Faculty of Agriculture
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Browsing Faculty of Agriculture by Author "Cloete, S. W. P."
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Item Genetic parameter and relationships of faecal worm egg count with objectively measured wool traits in the Tygerhoek Merino flock(South African Journal of Animal Science, 2013) Cloete, S. W. P.; Matebetsi-Ranthimo, P. A. M.; 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. __________________________________________________________________________________Item The relationship between faecal worm egg count and subjectively assessed wool and conformation traits in the Tygerhoek Merino flock(South African Journal of Animal Science, 2013) Cloete, S. W. P.; Matebetsi-Ranthimo, P. A. M.; Olivier, J.J.; van Wyk, J. B.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. ________________________________________________