Factors affecting the results of two-stage selection in open nucleus breeding

The objective of this research was to study the factors affecting the annual genetic gain resulting from an open nucleus breeding system with selection of base-born nucleus replacement ewes in two stages using base and nucleus indices, respectively. For this purpose, an open nucleus breeding system was simulated using a deterministic approach with information from Lori-Bakhtiary sheep population. First, the genetic gain from two-stage selection in basic conditions was estimated. Then, different levels of accuracies in base and nucleus and two different levels of fertility were envisaged. In basic conditions, advantage of two-stage selection over single stage selection using base index was 7.65 %. Correlation between base and nucleus indices in this situation was 0.627. Advantage of two-stage selection depended on the accuracies of selection in the two stages and varied from 4.6% (with accuracies of base and nucleus of 0.5 and 0.65, respectively) to 15.45% (with accuracies of base and nucleus of 0.2 and 0.95, respectively). Maximum advantage of two-stage selection (15.45%) resulted from low fertiliry rate (0.7) and low correlation (0.211) between indices of the two stages. In general, results showed that with lower correlations of base and nucleus indices and low fertility rates, advantage of two-stage selection is greater.