A good, thick stand of wheat, such as this one near McCook provides a warmer microclimate near the soil surface that reduces the potential for freeze injury. (Photos by Robert Klein)
Winter wheat development is ahead of normal in Nebraska. At the University of Nebraska–Lincoln West Central Research and Extension Center Dryland Farm south of North Platte, the average temperature since January 1, 2017, was 37.9° F. This compares to an average temperature of 34°F during the 1907-2017 period. In 2012, when winter wheat was as much as two weeks ahead of normal growth stage, the average temperature from January 1 to this point was 40.2°F. The growth stage this year is about a week ahead of normal.
Low temperature reports for the early morning hours on April 26 include 25° F for 10 hours in western Box Butte County. Table 1 shows lows for both April 26 and April 27 at a number of Nebraska sites.
Additional temperatures below freezing are expected across western Nebraska through Saturday morning. 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.
Given most of Nebraska’s winter wheat is in the jointing to pre-boot growth stage, the following points relative to low temperatures should be considered. Jointing wheat can usually tolerate temperatures in the mid to upper 20s with no significant injury. The lowest official readings were all in that range or above on April 26. However, if temperatures in some low-lying areas fell into the low 20s or even lower for several hours, the lower stems, leaves, or developing head may have sustained injury.
The ligule of the flag leaf is just visible in this wheat plant, corresponding to stage 9 on the Feekes Growth Scale. This field is near McCook and photographed April 27.
If the tillers are in this stage or earlier at the time of the freeze and the tillers are green and growing actively, the heads should be fine. If the head is killed, the tiller would not be green and actively growing. If the leaves coming out of the whorl are chlorotic, the head on that tiller is dead.
Winter wheat is most susceptible to freeze injury just after heading. This is when it contains the highest moisture content and does not have any protection from the plant. In 1981 Red Willow County (McCook) experienced low temperatures May 9-10 and suffered the highest 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 highest losses in that state.
Figure 1. Temperatures that cause freeze injury to winter wheat at different growth stages. Winter wheat rapidly loses hardiness during spring growth and is easily injured by late freezes. (Graph, which links to a larger version, is from EC132, Freeze Injury to Nebraska Wheat)
Table 1. Nebraska locations where temperatures dropped below 32°F either Wednesday, April 26, or Thursday, April 27, or both, based on information from the National Weather Service.
Location
4/26
4/27
Ainsworth
30°
26°
Albion
34°
27°
Alliance
27°
28°
Aurora
34°
25°
Beatrice
37°
31°
Broken Bow
32°
25°
Chadron
28°
28°
Columbus
36°
28°
Grand Island
34°
25°
Hastings
34°
27°
Hebron
37°
28°
Holdrege
32°
25°
Imperial
28°
33°
Kearney
34°
27°
Kimball
30°
36°
Lexington
33°
26°
McCook
33°
29°
Norfolk
33°
29°
North Platte
31°
31°
O’NeilL
32°
26°
Ord
32°
26°
Scottsbluff
31°
36°
Sidney
27°
36°
Thedford
31°
24°
Valentine
31°
21°
York
36°
27°
Additional information on assessing freeze injury follows. Figure 1 shows the temperatures that cause freeze injury to winter wheat at different growth stages. Table 2 lists the temperatures that cause injury to wheat at spring growth stages, symptoms and yield effect of spring freeze injury.
Table 2. Temperatures that cause injury to wheat at spring growth stages and symptoms and yield effect of spring freeze injury.
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
The following section provides information on how freeze injury occurs at various growth stages of winter wheat and what the implications are for the crop.
Figure 2. Burned and yellowing leaf tips are common spring freeze symptoms at the tillering stage.
Figure 3. More severe freeze damage causes the entire leaf to turn yellowish-white and the plants to be limp or flaccid. A silage odor may be detected after several days.
Winter wheat is protected by a microclimate. Moist soil cools and warms six times slower than dry soil. With the precipitation we received in many areas of the state, this reduces the temperature lows experienced by the wheat plant. Also, good stands and dense canopies reduce the lowering of temperatures to which the plant is exposed.
