Diagnosing damage to wheat from the snowfall and freezes of April 29 & May 1, 2017
Damage potential from the snowfall during April 29 – May 1 will be field specific and depend on the stage of crop development, amount of snowfall and consequent severity of stem damage, and the number of hours the crop was exposed to below freezing temperatures. While it is still early to try to estimate yield loss resulting from the snowfall, producers can start to look for the first signs of damage once the snow melts away by looking for broken stems. In two or three days, producers will be able to look for damage anthers by the cold temperatures in fields that were near or at anthesis. Other symptoms, such as white heads, might take another week or ten days to appear.
Here are more details for freeze injury by the most common stages of growth in the areas of the latest freeze:
In this stage, wheat can be injured if cold temperatures drop down into the mid to upper 20’s for several hours. Injury is more likely if this occurs repeatedly and if it is windy at night -- conditions experienced over the course of April 29 – May 1 when below-freezing temperatures were experienced during three consecutive nights for the western third of Kansas (please see accompanying article in this issue of the Agronomy eUpdate). In addition to the cold temperatures experienced, the heavy and wet snow fall also caused stem bending in wheat at boot stage (Fig. 1), which might further exacerbate yield loss. While yield loss from stem bending alone is not as severe during boot stage as in more advanced stages of development, the extent of yield loss would be worsened by the long period of time under below-freezing temperatures.
To detect injury to wheat at boot stage, producers should wait several days then split open some stems and look at the developing head. If the head is green or light greenish in color and seems firm, it is most likely going to be fine. If the head is yellowish and mushy, that’s a sign of freeze injury. Freeze injury at the boot stage causes a number of symptoms when the heads are enclosed in the sheaths of the flag leaves. Freezing may trap the spikes inside the boots so that they cannot emerge normally. When this happens, the spikes will remain in the boots, split out the sides of the boots, or emerge base-first from the boots.
Sometimes heads emerge normally from the boots after freezing, but remain yellow or even white instead of their usual green color. When this happens, all or part of the heads have been killed. Frequently, only the male parts (anthers) of the flowers die because they are more sensitive to low temperatures than the female parts. Since wheat is self-pollinated, sterility caused by freeze injury results in poor kernel set and low grain yield.
Usually, it is possible for some of the spikelets to be alive and a healthy dark green while other spikelets on the same head are damaged due to the difference in pollination timing within the wheat head. This is especially true following one single freezing event. If a spikelet flowers normally and the kernels on that spikelet develop normally, then the head is at least partially viable and will produce grain (unless it freezes again, of course). However, the three consecutive nights with below-freezing temperatures during the April 29 – May 1 period might decrease the chances that earlier of later pollinating spikelets survive.
Figure 1. Broken stems of wheat at boot stage as a consequence of the April 29 snowfall. Photo by Rick Horton, wheat producer in Wichita and Finney counties.
Awns beginning to appear
If the awns have begun to appear, there can be significant injury to the heads if temperatures reach about 30 degrees or lower for several hours. The heads may fully exert from the boot, but few, if any, of the spikelets may pollinate normally and fill grain. Damaged heads from a freeze at this stage of growth may seem green and firm at first glance, but the floral parts will be yellowish and mushy. Additionally, yield loss from stem damage (Fig. 2) will be greater at this stage than at boot stage. Many fields in the western portion of Kansas, particularly southwest, were at or past this stage of development.
Figure 2. Broken stems of wheat with awns beginning to appear. While the above picture was not resulting from snowfall, similar symptoms might be experienced due to the mechanical damage imposed by the heavy snow on the wheat canopy. Photo by Romulo Lollato, Extension Wheat Specialist with Kansas State University. Photo from a stem-bending simulation study conducted near Manhattan during 2015-16.
Several fields in the southwest portions of Kansas were flowering as of April 29 – May 1, especially in the southern counties (Meade Co. and surrounding area). In this stage of development, wheat is particularly vulnerable to damage from freezing weather. Temperatures of 30-32 degrees or lower for about 2 hours or more, can damage anthers.
If the wheat was in the flowering stage at the time of the freeze, you can determine if the anthers are damaged by examining them with a magnifying lens. Healthy anthers will first be lime green, then yellow (Figure 3). If they are damaged by a freeze, they will begin twisting within 2 to 3 days (Figure 4). Shortly afterward, they will begin to turn whitish or brown (Figure 5). The stigma in the florets may or may not also be damaged by a freeze. If the anthers are damaged by freeze, the flowers may fail to develop a kernel.
Wheat doesn’t flower all at the same time on the head. Flowering proceeds from florets near the center of wheat spikes to florets at the top and bottom of the spikes over a 3- to 5-day period. This small difference in flowering stage when freezing occurs can produce some odd-looking heads. The center or one or both ends of the spikes might be void of grain because those florets were at a sensitive stage when they were frozen (Figure 6). Grain might develop in other parts of the spikes, however, because flowering had not started or was already completed in those florets when the freeze occurred.
Unfortunately, temperatures below 32 degrees were experienced in a great portion of western Kansas consecutively during the April 29 – May 1 period, and temperatures were below freezing for more than 10 hours each day (please see accompanying article in this issue of the Agronomy eUpdate on actual minimum temperatures and duration of below-freezing temperatures). The combination of low minimum temperatures, long exposure to temperatures below 32F, and wheat at flowering indicates that severe freeze damage may have been sustained in the southwest region. Additionally, if some of the anthers do survive the cold spell, stem breaking at flowering wheat will induce more severe yield losses than in wheat at earlier stages of development.
Figure 3. Healthy wheat anthers are trilobed, light green and turgid before pollen is shed. Each wheat floret contains three anthers. Healthy stigmas are white and have a feathery appearance. Photos from Spring Freeze Injury to Kansas Wheat, K-State Research and Extension publication C646.
Figure 4. Anthers become twisted and shriveled, yet they are still their normal color within 24 to 48 hours after a freeze. A hand lens is necessary to detect these symptoms.
Figure 5. If damaged, anthers become white after 3 to 5 days and eventually turn whitish-brown. The anthers will not shed pollen or extrude from the florets.
Figure 6. Damage may occur in different areas of the spike because flowering, which is the most sensitive stage to freeze, does not occur at the same time in all florets.
If you are unsure whether there has been freeze damage to the anthers, wait several days and determine whether kernels are developing normally. A week after flowering, kernels should be well-formed up and down the head under normal conditions.
In addition to this, be watching for any freeze damage to lower stems. If the damage is severe enough, the plants will eventually lodge.
The comments above are general guidelines. Actual damage, if any, will not become apparent until temperatures have warmed back up for several days and growth has resumed.
For more information on freeze damage to wheat, see Spring Freeze Injury to Kansas Wheat, K-State Research and Extension publication C646, at: http://www.ksre.ksu.edu/bookstore/pubs/c646.pdf
Access to real-time Kansas Mesonet weather data is available here: mesonet.ksu.edu
Romulo Lollato, Wheat Production Specialist
Erick DeWolf, Wheat Extension Pathologist
Mary Knapp, Weather Data Library
Chip Redmond, Kansas Mesonet