Wheat development update and initial freeze damage assessment
Wheat stage of development
An estimated map of average wheat developmental stage by county -- created with the contribution of county Extension agents, area Extension agronomists, crop specialists, and calculations of accumulated growing degree days -- is shown in Figure 1. The actual wheat developmental stage is field-specific, varying within county by variety and planting date (among other management practices). Therefore, there may be differences between Figure 1 and actual wheat development from different individual fields.
Wheat is well advanced along the southern border of Kansas. The majority of the fields from Cherokee County in southeast Kansas to Comanche County in western south central Kansas are now past the second node, rapidly approaching flag leaf emergence or already showing flag leaves emerging out of the whorl.
The majority of the fields in the region immediately north of the south border counties is between jointing (Feekes 6) and second node, and some more advanced fields are approaching flag leaf emergence. Wheat development is not as advanced in northern and western regions of Kansas, where the majority of the crop is either just now at the point where the growing point is reaching ground level or is approaching jointing. A few isolated portions of northwest and north central Kansas have not yet reached jointing due to colder temperatures experienced in these regions.
Figure 1. Estimated wheat growth stage by county. Actual growth stage will vary within county, depending on variety and planting date.
Initial freeze damage assessment
Our two latest reports on potential of freeze injury to wheat in Kansas suggested that most of the freeze damage following March 19-20 and March 27-28 freezes would be expected to occur in south central and southwest regions of the state (see Agronomy eUpdate articles in issues 556 and 558). These regions had a combination of more advanced crop development (usually past jointing at the time of the freeze) and experienced air temperatures below 24 degrees F for a prolonged period of time.
Assessments of freeze damage performed by the K-State Extension team has shown consistent leaf tissue damage (Figure 2) throughout the region between Dickinson, Sumner, Meade, and Finney counties. In some cases, leaf damage was worse in heavy-residue no-till situations (Figure 3) where seed-soil contact at sowing may have been impaired by the heavy residue.
Additionally, varieties differ in their sensitivity to cold stress. Figure 4 shows that WB-Cedar, a variety that is released early from winter dormancy, showed more leaf injury symptoms to the freezing temperatures than WB-Grainfield, which greens up later. In most cases, though, the freeze damage to the leaf tissue did not affect the growing point. Leaf injury should be mostly cosmetic damage if the growing point was not affected.
In the southern-most counties of Kansas, bordering Oklahoma, the wheat sustained some level of tiller loss following the last two freeze events (Figure 5). Damaged growing points were observed at different incidence levels from Sumner through Meade counties. On average, the growing point was dead in 1 to 3 tillers in every about 15 tillers from Sumner through Barber Counties, and the incidence of dead tillers slightly increased west of Barber County. From Comanche County to Meade County, an average of about 3 to 5 of every 15 tillers were dead. In some cases, the newly emerging leaf was already dead, which is a good indicator of tiller loss (Figure 5). Very little tiller loss, if any, was found in Sedgwick, Reno, Barton, Barton, Hodgeman, or Finney counties.
It is still early try to predict any yield losses associated with freeze damage in the southern counties at this point, but most of the scouted fields should not sustain significant yield losses. It is important to keep in mind that the wheat plant naturally produces more tillers than the number of tillers the plant actually uses to produce grain yield. Therefore, some level of tiller loss is normal and occurs on a yearly basis. Some tiller loss may even benefit the wheat crop in years when the spring turns out dry and there is not enough moisture to sustain a lush canopy.
If weather conditions are cool and moist, favoring wheat recovery, remaining healthy tillers will be more than enough to sustain yields relative to a crop free of freeze injury. Unfortunately, the weather so far this spring has been drier than normal for most of western and northern Kansas, and the 10-day forecast does not predict substantial precipitation in the region. At this point, producers should probably be more concerned with the dry conditions experienced in great portion of Kansas than with possible yield losses from the latest two freeze events.
Another issue of concern for Kansas wheat producers at this point is the increasing incidence of stripe rust across the state. For more information about stripe rust conditions in Kansas and the need for foliar fungicides, please see the accompanying article in this issue of the Agronomy eUpdate: “Update on the risk for wheat stripe rust.”
Figure 2. Leaf tip burn caused by below-freezing temperatures. Photo by Romulo Lollato, K-State Research and Extension.
Figure 3. Effect of residue amount in the severity of freeze damage. Areas in the field where the wheat is greener (less leaf damage) had considerably less residue than the remaining portions of the field. Photos from Dickinson County by Romulo Lollato, K-State Research and Extension.
Figure 4. Effect of variety maturity in the severity of freeze damage. The greener variety in the left side of this picture (WB-Grainfield) was less vulnerable to below-freezing temperatures because of its later stage of growth than WB-Cedar (right side), which is showing apparent damage to the leaf tissue. Photos from Riley County by Romulo Lollato, K-State Research and Extension.
Figure 5. Typical symptoms of freeze damage to the newly emerging leaf (upper left) and growing point (upper center and right) versus healthy developing wheat heads (bottom photos). Photos from Meade, Harper, and Barber counties by Romulo Lollato, K-State Research and Extension.
Romulo Lollato, Wheat and Forages Specialist
Erick DeWolf, Extension Plant Pathologist