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First hollow stem occurs just prior to jointing and is the optimal time to remove cattle from wheat pasture. Given the warm forecast for the next two weeks, it is likely that we will start seeing first hollow stem in Oklahoma wheat fields. Grazing past first hollow stem can reduce wheat grain yield by as much as 5% per day and the added cattle gains are not enough to offset the value of the reduced wheat yield.
Similar to previous years, we will monitor occurrence of first hollow stem in our wheat plots at Stillwater and report the findings on this blog. There is also a new first hollow stem advisor available on the Oklahoma Mesonet that can assist in determining when to start scouting.
Checking for first hollow stem is fairly easy.
- You must check first hollow stem in a nongrazed area of the same variety and planting date. Variety can affect date of first hollow stem by as much as three weeks and planting date can affect it even more.
- Dig or pull up a few plants and split the largest tiller longitudinally (lengthways) and measure the amount of hollow stem present below the developing grain head. You must dig plants because at this stage the developing grain head may still be below the soil surface.
- If there is 1.5 cm of hollow stem present (see picture below), it is time to remove cattle. 1.5 cm is about the same as the diameter of a dime.
- Detailed information on first hollow stem can be found at www.wheat.okstate.edu under ‘wheat management’ then ‘grazing’
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.
There are a few cattle already on wheat pasture in Oklahoma and more that will be turned out in the coming weeks. An adequate crown root system is required to anchor wheat in place prior to grazing.
Without these roots, wheat can be pulled from the soil during grazing, thinning final stands. Aboveground appearances can be deceiving, so it is important to pull plants from the soil to ensure the crown roots are there even if the plants appear large enough for grazing.
It is also important to make preparations for measuring first hollow stem prior to turning cattle out on wheat pasture. Grazing delays plant development; therefore, first hollow stem has to be checked in a nongrazed area. The nongrazed area does not have to be large and can be achieved through a panel exclosure or by simply moving one of the posts for an electric fence in a few feet. The key is to plan for the nongrazed area now so you can measure first hollow stem in February.
By Dr. Bob Hunger, OSU Extension Plant Pathologist
My counterpart in Arkansas, Dr. Gene Milus (University of Arkansas), just sent out an important message about being careful to not exceed the maximum amount of a fungicide applied to a crop in a single year. Such a consideration could be an issue where more than one fungicide application is made. This is especially true if a generic of tebuconazole is applied, as this chemical also is in Prosaro; however, multiple applications of the same fungicide may also exceed the maximum amount of chemical that can be applied in a single year. Reading the label is the best place to determine the maximum amount of a chemical that can be applied in a single season and the exact amount of a chemical(s) that is in a fungicide. A quick reference to what chemicals are in the typical fungicides used on wheat in Oklahoma is the OSU Current Report (CR-7668) that Dr. Jeff Edwards and I recently updated (available at www.wheat.okstate.edu)
In Arkansas and many states through the mid-western region of the U.S., two or even three fungicide applications on wheat are more common, with the last application typically targeted toward Fusarium head blight (scab). In Oklahoma, where scab usually is not a concern, deciding to apply one fungicide application typically has been the only consideration. However, in recent years making two fungicide applications have become more common because of higher wheat prices, lower cost of fungicides, and increased no-till wheat acres that favor early diseases such as powdery mildew, tan spot, and septoria/stagonospora. It is in these situations that care must be taken to insure label compliance. The following message from Dr. Milus addresses this point. The report below is also at http://www.arkansas-crops.com/ along with other ag news from Arkansas.
Given the early onset of stripe rust and the cool rainy weather in recent weeks, some wheat growers are considering two or three fungicide applications. This is a new phenomenon for Arkansas growers and requires some planning to stay within the legal limits for total amounts of particular fungicides that can be applied to wheat fields. The fungicide label lists to total amount of each active ingredient that can be applied per acre per year. These amounts usually are given in pounds of active ingredient (lb ai) which require some math to translate into fluid ounces (fl oz) of particular products. The total amounts and usual application rates below are for fungicides most likely to be used in Arkansas.
Tebuconazole: total amount = 0.11 lb ai/A = 4 fl oz/A.
