Predicting genetic gain of backward and forward selection in forest tree breeding. Silvae Genetica 47

In several forest tree breeding operations either backward or forward selection is used, although a proper combination of both might be more advantageous. Using calculations based on quantitative genetic theory, comparisons were made between the relative merits of backward and forward selection for individual families. In backward selection the mother was chosen based on her offspring and in forward selection the best offspring was selected from the family consisting of the mother and her children. A range of heritabilities and selection inten-sities in natural forest and progeny test (the latter is a function of progeny size) were compared. It is the more favourable to select backward the higher the mother ranks. Depending on the combination of parameter values, backward selection was superior to forward selection for open pollinated progeny in 1 % to 57 % of the top ranking families. High intensity of selection in the forest, low herita-bility and small progeny size favoured backward selections. Backward selection for families with plus tree fathers was usually not favourable for more than a few percent of the families. Only when heritability was low it could be favourable to select 10 % of top-ranking mothers. The calculations show that it can often be optimal to apply backward selection for the best families and forward selection for the lower ranking families side by side. However, the difference is often small and then other considerations can be more important. Key words: backward selection, forward selection, breeding strategy, intensity of selection, progeny size, heritability, genetic gain. FDC: 165.3; 165.6. 1