Late maturing soybeans face damage from early killing frost and need special attention for harvest and storage, said University of Missouri Extension soybean specialist Bill Wiebold.
Harvest frost-damaged soybeans based on seed moisture content rather than how plants look after frost, Wiebold said.
Delaying harvest after damaging frost results in grain shatter and subsequent yield loss. Soybean breeders select for shattering tolerance and normally the seam of the soybean pods holds together. However, the halves rupture if left in the field too long after maturity or damage.
Ice crystals form inside plant cells when temperatures quickly drop below freezing. Freezing water expands and tears plant cells and membranes. This causes cell contents to leak outside the cells and plants die quickly.
When harvesting, adjust combine settings to allow for plants with wetter than normal stems and leaves, Wiebold said. Seeds also will shrink to smaller than normal size and seed will be more oblong in shape than normal. Frost-damaged soybean should store almost as easily as normal soybean, Wiebold said. He also recommended aeration when storing.
When soybean plants die before maturity, seeds on those plants do not mature normally. Plants quit producing chlorophyll when they reach physiological maturity. Seeds from immature plants contain chlorophyll and remain green.
Intensity of the pigment depends on how mature the soybean plant was prior to frost. The seed coat stays green if death occurs late in seed filling. If this is the case, the color may bleach from sunlight or natural metabolism or lessen during storage.
If plant death occurs earlier in the life cycle, the inner part of the seed may remain green, even after long-term storage. Immature beans also may require longer drying times.
Nearly all USA soybean grain is classified and sold as yellow soybeans, according to the “Official US Standards for Grain.” “Seeds with green seed coats should be classified as yellow soybeans and not docked. However, seeds in which less than 90 percent of the cross-section is yellow will likely be classified as “soybeans of other color.” If the grain lot has more than 10 percent seeds of other color it may be graded as “standard” and may receive substantial dockage, Wiebold said.
Below-freezing temperatures will not affect plants that showed signs of maturing, such as yellow leaves and leaf drop, before freezing.
However, many Missouri producers likely will see damaged slow-maturing soybean after recent frosts. Because of the large number of delayed planting acres this year, recent frosts will affect more soybean than usual.
Damage can occur even when temperatures are too warm for ice to form, Wiebold said, because cool temperatures can damage the plant’s enzymes.
“Killing freeze” usually means a temperature as low as 28 degrees Fahrenheit for four hours. However, the freezing point of water within leaves is less than 32 degrees Fahrenheit. This makes leaves most susceptible to freeze damage. Ice crystals form inside plant cells when temperatures drop quickly below freezing. Freezing water expands and tears plant cells and membranes. This causes cell contents to leak outside the cells.
Damaged leaves or parts of the leaves die and do not change color. If temperatures drop enough, the entire plant can die.
Wiebold said damage may not be uniform through the field. Cold air is heavy and drains down slopes to pool in lower parts of the field. Even small dips can catch and hold cold air and put plants at risk of freezing.
Soils lose heat more slowly than air, so damage varies in different areas of the canopy. Leaves, stems and pods near the top of the plant are most vulnerable while those closer to the warmer ground may receive less damage.