The 2017 Kansas wheat crop continues to develop fast with the warming temperatures and available moisture. Our estimates of crop development for different portions of the state are given in Figure 1.
The most advanced fields in far southeast corner of the state are between boot and flowering, and the majority of wheat in that region is already at or past flag leaf emergence (Fig. 1). The majority of the fields in south central Kansas as well as those fields that emerged last fall in southwest Kansas and the majority of fields in the central portion of the state, are past the second node and approaching flag leaf emergence. Northern Kansas and northwest Kansas are slightly behind in development as compared to the rest of the state, as would be expected based on temperature accumulation. The majority of the fields in those areas are now at the jointing growth stage or slightly past it.
Figure 1. Estimated wheat growth stage as of April 7, 2017. Growth stage is estimated for each county based on temperatures accumulated in the season and adjusted by observations of crop stage by K-State personnel. Local growth stage may vary with planting date and variety.
Risk of freeze injury from the temperatures in early morning hours of April 7
The risk of damage to wheat is a function of the stage of crop development, the minimum temperature, and the duration of time spent at potentially damaging temperatures.
Minimum temperatures during the morning of April 7th, 2017 reached anywhere from 27 to 41 degrees F across the state (Figure 2). Low temperatures were above 32 F in the majority of the state, although there were some areas in eastern and north central Kansas that got below freezing. Fields that are near boot, heading, or flowering stages of growth are at greater risk of freeze injury in the coming weeks. Generally, temperatures below 28-30°F can cause damage to booting or heading and flowering wheat, respectively.
Based on the minimum temperatures observed, and on the estimated wheat growth stages across Kansas, the southeast corner of the state was the region exposed to the greatest risk freeze damage potential. In southeast Kansas, the wheat is further along in development and many fields are past boot and reaching heading (Figure 1). That’s also the region with some of the lowest minimum temperatures across the state (Figure 2). These temperatures were measured at weather stations, and temperatures might actually have reached lower values in some fields or portions of fields, depending on the micro-climate and relief in each field. Below, we discuss some of the expectations and possible symptoms based on the growth stage of wheat following the cold temperatures from April 7.
Figure 2. Minimum temperatures during the 24-hour period of April 6-7, 2017. Source: Kansas Weather Data Library.
Wheat at boot stage – Temperatures needed to sustain damage when the wheat is at boot stage are generally about 28F or less. The temperatures measured at the weather stations in southeast Kansas (31 and 32 F) were most likely not low enough to cause damage to wheat at boot stage. Still, if temperatures reached the 28-degree threshold in particular fields due to individual micro-climate or field position on the landscape, some damage is possible to be sustained. Freeze injury symptoms to wheat at boot stage would include heads trapped inside the boot (heads cannot emerge from the whorl), which might result in twisted heads emerging from the side of the boot (Figure 3), yellow or white heads after head emergence indicating that heads were killed (Figure 3), or male floret sterility.
Figure 3. The twisted spike on the right was trapped in the boot and split out the side of the sheath. The awns of the middle spike were damaged while it was still in the boot stage. The spike on the left had partially emerged when freezing occurred so only the upper portion of the spike was damaged. Source: K-State Research and Extension publication C646.
Wheat between heading and flowering stages – These are the most sensitive stages to freeze injury. Temperature thresholds for cold damage at these stages is about 30F, which is still below the temperatures measured at the weather stations. Still, just as described for wheat at boot stage, individual field micro-climates might have resulted in lower temperatures and the crop might have sustained some freeze damage in some fields. The magnitude of the actual damage will depend on several factors and will be individual to each field. Symptoms include floret sterility (Figure 4), leaf desiccation or drying, bleaching of the awns (Figure 3), and damage to the lower stem. As wheat flowering begins in the middle of the head and proceeds to the top and bottom, a freeze event occurring during flowering might only partially damage the wheat heads. Thus, depending on flowering and freezing time, the center of the wheat head might be more affected than the top and bottom, or vice-versa, with grain developing normally in portions of the head not affected by the freeze damage.
Figure 4. After a freeze, affected anthers will become twisted and shriveled, although they will still hold their normal green color (left panel). After 3 to 5 days, affected anthers will become whitish-brown (right panel). Source: K-State Research and Extension publication C646.
For more information on symptoms of freeze to wheat, see “Spring Freeze Injury to Kansas Wheat”, K-State Research and Extension publication C646, available at: http://www.ksre.ksu.edu/bookstore/pubs/C646.pdf
Romulo Lollato, Wheat and Forages Specialist
Erick DeWolf, Extension Plant Pathologist