Will a Freeze Hurt Beans?

When Cold Temperatures Threaten Soybeans

Pamela Smith
By  Pamela Smith , Crops Technology Editor
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The forecast for frost is putting a chill in the air regarding late-planted soybeans. (DTN photo by Pamela Smith)

DECATUR, Ill. (DTN) -- Frost on the pumpkin might be the sign there's a chill in the air, but soybean farmers are hoping they don't have a serving of frozen green beans to go with it.

DTN Ag Meteorologist John Baranick warns that cold, near-artic air from Canada could bring frost to portions of the soybean belt, and that could also bring a chilling effect to late-planted or late-maturing soybeans. Temperatures below 32 degrees Fahrenheit can damage soybean leaves, and temperatures below 30 degrees for extended time periods can damage the stems, pods, and seeds. A killing freeze is 28 degrees for soybeans, according to an Iowa State University release.

While the recent hot, dry conditions sped up maturation in many soybean fields this fall, there is a lot of variability in the crop, noted Mike Staton, Michigan State University soybean educator.

"We had optimal planting conditions in late April and early May. But then we dried out, and one of our biggest challenges has been uneven emergence," he told DTN. "In many fields planted after mid-May, that seed just sat there for five weeks or more until we got rain in early July.

"Our challenge in this region is we have almost two different maturities in the same field," Staton said. Late-planted double-crop soybeans could also be at risk as the weather switch flips.


Soybeans that are grass green and still have a dense canopy should escape significant damage from the first frost/freeze event, he said. "In those cases, the leaves almost act like a cover. The first frost might take out the upper leaves, but not get too deep into the canopy. However, once the upper leaves have been damaged, the crop is more susceptible to subsequent freeze events that will penetrate deeper into the canopy.

Frost-damaged soybeans are generally considered salvageable as grain as long as the plants reach the R6 growth stage prior to the time the killing frost occurred. The R6 growth stage, often called the "green bean stage," occurs when the beans completely fill one pod at one of the upper four nodes on the main steam on 50% of the plants in the field.

He added that by the time plants reach the R7 growth stage, yield reductions due to frost or freeze will be minor. The R7 growth stage is defined as when one pod on the main stem has attained mature color on 50% of the plants in the field.

Green, frost-damaged soybeans can be harvested for forage. If that's not an option, they can be left as a cover crop or incorporated into the soil. Staton estimates using soybeans as a green manure crop gives growers a 20- to 40-pound nitrogen credit to apply to next year's corn crop.


Frost-damaged soybeans are going to be harder to harvest. Staton recommends reducing the combine concave clearance. If that doesn't work, increase the speed of the cylinder.

Frost-damaged beans are also going to dry down slowly. Green beans and immature beans require drying time adjustments. "In fields where only the upper leaves were damaged by frost, producers should wait and allow the beans to mature to 14% to 15% in the field," Staton suggested.

However, if entire plants were killed by freezing temperatures, the beans may need to be harvested at 16% to 18% moisture to avoid excessive shatter losses. He noted that University of Wisconsin research showed shatter losses of 0.2 bushel per acre per day occur after the beans reach 16% to 18% moisture.

Frost-damaged soybeans are included in the total damage factor in the U.S. soybean grading standard, so expect scrutiny at the elevator. Staton said small beans can be screened out, and the green and immature beans can be dried to 12% moisture and stored in aerated bins for later sale. Sometimes the green color fades.


Those warm conditions and recent rainfall of late may help avoid frosty conditions, at least temporarily. The greatest risks for frost are currently in the Northern Plains for Friday and Saturday mornings and in the northern Midwest Saturday, Sunday and perhaps Monday morning as well. However, if conditions change, we could see some of that frost risk as far south as southeastern Colorado and Kansas and even the Ohio Valley.

"Temperatures next week will slowly moderate, and we could see some patchy frost in northern areas early next week as well, though that is more likely for northern Wisconsin and Michigan than any other area," Baranick said.

Find more about upcoming weather patterns from John Baranick at https://www.dtnpf.com/….

For more information on using soybeans for forage, visit https://www.canr.msu.edu/….

More harvesting, drying and storage tips for frost-damaged soybeans are available at http://msue.anr.msu.edu/….

An Iowa State release on handling frost-damaged soybeans can be found at https://crops.extension.iastate.edu/….

Pamela Smith can be reached at pamela.smith@dtn.com

Follow her on X, formerly Twitter, @PamSmithDTN

Pamela Smith

Pamela Smith
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