STUDY OF EARLY SELECTION IN TREE BREEDING - 1 - ADVANTAGE OF EARLY SELECTION THROUGH INCREASE OF SELECTION INTENSITY AND REDUCTION OF FIELD-TEST SIZE

There are three main advantages for early selection in tree breeding: 1.) increased selection intensity or reduced field-testing size; 2.) a shortened generation interval; and 3.) genetic information from early testing can be used to enhance selection efficiency at mature age. Th e first advantage is realized when early testing results can be used f or culling families with the poorest performance prior to field testin g. The expected genetic gain formula is derived for early plus mature two-stage successive selection. This formula is used to study the firs t advantage of early selection, which results in an increase in total selection intensity or reduction of field-testing size. The gain incre ase from early selection for a larger base population and gain decreas e from early culling of the poorest families is a function of heritabi lities, selection intensities on early and mature traits and their phe notypic and genetic correlation. Both early-mature genetic correlation and heritability of the early trait affect the magnitude of genetic g ain increase for the mature trait from early selection. The formula is also used to answer the following three questions: (1) is it possible that early selection can be used to reduce the size of field testing without any loss in ultimate gain for the mature trait? (2) are there any conditions where more gain can be obtained when both early and mat ure selection are practiced than when selection is only practiced at t he mature stage? (3) what is the condition where any selection at the early stage will result in less gain than if all selection is postpone d to the mature stage? Depending on genetic parameters, all above thre e conditions are possible. The relationships of genetic parameters for satisfying one of the three conditions were derived from the formula and the theory is applied to a lodgepole pine retrospective early sele ction study.