TABLE OF CONTENTS
CONTENT PAGE
Title page ………………………………………………………………………………………………… i
Declaration ……………………………………………………………………………………………… ii
Certification …………………………………………………………………………………………… iii
Acknowledgment ……………………………………………………………………………………. iv
Abstract …………………………………………………………………………………………………. v
Table of contents …………………………………………………………………………………….. vi
List of figures ………………………………………………………………………………………….. x
List of tables ……………………………………………………………………………………………. x
List of appendices……………………………………………………………………………………. xi
CHAPTER ONE
1.0 INTRODUCTION ……………………………………………………………………….. 1
1.1 Justification ………………………………………………………………………………… 3
CHAPTER TWO
2.0 LITERATURE REVIEW …………………………………………………………….. 5
2.1 Genetic Improvement of Livestock …………………………………………………. 5
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2.2 Artificial and Natural Selection ………………………………………………………. 5
2.3 Reviewed Defects in Animals Undergoing Artificial Selections …………..10
2.4 Concept of Relaxed Selection ………………………………………………………..12
2.4.1 Contexts in which selection is relaxed ……………………………………………..13
2.5 Poultry Breeding ………………………………………………………………………….17
2.6 Improvement for Egg Production in Chickens …………………………………..19
2.7 Age at Sexual Maturity …………………………………………………………………21
2.8 Egg Number ……………………………………………………………………………….22
2.9 Average Egg Weight …………………………………………………………………….23
2.10 Sexual Maturity and Body Weight of Pullet ……………………………………..24
2.10.1 Variability in sexual maturity …………………………………………………………24
2.11 Body Weight and Selection …………………………………………………………..27
2.12 Rate of Lay …………………………………………………………………………………28
2.13 Egg Quality Traits ……………………………………………………………………….29
CHAPTER THREE
3.0 MATERIALS AND METHODS ……………………………………………………31
3.1 Location of the Experiment …………………………………………………………..31
3.2 Description of the Original Stock ……………………………………………………31
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3.3 Experimental Animals ………………………………………………………………….31
3.4 Management Practices …………………………………………………………………32
3.4.1 Nutrition …………………………………………………………………………………….32
3.4.2 Brooding and rearing …………………………………………………………………..32
3.5 Traits Studied ……………………………………………………………………………..32
3.5.1 Growth and egg production traits ……………………………………………………32
3.5.2 Egg quality characteristics …………………………………………………………….33
3.6 Data Analysis ……………………………………………………………………………..34
CHAPTER FOUR
4.0 RESULTS …………………………………………………………………………………….35
4.1 Descriptive Statistics of Production and Egg Quality Traits in
the Pooled and Individual Strains …………………………………………………..35
4.2 Means ± Standard Error of Means for Production and Egg
Quality Traits ………………………………………………………………………………42
4.3 Pearson Correlation for Production and Egg Quality Traits …………………45
CHAPTER FIVE
5.0 DISCUSSION …………………………………………………………………………….54
5.1 Differences between Male and Female Lines in Egg Production
Traits …………………………………………………………………………………………54
5.2 Differences between Male and Female Lines in Egg Quality
Traits …………………………………………………………………………………………55
5.3 Phenotypic Correlation Coefficients between Egg Production Traits …….56
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5.4 Phenotypic Correlation Coefficients between Egg Quality Traits …………57
CHAPTER SIX
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION ……………..59
6.1 Summary ……………………………………………………………………………………59
6.2 Conclusion………………………………………………………………………………….59
6.3 Recommendation …………………………………………………………………………60
REFERENCES ………………………………………………………………………………………61
CHAPTER ONE
1.0 INTRODUCTION
The goal of breeders is to improve the performance of their flock. One of the major
tools available for this improvement is selection. Selection is the act of choosing those
individuals that will become parents of the next generation with the aim of improving
the performance of the flock or population. The various selection methods used based
on phenotypic measurement includes: mass selection, progeny testing, tandem selection,
independent culling level, and selection index (Bourdon, 2000).
