Temperatures falling into the low- to mid-20 degrees F range for several hours in many western Kansas counties may have hurt the 2006 wheat crop, says Jim Shroyer, K-State Research and Extension agronomist. An earlier freeze April 18-19 injured wheat in north central Kansas.
"Most of the wheat, except that in far northwest Kansas, was in the early boot to heading stages of growth at the time of the freezes," Shroyer says. "Damage can be especially severe if the freeze occurred in the late boot to flowering stages, but wheat can also be injured by a hard freeze at other growth stages."
Don't be too quick to assess injury, Shroyer says: "The full extent of the damage, if any, won't be known for at least a week or so, until we've had time to see if the wheat resumes growth or sets kernels. I expect we'll see the most damage on wheat that was already under drought stress and wheat in low-lying areas. In some areas, the late-planted or no-till wheat planted after row crops seems to have been hurt the worst."
To assess damage, producers should look for limp, blue leaves and a silage smell within three to four days, depending on how warm temperatures are after the freeze. In more serious cases, producers may find split or collapsed stems. If there was ice in the stems, that sometimes causes damage if the stem ruptures. But if the stem is not ruptured, ice may not injure the wheat.
In northwest Kansas, wheat development ranged from the jointing stage in the far northwest counties to the heading stage near Hays at the time of the April 26 freeze, says Brian Olson, Northwest area Extension crops and coils specialist. Some of the wheat in the Colby area had ice in the stems.
Young, pre-jointing wheat may show some leaf burning, limp plants and collapsed stems from a hard freeze. With jointing wheat, the biggest concern is for damage to the growing point, which contains the developing grain head, Shroyer adds.
Cold damage at tillering will slow plant growth and reduce tiller numbers, but new leaf and tiller growth usually resumes with warmer weather.
"As a general rule, jointing wheat can only withstand a couple of hours of 20- to 24-degree F temperatures before damage occurs," he says. "Temperatures that cold at the jointing stage can kill the growing point, cause leaf burning and plant lesions, and split or bend lower portions of the tillers."
For wheat that was not yet in the boot stage at the time of the freeze, Shroyer advises producers to cut open a few main tillers after warmer weather returns, to look for grain heads that are limp and off-white colored. Healthy, undamaged plants have crisp translucent, green-colored grain heads. A yellow leaf emerging from the whorl is a sign that the growing point is dead. In time, those tillers will be covered up by the undamaged tillers so they will be difficult to see.
"In southwest Kansas, wheat was generally in the flagleaf emerging to the fully headed and flowering stage at the time of the April 26 freeze," reports Curtis Thompson, southwest area Extension crops and soils specialist. "We were cool enough to have potential freeze damage in a large area. Evidence of freeze damage from an April 19 freeze was also appearing in wheat across with varying levels of damage. This current freeze will only add insult to an already injured wheat crop from the present drought stress and wheat streak mosaic virus problems."
"Wheat is particularly vulnerable to damage from freezing weather from the late boot stage, as the head starts to emerge, through the flowering stage," Shroyer says. "Wheat starts to flower about five to seven days after heading. A freeze at that point can kill the (flower) anthers outright or damage them. It can cause sterility if it happens at the right point in the wheat's development. There can also be stem damage at this stage of growth."
Wheat that has headed and is beginning to flower is in the most vulnerable stage to be damaged by freezing temperatures because the floral structures - the anthers and stigma inside the floret - are very sensitive to temperatures below 32 degrees F, he says.
The best advice right now is to be patient and wait for warmer weather to see if symptoms appear, concluded all three agronomists. "Don't write off the crop until you have a chance to scout for cold damage," Shroyer says.
For more information about spring freeze damage to wheat, see K-State publication C-646, "Spring Freeze Injury to Kansas Wheat," or check the Web at http://www.oznet.ksu.edu/library/crpsl2/C646.pdf.