Evaluation of Production Traits in Two Strains of Layer Chickens Under Relaxed Selection
CHAPTER ONE
The Objectives of this study therefore are;
- To determine variations in production and egg quality traits between two strains under relaxed
- To estimate phenotypic correlation among production and egg quality traits in the two strains of birds under relaxed
CHAPTER TWO
LITERATURE REVIEW.
GENETIC IMPROVEMENT OF LIVESTOCK
Genetic improvement of livestock and poultry is based on two alternative approaches: crossbreeding and artificial selection (Szwaczkowski et al., 2003). Crossbreeding leads to the creation of more heterozygotes and in consequence to greater genetic variation of population. By contrast, selection determines both genetic gain and inbreeding rate. The inbreeding effects include increased homozygosity, a higher risk for the incidence of lethal or deleterious recessive alleles, and decrease in performance and fitness traits (Milgior et al., 1995).
CHAPTER THREE
MATERIALS AND METHODS
LOCATION OFEXPERIMENT
This study was carried out at the Poultry Research Programme breeding Farm of the National Animal Production Research Institute (NAPRI), Shika, Zaria, Kaduna State. The Poultry Research Programme Farm lies between 11 □ 12□ 42□ N and 7□ 33□ 14□ E at an altitude of 691m above sea level (Ovimaps, 2012). The area falls within the Northern Guinea Savannah having an average annual rainfall of 1100mm, which starts from late April or early May to mid-October, followed by a dry period (which is divided into early and late dry periods). The early dry period is characterized by cold dry weather and lasts from October to January. While the late dry period is characterized with hot-dry weather and lasts from February to April. The mean temperature is about 24.4 0C (14.5-39.5 0C) with the lower temperatures occurring during the early dry season and the higher temperatures during the late dry season. The relative humidity during the dry and wet seasons is 21% and 72% respectively. The description is as reported in Akpa and Jokthan (1996).
CHAPTER FOUR
RESULTS
DESCRIPTIVE STATISTICS OF PRODUCTION AND EGG QUALITY TRAITS IN THE POOLED AND INDIVIDUAL
Tables 4.1 to 4.6 show the descriptive statistics of all the studied traits, for the two strains and individual strains. Overall mean for ASM was higher in strain 1 than strain 2, while the coefficient of variation (CV %) for ASM was higher in strain 2 (7.44%) than strain1 (6.72%). Also the means for EN, BW18 and BW20 were higher in strain 2 than in strain 1 while AEW and RL were lower for strain 2 compared to strain 1. The degree of spread about the mean was greatest with EN, with a value of 39.70 %, 38.93% and 40.62 % in the combined strains, strain 1 and strain 2 respectively.
CHAPTER FIVE
DISCUSSION
DIFFERENCES BETWEEN MALE AND FEMALE LINES IN EGG PRODUCTION TRAITS
Observed significant difference in ASM between the two strains agreed with several authors who have reported that significant differences exist in egg type chickens for ASM across strains (Abubakar et al., 1991; Orunmuyi, 2006; Nwagu et al., 2007; Yahaya et al., 2009). The result obtained did agree with the findings of Orunmuyi (2006) in both generations, Nwagu et al. (2007) and Yahaya et al. (2009) where the dam line (Strain 2) attained sexual maturity (158 days) earlier than the sire line 161 days (strain 1). It disagrees, however, with the finding of Abubakar et al. (1991) who reported the male line as the better performing line. Values obtained were within the range reported by Omeje (1987); he reported a range of 149 days to 202 days for the sire line and 154 days to 214 days for the dam lines across grandparent and parent lines. According to Lahti et al. (2009) relaxed selection may result in the broadening of the selection peak without concomitant increase in trait mean. It appears that ASM has not changed drastically in a population where selection has been relaxed compared to population where selection was carried out. This could mean that the selection peak is gradually being widened, but no obvious reduction or increase in ASM as compared to literature report has occurred since selection was relaxed.
Differences observed in AEW did not agree with the report of Orunmuyi (2006) who reported the sire line to have a significantly higher value for AEW than the dam line. However, the values obtained in this study were lower than those reported by Ajeigbe (1992), Orunmuyi (2006), Nwagu et al. (2007) and Yahaya et al. (2009) probably due to the age at which the weight was recorded. Also significant differences between the two strains observed for BW20 was in agreement with the observation of these authors but was at variance with the report of Abubakar (1991). BWT 20 values were higher than the value reported by Orunmuyi (2006) and Yahaya et al. (2009) in both generations of selection for both strains. It may be safely assumed within the scope of this study that the relaxation of selection pressure in these strains is tending towards the stabilization of the strains for the studied traits. This is in agreement with the report of Siegel and Honaker (2009) that Chickens, like other animals, allocate nutritional resources to different physiological activities including maintenance, growth, reproduction, immunity and social interactions. Genetic selection can cause changes in the allocations of these resources, favouring some physiological traits or functions while disadvantaging others (Gross et al., 2002).
CHAPTER SIX
SUMMARY, CONCLUSION AND RECOMMENDATION
SUMMARY
- The maintenance of the variation between strains for most of the production and egg quality traits within the range reported during the initial selection period may indicate the broadening of the selection peak without concomitant increase in trait
- Observed phenotypic correlations between egg production and egg quality traits for each of the strains was similar in pattern to the pooled correlations for most economic and egg quality traits. This may point to the fact that relaxed selection is maintaining the relationship between traits at a constant
- From basic statistics mean and standard error of mean for most of the traits evaluated, it became clear that strain 2 had better performance than strain 1
CONCLUSION
- The female line (strain 2) had better performance than the male line in terms of ASM (158.79 Vs 50 days respectively), BW18 (1176.87 g Vs 1171.62 g) and BW20 (1370.17 g Vs 1327.39 g). The Rhodes Island chicken from which these strains were developed are known to be dual purpose breeds, in the absence of selection pressure for egg number, resources were redistributed to meat production . This explains why higher values for BW20 were obtained than previous reports (987.92 Vs1167.06) when selection was carried out .So it becomes evident that relaxed selection gradually altered selection responses previously obtained in these strains.
- The coefficient of variation of most of the studied traits within the individual strains had values higher than 20% therefore selection can still be resumed and appreciable response to selection can be
- Phenotypic correlations between traits studied in the two strains vary both in magnitude and direction implying stability of the production and egg quality traits. Relaxed selection is gradually influencing these strains and broadening selection peak but due to phenotypic correlations among traits within strains the mean value for most traits are still being maintained (149 – 202 day for ASM)
RECOMMENDATION
The coefficient of variation of most of the studied traits within the individual strains had values higher than 20% therefore selection can still be resumed and appreciable response to selection can be obtained.
REFERENCES
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