Temperatures are predicted to drop well below freezing tonight (14 April 2014), and there is high potential for freeze injury to Oklahoma wheat. I have posted an excerpt from K-State Extension Publication C-646 Spring Freeze Injury to Kansas Wheat along with a map which provides some rule of thumb temperature thresholds for the current Oklahoma wheat crop. Keep in mind these temperature thresholds are not exact, and temperatures closer to the soil surface might be higher than those reported by weather stations one meter above the soil surface, especially if moisture is present. Wheat in Oklahoma ranges from just past jointing to late boot and if forecasts are correct we will drop below the threshold temperatures where injury might be observed. The extent of injury will depend on how cold we get and how long we stay there. We can lose a few main tillers at this stage and still recover. Given our limited moisture and limited time prior to harvest, though, it is not likely that we will recover from a complete loss of tillers as we have after some March freezes in the past.

Excerpt from KSTATE publication C-646 Spring Freeze Injury to Kansas Wheat

Approximate temperature thresholds for freeze injury to Oklahoma wheat on 04/14/2014

Freeze injury is not clearly identifiable until 7 – 10 days after the freeze event. So, the best advice for a wheat farmer after a freeze event is to find something else to do for a week or two and then check your crop. I have provided some pictures below with typical injury symptoms and rules of thumb regarding the extent of the injury. Fields should be checked at several random locations by splitting 10 – 20 stems at each location and looking for injury. Don’t focus solely on the large stems. Split a random sampling and determine the percent damage. A good reference for evaluating freeze injury to wheat is K-State Extension Publication C-646 Spring Freeze Injury to Kansas Wheat (access online by clicking here).

This is a healthy wheat head at approximately growth stage 6 – 7. Note the light green color and healthy, turgid appearance.

Freeze injury just after jointing. Note the pale, milky color of the head. Freeze injury to wheat heads at this growth stage is all or none, so this head is a complete loss.

Leaf tip burn from freeze injury will have no impact on final grain yield

Yellowing is a common reaction to light freeze injury. Wheat will recover quickly from this injury.

Severe freeze injury at or just after jointing can turn the entire plant brown and fields can exude an odor similar to fermenting silage. If conditions are favorable, the plant can produce new tillers (as shown here) and make a partial recovery. It will take a few weeks after a freeze event to determine if the plant will recover from this type of injury

It is common for sub-lethal freeze injury to result in bent or weak lower nodes. These plants might look fine, but will lodge during grain fill.