Simulation of the open nucleus group breeding scheme for genetic improvement of six-month weight of Kermani sheep breed

The main objective of the present simulation study was to determine the most appropriate relative nucleus size and transfer rate of the replacement ewes from the base commercial flocks to the nucleus in an open nucleus group breeding scheme for improvement of six-month weight of Kermani sheep. Modeling the structure of the breeding pyramid including flocks in nucleus and base tiers including 5000 ewes and 143 rams with overlapping generations, 6 ewe age groups and 3 ram age groups in both nucleus and base tiers, the birth of first progeny at 2 years old, selection of replacements by the method of "progeny selection" for 6-month weight, transfer of replacements between two tiers in 5 and 6 selection paths, provision of all replacement rams in two tiers from nucleus, without- and with the transferring of ewes from nucleus was carried out, and it was simulated using a deterministic approach with account of the selection effect on the genetic variance and progress. Eight relative nucleus size considered at intervals of 0.02, from 0.04 to 0.18, for each of which the optimal transfer rate from the base to the nucleus was obtained. The annual genetic progress and the genetic lag of base flocks with optimal ewe transfer rate to nucleus calculated for each relative nucleus size and the most suitable relative nucleus size was determined. The best relative nucleus size was 0.12, but the most appropriate economic relative nucleus size was 0.08, 400 ewes in nucleus and 4600 in base flocks with 64 young ewes to be transferred to nucleus and 3.51 years genetic lag for base compared to nucleus predicted. When reducing the relative nucleus size to 0.08, 33% of relative nucleus size reduced and 5% was added to the base relative size. The increase in number of young ewes for transferring to nucleus was 3 heads. The genetic lag of base flocks increased by 0.36 years. The advantage of selection in the open nucleus scheme at the relative nucleus size of 0.12 in comparison to selection in all community flocks without a nucleus investigated. Deterministic simulation was quite effective for determining the most suitable nucleus size leading to genetic improvement of six-month weight quite close to optimal state, reducing costs of scheme and including as many sheep as possible in base.