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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.
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.
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.
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.
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 firstname.lastname@example.org
First hollow stem occurs just prior to jointing and is the optimal time to remove cattle from wheat pasture. A new first hollow stem advisor tool available on the Oklahoma Mesonet provides Oklahoma wheat farmers a real time assessment of the current first hollow stem situation in the state and a forecast for the next two weeks. While the first hollow stem advisor is a valuable tool, it is not a substitute for scouting, as conditions in your field may vary from the estimates provided.
The advisor uses a mathematical model that predicts the probability of first hollow stem based on soil heat units and wheat first hollow stem category (early, middle, or late). The model was developed by J.D. Carlson at OSU using first hollow stem data from the wheat variety testing program, and model development was made possible through a grant from the Oklahoma Wheat Commission.
You can navigate to the first hollow stem advisor from www.mesonet.org by clicking on “Agriculture” then “Crop-Wheat” and looking for First Hollow Stem Advisor on the lefthand menubar. Or you can click here.
Once you are at the first hollow stem advisor page, you will need to make a few selections. First, you have an option of viewing a statewide map or you can view data for a particular site in a table or graph. Next, you can select whether you want to view the current situation or a projection for the next one or two weeks. Finally, you will need to indicate if your variety falls into the early, middle, or late category. Click on the “look up by category” link if you are unsure where your variety falls.
Above is the statewide map for current conditions as of 14 February 2014. Other than a hot spot near Ardmore, there is less than 5% probability that we are at first hollow stem in Oklahoma. Note, however, that many locations are near the 576 heat unit threshold for 5% probability of first hollow stem. This is where the projection tools come in handy.
The map above is the two-week first hollow stem projection through 28 February 2014 (i.e the map was created on 14 February 2014). These projections are based on historical weather data for the next two weeks, and do not take into account the current forecast which might be warmer or colder than the historical average. Note that almost the entire state up to I-40 is predicted be at or above the 25% probability level for first hollow stem by February 28. It is recommended that you start scouting once the advisor predicts a 5% probability of first hollow stem in your area. If you are going by the first hollow stem advisor alone (not recommended) cattle should be removed no later than when a 50% probability of first hollow stem has occurred.
I thought I would share a couple of visual greenness maps sent to me by JD Carlson in Biosystems and Ag Engineering. These are from the Oklahoma Mesonet fire danger model site and provide a visual estimate of green vegetation for mid January 2012 (top) and 2013 (bottom). Note the stark contrast in greenness throughout the wheat belt of western Oklahoma. As indicated in the 2013 map, there are some small pockets of wheat with ample stands and average yield potential, but these are not large enough to make up for all of the red.