By Dusty Sonnenberg, CCA (Adapted from Crop & Soils Magazine, July-August 2021)
Names like Kip Cullers from Stark City, MO, or Randy Dowdy from Pravo, GA are legends in soybean yield contests. In 2010, Cullers raised 160.6 bushel per acre soybeans. In 2019, Dowdy raised 190 bushel per acre contest soybeans. While many sales agronomists have worked alongside of Cullers, Dowdy and other top soybean producers across the country, academia has not thoroughly evaluated the production until recently.
An examination of high-yield practices was undertaken by Larry Purcell, University of Arkansas soybean physiologist, Distinguished Professor of Crop Physiology and Altheimer Chair for Soybean Research. Also in 2020, 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. So what did the research show?
Agronomic practices were tested in 20 field locations across 9 states. Results found that 90% of high-yield practices were not profitable, even at $15-per-bushel soybeans yielding 75 bushels per acre. The high-yield practices did increase yield in 6 of the 9 states in the Midwest, 3 states did not observe yield increases.
Soybean nodules fix enough atmospheric nitrogen to produce about 80 to 85 bushel per acre soybeans according to research by Purcell.
“If you apply N, it shuts down soybean N fixation,” Purcell said. “I don’t know any way to apply N without shutting down N fixation.”
Further research shows there is no support from an economic standpoint to apply nitrogen. With the current price of N, it would take a very large yield response to pay for itself.
Finding the best varieties for the environmental conditions on each farm is key.
“Typically, the best-yielding varieties produce between 20% to 40% greater yields than those at the bottom,” Purcell said.
Farmers getting the record yields have identified the varieties the perform best on their farms.
Seed number and weight
Soybean yield is determined by the number of seeds and weight. Purcell says that the 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 work to 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. This means achieving full canopy closure by flowering and reducing stress.
“You want to increase the flowering period, capture that extended photoperiod at summer solstice, which results from planting early,” Purcell said.
Optimize seeding rates
The most profitable seeding rates vary by region and agronomic productivity category and are driven by variations in seed costs, grain prices, seed treatment use, and the environment’s productivity. In a seeding-rate study, researchers found that soybeans can adjust to a wide range of planting populations. Growers should increase seeding rates in less productive growing areas and decrease rates to be closer to the optimal seeding rate in more productive areas.
Soil test results
Soybeans can typically utilize residual P and K from a well-fertilized corn crop. High applications of P and K to attain record-setting yields are unaffordable practices for mainstream farmers. High application rates of N, P and K to soybeans is cost prohibitive and could also have negative environmental impacts.
Plant early and even
Getting the soybeans planted early and fast emergence (within 24 hours) is a factor in achieving high yields.
Seed to soil contact
Good seed to soil contact is important, especially when following a high-residue corn crop.
“The soybean seed needs to be planted in the soil, not the residue,” Purcell said.
Reducing plant stress
Moisture stress, weeds and disease pressure all negatively impact soybean yield. Those with high-yielding contest soybeans typically irrigate. Weed control starts early.
“Two-inch pigweed is way too long to wait before spraying,” Purcell said.
Weed control programs should include both a pre- and post-emerge herbicide application. Diverse modes of action should also be incorporated.
There is still a good deal largely unknown about inputs. The timing of application and rates to use are not yet an exact science. Slow-release fertilizers, micronutrients, and biologicals all are being used at different rates and times by many high-yield contest soybean growers. According to Purcell, soybean inoculants are not necessary if soybeans are planted at least every 3 years.
Asking questions and a willingness to try new things are key factors in achieving higher yields.
“Curiosity and a focus on soybeans tend to serve high-yield soybean contest growers well,” Purcell said.