By Jonathan Kleinjan
Low temperatures during the early morning hours of May 19 may have had detrimental effects on winter wheat in some areas of South Dakota. According to the National Ag Statistics service, only 1% of winter wheat in South Dakota was in the heading stage as of May 17. Injury will probably be most severe in early-maturing varieties or in areas that have shown drought stress. Dry growing conditions can limit canopy development, which increases susceptibility to low temperatures.
Winter wheat has steadily decreasing resistance to low temperatures after it begins rapid growth in the spring and advances toward maturity. Wheat is most sensitive to low temperatures during the reproductive period, which begins with pollination during the late boot stage. The temperatures required to cause injury throughout the winter wheat life cycle are shown in Figure 1. Both the temperature reached and the duration of the low temperature can be a factor in the amount of damage inflicted. Longer durations of freezing temperatures will cause more damage than a brief exposure.
Figure 1. Two-hour temperature exposures that cause freeze injury to winter wheat at various growth stages.
Local weather stations may not accurately represent the temperature levels for all surrounding areas. If a producer suspects damaging temperatures may have occurred, it is best to scout individual fields, beginning with low-lying areas, as cold air tends to flow to these areas. Other than a dark green color or “water-soaked” appearance of the foliage, symptoms may not show up for a week or more, depending on the growing conditions following the event. Warmer temperatures stimulate the decomposition of injured plant tissue and accelerate symptom development. Easily observed symptoms such as dead leaf tissue and white awns do not usually cause significant yield reductions by themselves. They may, however, be indicative of a more severe problem. A summary of the temperatures that cause injury, along with expected symptoms and effect on grain yield, is shown in Table 1.
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