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osunpk

osunpk

Since 2008 I have served as the Precision Nutrient Management Extension Specialist for Oklahoma State University. I work in Wheat, Corn, Sorghum, Cotton, Soybean, Canola, Sweet Sorghum, Sesame, Pasture/Hay. My work focuses on providing information and tools to producers that will lead to improved nutrient management practices and increased profitability of Oklahoma production agriculture

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Its dry and nitrogen cost a lot, what now?

The title says a lot about the primary question I am receiving right now. And the latest long range “forecast” does not make me feel any better about the current situation. But it is what it is and many great plains wheat farmers are having to make a decision.

The current situation in the wheat belt is that we are dry to depth, when the 32 inch PAW is on short supply and this comes from a combination of no rain and above average temperatures.

Average 32-inch Plant Available Water. Graph retrieved from Mesonet on January 20th, 2022.
120-Day rainfall accumulation across Oklahoma. Graph retrieved from Mesonet on January 20th, 2022. Start data of this time frame is September 22, 2021
The daily average temperature departure from the 15 year mean temperature for the Lahoma Research Station. The Mesonet long-term averages utilize 15 years of daily data (e.g. daily average, daily maximum/minimum, or daily total) for every current and past Oklahoma Mesonet station.

Fertilizer prices are holding fairly strong, at expensive, and the wheat crop currently seems to be going in reverse. So what is a wheat farmer to do? If we are looking on the bright side the lack of moisture in the surface will help reducing any potential losses through urea volatilization. It does not make the potential for loss zero though. If I am bound and determined to fertilize now, I would be very selective of the source and method of application. The biggest driver, tillage and residue amounts.

  • Conventional Till / No residue (plenty of bare soil showing) and small wheat-
    • UAN via Streamer nozzles
      • Why: With UAN (urea ammonium nitrate) you have a liquid N source that will get onto and into the soil and readily available nitrate. Streaming on will help concentrate the fertilizer and potential reduce any urea volatilization if any dews were to occur. Urea would sit until dissolved and lead to potential losses if the first moisture was heavy dew and not a incorporating rainfall.
  • No-till / high residue (no bare soil showing)-
    • Dry Urea
      • Why: If Our residue is dry when the urea is spread the wind will help push it below the residue surface providing protection until a good rain. If UAN is applied to this dry or even slightly damp residue and not washed off with a rainfall in a week or so the amount of N tied up in that residue will likely be significant.
  • The big wheat (very little bare soil, lots of wheat tissue.
    • Urea or UAN Streamer
      • Why not Flat fan. At least with the current status the wheat is not growing and bigger wheat has increasing levels of tip die back. So while UAN sprayed on actively growing wheat can be absorbed foliarly, stressed wheat can not do it as well. Plus the UAN that hits dead or damaged tissue will not make it into the plant. The UAN applied via flat fan will need incorporation via rain in a couple days.

You may have caught in the paragraph above I said, “If I was bound and determined”. If I had the option I am not pulling the trigger until after I have received some good moisture. I fully expect and have already seen rigs running before every decent chance of rain. Unfortunately many of those chances have not panned out and that will remain my concern moving forward. I want to make sure we have some water in the tank before investing in the system.

But now we increase the risk/fear by waiting and the question I get is what if we don’t get good rains or don’t get good incorporating rains. The short answer is, if we don’t get rains the N application is the least of our concerns. If we approach March 15th and we have not had the rains needed to put a little water in the tank and incorporate the N then we are not likely looking at a bumper crop which will need N. What survives in that scenario will be living off deep soil water, and where there is deep soil water there is a good chance of deep N. The shallow soils will be so stressed that nutrient demand will be very little.

