Case study: The effect of inbreeding on the production and reproduction traits in the Elsenburg Dormer sheep stud |
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| Authors: | JB van Wyk MD Fair SWP Cloete |
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| Institution: | aDepartment of Animal, Wildlife and Grassland Sciences, University of the Free State, P.O. Box 339, Bloemfontein, 930l, South Africa;bInstitute for Animal Production, Elsenburg, Private Bag X1, Elsenburg, 7007, South Africa |
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| Abstract: | Data of the Elsenburg Dormer sheep stud, which was kept closed since inception, were collected over a period of 62 years (1941–2002). The breed is a composite, resulting from a cross of Dorset Horn rams with South African Mutton Merino ewes. These data were analysed to quantify the increase in actual level of inbreeding and to investigate the effect of inbreeding on phenotypic values, genetic parameters and estimated breeding values. After editing 11954 pedigree, 11721 birth weight (BW) and survival, 9205 weaning weight (WW) and 7504 reproduction records were available for analysis. The mean level of inbreeding (F) of all animals over all years was 16%; 14% for dams and 16% for sires. Mean, minimum and maximum F for the lambs in 1997 (when 3 rams from outside were introduced) were 22%, 21% and 24% respectively. Estimates of inbreeding depression for individual inbreeding of 1% were − 0.006 kg for birth and − 0.093 kg for weaning weight respectively. These were the only estimates that were significantly (P < 0.01) different from zero. No significant effects of inbreeding on the other traits were found. There were virtually no differences in the genetic parameters estimated when fitting the two models (inclusion or exclusion of inbreeding coefficients as covariates). Estimates of the phenotypic variance differed slightly for WW between the two models. Ranking of animals were studied for weaning weight when the two models were considered. The high correlation coefficients (0.990) indicate that the use of inbreeding coefficients did not cause important changes in ranking of animals and sires for WW. It was concluded that slow inbreeding (rate of inbreeding of approximately 1.53% per generation over 19 generations) allows natural selection to operate and to remove the less fit animals. At any given mean level of F, less inbreeding depression would then be expected among the individuals who accumulated the inbreeding over a larger number of generations. Nevertheless, inbreeding coefficients should be considered when mating decisions are made, to limit the possible deleterious effects of inbreeding on productive and reproductive traits and to detect animals “resilient to” higher levels of inbreeding. |
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| Keywords: | Inbreeding depression Dormer sheep Genetic parameters Early growth Survival and reproduction traits |
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