Products containing only tebuconazole include Folicur (no longer being sold), Orius, Tebucon, Tebustar, Tebuzol, Tegrol, and Toledo.
Prosaro: total amount = 8.2 fl oz /A = 0.11 lb ai each of prothioconazole and tebuconazole.
(Note that no Prosaro can be applied if 4 fl oz of a tebuconazole product was applied earlier because Prosaro is half tebuconazole.)
Propiconazole: total amount = 0.22 lb ai/A = 8 fl oz/A.
4 fl oz/A = 0.11 lb propiconazole.
Products containing propiconazole include Tilt, Bumper, Fitness, Propiconazole E-AG, and PropiMax
Quilt Excel: total amount = 28 fl oz/A = 0.22 lb propiconazole + 0.26 lb azoxystrobin
14 fl oz Quilt Excel = 0.11 lb propiconazole + 0.13 lb azoxystrobin.
(Therefore 4 fl oz of a propiconazole product + 14 fl oz of Quilt Excel can be legally applied.)
Pyraclostrobin (Headline): total amount = 18 fl oz/A = 0.29 lb ai/A
9 fl oz = 0.147 lb Pyraclostrobin (Note the slight discrepancy between the total amounts expressed as fl oz/A and lb ai/A.)
Metconazole (Caramba): total amount = 34 fl oz/A = 0.20 lb ai/A
17 fl oz = 0.10 lb Metconazole
Twinline: total amount = 18 fl oz/A = 0.10 lb metconazole + 0.15 lb pyraclostrobin
9 fl oz/A = 0.05 lb metconazole + 0.076 lb pyraclostrobin
(Note the discrepancies for total amounts metconazole and pyraclostrobin depending on which products are used.)
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.
On April 4th I toured southwest Oklahoma and surveyed freeze injury to wheat. In my experience, most freeze events are overhyped; however, this one was the real deal Holyfield. I traveled a route from Faxon to Chattanooga to Altus to Blair and ended up at Apache. Damage was similar at all sites, with injury ranging from 50 to 80%.
The best looking wheat was the hardest hit. Particularly troubling are some fields in the Altus area that easily had 80 bushel potential prior to the freeze. In most of these fields we are too far past the tillering stage to have yield compensation from secondary tillers. Late-emerging fields that were jointing or smaller escaped the freeze with little injury. Fields that had been heavily grazed and/or under-fertilized also escaped with relatively minor injury. Conditions improved slightly when I checked wheat in the Chickasha area and injury was more in the 10 – 30% range.
I am frequently asked if the injured wheat head will go ahead and “push through” as the season progresses, and the answer is no. So, if you see heads emerging out of the boot in a few weeks, they are likely not damaged and a head count at this stage will be a reasonable estimate of fertile heads. Since there will not be additional stem elongation in freeze injured wheat, it will not accumulate as much tonnage as in a ‘normal’ year.
I have posted a few pictures below showing freeze injury symptoms. Freeze injury can vary greatly among fields and even within a field. So, it is important to check several sites within a field and split several stems when determining the percent injury. Check early maturing varieties such as Jackpot, Billings, and Everest first, as they are most likely to have injury.
Temperatures across Oklahoma dipped into the teens and 20’s March 25 and 26 (see maps below). The rule of thumb is temperatures below 24F will damage wheat at or past the jointing stage, so it was certainly cold enough to injure wheat that was not delayed due to drought stress, grazing, or late emergence.
Freeze injury is not an exact science, and it remains to be seen whether or not we actually have widespread wheat freeze injury in the 2013 wheat crop. My best guess is that we will have some injury and might lose our primary tillers in more advanced fields. Wheat that is at Feekes growth stage 6 – 7 generally has the ability to compensate for primary tiller loss by keeping secondary tillers that would otherwise be sloughed off in April. In this scenario, the effect on final grain yield would be minimal. Wheat that has already aborted secondary tillers does not have this flex ability and will not recover from freeze injury. This is why March freezes are generally yield reducing and April freezes are yield eliminating. I will check fields late next week and post the results on this blog.
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).