Study of early selection in tree breeding. 2. Advantage of early selection through shortening the breeding cycle.

Summary Three main advantages from early selection in tree breeding have been recognized: 1.) increased selection intensity or reduced field-testing size; 2.) shorter generation interval; and 3.) genetic information from early testing can be used to enhance selection efficiency at later ages. The second advantage is obtained through quicker realisation of genetic gain or by breeding several generations within a conventional breeding cycle from mature selection. To quantify the second advantage from early selection it is necessary to estimate genetic gain from indirect selection over several generations. In this paper, a method is derived to estimate genetic gain from several generations of early indirect selection and is used to study the advantage of early selection through shortening the tree breeding cycle relative to mature selection. The results show that genetic variance, heritability and selection response for the correlated (mature) trait as well as genetic correlation between directly selected (early) and correlated (mature) traits will decline after each generation of selection. When the number of generations approaches infinity, genetic variance, heritability and selection response for the correlated trait and the genetic correlation between directly selected and correlated traits each approach corresponding limiting values under FISHER’s infinite genetic loci model. The reduction in genetic variance, heritability and selection response for the correlated trait is slower than the reduction of genetic variance for the trait under direct selection. The method is applied to a lodgepole pine early selection study.