Spring frost damage to soybean is relatively rare in Minnesota, as the last average frost dates usually occur before soybeans are normally planted. However, soybean planting and emergence is well ahead of the 5-year average, leaving the crop more vulnerable to early season frost events. Temperatures dropped into the low 30s and upper 20s (F) overnight in the west-central and northwestern parts of the state, likely resulting in some degree of frost injury to emerged soybeans in select areas.
In planting date studies conducted at the U of M SWROC since the 1980s, April and later planted stands have rarely been injury by frost. However, when injured, stands were not reduced to a level that would have indicated replanting and they still yielded as well as later, unfrosted dates.
Temperatures at or below 28-30° F for several hours are usually needed to kill soybean tissue. However, an air temperature of 28° F does not guarantee that a soybean crop will freeze.
Corn seedlings are at a lower risk of death from freezing temperatures than are soybeans, because the growing point of corn remains below ground until the V5-6 stage. In soybean, the growing points are above ground and are exposed after the cotyledons open. Freezing of all growing points is fatal. However, soybean is better able to compensate for partial stand losses than is corn.
Newly emerged soybeans are protected by the nearby warm soil, and small, emerging and cotyledon stage soybeans can be a bit more tolerant to freezing temperatures than older soybean or corn leaves. For example, in a 2001 study at NDSU, the temperature required to kill 1/2 of the seedlings was as low as 24°F. Older soybeans are less freeze tolerant.
Crook stage soybeans will be killed if the crook tissue below the cotyledons is killed. Likewise, frozen tissue below the cotyledons of any older soybean will result in death. However, if the frost only affects the tops of the young soybean, those with one or more intact cotyledons might recover from surviving axillary buds. In more advanced early season soybeans, regrowth may occur from one of the vegetative buds in the leaf axils. If leaf axils haven't been frozen, the frosted soybean should regrow from one of these growing points.
What are the risk factors? Cold air settles into low-lying areas, heavy residue tends to keep rising soil heat at or below the soil surface, and dry soils tend to lose heat more quickly than moist soils; these environments are more likely to produce freeze injured soybeans. Many other factors like cloud cover, wind, soil temperature, soybean stage, previous weather and genetics influence injury from frost. This often leads to very spotty injury across the landscape.
Assessing frost injury
Soybean frost injury appears as water-soaked lesions on the cotyledons, leaves, or hypocotyl that dry and turn brown after several days. Examples of injury can be seen in the Purdue photo gallery, Symptoms of low temperature injury to corn and soybean. Assessing frost damage should be delayed 3 to 5 days after the event to allow the soybean plants to show signs of new growth. Check for firm, healthy stems, cotyledons and growing points. By this time, it should be evident whether the soybeans are recovering or are dead. If a significant proportion of the population is dead, replanting may be justified. For more information on replanting decisions, see The soybean grower's guide for evaluating crop damage and replant decisions.
Managing frosted soybeans
Freeze injury is a traumatic physiological event for the plant and can slow development of soybeans for several days. Affected areas of the field with significant stem and cotyledon damage should be replanted if recovery remains slow. Areas with greatly reduced stands can be replanted by spiking in a full seeding rate alongside the old rows, when replanting can be accomplished by late May.
Consider delaying any post-emerge herbicide applications (another reason for pre-emergence herbicides) on frost-damaged beans until they have started to recover. An additional, unfortunate side-effect of frost injury is the increased difficulty in evaluating early season insect and disease damage. Fortunately, the injury does not necessarily increase soybean susceptibility to pests and pathogens. Although some may suggest otherwise, fungicide and insecticide applications will not help frost-damaged soybean seedlings.