Air temperatures dropped to an average of about 25-32 degrees on at least two nights over the last few days. Understandably, some wheat producers are concerned that these temperatures may have caused some damage to their crop. We will have to see what happens over the next few days. Based on information coming out of a Kansas State University publication http://www.bookstore.ksre.ksu.edu/pubs/c646.pdf, at our current growth stage, between Feekes 9 and 10, in northern counties and between heading and flowering in southern Ohio, the yield effect of frost can range from moderate to very severe if temperatures drop to 24 - 28° F for two or more hours. It all depends on the growth stage, how cold it was, and the length of time plants were exposed to the cold temperatures. The amount of damage is a function of both time of exposure and the temperature, but we are unsure how modern wheat varieties growing under conditions in Ohio will react. For example, 28 degrees for 30 minutes may be as bad as 31 degrees for a long period. We are currently in the second year of a three year study to evaluate wheat freeze damage in modern soft red winter wheat cultivars.
Freezing temperatures between boot and flowering may cause leaf discoloration, spikes to be trapped in boot, floret sterility, and damage to the lower stems. Bent heads may also be the result of rapid growth with warm temperatures followed by slower growth with low temperatures as we observed bent heads prior to freezing temperatures. The damage tends to be most severe with the greatest yield impact between heading and flowering. The head has some protection from cold temperatures until it emerges, but is easily damaged after emergence. Sterility and stem damage may lead to yield loss, however, since it is highly unlikely that all the plants in a field were at the same growth stage and were equally exposed to temperatures below 30° F, the overall damage may be minimal and restricted to low areas of the field. At most there may be some leaf tip burn on more sensitive varieties. Wheat is a winter crop and can tolerate cold temperatures.
To determine if your wheat has suffered freeze injury, walk the field and observe leaves and stems for discolorations and deformations. Between Feekes 6 and 8, leaves and stems on freeze-damaged plants become twisted and turn light green or yellow, with necrosis (darkening) of the leaf tips (Figure 1). These symptoms usually appear about two to three days after freezing. At Feekes 8, the emerging flag leaf appears yellow (Figure 2B) or necrotic (Figure 2C) instead of healthy green (Figure 2A), indicating that the growing point is damaged or killed. Secondary, unaffected tillers will develop and produce grain, but tillers with damaged growing points will stop growing and will not produce a head. The visual symptoms of frost injury to the heads appear as bleached glumes (and can be confused with scab or take-all). Additionally, freeze damaged florets appeared to be lighter green in color than unaffected florets on the heads. Remember, you cannot detect damaged fields from the roadway; you will need to walk the field and inspect individual heads to see if there is any damage.