Soybean yield response to narrow rows is largely due to enhanced early growth

Researchers in the southern USA have shown that the increased yield associated with narrow rows for late-planted determinant soybean [Glycine max (L.) Merr.] is due mainly to a greater crop growth rate during vegetative and early reproductive stages which allows for an increase in the number of fertile nodes and increased number of pods per fertile node. The objective of this study was to determine if a similar explanation is applicable to the narrow-row yield response reported with full-season narrow-row production of indeterminate soybean cultivars in the midwestern USA. To test this hypothesis field experiments were conducted in 1992 and 1993 on a Flanagan silty loam soil (fine, montmorillonitic, mesic, aquic, argiudoll, mollisol). Soybean was sown in row widths of 38, 76, and 114 cm and thinned to 450 000 plants per ha at V1 to V2. A linear decrease with increasing row width was observed for grain yield (3497, 3250, 2920 kg/ha), number of pods per plant (93, 89, 85), plant height (99, 96, 93 cm), number of branches per plant (3.3, 3.1, 2.9), and harvest index (33, 31, 30). Crop growth rate increased with decreasing row width until about R5 after which row width had no significant affect on crop growth rate. These results support our hypothesis that the yield increase noted for narrow-row indeterminate soybean production in the midwestern USA is due predominantly to beneficial effects experienced prior to the main grain-fill periods in a manner similar to that reported for late-planted narrow-row determinate soybean production in the southern USA.