Effect of row spacing and seeding rate on soybean yield
Soybean [Glycine max (L.) Merr.] yield response to narrow row spacing has been consistently positive in the upper Midwest and new split-row planters have made narrow row soybean production feasible, yet adoption has been slow in Iowa. Wide (76-cm) and narrow (38-cm) row spacing and four seeding rates (185,000; 309,000; 432 000; and 556,000 seeds ha–1) were evaluated at three locations during 2004, 2005, and 2006 to determine seed yield in wide and narrow row spacing and four seeding rates and evaluate economic advantages associated with changes in row spacing. Soybean planted in 38-cm row spacing yielded 248 kg ha–1 greater than soybean planted in 76-cm rows after adjustment for differences in final plant populations. Maximum yield at all locations was attained at a final harvest population of 462,200 plants ha–1 but >95% of the maximum yield was achieved with final populations as low as 258 600 plants ha–1. Increased production costs associated with greater seeding rates removed the yield benefit from greater harvest plant populations. Farm size of 144 ha with at least 50% of the land base dedicated to soybean production would benefit from conversion from wide to narrow rows. To break even on the investment in a split-row planter a yield increase of 124 kg ha–1 was necessary for farms with 30% of 288 ha dedicated to soybean production. These data indicate that yield and economic benefits are sufficient to support the production of soybean in narrow rows and at seeding rates below current seeding rate recommendations.