ABSTRACT
The objectives of this study were to estimate response to selection after
five generations of selection for egg production to 280 days and to estimate the
genetic and phenotypic parameters for egg production traits in layer type
chickens under selection.
Data on part year egg production of chickens were used in this study.
The chickens were grouped into selected and control within each strain of male
and female lines. Response to selection in five generations was calculated for
male and female lines. Effective parents for both lines were also calculated
using two methods of estimation. The traits considered in the estimation of
response to selection and genetic parameters were egg number, age at sexual
maturity, average egg weight, body weight at 40 weeks, eggmass to 280 days
and rate of lay.
Direct genetic response to selection in part year egg number was higher
in the female line than in the male line. There was little or no genetic response
in the male line population. This was only 0.41 egg per generation. The value
for the female line was 3.4 eggs for the genetic response. From these results,
it could be concluded that selection had teen effective in improving the part year
egg production in the female line but not in the male line.
Correlated responses in age at sexual maturity, egg weight, egg mass,
rate of lay and body weight at 40 weeks were also calculated. In the female line,
the correlated response in age at sexual maturity as a result of direct selection
v i i i
for egg production to 280 days was negative. This was however positive in the
male line. Similarly, the correlated response in the rate of lay was positive in the
female line and negative in the male line. In fact, in the male line except the body
weight at 40 weeks, which showed a positive correlated response of 34g per year,
all other traits considered in this study showed a negative correlated response to
selection for egg number to 280days. However, in the female line, there was a
negative correlated response in the body weight due to selection for increased egg
number to 280days. There was a reduction of 0.89g per year in egg weight due to
selection for increase in egg number. The correlated genotypic responses in
eggmass and rate of lay were very low.
Genetic parameters were pooled over the five-year study period and also
estimated after correcting the data for hatch effects. The estimates of heritability
and correlations varied from low to high depending on the traits under
consideration. The pooled heritability estimates were 0.18, 0.15, 0.24, 0.16, 0.09
and for age at sexual maturity (ASM), egg number (EGGNO), egg weight (EWTAV),
body weight at 40 weeks (BWT40), rate of lay (RATE) and egg mass (EGGMASS)
respectively among the male line under selection. The corresponding values were
0.20, 0.16, 0.29, 0.21, 0.10 and 0.17 for the female line. The standard errors
associated with these estimates were very low which is an indication of their
reliability.
The pooled genotypic correlations were also computed. Age at sexual
maturity was highly negatively correlated with egg production to 280 days in both
lines. The pooled genotypic correlation of egg production with eggmass and rate
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