May 4, 2011

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Temperatures in western Nebraska dipped into the low to mid 20s this past weekend (see maps). A good deal of the wheat in western Nebraska was jointed, that is the wheat head had started to move up the elongating stem and was no longer below the soil surface. 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).

Figure 1. After a hard freeze, wait for three or four warm days to assess wheat damage. Split the stem lengthwise with a sharp knife to view the head. Shown are a healthy growing point with a crisp, whitish green appearance (left) and one that has freeze injury. A growing point that has been damaged loses its turgidity and greenish color within several days after a freeze and may turn white or brown and appear water soaked. A hand lens may be needed to help detect subtle freeze damage symptoms.

 

  Figure 2 (left). Discoloring and roughening of the lower stem are symptoms of spring freeze damage.
Figure 3 (right). The stem can split with severe freeze damage.

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 24°F for two hours can result in death of the growing point (head); leaf yellowing and burning; and lesions, splitting, or bending of the lower stem. 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 the most likely to experience injury.

Air temperatures recorded at about 5 feet above the soil surface dipped below freezing for more than 8 hours on the night of April 30 and morning of May 1 at several locations in the Nebraska Panhandle. West of Alliance and northwest of Scottsbluff, temperatures were below 25°F for about two hours. Some crop injury is likely, especially in weak stands where the crop canopy did not protect to the lower portions of the plant. Temperatures in the southern Panhandle, Sidney and Brule, did not dip below 25°F, although they were below freezing for eight or more hours.

Assessing Damage

To check for head damage, wait for three or four warm days and then go out and split some plant stems lengthwise with a sharp knife. A normal, uninjured head is bright yellow green and turgid (firm), whereas freeze injury causes the head to become white or brown and water soaked in appearance (Figure 1). This injury can occur even in plants that appear otherwise normal because the head is more sensitive to cold than other plant parts.

Stem growth stops immediately when the head is injured, but growth from later tillers may obscure damage. Partial injury at this stage may cause a mixture of normal tillers and late tillers and result in uneven maturity and some decrease in grain yield.

Freezing temperatures at this stage of development 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. Leaf injury does not usually result in significant yield losses, as new leaf and tiller growth resumes with warmer 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 (Figures 2 and 3). 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), which can be found by visiting CropWatch — Wheat.

Drew Lyon, Extension Dryland Cropping Systems Specialist, Panhandle REC, Scottsbluff
Bob Klein, Extension Western Nebraska Crops Specialist, West Central REC, North Platte
Greg Kruger, Extension Cropping Systems Specialist, West Central REC, North Platte