Freeze Hits Wheat Just as Heads Emerging - May 16, 2012, UNL CropWatch

Freeze Hits Wheat Just as Heads Emerging - May 16, 2012, UNL CropWatch

May 16, 2012

Assessing Damage to Your Crop

Table 1. Daily lows for selected sites in western Nebraska May 12-14.  (Source: High Plains Regional Climate Center)
  Daily low (°F)
May 12
Daily low (°F)
May 13
Daily low (°F)
May 14
Agate 19  26 27
Alliance 24  29 30
Bushnell (15 m S) 32  26 28
Chadron 24  30 31
Gordon 26 34 36
Harrisburg (12 m WNW) 31  27 30
Kimball 36 30 32
Lodgepole (8 m N) 37  28 28
Scottsbluff 34 32 32
Sidney 36  29 32

Table 2. Duration of lows at five western Nebraska reporting stations May 11-13, 2012. (Source:  High Plains Regional Climate Center)
  Duration of
lows (hrs)
May 11
Duration of
lows (hrs)
May 12
Duration of
lows (hrs)
May 13
  <= 32°F <= 28°F <= 32°F <= 28°F <= 32°F <= 28°F
Alliance 1 0  5 0 5 2
Chadron 0 0  6 4 0 0
Kimball 0 0  0  0 3 0
Scottsbluff 0 0  0  0 0 0
Sidney 2 0  0 0 2  0
Photo - Freeze damaged stem Photo - Freeze damaged wheat stem
(Left) Discoloring and roughening of the lower stem are symptoms of spring freeze damage.(Right) The stem can split with severe freeze damage.

Temperatures in the northern Nebraska Panhandle dipped into the low to mid 20s this past weekend (Tables 1 and 2). Some reports of temperatures in the teens were made by growers in the area. A good deal of the wheat in the northen Panhandle was in the boot and some had already headed. The further up the stem the head goes, the more susceptible it is to freeze damage. Damage to the developing head can result in significant yield losses. Significant yield losses from spring freezes are most common after the head has emerged from the stem (boot).

The amount of damage observed will depend on the minimum temperature experienced by the plants in the field and the duration of freezing temperatures. A crop canopy temperature of 28°F for two hours can result in floret sterility, trapped heads, damage to the lower stem, leaf discoloration, and odor. A good crop canopy and moist soil will reduce the likelihood of crop injury. Low spots in fields or places where canopy cover is thin are most likely to experience injury.

Once the heads have emerged, an air temperature of 30°F for two hours can result in floret sterility, white awns or heads, damage to the lower stem, and leaf discoloration.

Assessing Damage

To check for head damage, wait for three or four warm days and then go out and evaluate plants in the field. Heads that have turned yellow or white have been killed. Frequently, only the male flower parts (anthers) are killed, which results in poor kernel set and reduced yields. Anthers are normally light green and firm when young. They become yellow about the time they are extruded from the florets after flowering. Freeze injury causes anthers to become white and shriveled; they may not be extruded from the floret.

Freezing temperatures also can cause leaf injury, which is typically expressed as twisted leaves and a change in leaf color from dark green to light green or yellow. Leaf tips may become necrotic or “burned” by freezing temperatures.

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. Injured plants may 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. Lodging, or falling over, of plants is the most serious problem following stem injury. Wind or hard rain near maturity will easily lodge the plants, decreasing grain yield and slowing harvest.

For more information on how to assess freeze injury in winter wheat, see Freeze Injury to Nebraska Wheat (EC132) in the CropWatch — Wheat section.

Drew Lyon
Extension Dryland Cropping Systems Specialist, Panhandle REC, Scottsbluff