A 15-year Farm Journal study shows you need to push seeding rates if planting runs unusually late. The study also suggests that narrowing rows to 20" or less provides some yield insurance in an adverse growing season.
In the study, with normal planting dates, 120,000 viable seeds per acre were sufficient in 10" and 20" rows. That rate was competitive with 160,000 viable seeds per acre in 30" rows. "The data got a little fuzzy as to whether 120,000 seeds was enough in 20" and 30" rows," says Farm Journal Field Agronomist Ken Ferrie. "It did not raise any yield concern, but we always saw more scattered weeds in the wide-row plots at low populations."
Bumping the seeding rate to 200,000 never produced the highest yield in any row width. "That suggests that, under normal conditions, we cannot push yield by pushing seeding rate to very high levels," Ferrie says.
However, in 2008, which had an unusually wet spring when many soybeans were not planted until the end of June, planting only 120,000 viable seeds per acre carried a significant yield penalty. The highest yields across all row widths usually occurred with 160,000 seeds per acre. Planting 200,000 viable seeds per acre produced a yield advantage in 30" rows, "but it did not run away from 160,000," Ferrie notes.
With late planting, plants are shorter, so you need more of them, Ferrie explains. "The study tells us to push seeding rates if the planting season is unusually late," he concludes. "If you wind up with 150,000 plants per acre, you’ll probably be OK in row widths from 7½" to 20", but you may pay a yield penalty with 30" rows."