otentially lethal low temperatures (relative to corn and soybean) occurred in locations throughout Indiana during the past several nights. Low temperatures during the morning hours of April 17 were 26 F near Wanatah, 25 F near Columbia City, 26 F near Farmland, 26 F near Terre Haute, 28 F near West Lafayette, 29 F near Butlerville and 31 F near Oolitic. The Wednesday morning low temperatures were a little higher in northeastern Indiana and a little lower in southwestern Indiana. Columbia City, Dubois, Oolitic and Wanatah recorded low temperatures of 28 F with Greencastle, New Castle, and Wheatfield coming in at 25 F. West Lafayette and Butlerville had low temperatures of 27 F and all other stations reporting had temperatures of 29 F or above. Remember that official reporting stations measure temperatures at 4.5 feet above the soil surface. On a clear-still night, temperatures at the soil surface can be 2 to 5 degrees colder.
The temperatures themselves were not unusual for this time of year. What is unusual is that there are fields of corn and soybean already emerging due to some planting earlier in the month (albeit limited acreage statewide). Consequently, some farmers are wondering about the likelihood of having to replant fields that may be severely damaged by frost and/or lethal cold temperatures. Lethal cold temperature for corn is typically considered to be 28 F, while soybean can typically withstand somewhat cooler temperatures.
Early planted corn and soybean plants were examined at the Agronomy Research Center, near West Lafayette, at noon on Wednesday to determine the extent of the freeze damage. Soybean plants at the VE and VC stages of development were examined. Nearly all of the growing points were frozen and about 2/3 of the plants had frozen hypocotyls. The region of the hypocotyl just below the cotyledonary node had already lost turgor pressure and was becoming soft and shrunken. Within two or three days, these plants will shrivel to point that only the cotyledons will be identifiable. It was not possible to determine so soon after the damage the fate of those plants without frozen hypocotyls but with possibly frozen growing points.
Corn plants at the VE to V1 stages of development were severely damaged above ground, with leaves already drooped over and turning greenish-black. Such damaged leaves will slowly bleach to a straw color as the tissue dries out. As the frosted leaf tissue in the whorl dries, the whorl will often develop a constricted 'knot' that may restrict expansion of the undamaged whorl tissue later on. Usually, knotted corn plants will successfully recover as the expanding whorl tissue breaks these knots. Once in a great while, it may be necessary to mow a frosted corn field to cut off severely knotted leaf tissue. The key to deciding whether to mow or not is to allow the damaged field three to five days to show you how well it is recovering.
As with most early-season injuries to corn, the recovery of frosted corn depends greatly on whether the internal growing point region was damaged. The good news is that the growing point region of corn younger than growth stage V6 (six leaves with visible leaf collars, roughly knee-high) is below the soil surface and protected from aboveground frost damage. Inspection of the growing point regions of the plants at the Agronomy Research Center was inconclusive, although there was evidence of external tissue damage to the pseudo-stem (the rolled leaves that constitute the stem on such young plants). The uncertainty is due to whether the temperature at the growing point dropped to lethal levels.
The bottom line on diagnosing the severity of frost or low temperature injury to corn or soybean is that you generally need to wait three to five days after the weather event before you can accurately assess the extent of damage or recovery. Injury to either crop can look very serious the day after the event, but recovery may be possible if the growing points are not damaged. These three to five days will be better spent continuing to plant the remainder of your crop acres, assuming that most growers are not yet finished with corn and soybean planting.
After three to five days, surviving corn plants should be showing new leaf tissue expanding from the whorls, while dead corn plants will still look dead. Yield loss to frost damage in corn younger than V6 is related primarily to the degree of stand loss, not to the degree of leaf damage. Surviving soybean plants will show new leaves emerging from one or both nodes at the cotyledons, while dead plants will still look dead. If recovery is evident after three to five days, then replanting is not justified. If a significant proportion of the population is obviously dead after this same period of time, then replanting may be justified.