Amanda de Oliveira Silva, Small Grains Extension Specialist
It has been almost one month since the freeze event on April 21, and we are now obtaining a better picture of potential freeze damage on wheat fields across Oklahoma. As I have indicated, the extent of the freeze damage will depend on several factors, including the growth stage of the plants, how low the temperature will get, and how long it stays at those cold temperatures. Wheat growth stage ranged from flag leaf emergence to heads starting to or fully emerged when the freeze occurred, and number of hours and temperature varied across the state (Figure 1).
Traveling around the state for plot tours these past weeks, I have seen and heard about damage ranging from minimal to quite severe. Some fields seem to be fine with only scattered damaged heads and the grain appears to be filling as expected. Other fields however, show much more significant damage with discolored and sterile heads.
At the plot tour at Chickasha on April 30, there was a mix of freeze and hail damage with several abnormally growing heads (due to head trapping). Anthers seemed to be fine at that point (Figure 2).
On May 7, I checked some wheat fields around Sentinel with Gary Strickland (Jackson County Extension Educator and SW Regional Agronomist) and we observed almost no freeze damage with a few heads in the field showing a pale color and partial sterility (Figure 3). I have observed this symptom commonly in fields I have visited, and many producers have described this to me as well. Although common, it typically has been found at a low incidence.
We observed a few spots with freeze damage while at Alva on May 12, especially in low spots of the field and on the tops of terraces. Again, relatively few spots in the field showed damage. The wheat was looking good at Alva, but really needing a rain (Figure 4).
The most severe freeze damage that I have seen so far was yesterday near Morris in eastern OK. The heads were green but there was no grain present (i.e. sterile). Damaged heads had glumes with a chocolate discoloration, which is similar to the discoloration caused by the bacterial disease called black chaff. In some cases, Dr. Hunger and I felt these discolored heads were the result of this bacterial disease but that the majority of the heads showing these symptoms were the result of freeze damage (Figure 5).
At the plot tour at El Reno today, we also observed a few varieties with pale colored and “empty” heads due to the freeze (Figure 6). We observed more freeze damage on wheat that was planted earlier and grazed as compared to the grain-only (ungrazed) wheat in that same field. Another noteworthy item is related to my earlier observation that the February freeze hurt some of the varieties in the dual-purpose plots by severely reducing tillering that would cause a loss of stand. That observation in those varieties was confirmed as the stand loss was quite evident today.
In conclusion, the freeze damage I am seeing is variable within and across fields, but overall I would say is minimal in most of the state. However, continue to keep scouting as it will now be easier to identify freeze damage.
Amanda de Oliveira Silva, Small Grains Extension Specialist
Temperatures dropped below freezing in the past hours in northwestern Oklahoma and Panhandle (Figures 1 and 2), and freezing temperatures are expected across most of the state tomorrow morning (April 21) (Figure 3). There is a potential for freeze injury to Oklahoma wheat. The extent of that will depend on several factors, including the growth stage of the plants, how low the temperature will get, and how long it stays at those cold temperatures.
What are the temperatures that can damage the wheat plants?
This will depend on the growth stage of the plants. Anecdotal evidence suggests varietal differences in resistance to spring freeze injury, but this is likely due to differences in plant growth stages when the freeze event occurred. Earlier maturing varieties are more likely to be injured from these recent freeze events than later maturing varieties because they are likely more advanced. The susceptibility of wheat plants to freeze injury steadily increases as we progress through the spring from jointing to heading and flowering. Figure 4 below is a general guide to the minimum temperature threshold and its impact on yield. Keep in mind these temperature thresholds are not exact but provide a decent rule of thumb. 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. It is difficult to have exact numbers because each freeze event is unique. While a field at the jointing stage could spend two hours at 24 F, it is possible that the same amount of injury could occur at a 28 F temperature that was sustained for a more extended period.
How long should I wait to assess the injury?
Another important thing to keep in mind is that we need to be patient before assessing freeze injury. The extent of a significant freeze event may not be apparent 1 or 2 days after. If warm temperatures return quickly, you should wait about 5-7 days before determining the injury. Suppose temperatures remain cool after the freeze event. In that case, it may take 10-14 days before the extent of the injury can be fully assessed.
What are some freeze injury symptoms to look for?
