Recently, university researchers conducted a three-year, six-state study nicknamed the Kitchen Sink study because of the vast array of inputs and agronomic practices they analyzed. When all the data was tallied, one factor stood head and shoulders above the rest when it came to improving yields in a single move.

Narrow rows. That’s it. No bags, no software, no mixing or blending. Just switching from 30-inch rows to spacing of 20 inches or less boosted soybean yields an average of 2.9 bushels per acre. That is greater than the 2.3-bushel yield bump that resulted from implementing a full program of fertilizer (both dry and foliar), inoculants, seed treatments and foliar fungicides on beans planted in 30-inch rows.

“Of all the newfangled things we looked at, row spacing gave us the biggest increase,” marvels Seth Naeve, the University of Minnesota Extension agronomist who led the Kitchen Sink study. “I think that’s really provocative for farmers.”

Data from Cornell University agronomist William Cox helps explain the phenomenon. In his New York population and row-width trials, Cox determined that soybeans planted in 7.5-inch rows had 15% more biomass, 14% more pods, 9% more seeds and 15% greater yield than the same variety growing in 30-inch rows.

That’s a lot more conversion of sunlight and carbon dioxide into soybeans. It’s also a great way to make sure your canopy closes promptly, shading out weeds and capturing moisture.

Despite the growing body of evidence that narrow-row beans out-perform wider-planted ones, the acreage of beans on 30-inch rows is actually increasing. Part of that shift to wide rows could result from the need to return to cultivating herbicide-resistant weeds like pigweed in some areas.

In other areas, the switch is presumably due to the convenience of maintaining a single planter for both corn and beans. Though that sounds like an economically understandable move, a 2008 study by Iowa State University found that the benefits of narrow rows justified the added expenses on as few as 355 acres with a 50-50 corn-soybean rotation.

Cut back plant populations

Another surprise is that Shaun Casteel, state soybean specialist at Purdue University, and his colleagues have determined that soybean yields do not effectively increase at populations higher than 120,000 plants per acre, or ppa. Populations in the 100,000 to 120,000 ppa range deliver the best return on investment, he notes.

“This is lower than traditional thoughts of soybean plant stands, and certainly lower than the target of many farmers,” Casteel points out.

Fine-tuning plant populations requires several straightforward but important steps:

  • Check your work. Scout fields and count stands so you know if you’ve planted accurately.
  • Compensate for seed germination rates. Check the seed tag and increase seed numbers to account for the percentage that won’t germinate.
  • Adjust planter based on seed size. Each year’s growing conditions impact the number of seeds per pound — which means you need to adjust your planter based on seed size and number, not weight.

Adjustments can help

Researchers at Ohio State University recommend increasing seed numbers at the end of May and first couple of weeks in June. They note that stands of 125,000 seeds per acre and 225,000 seeds per acre planted April 25 and May 11 yielded similarly, but yields significantly favored higher populations when planted in June.

Dial back to thinner stands in fields where white mold has been a problem. Shawn Conley, University of Wisconsin Extension state soybean and small-grains specialist, recommends populations not much more than 100,000 ppa in those fields to alleviate the heavy canopy and tight spacing that favor white mold.

When it comes to soybean planting, it looks like thinking big starts by thinking small — lower populations and narrow rows.

 

Digging into row-width research

Soybeans are adaptable, branching out when you give them room, but how much elbow space does a soybean plant need? Iowa farmers have been growing soybeans in widths from 6 to 40 inches with varying results, but is there an optimum?

For Mark Licht, Iowa State University field agronomist, and Andy Lenssen, ISU soybean systems agronomist, that row-width question offers some interesting answers. Looking at the difference between 15-inch and 30-inch rows with four seeding rates at three Iowa locations from 2004 to 2006, in work done by ISU researchers, for example, they found some noteworthy results. Seed weight and plant height were largely unaffected by row spacing, but yield was 4 bushels higher per acre in narrow rows. There was more production of seed per unit area in the narrow-row setting.

Interestingly, seeding rates didn’t make much of a difference in the results. The narrower rows produced higher seed yields across a range of planting rates.

Another ISU study compared four row widths — 5 inches, 10 inches, 20 inches and 40 inches. In that study, maturity date, plant height and lodging were relatively unaffected across the range of widths. Seed yields were highest in the narrower 5- and 10-inch-row plots. The 20-inch plot was a middle-ground performer, and the 40-inch rows produced the least.

Licht and Lenssen also looked at data from Indiana and New York and found similar results. Soybeans like narrower rows, and across a range of studies, consistently outperform wide-row plantings.

One worry for farmers who stay with wide rows is white mold. However, new technology in the form of a new fungicide — Aproach from DuPont — could alleviate that concern.

While narrow rows were the closest determining factor for rising yield, work has also been done at ISU regarding soybean plant populations. And while you often hear about 150,000 plants per acre and higher as the goal for an emerged stand, turns
out the most important number may be a little lower.

ISU research shows that a uniform final stand higher than 100,000 ppa didn’t increase yield, but if your stand is lower than 100,000 ppa, you would see significantly lower yields.

So how much do you have to plant? Take a look at your stand counts versus planting rates. For example, if you planted at 160,000 ppa and got a final stand of 130,000, that’s an 18.75% reduction. To hit that optimum 100,000 ppa, given that loss, you’d need to plant 125,000 plants per acre.

If you’re looking at a new soybean planter for 2014, thinking narrow and fine-tuning plant populations could put more soybean profit on your bottom line.