Determining extent of freeze injury in wheat
Freezing temperatures have affected most wheat fields with the Colorado High Plains Region. The good news is that the wheat crop is not nearly as far along in development as it was at this time last year due to the drought and cool temperatures this year, but any wheat at the jointing stage or later will probably lose some tillers where temperatures were in the teens for an extended time. Where only some of the tillers have been damaged, there is still plenty of time for undamaged tillers to compensate and minimize any potential yield loss, but that will depend on having adequate moisture, which is uncertain this year.
Important factors determining freeze damage
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 amount of residue on the soil surface, the extent and duration of low temperatures, temperature gradients within the field, soil moisture, and the wind speed.
Wheat that has greened up but hasn’t started to joint yet will probably suffer damage to the existing foliage, but the growing points will be protected by the soil and should escape injury. This wheat will have cosmetic damage to the leaves that will show up almost immediately. If new leaves emerging over the next few weeks are green, that will indicate that the growing points survived and the plants will still produce tillers. If the new leaves are yellow, the growing point of that particular tiller was killed by the freeze.
Jointing wheat can usually tolerate temperatures in the mid to upper 20s with no significant injury. But, if temperatures fall into the low 20s 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-black and have a water-soaked appearance, then bleach out. This usually results in the field’s having a “silage smell.”
If the stand is thick, that will tend to reduce the extent of freeze damage. On the other hand, well-fertilized succulent wheat has often sustained more freeze injury than wheat that is not as well fertilized. Thin stands, which are common this year, are at higher risk of injury because the air can penetrate the stand more easily. If the plants were wet before the freeze, this can result in a coat of ice on the plants that may protect the growing point to some extent. If temperatures get too low, however, the cold will go through the ice.
Many times we see more freeze damage in no-till fields because the residue acts as a blanket and doesn’t allow the heat from the soil to radiate up into the plant canopy.
Significant injury becomes much more likely if the temperatures in the damaging range last for two hours or longer.
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.
Windy conditions during the nighttime hours when temperatures reach their lows will increase the chance of injury.
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.
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. Do not take any immediate actions as a result of the freeze, such as destroying the field for recropping. 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.
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. When inspecting a field, flag the areas where you find ice in the stems, and tag individual tillers with suspected damage. Then come back to those areas after three days and see if the stems are crimped and damaged. If so, that tiller will probably not produce a head. If the tagged tillers continue to grow and put out nice green leaves, then they are fine. If not, they probably had injury. Freeze injury to the lower stem at this stage of growth can be a significant problem. This kind of damage may take a little longer to detect, but producers will eventually be able to find soft “lesions” on the lower stems. The damaged tillers may lodge. Even if they don’t lodge, however, the heads will produce little or no grain.
Lodging. If the wheat lodged immediately after the freeze, that indicates stem damage. Later tillers may eventually cover the damaged tillers.
If the main tillers are injured, secondary tillers may begin growing normally and fill out the stand. The wheat may look ragged because the main tillers are absent, but enough tillers may survive to produce good yields (if spring growing conditions are good). If both the main and secondary tillers are injured, the field may eventually have large areas that have a yellowish cast and reduced yield potential.
Tillers damaged during early jointing may stop growing, so the head will never emerge. In the boot stage, the heads will go ahead and emerge even if they are severely freeze damaged. 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.”
If the lower stems are damaged by freeze injury, the wheat plants will likely lodge at some point. Lodging could also be caused by other factors, however, so it will be important for producers to examine the lower stems on lodged plants to determine the cause. Plants may have simply leaned over due to environmental factors, such as a hard rain or high winds, after a freeze and will eventually come back up if the lower stem isn’t damaged.
More information on freeze damage to wheat is available in “Spring Freeze Injury to Kansas Wheat,” K-State Research and Extension publication C646, available at county and district Extension offices and on the web at http://www.ksre.ksu.edu/library/crpsl2/c646.pdf.