US soybean yields have steadily increased, largely due to genetic factors. In contrast, Japanese yields remain low, partly due to a lack of genetic yield gain, but the yield potential and associated traits in Japanese cultivars have not been fully evaluated. The objectives of the present study were to estimate genetic gains in soybean yield and agronomic traits in Japan, and to determine whether Japanese cultivars are inferior to US cultivars in yield and agronomic traits; if so, which traits should be improved? We conducted a 4-year field trial to compare yield and agronomic traits of cultivars released between 1953 and 2014 in northern Japan and the Midwestern US. Japanese cultivars had lower yield. New US cultivars showed genetic yield gain of 9.86 kg ha^–1 year^–1, with the highest contributions from seed number. The increased seed number decreased 100-seed weight, but the increased seed number outweighed that effect, resulting in consistent yield gain. Increased node number contributed most to the increased seed number. Old and new Japanese cultivars showed no significant yield difference. Their yield did not increase over time because 100-seed weight increased but seed number decreased (i.e., there was a strong trade-off). Only 100-seed weight increased in new Japanese cultivars, as this trait is important to Japanese breeders; it determines the processing quality for tofu and the esthetic quality for nimame (boiled beans). We could not confirm a genetic yield gain, but variations in 100-seed weight and seed number contributed similarly to yield in Japanese cultivars. Japanese breeders also focused on improving seed number, but it has been difficult to achieve high yield by simultaneously improving 100-seed weight and seed number. Innovation will be needed to resolve the trade-off between these factors.

美国大豆产量稳步增长，主要是由于遗传因素。相比之下，日本的产量仍然很低，部分原因是缺乏遗传产量增益，但日本品种的产量潜力和相关性状尚未得到充分评估。本研究的目的是估计日本大豆产量和农艺性状的遗传增益，并确定日本品种在产量和农艺性状方面是否不如美国品种；如果是这样，应该改进哪些特征？我们进行了一项为期 4 年的田间试验，以比较 1953 年至 2014 年间在日本北部和美国中西部发布的品种的产量和农艺性状。日本品种的产量较低。美国新品种的遗传产量增加了 9.86 公斤公顷^–1年^–1，种子数的贡献最高。增加的种子数量减少了 100 粒种子的重量，但增加的种子数量超过了这种影响，导致产量持续增加。增加的节点数对增加的种子数贡献最大。日本新老品种产量差异不显着。它们的产量并未随着时间的推移而增加，因为 100 粒种子重量增加但种子数量减少（即，存在强烈的权衡）。日本新品种的种子重量仅增加了 100 粒，因为这一性状对日本育种者很重要；它决定了豆腐的加工质量和nimame（煮豆）的美学质量。我们无法确认遗传产量增加，但 100 粒种子重量和种子数量的变化对日本品种的产量有类似的贡献。日本育种者也注重提高种子数，但很难同时提高百粒重和种子数来实现高产。需要创新来解决这些因素之间的权衡。