Genetic Contributions of Thai Soybean Cultivars to Annual Yield Increases

Abstract

During 1965-1987, at least 6 soybean cultivars have been released by the Department of Agriculture (DOA), Ministry of Agriculture and Cooperatives, This breeding efforts may have increased the average yield of soybean. The objective of this study was to estimate the degree of genetic contribution to yield gain of soybean. Six soybean cultivars, namely; SJ2, SJ4, SJ5, NW1, SK 1 and CM 60 were plated in a randomized complete block design with 4 replications during the late rainy season of 1988 and the dry season of 1989 at the National Corn and Sorghum Research Center, Kasetsart University, Amphoe Pakchong, Nakhon Ratchasima Province. Grain yields were recorded and averaged across the seasons. To estimate the annual genetic gain achieved in soybean cultivar improvement from 1965 to 1987, cultivar means of grain yields (averaged across seasons) were regressed on year of cultivar release. Cultivar release dates were obtained from published report by the DOA. The annual yield gain in soybean production was obtained by regressing annual soybean yield estimates on year of production using yield estimates from 1967-1987. This annual gain reflected the combined contribution of genetic and agronomic practice improvements. The results revealed that the annual yield gain was 17.33 kg/ha. And the annual genetic was -2.85 kg/ha. However, when NW 1 was excluded from the linear regression as its adaptation is limited only to the lower northern region the result revealed the annual genetic gain of 1.32 kg/ha. The breeding effort also reduced the number of pods per plant. This study indicated that improvements in agronomic practices contributed to annual yield gain more than improvement in genetic yield potential.