The old adage of planting soybeans as thickly as you can afford gets expensive as today’s seed prices can be five times greater than 15 years ago. But higher prices aren’t the only reason soybean growers consider lower seeding rates. It still boils down to yield, and how much you need to plant to get to that target density of 100,000 plants/acre to maximize it.

“Go back 10 years, and planting soybeans with drills in 8-in. rows was basically controlled spillage; we didn’t seed beans very well,” says Scott Eversgerd, a Pioneer agronomist in southern Illinois. “Today, a majority of beans are put in with 15- or 30-in.-row planters, or drills with good metering devices.”

Eversgerd says that because of better equipment, improved varieties, more options in seed treatments and higher seed costs, soybean seeding rates have been reduced in recent years. It was common to recommend 200,000 seeds/acre 10 years ago, vs. what Eversgerd recommends today as a starting point: 160,000-165,000.

University of Illinois (U of I) studies suggest that number can go even lower. Vince Davis, U of I Extension soybean specialist found that reduced soybean seeding rates do not directly increase yields. With funding from the soybean checkoff, he’s studying the interaction of seeding rates by planting date and by row spacing.

His trial included six Illinois locations, four planting dates, 15-in. and 30-in. row spacing and three seeding rates (75,000, 125,000 and 175,000 seeds/acre).

“We were interested in what the interactions of these three factors would be, and the suprising results from our first year of data in 2010 is that there wasn’t any,” he says. “For every one of those individual factors, the yield response was the same regardless of the combination of the other two factors.”

The team found there was no yield advantage to more narrow, 15-in. rows, when typically you see a 2-bu. advantage. And there was very little response across the range of seeding rates, which means increasing seeding rates from 75,000 to 175,000 did not directly increase yields.

“I’ve been looking at other data, as well, and I’m not surprised to find the lower seeding rates doing well,” says Davis, who recently analyzed 50 site-years of data that showed there is just as much yield variation at higher seeding rates as at the lower rates. “An optimum seeding rate may be 100,000-130,000 seeds/acre, which is low compared to what we’ve recommended in the past.”

Jeff Keifer, agronomist with Elburn Cooperative, works with a small group of growers in northern Illinois to manage soybean trial plots. Their simple goal is to figure out practical ways to get the highest yields possible. After finishing the program’s third year, he says he’s found some interesting results regarding seeding rates.

“The days of 200,000 seeds/acre are over,” Keifer says, whose own farm is one of the test plots, which he manages with his two brothers. “We try to get the cost per acre down by lowering seeding rates, but it’s also important to properly manage inputs and stay on top of weed pressure.”

Based on the test plots’ 2010 harvest results, Keifer found that 150,000-152,000 was the optimal seeding rate, with one plot yielding 83 bu./acre. The plots that seeded as low as 120,000 took in 10 bu. less.

Eversgerd urges growers to talk to the agronomists and seed-company representatives in their areas, consider their recommended starting range and then adjust that based on variety selection, field and weather conditions, past results and see what works for them.

February 2011