A typical freeze injury symptom is leaf tips turning yellow and necrotic (Figure 5). This is very often just cosmetic and will not hurt yield in the end. More severe damage can result in the entire leaf turning yellow to white, and the plants become flaccid (Figure 6). You may even notice a “silage” smell after several days.
The most important plant part to check is the developing head (i.e., growing point)
This will be important for areas of the state with fields with plants at flag leaf emergence stage. Sometimes we can see what look like healthy plants overall, but the developing head has been damaged or killed. To get a look at the developing head, you can slice the stem open lengthways. A healthy growing point will have a crisp, whitish-green appearance and be turgid (Figure 7). Often, you can lightly flick the head, and if it bounces back and does not break, it is still healthy. If it is mushy, limp, and breaks or parts of it break off when you lightly flick it, it has been compromised. It may also have a brown color (Figure 8, right). Another indication that the growing point has been compromised is that the next emerging leaf is necrotic, and the lower stems are discolored, with lesions and enlarged nodes.
Freezing at the boot stage may cause the head to be trapped by the sheaths of the flag leaf resulting in issues with head emergence (Figure 9). The whitish tips of the awns indicate that it was exposed to freezing temperatures and that the flower parts could have been compromised. Freeze during the flowering stage may result in flower sterility via the death of the anthers (male organ) and consequently poor kernel set and grain yield losses (Figure 10).
Also, the percent of damaged heads may not translate into percent yield loss. For example, there is still an opportunity for wheat to produce additional tillers and/or retain secondary tillers at the jointing stage. Whether or not these tillers can compensate for larger tillers that were lost due to freeze will depend on the subsequent weather. If conditions are favorable, there is a chance for late-emerging tillers to have a shot at producing grain. If the wheat is more advanced (which is the case for most Oklahoma wheat), it will be more challenging to make this type of recovery.
A few points to consider:
Every freeze event is unique and freeze injury needs to be checked on a field by field basis – the temperatures and time durations we use regarding freeze injury are rules of thumb and are not exact. I have seen instances where conventional wisdom would indicate complete crop loss, and we skate through with minimal damage.
The amount of injury observed will depend on – the growth stage of the plants, how low the temperature got, and how long it stayed at those cold temperatures. Other factors such as elevation, residue cover, and moisture can influence the observed temperature within the canopy as well. Because of the number of influential factors, it is important to check each field. It is possible to have variability in injury symptoms among fields and even within fields.
It will take a few days to see how bad things are – Symptoms may start to appear mid-next week and will likely be identifiable by the end of the following week. Healthy wheat heads will remain turgid with a green color. Damaged wheat heads will be bleached, yellow, or brown and will easily break when pushed against.
Temperatures over the weekend were cold enough to cause injury to the Oklahoma wheat crop. As shown in the figure below from the Oklahoma Mesonet many areas of Oklahoma spent several hours below 28F. While temperatures in the wheat canopy might have remained slightly higher than reported air temperatures, they were still probably low enough to result in significant injury to wheat.
Hours spent below 28F March 18 – 20
A few points I would encourage everyone to consider:
Every freeze event is unique – the temperatures and time durations we use regarding freeze injury are rules of thumb and are not exact. I have seen instances where conventional wisdom would indicate complete crop loss and we skate through with minimal damage. It will take a few days to see how bad things are – Symptoms may start to appear later this week and will likely be clearly identifiable by the end of this week. Healthy wheat heads will remain turgid with a green color. Damaged wheat heads will be bleached, yellow, or brown and will easily break when pushed against. I anticipate that we will not have any partial “blanking” of wheat heads and that most wheat heads will either be okay or a complete loss. This post from last year has some pictures showing tell tale signs of freeze injury. The linked post also serves as a reminder that while freeze is the concern of the day, the potential worsening of drought conditions in NW Oklahoma has the potential to do far more damage.
% damaged heads might not = % yield loss – It is still relatively early in the growing season and there is still opportunity for smaller (two nodes or less) wheat to produce additional tillers and/or retain secondary tillers. Whether or not these tillers are able to compensate for larger tillers that were lost due to freeze will depend on moisture and weather. IF (and that is a big if) weather conditions remain favorable, late emerging tillers in central and northern Oklahoma might still have a shot at producing grain. It will be tougher for more advanced wheat in southern Oklahoma to make this type of recovery.
