Zero in on practices that add more bushels
Soybean yields flat-lined in much of the U.S. during the past decade as farmers focused their time and resources on growing corn. With soybean prices more favorable this year, however, farmers are looking to resuscitate soybean performance and get that trend line moving upward again.
A good way to start the process is to understand the various components that drive soybean yield, explains Missy Bauer, Farm Journal Associate Field Agronomist.
"You have to understand how the bean plant works—when it sets yield potential and what can alter yield potential," she says. "You can then use that knowledge to refine your entire production system."
Right time for yield. Soybeans are developed in two stages: vegetative and reproductive. Soybeans are known for their ability to withstand high levels of stress in the vegetative stages and still produce a decent crop if they get adequate rain and nutrition in the reproductive stages.
Most likely, that’s because the three main soybean yield components—total number of pods per plant, number of beans in the pod and bean weight at maturity—reach their full development during the reproductive stages.
Bauer says farmers saw that fact play out in south-central Michigan during the tough growing conditions of 2012. "The drought conditions had allowed spider mite infestations to increase, and by the end of July, everyone here wanted to give up on their bean crop," she recalls. "But then we had good rains starting in August, so the plants were able to put on a lot of pods and fill them."
The result was the farmers who sprayed for spider mites ended up seeing good yields, she adds.
That was Scott Simington’s experience. He recalls that his soybeans perked up after catching an initial rain on Aug. 15. A couple of more timely rains fell, and by harvest, he says the puny knee-high crop had turned into a yield powerhouse.
"That made me a believer in using management practices that can help my crop," says Simington, who farms near Union City, Mich.
Simington ended up with a 52-bu. per acre average yield across his farm in 2012. By comparison, the Michigan state yield average was only 39 bu. per acre that year, according to figures from USDA–National Agricultural Statistics Service.
Increase pod numbers. A variety of agronomic practices can impact soybean yield components, for better or worse, during the growing season. Some of the key influencers include variety selection; planting date; row spacing; planting practices; soil fertility; and weed, insect and disease control.
Of the three yield components, Bauer says farmers do the best job today of influencing seed size, mainly through the strategic use of fungicides and insecticides when thresholds have been met. "Plot data has shown when controlling foliar diseases and/or insects, once thresholds have been exceeded, the resulting improvement in yield can often be attributed to an increase in seed size," Bauer says. "Protecting seed size can influence soybean yield as much as 10 bu. to 15 bu. per acre."
However, Bauer believes that the upper limit on seed size and the number of soybeans per pod are genetically set and won’t budge much beyond what genetics will allow. Instead, she says the majority of yield gains that farmers stand to capture will likely come from increased pods per plant.
"To get more pods, we need more nodes and axillary buds that have the potential to turn into pods," she says.
One of the challenges standing in the way is flower and pod abortion. Soybeans abort between 60% and 75% of all flowers each season, which are unable to contribute to yield.
"About half the abortion occurs before the flowers develop into young pods, while the other half occurs as a result of pod abortion," Bauer says.
Soybean flowers also pollinate at different times within the node. Bauer’s theory is if the soybean plant could achieve uniform flower pollination within the node, it would have less pod abortion. Unfortunately, there is no specific practice to help farmers reduce pod abortion at this time.
"Reducing pod abortion in the node may be a way in the future to increase yields," she says. "That means growers are going to have to think outside the box and try some new practices."
Delays cost money. One of the most important practices Bauer says farmers can implement is timely planting for their geography. Research at the University of Wisconsin Arlington Agricultural Research Station shows an average yield loss of 0.4 bu. per acre per day occurs when soybean planting in the state is delayed past the first week of May.
Similar results were determined by Michigan State University (MSU) Extension research, based on a three-year study funded by the Michigan Soybean Promotion Committee to determine the effect of planting date on yield results. The trial was planted in the same field using a full-season variety planted in 20" rows.
Yields averaged approximately 1⁄3 bu. per acre less for every day planting was delayed beyond prime dates, says Phil Kaatz, MSU Extension educator.
"Considering the average price of soybeans at harvest for the past three years was $12.66 per bushel, the amount of loss was $4.18 per acre per day for every day planting was delayed," he says. "For a 12-day delay, the gross income lost was $50.21 per acre. A 27-day delay resulted in a loss of $124.06 per acre."
Along with early planting, stand density can also influence pod numbers per plant. Bauer tells farmers to evaluate their planting populations to improve their pod numbers.
"We find plant populations can be decreased with seed spacing and planting depth. If the ratio of seeds per acre versus plants per acre is close, consider evaluating reduced populations on your farm," Bauer advises.
She notes that soybeans planted at too high a density tend to grow tall but produce fewer branches, pods and seeds per plant than those planted at low densities.
"Now we have good seed treatments, inoculants and improved seed-to-soil contact with our planters, so that the number of seeds planted versus the number that emerges is getting to be closer to each other," she says.
Simington has taken steps to lower his populations. Based on Bauer’s recommendation, he sold his drill and purchased a planter with 15" spacings. Then, over the course of a couple of years, he transitioned from planting 175,000 seeds per acre down to 155,000. He now achieves a final stand of about 145,000 to 150,000 plants per acre.
For more soybean management skills to increase yield and tips for management after plant emergence, visit www.FarmJournal.com/bean_yields