Genetic improvement in animals and plants for food consumption has been practised
and realised since domestication. Intensive improvement, increasingly with
incorporation of genetic principles, has been undertaken for a century or more (Hill,
2008). Much of this has been highly intensive and effective, notably in poultry. There
are consequently concerns that genetic variation is being exhausted and continued gains
cannot be expected (Hill, 2008). The genetics of a metric character centres round the
study of its variation (Falconer and Mackay, 1996). It is assumed that genetic variances
either remains constant during selection or that any changes in variance can be predicted
solely from the variance components of the base population. Selection experiments have
been the basis for testing theories, hypotheses, and predictions of quantitative genetic
theory. In addition, selection experiments enable researchers to estimate genetic
parameters, test alternate breeding schemes, and study the causes of selection limits and
plateaus and develop means to overcome them (Reddy, 1996).
Continued genetic progress, especially in traits of growth efficiency, with little evidence
of selection limits surprised many breeders in the past; however, with quantitative
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genetic theories developed by many researchers, poultry breeders came to realize that
although one-half of total response to selection occurs early in the selection process, one
can predict the number of generations in which response can be obtained from the
knowledge of effective population size (Ne). The probability of fixation of an allele, and
thus a selection limit, was postulated by Robertson (1960) as a function of effective
population size, initial gene frequency, and selection coefficient.
In long-term selection experiments, it is often noticed that some traits reach a plateau in
relatively few generations. A classical selection response represents a linear change
over generations of selection, followed by decreased response until some selection limit
is reached. A further decrease in response occurs because of random drift in finite
populations when dominance is present; inbreeding effects lower responses further. This
reduction in response is especially true for fitness traits (Falconer and Mackay, 1996).
Response to selection is also influenced by negative genetic correlations among selected
traits and fitness; however, breeders should be aware of the approximate half-life
generation and have ready suitable and alternate scheme to test. The approximate halflife
generation could serve as a reference point at which to cross to another unrelated
line or relax selection pressure rather than waiting for the actual selection limit to occur.
This scheme would increase additive genetic variance at a time when it may be
beginning to decline significantly (Eisen, 1980).
In a review of artificial selection, Hill (2011) stated that part of the uses and need for
conventional selection despite recent advances are that it will continue to furnish raw
materials needed to estimate the magnitude of variation arising from mutations due to
selection; to estimate numbers and effects of quantitative trait loci (QTL) by mapping
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the selected lines and as a demonstration to breeders that selection using modern
methods could be effective. There has been tremendous progress in productive poultry
breeding in the past few decades. Industrialized battery-caged hens lay up to 300 eggs
annually compared to 30 eggs per year by indigenous scavenging hens commonly found
in most African countries (Riise et al., 2005). Various aspects of egg production have
been brought under practical genetic control by the commercial sub-sector dominated
by international companies. Many other economic important traits have been improved
by selection. Further improvement will depend on reevaluation and refinement of
selection methods and practices. One of the methods of reevaluating selection and
selection response is relaxed selection. Relaxed selection is a selective phenomenon that
occurs when selective pressures are either eliminated or dramatically reduced resulting
in the absence of pressure and the evolutionary pathways that comes into play as a result
of imposed directional selection.
1.1 JUSTIFICATION
The concept of relaxed selection was studied at earlier times, but there exist few current
works on its impact on already selected populations. This work was designed to
evaluate differences between two strains of layer chickens where relaxed selection
exists.
Consequently the following hypotheses are put forward:
H0: There is no difference between two strains of layer type chickens from populations
with relaxed selection.
HA: There is difference between two strains of layer type chickens from populations
with relaxed selection.
The Objectives of this study therefore are;
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ï‚· To determine variations in production and egg quality traits between two strains
under relaxed selection.
ï‚· To estimate phenotypic correlation among production and egg quality traits in the
two strains of birds under relaxed selection.
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