Dr. Hunger traveled southwest Oklahoma this week, so I made a trip out Hwy. 60 yesterday to evaluate freeze injury and assess the overall condition of the wheat crop in northwestern and north central Oklahoma. Last week’s warm temperatures and wind have taken their toll on wheat in Kay, Grant, and eastern Garfield Counties. It is not too late for rain to save a partial wheat crop in these areas, but the “full yield potential” ship sailed long ago. Wheat sown behind summer crops is the hardest hit, and wheat in these fields could best be described as yellow and thin. If the weather turned and we received rain in the next week, I would predict that yield potential in these fields would still only be around the 15 bushel mark. Without rain, subtract around 15 bushels. Wheat planted behind summer fallow has held on a little longer, but is clearly showing the signs of extreme drought stress. If we receive rain in the next week (and continue to see rain) these fields could still make 20 – 30 bushels per acre. In the absence of rain in the near future, they will be 10 bushels per acre or less.
Wheat in the Lamont test plot was approximately GS 7 – 8. Flag leaves were rolled and plants were starting to abort tillers.
In addition to drought stress, we found freeze injury and greenbugs at Lamont. I was a little surprised to find freeze injury and even more surprised to find the greatest injury in the later-maturing varieties. We split several stems of early varieties such as Ruby Lee and Gallagher and did not find any injury. These varieties would have likely been at approximately GS 7 – 8 when the freeze occurred. We found significant injury in later-maturing varieties such as Endurance, but these varieties were likely only GS 6 – 7 when the freeze occurred. Conventional wisdom regarding freeze injury is that the more advanced the variety, the greater the likelihood of freeze injury. After seeing the same phenomenon last year (i.e. the greatest injury in later maturing varieties) I am changing my thinking on freeze injury and now say that all bets are off when it comes to freeze injury in drought stressed wheat.
Freeze injury was greatest in late-maturing varieties at Lamont.
Overall wheat condition started to improve around Nash and Jet, I would say that much of the wheat in this area is CURRENTLY in fair to good condition. I emphasize the currently in the previous sentence, as the only difference between wheat in the Cherokee area and wheat to the east was about one week’s worth of moisture. Some terrace ridges had already started turning blue and moisture was starting to run out. Without rain wheat in this area will rapidly deteriorate from good to poor. One consistent theme throughout the day was greenbugs. Many sites had evidence of parasitic wasp activity (i.e. aphid mummies), but the presence or absence of parasitic wasp activity varied field by field. Dr. Royer has indicated that greenbugs still need to be controlled in drought stressed wheat. If parasitic wasps are active, the best decision is to let them do the aphid killing for you. If no mummies are present, then insecticide control could be justified. The only sure way to make this determination is to use the glance-n-go sampling system.
Greenbugs were alive and well at Lamont
Parasitic wasps were keeping greenbug populations under control in this field
Active and parasitized greenbugs on the same plant
Similar to Lamont, we found freeze injury in the Cherokee and Helena areas. Many of the worst looking fields (extensive leaf burn) had only superficial injury and should recover if moisture allows. Conversely, some plants that showed no outward signs of freeze injury had injured heads within. Most fields I surveyed had less than 10% injury, but one field was a complete loss. On the surface the 10% injury field and 100% loss field looked the same, so I cannot over stress the importance of splitting stems. I have received a few additional reports of freeze injury from Kay County this morning, so it is important for producers throughout northern Oklahoma to evaluate their wheat on a field by field basis.
Plants that look healthy on the exterior could contain damaged wheat heads
A closeup of the damaged wheat head from the picture above
Although freeze injury to plant tissue in this field was severe, the wheat heads were mostly left unscathed
A closeup of a head from the freeze-injured wheat shown above. Although tissue damage is severe, the growing point and wheat head are still viable
A final note on freeze injury. Freeze injury appeared to be worst in no-till fields and in areas where residue was heaviest. Based on my observations, this was not due to winterkill or poor seed to soil contact. My best explanation is that the lack of soil cover in conventional till fields allowed stored heat to radiate from the soil surface and slightly warm the crop canopy. The insulating effect of residue in no-till fields did not allow radiant heating to occur. Given the pattern of freeze injury in fields with varying degrees of residue across the field, I feel pretty confident in this analysis of what occurred.
