Winter wheat development is on schedule across Nebraska. This is a good thing since some level of freeze damage occurs almost every year after the wheat breaks dormancy. Wheat that responds too early to warming temperatures with growth is at greater risk of injury from freezing temperatures.
On the morning of March 20th, temperatures approached zero degrees across much of the wheat growing region in Nebraska. A low temperature of three degrees Fahrenheit was recorded in Sidney, NE. Fortunately, wheat in 2020 had been slow to emerge from dormancy due to cool temperatures and was estimated to be near a three on the Feekes growth scale.
The picture (Figure 1) shows how wheat looked on April 8, 2020 near Sidney, NE. New growth near the base of the plant has emerged. However, burned leaf tips remain from the earlier cold spell. The day after the freeze event, the exposed wheat leaves turned black and had a noticeable odor.
An approaching cold front will bring another round of freezing temperatures across most of Nebraska beginning Saturday night and will continue each morning for a few days. Given that most of Nebraska’s winter wheat is still in the tillering stage and has not yet jointed, the impact of the previous and coming freeze events are expected to be minor with little to no impact on the crop yield (Table 1). As the wheat further develops and begins jointing, it is still able to tolerate temperatures in the mid to upper 20s with no significant impact on yield apart from the cosmetic damage that may be observed.
Near the crop canopy and soil surface, a microclimate exists that may help alleviate temperature fluctuations. While the air temperature may drop and be low for several hours at a weather station, the actual microclimate of the crop may be several degrees warmer and create a "cushion" of protection to help moderate temperature swings. Factors besides air temperature affecting the potential for damage include:
- Crop condition — the advanced growth stage increases winter injury. On the other hand, winter wheat stands that are poor or thin have less of a microclimate and that makes the plants more susceptible to freeze injury.
- Soil moisture — generally, if the topsoil is moist, it helps limit soil temperature changes. Several years ago where there was a hard freeze in the Republican Valley in June, the cultivated corn with dry topsoil suffered significantly more damage than the non-cultivated corn.
- Duration of the chill — most charts and tables that reference freeze injury in wheat require the temperatures to maintain for a period of two hours duration. The wheat crop’s microclimate can reduce the amount of time the crop is exposed to damaging temperatures.
The many factors influencing freeze injury to wheat — plant growth stage, plant moisture content, and duration of exposure — often make it difficult to predict the extent of injury too quickly. This is complicated further by differences in elevation and topography among wheat fields and between the fields and official weather stations. It is not unusual, for instance, for wheat growers to report markedly lower temperatures than are recorded at the nearest official weather station. Areas that may have been particularly susceptible to the temperatures are low field areas, thin stands, and dry soil.
| Growth Stage | Approximate Injurious Temperature (Two Hours) | Primary Symptoms | Yield Effect |
|---|---|---|---|
| Tillering | 12°F | Leaf chlorosis; burning of leaf tips; silage odor; blue cast to fields | Slight to moderate |
| Jointing | 24°F | Death of growing point; leaf yellowing or burning; lesions, splitting or bending of lower stem; odor | Moderate to severe |
| Boot | 28°F | Floret sterility; head trapped in boot; damage to lower stem; leaf discoloration; odor | Moderate to severe |
| Heading | 30°F | Floret sterility; white awns or white heads; damage to lower stem; leaf discoloration | Severe |
| Flowering | 28°F | Floret sterility; white awns or white heads; damage to lower stem; leaf discoloration | Severe |
| Milk | 28°F | White awns or white heads; damage to lower stems; leaf discoloration; shrunken, roughened or discolored kernels | Moderate to severe |
| Dough | 28°F | Shriveled, discolored kernels; poor germination | Slight to moderate |
Winter wheat is most susceptible to freeze injury just after heading. This is when the head contains the greatest moisture content and has little protection from the plant. In 1981, Red Willow County (McCook) experienced low temperatures May 9-10 and suffered the greatest wheat loss for the state that year. Winter wheat had just headed when temperatures dropped to 28°F and slowly warmed to 37°F over four hours. Decatur County Kansas, which adjoins Red Willow County, suffered the greatest loss in that state.
Typically, it takes at least two hours at a given low temperature to cause injury. The longer the cold temperatures persist, the greater the chance of injury. As the wheat continues to progress and develop, it becomes more vulnerable to freeze events and also potential impacts on yield.
Remember it takes several warm days (a week or more, depending on temperatures) after a freeze before an accurate determination of injury can be made. Therefore, patience it required before deciding if the crop can be salvaged. Freeze damage can be observed by finding the developing head within the stem to check for irregularities.
Overall, remember that it is typical for winter wheat to experience freeze damage at some point each spring due to fluctuating weather patterns in Nebraska. However, wheat is a resilient crop and most varieties have been bred to handle the unique environment of Nebraska. Give the wheat a few days to recover before accessing the level of injury. If you suspect yield potential has been reduced, contact your crop insurance agent so the crop can be properly adjusted. Also, check with the Farm Service Agency before making any changes.
