Doctoral Degrees (Animal, Wildlife and Grassland Sciences)

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Browsing Doctoral Degrees (Animal, Wildlife and Grassland Sciences) by Author "Fair, Michael Denis"

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    The genetic and environmental modelling of production and reproduction in ostrich females within and across breeding seasons
    (University of the Free State, 2012-01) Fair, Michael Denis; Van Wyk, J. B.; Cloete, S. W. P.
    English: Pedigree and performance data of a well-documented ostrich breeding resource flock was used to (1) investigate the flock structure, (2) estimate variance and (co)variance components for egg and day-old chick weight (individual traits), (3) estimate genetic and environmental parameters for egg and chick production, mean egg and day-old chick weights and hatchability within breeding seasons (hen traits) and (4) investigate early identification of superior animals using random regression models for repeated measures (longitudinal) data over years. The average level of pedigree completeness of 40 074 birds of a pair-breeding ostrich flock maintained from 1978 to 2005 at the Oudtshoorn Research Farm, South Africa was high (99.3%) in the first generation and the average level of inbreeding (F) was low at 0.51%. The estimated measures of variability were: effective genome equivalents = 47.3, effective number of founders = 59 and the effective number of ancestors = 58. The numbers of ancestors responsible for 100%, 50% and 20% of the variation in the reference population (birds with both parents known), were equal to 254, 21 and 6 respectively. The generation interval in years calculated as the average age of parents when their offspring which were kept for reproduction were born, amounted to 7.72 ± 4.79 years. The linear regressions of rate of inbreeding on year of hatch for the two distinct periods, 1995-2002 and 2003-2005, were 0.08% and -0.07% per year respectively. The estimate of effective population size (Ne) computed via the increase in the individual rate of inbreeding was 112.7 animals. The results of this study indicated that the population under study was at an acceptable level of genetic variability. Pedigree and performance data for 71 147 individual egg records collected between 1991 to 2005 were used to estimate genetic parameters for egg weight (EWT), live day-old chick weight (CWT) and hatchability (H). Heritability estimates (±SE) were 0.12 ± 0.02, 0.14 ± 0.04 and 0.09 ± 0.04 for EWT, CWT and H. Corresponding estimates for maternal genetic effects were 0.27 ± 0.08, 0.38 ± 0.08 and 0.13 ± 0.02. The effects of common environment, permanent environment and breeding paddock were significant but relatively low for all traits. Egg weight and CWT were highly correlated at all levels, while H was mostly independent of the weight traits. These results indicated that genetic improvement in these traits would be feasible. Data involving monthly records of egg production (EP), chick production (CP), hatchability (H), mean egg weight (MEW) and mean day-old chick weight (MCW) were analysed as hen traits. Heritability estimates amounted to 0.04 ± 0.02 for EP, 0.05 ± 0.03 for CP, 0.44 ± 0.04 for MEW, 0.02 ± 0.02 for H and 0.67 ± 0.02 for MCW. Permanent environmental effects as a ratio of phenotypic variance (c2 pe) ranged from 0.08 ± 0.03 to 0.11 ± 0.02 for the first four traits (MCW did not exhibit a significant c2 pe effect). Monthly EP and CP were highly correlated at all levels, ranging from 0.74 for the temporary environmental correlation to unity for the genetic correlation. Hatchability was highly correlated with EP and CP at the genetic level (>0.94). Genetic correlations of EP and CP with MEW and MCW were variable and in some cases antagonistic. Genetic correlations of H with MEW and MCW were positive (0.52 and 0.47, respectively). Results indicate that selection for improved reproduction (reproductive output and product quality traits) is feasible. Selection for production is unlikely to be complicated by unfavourable correlations with H, MEW and MCW. Hen traits defined above were further analysed in single-trait mixed models with a random regression fitted as an intercept for the direct animal (a) and a quadratic polynomial with intercept for the permanent environmental (p) effect peculiar to each hen. Heritability (h2) estimates were moderate and remained relatively constant for EP and CP ranging from 0.13-0.14 and 0.07-0.08 respectively for 3- to 10-year old hens. Quality traits MEW and MCW had moderately high h2 estimates ranging from 0.49-0.61 and 0.37-0.45 respectively. Hatchability had h2 estimates ranging from 0.11-0.13 for the 10 hen-ages. Permanent environment variance ratio for EP, CP, MEW, MCW and H ranges were 0.28-0.42, 0.29-0.41, 0.17-0.33, 0.21-0.35 and 0.14-0.24 respectively. Selection of superior hens from three years onwards seems possible. Hens older than eleven years should be replaced with younger, genetically superior hens which would reduce the generation interval and improve EP and CP genetically, without adversely affecting MEW, MCW and H.