Record-setting soybeans: What CCAs should know

What can we learn from soybean yield contest winners? Larry Purcell has some answers based on three years of documenting how Kip Cullers, Stark City, MO, grew 160.6-bu/ac soybeans in 2010 and 139 bu/ac previously. Purcell is a University of Arkansas soybean physiologist, Distinguished Professor of Crop Physiology, and Altheimer Chair for Soybean Research.

While recent soybean yield king Randy Dowdy, Pavo, GA, has since raised 190 bu/ac contest soybeans in 2019, there’s no record of a soybean researcher independently assessing how he accomplished that yield.

Purcell replicated some of Cullers’ agronomic practices on campus plots. The highest soybean yields he attained reproducing some of Cullers’ practices were 118 bu/ac. He did not access every last detail of Cullers’ practices but focused for three years on major variables, especially light interception.

This article addresses the controversy of adding nitrogen (N) fertilizers to soybeans, as some yield contest winners are assumed to have done, and identifies some of Cullers’ important practices to increase yields and profits for more mainstream soybean growers. In addition, we examine the profitability of yield-increasing practices as verified by 60 site-years of data (see https://bit.ly/2SsbSjz).

Nix Nitrogen

Soybean nodules fix enough atmospheric N₂ to produce about 80–85 bu/ac soybeans, Purcell says. “If you apply N, it shuts down soybean N fixation. I don’t know any way to apply N without shutting down N fixation.”

Less than 10% of Cullers’ soil N was derived from fixation, Purcell says. “We assume that most of his N came from mineralization of chicken litter; it may have accumulated during previous seasons, or from this year’s application, there was no way to know. But it didn’t come from N fixation,” Purcell says. He used non-radioactive isotopes of N to determine Cullers’ N forms. Cullers applies a lot of chicken litter—about 500 lb N/ac—to achieve 100 bu/ac soybean yields.

“Soybean yield responses to N fertilizer are very small and very unpredictable,” says Seth Naeve, University of Minnesota Extension soybean agronomist who is part of a soybean agronomist team comprising the North Central Soybean Research Program. “Adding any N is unlikely to increase yield much,” he adds. “So, there is no support from an economic standpoint for N application. Even with very high soybean market prices, it takes a large yield response to N to pay for itself. If N is $0.50/lb, 500 lb/ac amounts to $250. That takes 20 bu/ac of yield to recoup added N cost.”

Shawn Conley, University of Wisconsin–Madison professor and Extension soybean and small-grain specialist and the North Central Soybean Research Program soybean agronomist, and 12 other university agronomists participated in a large collaborative research SOYA project to investigate a high-input system’s impact on soybean yield and profitability (see https://bit.ly/3x3gCLO). They tested agronomic practices in 20 field locations in nine states and found 90% of high-yield practices to be unprofitable, even at $15/bu soybean prices and 75 bu/ac yields.

Although high-input management systems increased yield, the probability of breaking even was less than 10% for the yield and sale price combinations analyzed.

However unprofitable, high-input management systems significantly increased average yield in Iowa, Illinois, Indiana, Michigan, Minnesota, Wisconsin; but not in Arkansas, Kansas, and Kentucky.

Variety Selection

“Cullers uses indeterminant Group IV varieties tested on his farm to see which thrive in his conditions,” Purcell says.

Minnesota’s Naeve adds, “Typically, the best-yielding varieties produce between 20 and 40% greater yields than those at the bottom.”

Seed Number, Weight Determine Yield

Soybean yield is determined by seed number and weight, Purcell says. “Seed number is determined during growth stages R1 to R5. Seed weight is determined by the length of stages R5 to R7. All practices should achieve optimal growing conditions from R1 to R7 to maximize sugar production from photosynthesis during seed set and to lengthen the seed-fill period in order to maximize yield.

“Stresses or gain in crop growth before R1 aren’t likely to have a large impact on final yield as long as full canopy closure occurs by flowering,” he says. “You want to increase the flowering period, capture that extended photoperiod at summer solstice, which results from planting early, about when you typically plant corn.”

Much of Purcell’s research on Cullers involved light interception. “Light is not a limiting factor for most Midwest soybean growers,” he says. “We could maximize soybean yield with half the normal amount of light available during the summer in the Mid-South.”

“It’s critical to completely close the soybean canopy and have more than 95% light interception by the beginning of flowering,” Purcell adds. “This intercepts more energy to produce more seeds and pods from photosynthesis. It often requires row spacing below 30 inches.”

Naeve adds, “Canopy closure by R1 is ideal, but regions north of southern Iowa can’t get there. Farther north, light interception and water limitations are equal yield limiters.

“Because light interception is so vital, narrow rows and early planting are key. You can expect approximately a 5% yield advantage for every 10 inches of narrowing, down to about 10 inches,” he says. “The seed-to-seed distance is greater in narrow rows, there is less competition, and they allow higher final stands at any given seeding rate. However, seeding rate does not interact with row spacing—do not change seeding rate when you change row spacing, assuming the same planting equipment.”

Fields with a history of white mold may still be planted in narrow rows, but populations should be managed carefully, Naeve says. In the SOYA high-yield research project, narrow row spacing (20 inches and below) produced the highest yields.

Cullers had a plant population of 140,000/ac planted on 9-inch twin rows on 30-inch centers, Purcell says. “That planting arrangement allows more equidistant spacing.”