Rainfed lowland rice breeding strategies for Northeast Thailand. II. Comparison of intrastation and interstation selection

There has been limited progress for grain yield of rainfed lowland rice in Northeast Thailand since the 1960s. The current breeding strategy operates as a series of six semi-independent pedigree programs, each at a different site. Each program has three major phases of selection: (1) intrastation selection, (2) interstation selection, and (3) on-farm selection. The expected selection response for grain yield based on intrastation and interstation selection was examined using a combination of experimental results, prediction equation theory and computer simulation. Experiments were conducted to estimate genetic, genotype-by-environment interaction and error components of variance as inputs for estimation of heritability on a number of bases and also to obtain estimates of realised response from selection. Estimates of line-mean heritability for grain yield based on intrastation evaluation of lines suggest that it is low, ranging from 0.07 to 0.13, for one to four replicates, respectively, at a single site in 1 year. Line-mean heritability for intrastation evaluation based on two replicates and 2 years was estimated to be 0.18, only slightly higher than for 1 year and four replicates. In contrast, estimates of line-mean heritability for interstation testing were intermediate, e.g. 0.32 and 0.48 for two replicates at six sites for 1 year and 2 years, respectively. Estimates of realised selection response for grain yield from intrastation and interstation selection were consistent with the low to intermediate heritability estimates. Interstation selection, based on two replicates, eight sites and 1 year, showed an advantage over intrastation selection, based on two replicates and 1 year, when response was measured as the mean yield of selected lines across environments. The present breeding strategy applies intense selection during the intrastation phase of the breeding programs. Consequently, only a small number of lines (ca. 70 lines from all stations) are advanced from the intrastation selection phase to the interstation selection phase. Therefore, for most lines generated by the breeding program there is limited opportunity to evaluate the contributions of broad and specific adaptation to higher yield. The presence of large genotype-by-environment interactions, in combination with limited yield evaluation of lines in multi-environment trials (until the final stages of testing), is identified as a major factor contributing to the slow genetic progress for grain yield. The proposed breeding strategy replaces the intrastation testing phase with a coordinated early generation interstation testing based on F bulks. Evaluation of the proposed breeding strategy by computer simulation demonstrated an advantage from modifying the current breeding strategy to give greater emphasis to interstation selection in place of intrastation selection