Genotypic Adaptation of Soybean to Late Sowing in Southwestern Japan

In southwestern Japan, late sowing of soybean is an option that may avoid the damage caused by excessive soil moisture during the rainy season. We investigated the adaptability to late sowing using 17 genotypes from 5 countries. The seeds were sown on 15 - 20 July, and 2 - 5 August in the normal and late sowing seasons, respectively, in an upland field in Karatsu, southwestern Japan in 2009, 2010 and 2011. Late sowing reduced the seed yield by 28.1% and 21.8% on average in the 2009 and 2011 experiments, respectively, whereas it had almost no effect in 2010, in which the temperature was high from sowing to flowering. Seed yield was not improved by increasing the growth period with a longer juvenile growth stage in the genotypes that originated in tropical areas. In the late sowing, seed yield was significantly correlated with the reproductive period from flowering to maturity and pod number, but not with the vegetative period from sowing to the end of leaf expansion or seed size. Soybean cultivars Caviness (USA), Parana and IAS-5 (Brazil), and Akisengoku and Akiyoshi (Japan) showed higher productivity in both types of sowing season, and their seed yields were less reduced by late sowing. These genotypes generally have larger pod number and seed number or longer seed filling periods, but they have medium-size seeds. Our results indicate that the seed yield in late sowing could be improved by the selection of adaptive genotypes that have larger seed number and/or longer seed filling periods.