Field experiments were conducted in 2020 and 2021 at the Field Science Center of Okayama Univ. (34°41’ N, 133°55’ E). Two Soybean cultivars ‘UA4805’ and ‘Akimaro’ were sown with two planting densities, 12.5plants m−2 (sparse, 80×10cm) and 25plants m−2 (dense, 80×5cm)on May 25 (early), June 29 (normal), and Aug. 3 (late) in 2020, and 80 and 30cm row-width, and 12.5 and 25 plant m−2 in 2021 on June 23. Seed yield was higher in ‘UA4805’ than in ‘Akimaro’ in 2020 and 2021. The later the sowing time, the higher the seeds/stem ratio. Both cultivars showed higher dry matter in dense planting. Dry matter was higher in ‘Akimaro’, while seed yield was lower than ‘UA4805’. In contrast, ‘UA4805’ showed lower dry matter with higher seed yield. The numbers of nodes, pods, and seeds were higher in ‘UA4805’ resulting in the higher seed yield. Lodging score is larger in ‘Akimaro’ especially in dense planting. The seeds/stem ratio is much higher in ‘UA4805’ than ‘Akimaro’ across 2 densities, 3 sowing times and 2 row width. Pods setting ratio was nearly two times higher in ‘UA4805’ compared to ‘Akimaro’. The greater seed yield of ‘UA4805’ compared to ‘Akimaro’ was due to the higher pod setting ratio, seeds/stem ratio, and lower lodging score, nevertheless the dry matter was larger in ‘Akimaro’. If late sowing is applied, higher planting density is recommended for better seed yield. Narrow row is an effective way to improve seed yield in soybean.

2020年と2021年に岡山大学農学部附属山陽圏フィールド科学センター（34°41ʼN, 133°55ʼE）でダイズの栽培試験を行った．アメリカ品種‘UA4805’ と日本品種‘あきまろ’ の2 品種を供試し，栽植密度を12.5 株 m-2（疎植，80 × 10 cm）と25 株m-2（密植，80 × 5 cm）の2 段階として，2020年5 月25日（早期）， 6 月29日（普通期）， 8 月3 日（晩期）に播種した．2021年は畦幅80 cm（広畦）と30 cm（狭畦），栽植密度12.5株m-2と25株m-2の2 段階で栽培した．子実収量は，2020年と2021年ともに‘あきまろ’ よりも‘UA4805’ の方が高かった．播種時期が遅いほど，粒/茎比が高くなった．両品種ともに疎植区に比べ密植区で乾物重が大きくなった．‘あきまろ’ は‘UA4805’ よりも乾物重は大きかったが，子実収量は低かった．一方，ʻUA4805ʼ は乾物重が小さかったが，節数，莢数，子実数が多く，子実収量が高かった．‘あきまろ’ は特に密植区において，倒伏程度が大きかった．粒/茎比は，いずれの試験区においてもʻ あきまろ’に比べ‘UA4805’ が著しく高かった．結莢率は‘UA4805’ が‘あきまろ’ より2 倍近く高かった．‘UA4805’ の子実収量が‘あきまろ’ に比べて高かったのは，結莢率，粒/茎比が高く，倒伏程度が小さかったことによるが，乾物生産は‘あきまろ’ の方が大きかった．晩期栽培の場合，子実収量を向上させるためには栽植密度を高くすることが推奨された．ダイズの子実収量を向上させるには，狭畦栽培が効果的であった．