Spring frost rarely damages soybean in Minnesota, as the last average frost dates usually occur before producers normally plant soybeans.
However, when soybean planting and emergence is well ahead of the five-year average, the crop becomes more vulnerable to early season frost events. If temperatures drop into the low 30s and upper 20s, emerged soybeans will likely experience some degree of frost injury.
In University of Minnesota planting date studies conducted in Lamberton since the 1980s, frost has rarely injured stands planted April and later. However, when injured, stands were not reduced to a level where replanting would be recommended. The injured stands still yielded as well as later, unfrosted dates.
Factors affecting survivability
To kill soybean tissue, temperatures usually need to be at or below 28 to 30 degrees Fahrenheit for several hours. However, an air temperature of 28 does not guarantee a soybean crop will freeze.
Compared to soybean, corn seedlings have a lower risk of death from freezing temperatures. This is because the corn plant’s growing point remains below ground until the V5 to V6 stage.
In soybean, the growing points are above ground and exposed after the cotyledons open. It’s fatal when all growing points freeze. However, compared to corn, soybean is better able to compensate for partial stand losses.
Newly emerged soybeans
The nearby warm soil protects newly emerged soybeans. Plus, small, emerging and cotyledon stage soybeans can tolerate freezing temperatures a bit better than older soybean or corn leaves.
For example, in a 2001 North Dakota State University study, the temperature required to kill half the seedlings was as low as 24 degrees. Older soybeans are less freeze-tolerant.
Crook stage soybeans
Crook stage soybeans will die if the crook tissue below the cotyledons dies. Likewise, frozen tissue below the cotyledons of any older soybean will result in death.
However, if the frost only affects the tops of young soybean, those with one or more intact cotyledons might recover with new growth from the 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.
The following environments are more likely to produce freeze-injured soybeans:
Low-lying areas, as cold air settles into these areas.
Heavy residue, which tends to keep rising soil heat at or below the soil surface.
Dry soils, which tend to lose heat more quickly than moist soils.
Other risk factors that influence frost injury include cloud cover, wind, soil temperature, soybean stage, previous weather and genetics influence. 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. Photo gallery: Symptoms of low temperature injury to corn and soybean.
Before assessing frost damage, wait three to five days to allow the soybean plants to show signs of new growth. Check for firm, healthy stems, cotyledons and growing points. At this point, you should be able to tell whether the soybeans are recovering or dead.
If a significant proportion of the population is dead, replanting may be justified. Guide to evaluating soybean damage and deciding whether to replant.
Managing frosted soybeans
Freeze injury is a traumatic physiological event for the plant and can slow soybean development for several days.
If recovery remains slow, replant field areas with significant stem and cotyledon damage. You can replant areas with greatly reduced stands by spiking in a full seeding rate alongside the old rows, if you can replant by late May.
Consider delaying any post-emergence herbicide applications (another reason for pre-emergence herbicides) on frost-damaged beans until they’ve started to recover.
Interaction with PPO herbicides
While it’s possible that frost interacts with PPO inhibitor herbicides, it’s unlikely that it’s widely prevalent in Minnesota.
An interaction of frost with soil-applied PPO herbicides is possible because cold temperatures slow the soybean’s rate of emergence through the herbicide-treated soil. The soybean’s slow growth limits its ability to metabolize the herbicide.
Example frost event
As an example, take the low temperature conditions Minnesota experienced on May 19, 2015. Injuries targeted soybean plants in the crook stage, which appeared to have been planted in early May (May 2 to 4 are frequently mentioned). Soybeans planted in early May were just cracking from the soil at the time of the low temperature conditions and were vulnerable to freeze damage.
Many reported affected fields were treated with soil-applied PPO herbicides that include Authority-, Valor-, or Sharpen-based products. This is not definitive proof of an interaction but their frequency of use does make sense.
This is because many growers saw the need to use these herbicide products to address the greater risk of herbicide-resistant weed management and wisely chose to initiate their weed management program with a preemergence herbicide.
How to tell the difference between frost damage and PPO injury
PPO inhibitors are contact herbicides that can cause necrotic lesions on the cotyledons and crook of the plant. Lesions are reddish to purplish to brownish in color. More advanced symptoms result in plant-girdling below the cotyledons.
Because frost injury is due to radiation of heat away from the plant, we’d expect more uniform necrosis of the crook of the plant. We would not expect to see discolored lesions on the cotyledons.
Another unfortunate side effect of frost injury is that it’s harder to evaluate early season insect and disease damage.
Fortunately, the injury does not necessarily make soybean more susceptible to pests and pathogens. Although some may suggest otherwise, fungicide and insecticide applications will not help frost-damaged soybean seedlings.
Reviewed in 2018