Average row spacing in Illinois soybean fields has increased over the past decade after narrowing for several decades before that, according to University of Illinois crop sciences professor Emerson Nafziger.

This increase resulted from drilled soybeans – those with rows less than 10 inches apart - losing ground to 30-inch rows, which now occupy nearly one-third of the soybean acres. Just over 50% of Illinois soybean fields remain in 15-inch rows.

Planting 30-inch Soybean Rows Could Result in Slight Yield Penalty

Most corn planters are set to plant 30-inch rows, and 15-inch soybeans are generally planted by using split-row units placed between the 30-inch rows. The increasing width of planters – many are now 60 feet wide – means that splitting rows with additional units makes planters very heavy. Opting not to split large planters means that both corn and soybeans are typically planted in 30-inch rows.

Another reason given for moving from narrow soybean rows to 30-inch rows is white mold tends to be more severe when the ground and the lower crop canopy stay wet. Air moves better around plants in wider rows, so soybeans in wide rows may be less susceptible to damage by white mold. Wide rows are also easier to drive through to spray or cultivate.

A U of I study over the past two years, funded by the Illinois Soybean Association, compared 15-inch and 30-inch soybean rows, at different planting dates and seeding rates, over six sites in Illinois. Excluding one southern Illinois site where stands were not adequate, 15-inch rows yielded on average 1.5 bushels more than 30-inch rows. This difference varied among the sites, from no difference to up to nearly four bushels per acre.

"Because late-planted soybeans tend to produce smaller plants, we had expected to find that narrower rows would tend to yield more than wide rows when planting was late," Nafziger says. However, the data showed that response to row width was as likely with early as with late planting. Similarly, there was little interaction between row width and seeding rate.

According to Nafziger, these results suggest that choosing to plant 30-inch rows instead of narrower rows could bring a small yield penalty. "This doesn't mean that widening rows in order to use a larger planter is unsound, but those making the decision to do this should pencil in about 1.5 bushel less yield," he adds.

That amount is not very noticeable in the field, and some growers who make the switch may conclude that they suffered no yield loss. "We know from our research that such loss will not always occur," Nafziger notes. "But we have also found that it can, and there's no reason to expect that it won't, at least on average."

There may be ways to reduce yield loss related to changing to wider rows, but the results of recent research in this area are not conclusive. "We do not believe that changing the seeding rate is likely to help," Nafziger explains. "If incidence of disease that can be controlled by fungicide is decreased in wider rows, then we would expect fungicide application to produce less response in wide than in narrow rows."

Choosing taller, perhaps slightly later, varieties might help the crop capture light better in wider rows. Nevertheless, it is best to choose a soybean variety based on its productivity rather than on traits for which the benefits are not certain.