Browsing by Author "Olivier, J. J."

Now showing 1 - 6 of 6
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    Across flock genetic parameter estimation for yearling body weight and fleece traits in the South African Dohne Merino population
    (South African Society for Animal Science, 2008) Van Wyk, J. B.; Swanepoel, J. W.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Olivier, J. J.; Delport, G. J.
    Accurate genetic parameter estimates are needed upon which to perform multiple-trait across flock breed analyses. Genetic parameters for yearling body weight (BW), clean fleece weight (CFW) and mean fibre diameter (MFD) were estimated using records of 107 389 individuals (the progeny of 1 530 sires and 45 178 dams) collected between 1992 and 2004 in the South African Dohne Merino population. Fixed effects included in the model were flock-year-season-sex- management group (1 594 classes), type of birth (singles, multiples), age of dam (2 - 7+ years) and age at measurement, fitted as a linear covariate (385 ± 12 days). Six different single-trait animal models were fitted, where different combinations of the following random effects were fitted: direct additive, the sire-flock interaction, the sire-flock-year-season interaction (SFYS), the dam genetic effect, the direct-maternal correlation and the dam permanent environmental effect. These analyses were followed by a three-trait analysis structured according to the log likelihood ratios obtained for the single-trait analysis. This analysis allowed the calculation of relevant correlations among traits together with their respective standard errors. Direct heritability estimates from the three-trait analysis were 0.17 for BW, 0.19 for CFW and 0.45 for MFD. Maternal heritability estimates were 0.01 for BW and 0.006 for CFW, with corresponding dam permanent environmental ratios of respectively 0.03 and 0.02. The genetic correlation between animal effects amounted to 0.48 for BW. Derived proportions of the total phenotypic variance due to SFYS were 0.02, 0.02 and 0.02, respectively. Genetic correlations of BW with CFW and MFD were 0.11 and 0.13, respectively, and of CFW with MFD 0.16. It was concluded that the inclusion of some form of a genotype by environmental interaction as part of the national evaluation is essential, although it controlled only a modest portion of the overall phenotypic variation.
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    The adaption of the South Africa sheep industry to new trends in animal breeding and genetics : a review
    (South African Society for Animal Science, 2014-10-11) Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Olivier, J. J.; Sandenbergh, L.; Snyman, M. A.
    The history of sheep breeding research in South Africa can be divided roughly into four eras, namely the research and development phase, the commencement of recording and evaluation, the expansion of recording schemes, and, most recently, the adaptation of schemes to international benchmarks. The most recent era has presented scientists with the greatest challenges, namely the inclusion of genomic breeding values in routine sheep recording and of disease-resistance traits during routine evaluation. The establishment of reference populations for the major South African sheep breeds to estimate genomic breeding values is an immediate challenge. This process may be facilitated by a number of genetic resource flocks that are phenotyped for traits that are not routinely recorded in the national evaluation. A limited number of these animals are also genotyped. There is strong evidence that resistance of sheep to external and internal parasites is heritable, and may be improved by purposeful selection. Efforts should be concentrated on the inclusion of disease resistance traits in national analyses where appropriate. However, seen against the background that South African investment in research is appreciably less than in developed countries, lack of funding and high-capacity manpower may impede rapid progress. There thus seem to be many challenges for future generations of sheep breeding scientists.
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    Genetic analysis of faecal worm egg count in South African Merinos under natural challenge
    (South African Society for Animal Science, 2007) Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Olivier, J. J.; Du Toit, E.; Dreyer, F. H.
    Sheep from a Merino selection experiment at the Tygerhoek research farm in the Southern Cape provided material for this study. The selection lines involved included a line selected for clean fleece weight, a "Wet and Dry" line, a fine wool line and an unselected Control line. Rectal faeces samples were obtained from individual animals at 13 to 16 months of age, after drenching was withheld for at least 10 weeks. Nematode eggs in these samples were counted. Fitting the appropriate fixed effects, the heritability of untransformed, cube root transformed and log transformed faecal nematode egg count (FEC) was obtained from single-trait analyses. The effects of sex and birth year were involved in a significant interaction. Means for FEC were generally higher in ram progeny than in ewes, but the magnitude of the sex difference was not consistent. Multiple lambs had a slightly lower mean for FEC than singles, while FEC was unaffected by dam age. The heritability of FEC was estimated at between 0.14 for untransformed data and 0.18 for log transformed FEC. Genetic correlations of log transformed FEC with two-tooth staple strength (-0.49) and coefficient of variation of fibre diameter (0.30) were favourable. Clean fleece weight was unfavourably related to FEC on a genetic level (0.19). Selection for resistance to parasitic nematodes after natural challenge should thus be feasible in the Merino lines studied.
