Response of Soybean Cultivars to Narrow Rows and Planting Rates Under Weed‐free Conditions1

With continued development of new and better herbicides for chemical weed control in soybeans, the potential exists for growing soybeans in very narrow rows (17 cm) without cultivation. The objective of this research was to examine the potential yield of soybeans [Glycine max(L.) Merr.] in 17‐cm rows under weed free conditions and also to determine the optimum seeding rate and relative yield of this row width compared to greater row widths. Up to nine cultivars were grown in three different row widths and at five different populations in field plots in central Illinois over a period of 6 years. When seeding rate was low enough to prevent lodging, yield advantages of 10 to 20% (3 to 6 q/ha) were obtained from planting in 17‐cm rows compared to 50‐ or 75‐cm rows. Earlier maturing cultivars tended to be more responsive to 17‐cm rows, and under high yield conditions and irrigation, 30 to 40% (9 to 12 q/ha) yield responses were observed with one of the more responsive cultivars. The combination of early cultivars in 17‐cm rows provided the highest absolute yields. A seeding rate of 37.5 viable seeds per m2 was near optimum for most cultivars and for all three row widths tested. These results suggest that if an effective weed control program can be developed to control weeds in soybeans without cultivation, there would be a significant yield advantage from planting soybeans in 17‐cm rows.