Soybean Row Spacing Responses to Planting Date and Variety Selection.

The effect of row spacing on soybean [Glycine max L. (Merr.)] yield varies across the Upper Midwest. Recently, a study in Iowa documented that soybean produced in narrow row spacing (38-cm) yield 248 kg ha^-1 more than soybean in wide rows (76-cm). Despite this difference there are farmers in Iowa who claim they do not see any yield difference between wide or narrow row spacing. The objective was to determine if planting date and variety selection influenced the yield response of soybean to row spacing. Two separate studies were established at three locations with four replications in Iowa during 2007-2008. Study one was a randomized complete block design in a split-split-plot arrangement. Main plot was three planting dates planted with two week intervals, subplots were two row spacing (38-cm and 76-cm) and sub-subplots were two cultivars. Study two was set up as a randomized complete block design in a split-plot arrangement. Main plot was row spacing (38-cm and 76-cm) and the subplots were eight cultivars. Our results indicate that delayed planting interfered with soybean yield response to row spacing. Soybean in narrow rows yield greater than soybean in wide rows at the early planting date but no differences were observed due to  row spacing at the two later planting dates. Response to row spacing was not influenced by cultivar selection. Our data suggest that soybean yield response to narrow row spacing is maximized by planting a high-yield, adapted cultivar at an early date.