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Get Frosty? Don't Rush to Judgment Yet, Especially for Soybeans

The early verdict on the overnight cooldown in the northern Plains and Corn Belt: Some crops may need to be replanted, but on the whole, damage could have been much worse.

Expectations heading into the chilly overnight period were for frosting and freezing temperatures to stretch south as far as northern Nebraska in the Plains and down to around the Minnesota/Iowa border. However, the line of cold air did not extend as far south, with just a few isolated sub-32°F. temperatures in South Dakota reported Tuesday morning, and about the lower half of Minnesota staying above the freezing mark.

"Temperatures are once again quite cold in the northern Plains and northwest Midwest this morning, with lows well below freezing across much of North Dakota and northern Minnesota. This is likely resulting in some freeze damage to emerging corn in those areas, and replanting will be necessary," says MDA Weather Services senior ag meteorologist Don Keeney.

Early Tuesday morning, one farmer in southern Minnesota said he did, in fact, observe temperatures below freezing, though he says it's not likely to have been too widespread or severe to inflict much crop damage.

"My thermometer showed just under 30 degrees at 5 a.m., and at 7 a.m., the metal sheds and house shingles had white frost on them," says southern Minnesota farmer and Marketing Talk adviser Red Steele. "Didn't see any frost on the grass in the road ditches, though, so the warm ground probably prevented any frost damage to the young crops."


Farther north, more crop damage is likely imminent as more farmers get into the field to take stock of the damage from temperatures that dipped as low as 21 degrees in west-central North Dakota. Any spring-planted crops in the ground already in that state are likely candidates for replanting, says ag-?, a northern North Dakota farmer and Marketing Talk contributor.

"I am thinking any canola up will probably be toast after tonight; these low temperatures sure could hurt canola just trying to emerge. I would guess the ground will be a little stiff in the morning especially after last few cold days and having cold rain and snow yesterday. Any soybeans up will be toast and I guessing a lot of sugar beets could be hurt, too," he says. "I think wheat and barley will survive, but this won't do them any good."

If you think you've got damage to young corn and soybeans already in the field, make some time in the next few days to get into the field and scout before you make any decisions. At this juncture, if your temperatures didn't get below about 28 degrees for an extended period of time, your small corn is likely OK, says University of Minnesota Extension agronomist Seth Naeve.

"Temperatures at or below 28 to 30 degrees Fahrenheit for several hours are usually needed to kill soybean tissue. However, an air temperature of 28 degrees F does not guarantee that a soybean crop will freeze," he says. "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 soybeans, the growing points are above ground and are exposed after the cotyledons open. Freezing of all growing points is fatal. However, soybeans are better able to compensate for partial stand losses than is corn."

If you suspect potential freeze damage, think first about where temperatures may have slumped the most, taking into account both conditions on the ground and in the air, Naeve says.

"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," he says. "This often leads to very spotty injury across the landscape."

Getting a full picture of the damage your young soybean crop may have incurred from the frost or freezing temperatures will take a few days, so don't rush it, Naeve advises. Wait a few days so that any growth can resume, and you can see the visible differences between the living and dead plant tissue.

"Soybean frost injury appears as water-soaked lesions on the cotyledons, leaves, or hypocotyl that dry and turn brown after several days," he says. "Assessing frost damage should be delayed to three to five 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."

If you have some frost damage but want to ride it out with the seed that's already in the ground (some farmers say early frost damage can actually help a soybean crop in the long run), don't forget your fields may be a little slow and weak when it comes to things like applying herbicides later on.

"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," Naeve says. "Consider delaying any postemerge herbicide applications (another reason for preemergence 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."

Right now, don't get in too huge a hurry to get out and scout just yet, regardless of the time frame experts recommend for getting a full picture of frost and freeze damage, especially farther north. Though it's likely to stay warmer than last night, more freezing temps could be on the way to the northern Plains tonight.

"Temperatures should be cool again tomorrow morning, although not nearly as cold as this morning," Keeney says.

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