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    Genetic parameters and relationships of faecal worm egg count with objectively measured wool traits in the Tygerhoek Merino flock
    (South African Society for Animal Science, 2014-07-09) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
    The costs of internal parasite control and treatment are potentially very high in grazing sheep. Faecal worm egg count (FEC) has been suggested as a suitable criterion for selection for resistance to nematode infestation in livestock. Genetic parameter estimates for FEC and its relationship with wool traits were assessed in the current study, using data from Merino sheep from a selection experiment maintained at Tygerhoek Research Farm. Data consisted of ~7 100 animals born between 1989 and 2010. Rectal faecal samples were taken from individual sheep at 13 to 16 months of age, after drenching had been withheld for at least 10 weeks, generally in July to September. Nematode eggs were counted with the McMaster technique, with a sensitivity of 100 eggs per gram of wet faeces. The fixed effects of birth type, sex, birth year and sex x birth year interaction were included in the operational model for FEC. Only the effect of additive animal affected the data for FEC. Heritability estimates of FEC ranged from 0.10 for untransformed FEC to 0.16 for Log (FEC + 100). The genetic relationships of Log (FEC + 100) with staple strength (SS), staple length (SL), coefficient of variation of fibre diameter (CVFD) and standard deviation of fibre diameter (SDFD) were favourable. The genetic correlations of FEC with wool weight traits were unfavourable in absolute terms, but not significantly different from zero. Selection for FEC is unlikely to result in unfavourable correlated responses to wool traits in South African Merinos, with the exception of wool weights.
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    Relationship of ewe reproduction with subjectively assessed wool and conformation traits in the Elsenburg Merino flock
    (South African Society for Animal Science, 2017-11-06) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
    Subjectively assessed wool and conformation traits are widely used to select breeding ewes and rams in the sheep industry. Data from a Merino flock that is maintained at Elsenburg Research Farm were used to investigate animal model (co)variance components for ewe reproduction traits with subjectively assessed wool and conformation traits. Ewe reproduction traits were assessed at their first lambing opportunity at two years, or over a three-year period from their lambing opportunities at two to four years old. Relationships of ewe reproduction traits with subjectively measured wool and conformation traits were also investigated. All these traits were heritable, with a range from 0.16 ± 0.03 for topline (TOPL) to 0.64 ± 0.04 for woolly face score (WFS). Genetic correlations of number of lambs born (NLB1) with colour (COL), number of lambs weaned (NLW1) with COL and belly and points (BANDP) and total weight of lamb weaned (TWW1) with COL were negative and significant. Significant genetic correlations of ewe reproduction traits over three lambing opportunities were found between number of lambs born (NLB3) and WFS (0.23 ± 0.11) and between total weight weaned (TWW3) and face cover score (FCS) (-0.33 ± 0.16). Among these traits, the noteworthy favourable genetic correlation between total fold score (TOT) and NLB1 suggested that plainer ewes were more reproductive. This is important for the South African Merino industry as plainer sheep are more desirable because of their faster growth and higher lambing percentages and reduced chances of fly strike. Selection for improved ewe reproduction in Merino sheep thus would not result in marked unfavourable correlated responses in most of these subjective wool and conformation traits.
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    The relationships between faecal worm egg count and subjectively assessed wool and conformation traits in the Tygerhoek Merino flock
    (South African Society for Animal Science, 2014-08-02) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
    Subjectively assessed wool and conformation traits form part of the selection objective in wool sheep enterprises. The present study investigated the genetic, phenotypic and environmental correlations for nematode resistance (using faecal worm egg count (FEC)) with subjectively assessed wool and conformation traits. The Merino sheep flock (consisting of four lines) maintained on Tygerhoek Research Farm was used. Fixed effects of selection line, birth type, sex, age of dam in years, year of birth, and sex*birth year interaction had a significant effect on all subjective traits. At genetic level, log transformed FEC was significantly related to wool oil only at 0.18 ± 0.09, staple formation at 0.29 ± 0.10, and topline at −0.33 ± 0.11. These correlations suggested that sheep with high FEC are likely to have excessive wool oil, thicker and bulkier staples, and lower scores for topline. Selection for resistance to and resilience against nematodes in Merino sheep thus will not result in marked unfavourable correlated responses in the vast majority of these subjective wool and body conformation traits.