenarios
for Kuchi ecotype
Predicted direct response to selection in 16 weeks body
weight ranged from 70 to 81 g per generation. Compared
to gain per generation achieved in previous studies for
selection for body weights it is difficult to make direct
comparison due to differences in either selection proce dures employed, selection intensities used, or age at
which the body weight was intended to be improved. In a
study by Su et al. (1997) in broilers, gain per generation
for body weight at 6 weeks of age under intensive
management of about 45 g based on mass selection was
reported. On the other hand Bhusan et al. (1998) re ported predicted gain per generation in broilers resulting
from selection based on indices incorporating body
weight at 6 and 8 weeks of age and feed utilization
contrast, using similar traits, response of up to 56 g per
generation were reported in broilers by Singh et al.
(2000).
Considering the mean body weight at 12 weeks of age
for Kuchi averaged over both sexes of 954 g under
intensive management obtained in the study by
Lwelamira (2007), and correlated responses for 12 weeks
of age of around 280 g after the entire period of selection
(that is gain per generation x number of generations
required to achieve the desired gain) in all selection
indices considered, the expected average body weight at
12 weeks of age after completion of the selection process
is estimated to be 1234 g. Furthermore, considering local
chickens in the tropics are starting to be marketed when
their average body weight is around 1 kg and above
(Pedersen, 2002; Theerachai et al., 2003; Acamovic et
al., 2005), therefore after selection, Kuchi can start to be
marketed at 12 weeks of age under intensive
management, and at 16 weeks of age they would be
having a weight of more than 1500 g (that is 1800 g)
(Table 3) and hence fetching higher prices. In addition,
after the entire period of selection, on average, the
resultant stock would be able to attain age at sexual
maturity 12 to 15 days earlier and lays eggs weighs 3 to 4
g more (that is gain per generation x number of
generations required to achieve the desired gain)
compared to the average performance of the current
population.
Results from this study indicate selection index I3k takes
few years (3.6 years) compared to other breeding
scenarios considered for Kuchi ecotype. Although few
years are required to attain the desired gain in body
weight for this selection index, and it is cheap in terms of
recording as egg production has been ignored, however,
this selection index is predicted to result into noticeable
drop in egg number (approximately 1 egg per
generation), a trend which is undesirable. Therefore,
given the situation where resources are available, it could
be better to use those selection indices which control
response in egg number (that is selection index I1k and
I2k). An additional advantage of using selection index I1k is
that apart from improving body weight, humoral immune
response against NDV vaccine is also improved. This
breeding strategy could be recommended at the expense
of one more year of selection relative to selection index
I2k. Although the majority of farmers in the tropics rear
their chickens under extensive management (Tadelle et
al., 2003; Njenga, 2005), the stock obtained from
improving this ecotype under intensive management can
benefit those farmers who would be able to shift from
current system of management to at least semi-intensive
system of management.
Desired-gain selection indices/breeding scenarios for
Medium ecotype
Variable direct response per generation between some
selection indices with respect to some egg traits has
been obtained in this study. As with body weight, it is
difficult to make direct comparisons of responses per
generation obtained in the present study with those from
other studies due to differences in either selection
methods or selection intensities used. However, the
response obtained in the present study would definitely
be lower than selection based on single trait as selection
for antagonistic traits (that is egg weight and egg number)
are always expected to lower the magnitude of genetic
response (Pattanayak and Patro, 1995; Pakdel, 2004).
For example, values ranging from 1.2 to 2.2 g per
Lwelamira and Kifaro 139
generation in egg weight, which are much higher than the
response predicted in the present study, were reported
for single trait selection for egg weight in Scandinavian
egg type chickens (Kolstad, 1980; Liljedahl and Weyde,
1980; Sørensen et al., 1980).
The observed slight decline in body weight and
humoral immune response in this study when these traits
were dropped from the index could be attributed to the
favourable relationship existing between age at first egg
and egg weight with body weight, which were also
included in the indices, and the low genetic correlation
between humoral immune response and production traits
revealed in a previous study by Lwelamira (2007).
The purpose of various selection indices/breeding
programs in the current study as far as the Medium
ecotype is concerned was mainly to improve egg pro duction and related traits together with antibody response
(humoral immune response) against NDV vaccine. It
could also be desirable to improve these traits without
affecting other important traits negatively that is decrease
in body weight. Given that dropping of body weight into
indices that involve all three egg traits (that is AFE, EN 90, EW) did not result into significant change per
generation in this trait. Therefore, for the selection indices
given for Medium ecotype, it could be logical to
concentrate on those which ignore body weight from the
index (that is Selection index I3m, I4m, I7m and I8m). Apart
from some egg production and related traits, humoral
immune response against NDV vaccine is also improved
in selection indices I4m and I8m. Furthermore, since eggs
from local chickens can be marketed with the current low
egg weight without a problem (Mlozi, 2006; Lwelamira,
2007), then the two selection indices (I4m and I8m) could
be selection indices of choice among the four above (that
is those ignoring body weight in the index). However, for
selection index I4m it would require approximately 3 more
years of selection to achieve the desired gains compared
to selection index I8m. Therefore, the choice among these
two indices will depend on availability of resources. After
the entire period of selection average egg number in the
first 90 days after sexual maturity is expected to be
increased by 19 eggs, age at first egg (that is age at
sexual maturity) decreased by 14 days, and humoral
immune response increased by around 1(HIlog2) in both
selection indices (that is I4m and I8m) compared to the
average performance of the current population.
Furthermore, average egg weight is expected to be
improved by 8 g for selection index I4m. In this regard, egg
number in the first 90 days after sexual maturity, age at
first egg, humoral immune response and egg weight
would have been improved from their current population
mean of 49 eggs, 168 days, 5.1 (HIlog2) and 42 g to 68
eggs, 154 days, 6 (HIlog2) and 50 g, respectively, after
selection.
As stated earlier, the majority of farmers in the tropics
reside in rural areas and keep their chickens under
extensive management, therefore, breeding programs for
140 Afr. J. Agric. Res.
improving egg production under intensive management
can benefit those farmers who are ready to shift from
extensive system of management to semi-intensive
system of management as this will reduce the magnitude
of genotype by environment interactions (Sørensen,
1999; Ali, 2002).
Conclusion
The study conclude that, approximately 5 to 6 genera tions of selection which corresponds to around 4 to 6
years of selection would be required for Kuchi chicken
ecotype for improving Bwt16 either singly or together with
Ab from their current population mean of 1394 g and 5
(HIlog2) to the desired population mean of 1800 g and 6
(HIlog2), respectively. As for medium ecotype, it would
require approximately 8 to 11 generations of selection
which corresponds to around 8 to 12 years of selection to
improve either AFE and EN-90 or together with EW or Ab
or both of them from their current population mean of 168
days, 49 eggs, 42 g and 5 (HIlog2) to the desired
population mean of 154 days, 68 eggs, 50 g, and 6
(HIlog2), respectively. The number of years of selection
required to attain the desired gains for the studied
chicken ecotypes indicates that it will not take too long to
reach the target (desired performance) if selection
breeding programs would be initiated.
ACKNOWLEDGEMENT
The authors are very grateful for the financial support
from Production and Health of Smallholder Livestock
(PHSL) project funded by DANIDA which sponsored the
senior author in his PhD. Programme.
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