Soybean [Glycine max (L.)Merr.] is one of the major protein and oil crops in the USA and world. Soybean seed yield was increased by 272% during 1924 to 2011 in the U.S. To study the seed yield genetic gain of southern soybean cultivars and its association with other agronomic traits, 93 cultivars selected from maturity groups (MG) V, VI and VII were evaluated for yield and agronomic traits in 26 environments during 2010 and 2011. The soybean cultivars included in this study were released from 1927 to 2008. Linear regression of seed yield on the year of release showed an increase in seed yield by 18.2, 14.3 and 11.0 kg ha^-1 year^-1 in MG V, VI and VII cultivars, respectively. Modern cultivars had a greater response to superior environments, and greater yield stability among environments than the older cultivars. Lodging resistance was significantly correlated with seed yield in MG V and VI. Lodging resistance was increased 0.01 lodging unit year^-1 in the three maturity groups. In MG VI, a significant decrease in protein content of 0.24 g kg^-1 year^-1 was observed. There were no significant changes in oil content, plant height or seed size in these cultivars over years of release. The present results suggested that breeding for yield in MG V, VI and VII increased the seed yield by 0.84%, 0.71%, and 0.60% per year, respectively. During the selection for high seed yield, lodging resistance was increased ~0.35% year^-1.