Most of the 2017 Nebraska winter wheat crop has good stands with the exception of portions of southwest Nebraska where dry conditions limited growth, resulting in thin stands.
In 2005, winter wheat fields experiencing the most freeze injury were those low on soil water (under moisture stress) and which had poor stands and/or a poor canopy.
It takes a number of warm days (a week or more depending on temperatures) after freezing to determine the condition of the winter wheat crop, so don’t make any quick decisions after a freeze. The main tillers may be killed or injured, but other tillers may survive and help compensate for some of the lost yield potential. See NebGuide G1429, Estimating Winter Wheat Grain Yields.
A number of factors determine the degree of freeze injury to a winter wheat crop. They include, as previously mentioned:
air temperature,
soil moisture,
stand,
canopy density, and
the winter wheat growth stage.
Other factors include the length of time of the low temperature, wind speed, and temperature gradient across the field. Not all parts of the field will have the same amount of damage. Cold air often settles in low spots in fields, although wheat canopies in low spots are frequently better as the result of increased soil moisture.
Take time to thoroughly evaluate the winter wheat condition before making any decision to spray out the crop and seed or plant to another crop. Always consider that winter wheat crop residue provides a good foundation for succeeding crops.
Before destroying any crop or making changes, check with your crop insurance provider, Farm Service Agency, and any other representatives who need to be informed.
Assessing Freeze Injury by Crop Growth Stage
Following is information from EC132, Freeze Injury to Nebraska Wheat, to help determine injury in the tillering and jointed stage to winter wheat.
Tillering Stage
Spring tillering of wheat in Nebraska usually begins in March and continues through mid-April. The growing point is just below the soil surface during this stage and is protected against injury. Most damage occurs to leaves, which become twisted and light green to yellow in color and are necrotic (“burned”) at the tip within one or two days after freezing (Figures 2 and 3). A strong odor of dehydrating vegetation may be present after several days.
Injury at this stage slows growth and may reduce tiller numbers, but growth of new leaves and tillers usually resumes with warmer temperatures.
Jointing Stage
The jointing stage is when the internodes (stem segments between joints or nodes) are elongating in the wheat stem and the embryonic head is moving up through the stems. This usually occurs from early April through early May. Leaves of freeze-injured plants show the same symptoms as the tillering stage (Figures 2 and 3), but the most serious injury occurs to the growing points (Figure 4).
The growing points can be located by splitting stems lengthwise with a sharp knife. A normal, uninjured growing point is bright yellow green and turgid; freeze injury causes it to become white or brown and water soaked in appearance (Figure 5). This injury can occur even in plants that appear otherwise normal because the growing point is more sensitive to cold than other plant parts. Stem growth stops immediately when the growing points are injured, but growth from later tillers may obscure damage. Partial injury at this stage may cause a mixture of normal tillers and late tillers and result in uneven maturity and some decrease in grain yield.
Figure 4. A yellow necrotic leaf emerging from the whorl indicates the growing point may be damaged.
Figures 5a and 5b. A healthy growing point has a crisp whitish-green appearance (left). A growing point that has been damaged loses its turgidity and greenish color within several days after a freeze. A hand lens will help detect subtle freeze damage symptoms (right).
Figure 6. Discoloring and roughening of the lower stem are symptoms of spring freeze damage.
Figure 7. The stem can split with severe freeze damage.
Injury to the lower stems in the form of discoloration, roughness, lesions, splitting, collapse of internodes, and enlargement of nodes frequently occurs at the jointing stage and the following stages after freezing (Figures 6 and 7). Injured plants often break over at the affected areas of the lower stem so that one or two internodes are parallel to the soil surface.
Stem injury does not appear to seriously interfere with the ability of wheat plants to take up nutrients from the soil and translocate them to the developing grain. Injured areas might become infected by microorganisms, however, which can cause further stem deterioration. Lodging, or falling over, of plants is the most serious problem following stem injury. Wind or hard rain will easily lodge the plants, decreasing grain yields and slowing harvest.