Please use the comment section to share pictures or descriptions of wheat in your area.
I have been out in much of the state with wheat field days this week and wanted to share a few observations. Drought conditions are worsening in most wheat producing areas of the state and yield potential is declining fairly rapidly. An area roughly extending from Chickasha to Enid along highway 81 still has some potential, provided that that we receive rain soon. The same can be said for a few small pockets of wheat that received rain earlier this spring in Alfalfa, Grant, and Kay counties. With temperatures predicted to climb to the upper 90’s next week, however, the potential in these areas could decline rapidly. Most other areas of western Oklahoma have very limited or no yield potential remaining.
The effects of the April 15th freeze are still showing up in the Oklahoma wheat crop. We have several fields with lots of tillers but few heads. Most wheat south of Hwy 51 in Oklahoma is as fully headed as it is going to get. That is, the heads that were not killed by the freeze are fully emerged. Tillers that still look yellow or even green but are not headed out most likely have dead wheat heads inside. These can easily be identified by splitting the stem and examining the wheat head as shown in the pictures below.
Freeze injured wheat can still have green flag leaves but dead wheat heads. This tiller will eventually turn brown.
Freeze injured wheat heads will be brown. As indicated in the post above, wheat tillers that are not headed out south of Hwy 51 in Oklahoma likely has this type of freeze injury.
Injury symptoms from the April 14th freeze are now showing in the Oklahoma wheat crop. Robert Calhoun and Matt Knori madeTh a trip through north central Oklahoma yesterday splitting stems (some pictures are posted below). Their first stop was our wheat variety trials at Marshall Oklahoma where they found 20% injury in our grazed wheat plots and 51% injury in our non-grazed wheat plots. While planting date and management system clearly affected the level of injury, variety did not seem to have much effect.
Next stop was a grazed field north of Hennessy where they found little injury. The same was true for a field in the Waukomis area and the Lahoma variety trial where they found less than 5% injury. Not too far to the north, however, our Lamont variety trial sustained over 80% injury. I received similar reports of severe wheat freeze injury from Curtis Vap in the Blackwell area.
Late last week our team traveled to Apache to apply fungicides to the wheat variety trial, but never unloaded the sprayer. Freeze injury was severe and clearly visible without splitting stems. Our wheat at the Chickasha research station had little to no damage, and most wheat in the area seemed to dodge the freeze bullet. I will make a bigger loop into southwest Oklahoma later this week and report findings.
Injury symptoms should now be easily identifiable and growers can assess damage to individual fields. I recommend splitting 10 stems at four or five locations throughout the field and determining % injury from these numbers. If injury is extremely variable, increase sample size. While it is fairly easy to determine the extent of injury on individual fields, the hit or miss nature of freeze injury this year makes it difficult to estimate the total impact on the Oklahoma wheat crop as a whole.
The drought has severely limited resilience in our crop and we are entering late April, so I do not anticipate there will be much of a recovery or rebound in fields that were severely damaged. It is important to note that 50% injury does not necessarily mean 50% yield loss. In most cases the actual yield loss will be less than the % injury. So, it is reasonable to expect that 50% injury might only result in a 35 or 40% yield loss. Of course, this depends on several factors such as soil moisture and temperature.
Finally, a word on foliar disease and fungicide application. I would make decisions regarding fungicide application based on variety, current disease reports, and the yield potential of the crop as it stands right now. Our long-term data shows that fungicides protect yield potential to the tune of about 10%. Of course individual variety responses can deviate from this number but 10% is a good rule of thumb. I do not, however, recommend applying a fungicide to “assist the crop in recovery from freeze”. Again, make these decisions based on the remaining yield potential rather than an effort to attempt to nurse the crop back to health after freeze.
Freeze injured wheat from Marshall, OK
As evidenced by this picture from Marshall, Oklahoma, freeze injured wheat can still have a green appearance. You must split stems to accurately assess injury.
Freeze injured wheat at Lamont, Oklahoma. Freeze might have finished this plot, but drought had it with a standing eight count prior to the freeze.
Freeze injury in Kay County Oklahoma. Photo courtesy Curtis Vap.
