Reasons Why High Planting Density Dose Not Increase the Yield of Soybean Cultivar ‘Sachiyutaka’ in Nara Prefecture and Yamaguchi Prefecture, Japan

Abstract

Field studies were conducted in 2011 and 2017 in Nara and Yamaguchi prefectures in Western Japan to examine why the yield of the determinate soybean cultivar ‘Sachiyutaka’ is not improved by increasing the planting density. Branching, radiation interception (IR), radiation use efficiency (RUE), and dry matter production were evaluated in ‘Sachiyutaka’ planted at normal (<20 plants m⁻²) and high (≥30 plants m⁻²) densities. High densities were ensured by narrowing hill spaces and keeping row spaces constant. Yield and pod number at maturity were not significantly increased by increasing the planting density. The number of branching nodes and pod number were decreased by the high planting density at the Nara field site but not at the Yamaguchi site. Instead, the high planting density decreased the number of pods per node on the main stem at Yamaguchi. Although the number of nodes and pods varied with the site, pod number at maturity did not vary with the planting density at each site. The pod number may not have increased by the increase in the planting density because of the lack of dry matter production from flowering to early seed filling, probably due to the lack of IR and RUE. Thus, the lack of increase in yield in ‘Sachiyutaka’ at a high planting density may be due to the lack of change in IR, RUE, and dry matter production after flowering.