Now lets talk waiting and applying N. How late before we just say we are done. To answer I am going to draw from a data set I talk about a lot, the delayed N work by Dr. Souza. This study was started in the fall of 2016 and concluded with the 2020 wheat harvest. In all, twelve trials were established and achieved maturity. This study was designed to evaluate the recovery of winter wheat grain yield and protein after the crop was N stressed. Treatments included an untreated check, pre-plant application and ten in-season treatments. The application of in-season treatments was initiated when N deficiency was confirmed and treatments were applied in progressive order every seven growing days to the point of 63 growing days after visual deficiency (DAVD). A growing degree days is any day that the average daily temperature is at or above 40⁰ F.  Ammonium nitrate (NH4NO3) was applied at a rate of 90 lbs N ac-1 for all treatments.

With this data we can answer two questions, first at what point did we lose yield compared to pre-plant and second how late could we apply and still increase yield above the check. So comparing to the pre lets us know how long could we wait with losing yield. Across the trials we lost yield three times by waiting too long, at LCB2017b that was 4/19, Lahoma18 it was around 3/30, and then Newkirk2020 we lost yield by waiting until 4/6. This data is why I am pretty comfortable waiting until mid March when and if needed. Now if we look at the check, that will tell us if things start improving late can we get still get a yield bump with added N. Newkirk 2020 was the only time and place we could increase yield above the zero after the 4/14 additions.

Table 1. Date of nitrogen application. Each month is color coded.
Table 2. Evaluation of winter wheat grain yield and protein results compared to the Pre-plant Nitrogen treatment. Red boxes means the treatment yielded statistically lower results, Yellow is no difference, Green means the treatment has increased grain yield or protein. Perkins2018, LSC2018b, and LCB2019, did not have a grain yield respond to N (no red box in Zero check) and therefore will not be discussed.

Take Home Message

My recommendation is that if you are not required to take delivery or needing to cover a lot of acres, i.e. time limited, I would not get in a hurry to apply N on this wheat crop. I think if we combine weather by market this a good time to wait and see. Once we get a rain and have some soil moisture it will be time to run the rigs. The crop currently does not need a lot of N so why spend the $. If things don’t improve by mid to late march, consider the wheat a cover and look towards a summer crop with the hopes of rains in April. If you need to take the crop to yield, then you can wait a while longer and still get a return on the N, with hopes the price could come down a bit.

Finally, While I don’t suggest running fertilizer in front of the first chance of rain, I would make sure I had an N-Rich strip on each and every single field. Strips can go out well past green up and serve a great purpose. The N-Rich strip will help you determine if the crop is able to mine any soil N or if the N tank is dry.

Feel free to reach out with questions or comments.
Brian Arnall Precision Nutrient Management Specialist.
b.arnall@okstate.edu

Special thanks to EDC Ag Products Co LLC for suppling NH4NO3 used in the delayed N project.

Relevant past blogs for your reading enjoyment.

The Easy Button for Nitrogen…….

Brian Arnall, Precision Nutrient Management Extension Specialist.

The basics for nitrogen (N) fertilizer rate determination can be described in a mechanistic approach by the Stanford Equation NFert = ( NCrop – NSoil ) / Neff. This equations states that the N fertilizer rate is equal to the amount of nitrogen taken up by the crop minus the amount of nitrogen supply by the soil, divided by the efficiency of the nitrogen fertilizer used. I outline the importance of this equation in the blog “Components of a variable rate nitrogen recommendations“.

There are nitrogen “Easy Buttons” which utilizes averages collected over diverse environments to create accurate N rate recommendations. The best example of this is the yield goal rules of thumb such as wheats 2.0 lbs N per yield goal bushel minus soil test nitrate. Yield goals are generally calculated as the average of the best 3 out of 5 years, or the 5-year average times 20%. Also, the 2.0 lbs of N is more than what is in a bushel as it also adds in an efficiency factor or a 0.5 lbs per bushel cushion. This method and others like it provide an accurate N rate with slight probability of yield loss. However, the rec is often highly imprecise. Meaning that if I apply the method to 100 fields the average will be spot on, however if I look at the performance of the recommendation on a single field, I will likely be disappointed.

Illustration of accuracy versus precision.
Figure 1. Illustration of accuracy versus precision.

