Effects of different strategies for breeding towards scrapie resistance on inbreeding in small populations

All flocks with registered animals in the herd book have to breed towards scrapie resistance until 01. April of 2005. The objective of the following study was to analyse the effects of different strategies for breeding towards scrapie resistance on inbreeding, bottle neck effects and breeding costs by using simulation studies. For the simulations a programme was developed, in which population structures and selection strategies could be varied. Scrapie resistance could be reached up to 4 generations earlier and without loss of genetic diversity with higher (55%) ARR allele frequencies compared to lower (1%) ARR allele frequencies in the founder population. Moreover, this could be reached faster by switching from a strategy of selecting rams only (22 to 26 generations) to a strategy where both rams and ewes were selected (10 to 11 generations). But that was linked with higher inbreeding rates and laboratory costs. The inbreeding coefficient was 0.2 to 0.5 percent higher and the costs were twice as much as in a scheme where selection of rams only was conducted compared to a scheme of selecting of rams and ewes. The increase of inbreeding and costs could be minimized by changing the mating proportion towards a more balanced situation. The strategy for breeds with small population sizes and low ARR allele frequency should be to breed initially towards ARR heterozygous sheep until a threshold value of 45% for the ARR allele is reached. The loss of genetic diversity was kept to a minimum by using this strategy.

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