Source: Brian Olson, Northwest Area Crops and Soils Specialist, Kansas State University and Jim Shroyer, Extension Agronomy State Leader, Kansas State University
A hard freeze was predicted for the night of May 7 in parts of Kansas. The extent of that injury will depend on several factors.
There are a number of key factors in determining freeze damage: the stage of development of the wheat, the density of the stand and condition of the plants, the extent and duration of low temperatures, temperature gradients within the field, soil moisture and the wind speed.
The effect of these factors:
- Stage of development. Jointing wheat can usually tolerate temperatures in the mid to upper 20's with no significant injury. But, if temperatures fall into the low 20's or even lower for several hours, the lower stems, leaves, or developing head can sustain injury. If the leaves of tillers are yellowish when they emerge from the whorl, this indicates those tillers have been damaged. Existing leaves may also be damaged so severely that they turn bluish, then bleach out. This usually results in the field's having a "silage smell." At the boot stage, wheat can be injured if temperatures drop into the upper 20s for several hours. There can be damage to the flag leaf, stems, and developing head.
- Density of the stand and condition of the plants. If the stand is thick, that will tend to reduce the extent of freeze damage. On the other hand, well-fertilized wheat has often sustained more freeze injury than wheat that is not as well fertilized.
- Extent and duration of low temperatures. It usually takes more than just a momentary dip into the upper 20's to damage wheat in the jointing or boot stages of growth. Significant injury becomes much more likely if the temperatures in the damaging range last for two hours or longer.
- Temperature gradients within the field. Low spots in the field are almost always the first to have freeze injury. The coldest air tends to settle in the low areas, especially under calm wind conditions.
- Soil moisture. There is often less freeze injury at a given temperature when soils are wet than when dry. Wetter soils tend to radiate a little more warmth than dry soils.
- Wind speed. Windy conditions during the nighttime hours when temperatures reach their lows will increase the chance of injury.
- Tillage system. Wheat tends to suffer more freeze injury in no-till systems where residue covers the soil surface and keeps some of the soil heat from radiating into the plant canopy.
If a freeze occurs, the best thing producers can do for the first few days is simply walk the fields to observe lodging, crimped stems, and damaged leaves. Be patient. It will take several days of warm weather to accurately evaluate the extent of damage. After several days, producers should split open some stems and check the developing head. If the head is green or light greenish in color and seems firm, it is probably fine. If the head is yellowish and mushy, it may have freeze injury.
There are also a couple of early signs producers might have noticed right away.
- Silage smell. If a field of wheat is giving off the aroma of silage, that indicates that leaves have been damaged. Damaged leaves will likely turn black within a few days, then become bleached. If the flag leaf is killed, that tiller won’t produce much, if any, grain. Damage to lower leaves will not have such a drastic effect. Even if the flag leaf on the most advanced tillers is lost, less developed tillers can still come on and produce grain at this point in the season.
- Ice in the stems. If there was ice in the stems below the first node the morning of the freeze, those tillers will probably be damaged (although not always) and may not produce grain.
- Lodging. If the wheat lodged immediately after the freeze, that indicates stem damage. Later tillers may eventually cover the damaged tillers.
In the boot stage, the heads will go ahead and emerge even if they are severely freeze damaged, they added. However, that head may be partially damaged or completely dead. If the freeze damage is light to severe, heads may "back out of the boot."
Producers should remember that even if primary tillers are damaged, less-developed secondary tillers may be fine, they said. If there are enough secondary tillers that survive, these tillers should be able to compensate and keep yield losses to a minimum.