When it comes to nitrogen recommendations the Easy button method will use components which help ensure that the rate prescribed will maximize yield 90-95% of the time. For example, take the data presented in Figure 2. Over fifteen years of the long-term winter wheat fertility study near Lahoma, Oklahoma the average pounds of N per bushel to reach economic optimum nitrogen rate (EONR) was 1.6, however if 2.0 of N was applied per bushel yield would have been maximized 13 out of the 15 years. While 2.0 lbs. of N per bushel would have been quite accurate for maximizing yield, it would be highly imprecise as over the 15 years optimum pounds of N per bushel ranged from 0.0 to 3.2.

Figure 2. Grain yield (bushels per acre), economical optimal N rate (EONR), and pounds of nitrogen per bushel producer at the EONR, from 15 years of data from the long-term fertility trials located near Lahoma, Ok.

The trick to improving your N rate recommendation closer to a precise and accurate system is to obtain representative site-specific values for the Stanford Equation NFert = (NCrop – NSoil) / Neff.

Looking at the 15-year long-term data above the yields range from a low of 27 to a high of 88 bushels. Of those 15 years, I personally planted multiple years, usually sometime in October, and many of those years while sowing I could have guessed a range of 55-60 bushel, which just happened to be just above the 15-year average.   It was not until February and March when the yield potential really started to express itself. Why, well there is a lot of weather between Oct to March, a lot of environmental positive and negative impacts on that final grain yield. This is the best timing to go out with approaches, models, or techniques to estimate yield potential for N rate recs.

While I am a big fan of soil testing, pre-plant soil samples for N are just a snap shot in time. But the While I am a big fan of soil testing, pre-plant soil samples for N are just a snapshot in time, but the nitrogen cycle Figure 3, will roar on after the soil sample is collected. Organic matter (OM) is the central component of this cycle and drives availability of NH4 and NO3 in the system. For each 1% OM in the top 6″ of the soil there is approximately 1000 lbs of organically bound N. The amount of N going into and out of OM pool is driven by C:N ratio of residues, soil temperature and soil moisture. While we very well what the mechanisms of the cycle are and can model the reactions quite well. Our inability to predict long term weather patterns is the greatest factor limiting our ability to predict future availability of NSoil.

Figure 3. Complete Nitrogen Cycle. http://psssoil4234.okstate.edu/lecture

This is where the reader should be asking “how can we get better site specific data” and I begin the discussion on why I have been promoting the of the Sensor Based Nitrogen Rate Calculator (SBNRC) and N-Rich strip method.

Lets talk about how the approach follows Stanford’s mechanistic approach to N management. First the Yield Potential component of the SBNRC which is related to NCrop. In effect researchers have built models over the past two decades that can correlate the NDVI collected from a sensor, such as the GreenSeeker, with the crops biomass and chlorophyll content. If given the number of days the crop has been growing it is possible to use the NDVI collected from the crop as a tool to predict final grain yield. The closer the wheat gets to hollow stem, or the corn gets to tassel, the better the prediction. One reason is that we have allowed more “environmental influence” to happen. Dr. Bill Raun, a founder of the SBNRC concept kept great discussion and data sets on his NUE.OKSTATE.edu website. On the “NUE Website on YP” he provides information on how yield prediction work while on the “NUE Website YP Library” he has not listed every algorithm created, and the math behind them, but also a recipe book for how anyone can create their own algorithm. While there are a lot post sensing stresses that can bring down final grain yield, the models that have been built and continually improved, do quite a good job on predicting final grain yield in-season. Resulting a much more site specific value for NCrop. The blog”Sensing the N-Rich Strip and Using the SBNRC” goes into a further discussion of using the online SBNRC.

