With low temperatures plummeting to the teens over Easter weekend, farmers are nervous about the condition of the 2007 wheat crop.
"Normally, a hard freeze in early April is not a problem for wheat," says Kansas
State University agronomist Jim Shroyer. "This year, however, the crop is about two weeks ahead of normal development in all but northwest Kansas."
That means that some fields may be more vulnerable to freeze damage than they normally would be.
"It takes several days of warm weather to evaluate the condition of the crop and its yield potential. Even if some of the main tillers are injured or killed, producers should wait to see if enough other tillers survived to compensate for the lost yield potential," he says. "Patience is key. If it turns out after a week or so of warm weather that the wheat crop is, in fact, severely damaged, there still should be time left in April to destroy it, if necessary, and plant corn. There will be even more time to plant grain sorghum."
Farmers should keep three factors in mind as they determine whether their crop may be freeze-damaged: its stage of development, the temperatures, and the wind speed.
Whether actual freeze injury took place depends not only on how low the temperature fell, but also how long the readings stayed that low. Along with wind speed, it also depends on the temperatures gradients in a field, canopy density, and other microclimate factors.
Shroyer broke down the possibilities for freeze injury by stage of wheat growth:
* 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 some injury. Windy conditions during the nighttime hours when temperatures reach their lows will increase the chance of injury.
"Right now, soils are warm and moist, and will radiate heat into the canopy. This could help protect wheat from freeze injury, unless conditions are windy," Shroyer says. "But, when and how will producers know if their wheat in the jointing stage has been injured?
"If temperatures warm up rapidly after Easter, damage may be apparent by the end of that week. If temperatures stay cool for another week or two, it will take longer to notice any injury."
Injury symptoms will vary, he cautions:
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 the leaves of tillers are yellowish when they emerge from the whorl, this indicates those tillers have been damaged. 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. Tillers damaged later in the jointing stage may still head out, but the head may be partially or entirely blank.
Leaves in the whorl aren't the only ones at risk. For jointing wheat, a hard freeze can also damage the existing leaves so severely that they turn bluish, then bleach out. This usually results in the field's having a "silage smell."
When freezing damages the lower stems, the plants probably will lodge at some point.
"Other factors also can cause lodging, however, so it will be important for producers to examine the lower stems on lodged plants to determine the cause," Shroyer says. "For example, plants may have simply leaned over after a freeze, due to such environmental factors as a hard rain or high winds, so will eventually come back up if the lower stem isn't damaged."
* Boot stage wheat can be injured if temperatures drop into the mid to upper 20s for several hours. Again, injury is more likely if this occurs repeatedly and if it is windy at night.
To detect injury after a freeze, producers should wait several days, then 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.
Freeze injury to the lower stem at this stage of growth can also 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 not produce grain.
* Awns beginning to appear is a stage that can suffer significant injury to the heads if temperatures reach about 30 degrees or lower for several hours. Tillers may finish shooting up the head, but few, if any, of the spikelets will be likely to pollinate normally and fill grain. Damaged heads from a freeze at this stage of growth may seem green and firm at first glance, but the floral parts will be yellowish and mushy.
"But all may not be lost if the heads on the main tillers are damaged," Shroyer says. "If there are enough secondary tillers at an earlier stage of development when the freeze occurs, these tillers should be able to compensate and keep yield losses to a minimum – assuming temperatures did not get into the teens. In some past spring freeze events, secondary tillers have compensated so well, due to good spring growing conditions, the result was good yields."
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: www.oznet.ksu.edu/library/crpsl2/c646.pdf.