Predicted optimal selection age remains consistent in multiple rotations in the first-generation breeding seedling orchard of fast-growing Neolamarckia macrophylla in Central Java, Indonesia

Neolamarckia macrophylla is one of valuable fast-growing species that contributes in Indonesia’s timber supply. To support plantation productivity of this species, supplying genetically improved seed is indispensable. Breeding programs for forest tree species aimed at producing improved seeds require a long period of time, making it necessary to identify the most efficient timing for selection in N. macrophylla. Therefore, the objective of this research was to determine the optimal selection age in N. macrophylla within a breeding seedling orchard (BSO) to maximize predicted genetic gain across the breeding cycles. The research leveraged the first-generation BSO of N. macrophylla established in Wonogiri District, Central Java Province, Indonesia. Diameter at breast height (DBH) was measured periodically at seven periodic measurements. A growth model for DBH was constructed to predict the extrapolated unobserved data up to a 15-year rotation. Genetic parameters were estimated using the fitted model across all observed ages. Genetic gain efficiency was calculated for several rotation ages corresponding to its end product use. The results showed a slight downward trend in the genetic parameters for heritability as age increased, dropping from 0.65 at one year to 0.45 at twelve years. The modified Chapman-Richards equation yielded moderately high genetic correlations for DBH among ages, with an R2 value of 0.69. The optimal selection age was found to be most efficient when conducted at six years of age for an eight-to-fifteen-year rotation, which gave the opportunity for early selection. By contrast, a seven-year rotation was found to preclude early selection.