As we had feared, delayed planting of soybean in the spring and early summer resulted in delayed maturity this fall. Late maturing soybean plants are vulnerable to freeze and cold temperature damages. Parts of Missouri experienced several episodes of temperatures cold enough to damage soybean plants.
Plants that had expressed noticeable signs of maturity including yellow leaves and leaf drop will not be affected by temperatures below freezing. The rest of this article is focused on those soybean fields in which plants had expressed no or only slightly the normal maturity process.
"Killing freeze" has many definitions, but usually includes a low temperature such as 28F and a time amount such as 4 hours. The freezing point of water within leaves is lower than 32F because of the dissolved compounds such as sugars. But, even temperatures too warm to cause ice can affect plants. Life processes are mediated by enzymes and some of these enzymes can be damaged by cool, but not freezing, temperature. For example, photosynthesis rate may recover slowly after temperatures below 40F.
When temperatures drop quickly below freezing, ice crystals form inside plant cells. Because water expands upon freezing, plants cells and their membranes are literally torn apart. As temperatures rise and plant tissues thaw, cell contents leak outside the cells. Leaves are most susceptible to freeze damage. Damaged leaves or portions of leaves will die and will not proceed through normal change in color. If temperature is cold enough, the entire plants will be affected and die.
Damage may not be uniform throughout the field. Cold air is heavy and drains down slopes. It pools in lower portions of the field. Even small depressions can catch and hold cold air. Soils are still relatively warm and they lose temperature more slowly than air. This may create differences within the canopy for damage with leaves, stems, and pods near the top of the plant most vulnerable.
Predicting the effect of freezing temperatures on soybean plants is difficult because of all the variation within a field and within plants. But, even partially damaged plants may not mature normally and normal signals of approaching maturity such as yellowing of leaves and dropped leaves may be changed. Any leaf or portion of leaf killed by a freeze will not yellow, but will turn brown. Also, the sequence of maturity among leaves, seeds, and stem will be changed. Dead leaves may remain on the plant and stems may remain green long after seeds are mature. Harvest timing should be based on seed moisture not the appearance of the plant. Waiting too long to combine freeze damaged plants could result in grain shattering.
If soybean plants are killed before physiological maturity, seeds on those plants will not mature normally. Immature soybean seeds contain chlorophyll and are green because of this pigment. After plants reach physiological maturity, chlorophyll production in seeds ceases. Chlorophyll that is present in seeds and pods is broken down through natural metabolism. Premature death stops this natural degradation of chlorophyll and the seeds remain green. The extent of the green color depends on timing of premature death. If death occurs late in seed-filling, the green color is usually confined to the seed coat. This color may lessen over time with field drying or in storage. If death occurs during earlier in the life cycle, the green color remains throughout the interior of the seed. This color will probably not disappear even with long term storage.
Frost damaged soybean grain should store almost as easily as normal soybean grain, although aeration is recommended. Usual precautions of foreign material and damaged seed coats apply to all stored soybeans. Plants killed by frost will retain leaves and stems may remain green. This can add moisture to grain or make grain difficult to separate from other plant parts. Prematurely killed soybean seeds will shrink to smaller than normal size and the shape will be more oblong than normal. Combine settings should be adjusted to account for these differences.
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% of the cross-section is yellow will likely be classified as "soybeans of other color". If the grain lot has more than 10% seeds of other color it may be graded as "standard" and may receive substantial dockage.
REVISED: October 15, 2019