Although narrow rows and high plant populations brought significant yield gains in several Midwestern states (see above), Iowa State University researchers were unable to duplicate those results.
In three years of research at five locations, no consistent yield response was reported from narrow rows or established stands of more than 160,000 plants per acre.
Adapted, high-yielding varieties were planted in 7.5" or 10" rows with a no-till drill, and in 30" rows with a planter.
"We could find one location one year that showed a yield response to narrow rows, but another location showed a better response to wide rows," reports agronomist Keith Whigham. "And when we averaged across locations and years, we saw no difference."
He figures differences in weather, intensity of soybean production and other factors may explain why he and his colleagues weren't able to duplicate the narrow-row yield gains reported in other Midwestern states.
"If a farmer tells me he can do better with narrow rows, there's no doubt he probably can," says Whigham. "But that doesn't mean he can afford to buy a new planter to change row spacing. I get the question every year, 'If I buy a new drill for narrow rows, can I pay for it with the extra yield?' I have to tell them no, they better have another reason to buy the drill."
The Iowans also compared five seeding rates within the narrow and wide row widths, with the goal of establishing 80,000, 120,000, 160,000, 200,000 or 240,000 plants/acre. Higher seeding rates didn't produce higher yields. So they recommend that growers aim for established stands of 150,000-170,000 plants/acre to optimize yield potential vs. seed cost.
In three years of planting-date comparisons, they planted varieties with a range of relative maturities at six planting dates from late April to mid-July. They found that late April to mid-May plantings usually yielded best, with yields declining sharply with later plantings.
(John Lundvall, Keith Whigham and Dale Farnham, Iowa State University)