Masters Degrees (Animal, Wildlife and Grassland Sciences)

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Browsing Masters Degrees (Animal, Wildlife and Grassland Sciences) by Author "Delport, G. J."

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    A genetic evaluation of the Dohne Merino breed in South Africa
    (University of the Free State, 2006-05) Swanepoel, Jan Willem; Van Wyk, J. B.; Delport, G. J.; Cloete, S. W. P.
    English: Genetic (co)variances for yearling body weight (BW), clean fleece weight (CFW) and mean fibre diameter (MFD) in the SA Dohne Merino population were estimated using records of 107 389 animals recorded between 1992 and 2004. The data include records of 1 530 sires and 45 178 dams. An animal model with direct and maternal additive, maternal permanent and temporary environmental effects was fitted for all traits. Sire-flock (SF) and sire-flock-year-season (SFYS) was included as additional random effects. Fixed effects were (FYSSM) (1594 classes), type of birth (singles, multiples), age of dam (2 to 7+ years) and average age (± SD) at measurement fitted as a linear covariate (385 ± 12 days). Estimates obtained by single -trait analyses were used as starting values in three-trait analyses. The direct genetic heritability estimates for FD, CFW and BW obtained from the three-trait analysis were 0.447 (0.009), 0.216 (0.008) and 0.277 (0.008) respectively. The genetic correlations were 0.050 between BW and CFW, 0.100 between BW and FD and 0.139 between FD and CFW. The phenotypic correlations were 0.318 between BW and CFW, 0.129 between BW and FD and 0.180 between FD and CFW. Direct genetic correlations of BW, CFW and MFD were positive, which suggest that selection for bigger and heavier sheep would generally lead to a stronger MFD and higher CFW. Because these values are very low the subsequent effect of this phenomenon would be that the effect of selection for body traits on fleece traits would be very small and vice versa. Genetic trends were calculated using the annual average breeding value estimates (EBV) for each trait. According to the genetic trends derived, the selection policy followed did cause genetic change in the traits studied, even though it was slow. The level of inbreeding (F) in the South African Dohne Merino sheep population is very low. The proportion of animals that was inbred to some extent increased from 0% (average F=0) in 1980 to 38% in 2003 (average F=1.22%). No significant inbreeding depression on BW, CFW and MFD could be found. In general the results suggest that inbreeding at present is not a serious problem in the South African Dohne Merino breed.