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What’s Causing Higher Soybean YieldsWhat’s Causing Higher Soybean Yields
by Darrell Smith
11:30AM Nov 11, 2019
Soybean yields are climbing thanks to genetics, climate change and technology.
( Lindsey Benne )
There’s something good happening in soybean fields across the U.S. Since 2013, average soybean yields have exceeded the trend line, with an average above-trend yield of 3.7 bu. per acre, according to USDA’s National Agricultural Statistics Service. From 2013 to 2018, six states had average yields more than 6 bu. per acre above the trend. Farm Journal Field Agronomist Ken Ferrie has observed similar results among his own consulting clients. So what's causing higher yields?
Soybean Yields Are Climbing Thanks To ...
Genetics. “Breeders keep striving for higher yield,“ says Farm Journal Field Agronomist Ken Ferrie. “Improvements have been made in all maturity groups, from Canada to the southern U.S.” One example: Four-bean pods, unheard of years ago, are now fairly common.
Climate change. However you might feel about man-made versus natural climate variation, measurements by the National Oceanic and Atmospheric Administration show carbon dioxide levels in the atmosphere have been rising since the 1950s (and possibly longer).
Like 85% of plants, soybeans are C3 plants, meaning they respond to higher carbon dioxide levels. (C4 plants, such as corn and sorghum don’t respond to higher levels.) “If atmospheric carbon dioxide levels are rising, it could be beneficial for soybeans, Ferrie says.
Through the Farm Journal Test Plot program, Ferrie monitors carbon dioxide emissions from the soil. “Fields with soybeans after soybeans emit considerably less carbon dioxide than soybean fields in corn the previous year,” Ferrie says. “Some of the most consistent yield increases we have observed came in soybean fields that followed long-term continuous corn.”
While some yield increases are from rotational effects, Ferrie also has documented much higher carbon dioxide emissions.
Another potentially positive aspect of climate change is warmer weather in May and June. “This drives better early growth and the potential for plants to add more nodes and then pods,” Ferrie says.
Technology. “Thanks to technology, farmers have more tools, such as fungicides and insecticides, to protect soybeans from pests and keep plants healthier throughout the entire growing season,” Ferrie says. “For our clients, the biggest technological improvement is better seed treatments. Today we have options to attack diseases such as phytophthora root rot, pythium, rhizoctonia and fusarium.
“For central Midwestern farmers, sudden death syndrome (SDS) and soybean cyst nematodes are some of the biggest challenges. Formerly, their only tool was soybean genetics. But now a combination of fungicides and nematicides, along with genetic resistance, makes it possible to plant soybeans earlier.”
Since the 1980s, Ferrie has studied planting dates. Results show earlier planting produces repeatable yield increases.
“But then along came sudden death syndrome and soybean cyst nematodes,“ he adds. “SDS can quickly cut a 60-bu. soybean yield to 30 bu. or 35 bu. per acre. By the mid-1990s, the early planting option had been taken off the table for many growers. Technology that once again makes early planting possible is a game changer.”
How Early Planting Boosts Yield
Early planting’s impact on yield results from how a soybean plant develops. An internal clock, triggered by the phytochrome protein in leaves, tells it when to start flowering, based on the length of the hours of darkness.
“The length of night (the hours of darkness) must be long enough to trigger flowering,” Ferrie says. “Night length gets shorter until the summer solstice — between June 20 and June 22 in the Northern Hemisphere — then it gets longer. With traditional planting dates, the nights get long enough to trigger flowering for earlier varieties around July 1, and later varieties flower after that.”
But there also are nights before the summer solstice with enough hours of darkness to trigger flowering. “The objective is to plant early enough for plants to get big enough to start to flower on the front end of the solstice, as well as afterward,” Ferrie says.
Here’s an example from the Farm Journal Test Plots: “In 2019, soybeans planted on June 4, because of weather delays, did not flower until July 12,” Ferrie says. “Plants of the same variety, planted on April 23 before the rain set in, were big enough to start flowering on June 9, before the nights got too short.
“When a soybean flowers early, it lengthens the time between reproductive stages, adding more days to accumulate starch,” Ferrie says.
Farm Journal studies show with early planting, the first pods to set are in the third to sixth nodes. “Those pods tend to get big and mature enough so they are stable by the time the rows close,” Ferrie says. “With normal planting dates, the immature pods at the bottom of the plant often are lost when the trifoliates fall off.
“So another way early planting impacts yield is by adding more pods at the bottom of the plant. If we have 150,000 plants per acre, and there are 2,800 seeds per pound, adding two three-bean pods per plant increases yield by more than 5 bu. per acre. If we add six three-bean pods, we add about 16 bu. per acre.”
More Insights From Early Planting Test Plots
Under the guidance of Field Agronomist Ken Ferrie, Farm Journal has been studying soybean planting dates since the 1980s. Along the way, Ferrie has learned:
“The smallest plant to initiated flowering was at the V3 stage,” Ferrie says. “That suggests we must plant early enough to get plants to their third trifoliate before the night gets too short.”
When plants flower before the solstice, they lengthen the time between each reproductive stage, when the plants accumulate pods and starch. “Mid-Group II soybeans planted in mid-April spend more than 50 days in the R1 to R6 stages,” Ferrie says. “If we plant on May 10, that drops to 40 days. On June 10, it falls to 25 days.”
Early planted soybeans will usually outyield later plantings. While the bulk of soybean yield normally comes from pods on the middle two-thirds of the plants, in the Farm Journal Test Plots, early planted beans retain more pods near the bottom of the stems.
Climate conditions can simulate the effect of early planting. “In 2018, because of wet weather, we did not plant soybeans until May 10, which isn’t considered early,” Ferrie says. “Then we had record warm temperatures and an unusual number of growing degree days in May and June. Compared to the five-year average, the soybeans performed as if they had been planted on April 21. We ended up with V8 to V10 soybeans flowering before the solstice.”