This soybean plots' rows are planted on 15-inch centers. Researchers said they find that soybeans planted closer than 30 inches actually see a yield bump. Closer plants have earlier canopy cover, helping to control weeds earlier in the growing process.

Every year soybean producers make major decisions about the planting process. The first decision is variety selection, the second is planting date, and a third is seeding rate. Seeding rate has become more economically critical as soybean seed prices have increased with improved varieties, the addition of biotechnology traits, and the increasing popularity of more costly soybean seed treatments.

For the past three decades research has been ongoing studying the affects of planting soybeans in rows narrower than the standard 30-inches.

Vince Davis, an Extension soybean specialist with the University of Illinois at Urbana-Champaign, has been conducting the same research that was being performed by Palle Pedersen, an assistant professor at Iowa State University. Pedersen recently resigned from that position to take a research position with a private company.

Davis told Farm-News that information on narrow-row soybeans has been available showing there are advantages to planting in narrow rows, although the idea has not caught on in Iowa at the same rate as in Illinois.

Pedersen’s former research indicates that in Iowa, producers can get 4.5 bushels more in average yield with 15-inch rows as long as weeds and pathogens can be controlled.

Davis said that soybeans planted closer together form a canopy earlier, helping in controlling weeds.

In his state, Davis estimated that the majority of soybean acres are planted in 15-inch rows and upward to 20 percent in 7.5-inch rows. Roughly half of Iowa’s soybeans are planted in wider rows due mostly, Davis said, to the presence of white mold.

Those with white mold problems in their fields, Davis added, need the wider rows for air movement to keep the fungal disease under better control.

However, fields with white molds can be successfully planted in 15-inch rows with reduced seed populations, one Pedersen research account claims, which was published just prior to the 2007 season.

Fewer seeds?

“Several of my colleagues in surrounding states have looked at the relationship between soybean seeding rates and yield,” Davis said, “and have promoted lowering seeding rate recommendations in their states

” In the past decade, there have been a couple of large multiyear studies investigating desired soybean seeding rates in Illinois, and I was privileged to get these data to analyze.”

The data from these trials show, Davis explained, that even though soybeans can yield well with plant populations of less than 100,000 plants per acre, the risk of reduced yields would be higher when starting with lower seeding rates.

“I paid particular attention to the magnitude of changes in the group variances,” Davis said, “not just to the changes in mean yield while accounting for the group variances.

“From these two data sets, a reduction in variance did not occur in relation to seeding rates. Therefore, reducing risk by increasing seeding rate does not intuitively appear to be logical.”

The first study examined 125,000, 175,000, and 225,000 seeds per acre planted in row spacings of 7.5, 15, and 30 inches in Illinois test plots at Monmouth, DeKalb, and Urbana, from 1998 to 2000. There was no yield difference between the 7.5-inch rows (53.6 bushels per acre) and 15-inch rows (53.0 bushels per acre); however, 30-inch rows yielded 2.1 and 2.7 bushels per acre less than the narrower row spacings, respectively.

“An increase in seeding rate produced a modest, although significant, increase in yield.” Davis said. These included:

• The rate of 125,000 seeds per acre yielded 51.6 bushels per acre.

• An increase of 50,000 seeds per acre increased yield 1.2 bushels per acre.

• Another increase of 50,000 seeds per acre (175,000 to 225,000) increased yield only 0.2 bushels per acre.

Contact Larry Kershner at (515) 573-2141, Ext. 453 or by e-mail at kersh@farm-news.com.