Response of Determinate and Indeterminate Soybeans to varying Cultural Practices in the Northern USA

Determinate soybean [Glycine max (L.) Merr.] cultivars adapted to the northern U.S. soybean growing region have been developed and released in recent years. The purpose of this research was to compare the response of determinate and indeterminate soybean cultivars to varying cultural practices. The determinate cultivars, ‘Gnome’ and ‘Elf’, and the indeterminate cultivars, ‘Beeson’ and ‘Williams’, were grown at two Illiiois locations in planting dates ranging from early May to early July. The soil was Flanagan silt loam (Acquic Argiudoll) at Urbana and Cisne silt loam (Mollis Albaqualf) at Brownstown. Within each planting date, cultivars were planted in 80, 50, and 20 cm row widths. A separate seeding rate study was conducted with the determinate cultivar Elf, using seeding rates of 350,000, 500,000, and 650,000 seeds/ha in both 76 and 20 cm row widths. Planting date contributed most to variation in seed yield. Seed yield in central Illinois decreased an average of 33y0 as planting date was delayed from early May to early July. Seed yield of the indeterminate cultivars d e clined steadily after the early May planting date, whereas, seed yield of the determinate cultivars did not decrease until planting date was delayed past early June. The indeterminate cultivars showed a linear decline in plant height and main stem node number as planting date was delayed, whereas, plant height and main stem node number of the deternunate cultivars were greatest in early June plantings. Lowest pod height of the determinate cultivars planted after mid-June or in drought-prone soils was sufficiently close to the soil surface (less than 10 an) to cause concern about excessive harvest losses. Seed yields of both the determinate and indeterminate cultivars increased significantly (9 and 5% at Urbana and Brownstown, respectively) as row widths were narrowed from 80 to 50 cm, whereas, seed yields in 50 and 20 cm row widths did not differ. The determinate cultivar Elf exhibited the ability to perform well over plant densities at harvest ranging from 279,000 to 494,000 plants/ha. A seeding rate of 400,000 to 500,000 seeds/ha for determinate cultivars appears sufficient to maximize seed yield, reduce potential harvest losses due to low basal pod height, and insure adequate and uniform stands in unfavorable seedbeds.