Assessing freeze injury to wheat
Wheat growers in western Nebraska are concerned about possible injury due to abnormally low temperatures that occurred several times over the past weekend. Winter wheat is in the late boot to early heading stage at the present time but has been subjected to a number of stressors this growing season. Dry planting conditions and continued drought have reduced crop tillering and vegetative growth, and scattered hail has occurred in a handful of wheat-growing regions. It is recommended that growers wait four to six days and then scout intensively to assess damage and reduction in yields.
Injury does not always occur at the low air temperatures. Instead, injury and reduction of yield is influenced by both duration of the low temperature and growth stage of the wheat. If the head is not fully exposed, it benefits from the microclimate in the wheat canopy, and temperatures there may not be as low as the air temperatures reported. Once the wheat is headed, it is subject to the air temperature and injury is more likely to be observed.
While the air temperature may drop for several hours, the actual microclimate of the crop may be several degrees warmer and create a “cushion” of protection to help moderate temperature swings.
Factors besides air temperature affecting the potential for damage include:
Crop condition — The current growth stage (not yet heading) and a fuller stand and dense canopy helps create a warmer microclimate.
Soil moisture — Generally, if the topsoil is moist, it helps mitigate against temperature changes. Several years ago, when there was a hard freeze in the Republican Valley in June, the cultivated corn with dry topsoil suffered significantly more damage than the non-cultivated corn.
Duration of the chill — Research indicates that temperatures would have to maintain freezing temperatures for more than two hours to damage wheat at its current growth stage.
The many factors influencing freeze injury to wheat — plant growth stage, plant moisture content and duration of exposure — often make it difficult to predict the extent of injury right after the freeze events. This is complicated further by differences in elevation and topography among wheat fields and between the fields and official weather stations across the state and even across a county. It is not unusual, for instance, for wheat growers to report markedly lower temperatures than are recorded at the nearest official weather station.
Areas that may have been particularly susceptible to the temperatures are low field areas, thin stands and areas with dry soil.
To check for head damage, wait four to six warm days and then go out and collect stems from several places across the field. Split 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. 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. It’s therefore also important to look at samples from primary and secondary tillers.
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 a week or more after the freezing temperatures. Leaf injury alone 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. 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. Sometimes we see stems that have ruptured during a freeze and the head turns white on the next hot dry day.
For more information visit https://cropwatch.unl.edu/assessing-freeze-injury-wheat.
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