A strong negative trade-off between seed number and 100-seed weight stalls genetic yield gains in northern Japanese soybean cultivars in comparison with Midwestern US cultivars

Abstract

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. • Soybean cultivars in northern Japan had lower yield than in the Midwestern US. • Increased seed number contributed strongly to yield gain in recent US cultivars. • There was no evidence of genetic yield gain by recent breeding in Japan. • Recent breeding in Japan increased 100-seed weight but decreased seed number.