ABSTRACT
The high protein concentration and, consequently, high nitrogen demand by developing seed has been proposed as a possible explanation for the relatively small genetic improvement in soybean yields. Jeppson et al. reported that the harvest nitrogen index of soybean cultivars was closely associated with the dry-matter apparent harvest index, but not seed yield. Results from studies such as these indicate genetic improvement in soybean yield has been 10-18 kg/ha annually. Apparent harvest index has been reported to have lower variances than seed yield over years, soybean strains, and maturity groups. To evaluate the dependency of seed yield on vegetative growth, our group at the University of Illinois grew two old and two newer indeterminate soybean cultivars under drought stress during vegetative and early-reproductive development to reduce the amount of vegetative tissue produced. Seed yield improvements in soybean have been relatively small compared to other crop species. .