I have posted a few images from the Oklahoma Mesonet below. Most of Oklahoma spent at least four hours below freezing last night and some areas spent an extended period of time below 28F. While temperatures in the wheat canopy might have remained slightly higher than reported air temperatures, they were still probably low enough to result in significant injury to wheat.
Over the next few days growers will need to inspect fields closely to determine the extent of injury. Symptoms may start to appear later this week and will likely be clearly identifiable by early next week. Healthy wheat heads will remain turgid with a green color. Damaged wheat heads will be bleached, yellow, or brown and will easily break when pushed against. I anticipate that we will not have any partial “blanking” of wheat heads and that most wheat heads will either be okay or a complete loss.
What about new tillers? New tillers might emerge, but it is already April 15. In addition we have very dry soil conditions. For these reasons I am doubtful that newly emerging tillers will have much yield potential in areas south of I-40. IF (and that is a big if) weather conditions remain favorable, late emerging tillers in northern Oklahoma might still have a shot at producing grain.
I will survey some fields in a few days and report back with my findings. If you are interested in receiving weather maps and updates such as the ones posted below, subscribe to the OCS Mesonet Ticker by emailing email@example.com
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.
Freeze injured plants from Cotton County, OK. Note the green, healthy leaf coming through the desiccated leaves in the plant on the right. These plants will make a full recovery with adequate moist and fertility.
Our recent extreme shifts in temperature have resulted in moderate to severe freeze injury in some Oklahoma wheat fields. To be honest, the damage is not as widespread or severe as I thought it would be given that most of our wheat had not had an opportunity to harden off. The dry soil conditions in western and southern Oklahoma did not help the situation, as there was not sufficient soil moisture to buffer the temperature shift in the top few inches of soil.
Freeze injury at this stage of growth (tillering) rarely impacts grain yield, but, as always, there are a few exceptions. Wheat that was very small or late-sown is more susceptible to winter kill. Similarly, wheat that does not have a good root system or that was shallow sown due to crop residue is more susceptible to winter kill. It is best to wait until after a few days of favorable growing conditions to check for freeze injury. Plants with regrowth that is green and healthy should make a full recovery, and this will be the case for most Oklahoma wheat fields.
Freeze injury in late-sown wheat near Enid, OK. Some of the smaller plants might have a tough time recovering, but it is still too early to determine whether or not the field as a whole will adequate to produce a decent grain crop.
Large amounts of freezing rain, sleet, hail, etc. hit the Oklahoma wheat belt on April 10, 2013 and temperatures are expected to drop to the mid to upper 20’s this evening (I posted a couple of pictures below). Wheat development ranges from early heading in southern Oklahoma to just past jointing in northern Oklahoma and the Oklahoma Panhandle. If forecasts are correct, wheat tillers in southwest Oklahoma that escaped the first freeze have a good chance of being taken out by this freeze. Central and northcentral Oklahoma has quite a bit of ice-covered wheat. Ice-covered wheat will remain at approximately 32F and this might be just warm enough to escape severe injury. If the ice melts, however, and temps drop into the 20’s even wheat that is just past the jointing stage can be injured. At this stage it is certain that we will have some freeze injury to the majority of the Oklahoma wheat crop, but it will be a good 7 – 10 days before we can accurately assess the level of injury.
I have been asked if there is a 1:1 relationship between % freeze injury and % grain yield loss. Generally, the answer is no. DISCLAIMER — the values I am about to discuss are approximations and have huge margins of error — An otherwise healthy wheat crop that sustains 10% freeze injury prior to boot would probably suffer yield losses in the order of 0 to 5%. This is because the plant will divert resources to the remaining wheat heads. If damage is sufficient to reduce the final head count below a critical mass (around 400 heads per square yard) the relationship between % freeze injury and % yield loss will be much closer. So a 60% freeze injury might result in a 40 – 50% loss in grain yield. Again, these numbers are rough estimations and environmental conditions following the freeze will greatly impact the plant’s ability to compensate after freeze.
Advanced wheat partially covered by ice at Okarche, OK. Photo courtesy Mike Schulte, Oklahoma Wheat Commission
Mixture of ice and rain on wheat at Banner Rd. and I 40. Photo courtesy Mike Schulte, Oklahoma Wheat Commission
Ice covered wheat in the Alva area. Photo courtesy Woods County Extension Educator, Greg Highfill