That now leaves NSoil, which I will argue is at least as important as NCrop. As weather so greatly influences the nitrogen cycle it would be nice to have a weather station on every field paired with a 0-4 ft soil description which could be incorporated into a model. Given those might be out of reach we have found the the use of a reference strip, high N or low N, really provides an site specific estimate the of nitrogen the crop has access to. If the high N reference (N-Rich) strip is showing up that means the remainder of the field is N deficient. This may be due to losses or lack of mineralization, either way more N is needed. If the N-Rich strip is not evident then the crop is finding enough N outside of the reference strip to support its current growth. This could be that residual N or mineralization is high, or it could mean that crop growth and therefore N demand is low. Having the N check strip in each field allows for a season long evaluation. We can use NDVI to characterize how big or little of a response we have to N. We call this the Response Index (RI). An RI of 1.8 means that we could increase yield by 80% if we add adequate N, if the RI is 1.05 then we are looking at a potential increase of 5%. I have a previous blog which goes into the application of the reference strip. “Nitrogen Rich Strips, a Reminder

Finally we combine the two, YP and RI. By predicting the yield of the area out side the N-Rich strip we can determine environmental yield potential, YP0. Basically what can the field yield if nothing is added. We multiple YP0 by the RI to get the yield potential with added N, YPN. Then its as simple as N rate = (YPN – YP0 ) x N needed per bushel. So for example if YP0 is 40 bushel RI =2, then YPN is 80 bushel. I need to fertilize the additional 40 bushels of wheat and I can use the 2.0 N per bushel can come up with a top-dress rate of 80 lbs N per acre. We are now incorporating site specific in-season NCrop and NSoil data.

And just a reminder for those of you new to my blog, I have a lot of research documenting that it is not only OK, but often best if we wait on N application in wheat and other crops. Value of In-Season N blog.

Every step we take towards the easy button is often a step towards site specific imprecision due to the use of generalized terms or models. Depending on your goals this very well could be acceptable for your operation, but with nitrogen prices as volatile as they are, should we not be considering pushing the easy button to the side, for now. Let’s add a bit of site-specific data so that we can take advantage of the N the system may be giving us, or the yield we did not expect. Let the N-Rich Strip be that first step.

Relevant Peer Review Publications.

In-Season Prediction of Yield Potential Using Wheat Canopy Reflectance,  Agron. J. 93:131-138

Nitrogen Fertilization Optimization Algorithm Based on In-Season Estimates of Yield and Plant Nitrogen Uptake
  J. Plant Nutr. 24:885-898

Real-Time Sensing and N Fertilization with a Field Scale GreenSeeker Applicator

Identifying an In-Season Response Index and the Potential to Increase Wheat Yield with Nitrogen (pdf)

Nitrogen Response Index as a Guide to Fertilizer Management
 

Evaluation of Green, Red, and Near Infrared Bands for Predicting Winter Wheat Biomass, Nitrogen Uptake and Final Grain Yield 

Full List of NUE Publications

If you have any questions please feel free to contact me @ b.arnall@okstate.edu

Nitrogen Rich Strips, a Reminder

With the recent increase in fertilizer prices just prior to winter wheat planting season I felt it was a good opportunity to bring this older post back up and give it an update. Since the blog was originally written in 2013 there has been a lot of work done both to better understand the nitrogen fertilizer need / timing of winter wheat and efforts to updated and improve the algorithms behind the Sensor Based Nitrogen Rate Calculator.

The Nitrogen Rich Strip, or N-Rich Strip, is a technique/tool/process that I spend a great deal of time working with and talking about.  It is one of the most simplistic forms of precision agriculture a producer can adopt.  The concept of the N-Rich strip is to have an area in the field that has more nitrogen (N) than the rest.  In recent years we have been utilizing Zero-N strips in corn. The approach to some may be new but at one point most producers have had N-Rich Strips in their fields, albeit accidentally.  Before the days of auto-steer it was not uncommon, and honestly still is not, to see a area in the field that the fertilizer applicator either doubled up on or skipped.  In our pastures and dual purpose/graze out wheat every spring we can see the tell-tale signs of livestock deposits.  When over laps or “Cow Pox” become visible we can assume the rest of the field is behind in nitrogen.  The goal of an N-Rich Strip is to let the field tell you when it needs more N. Research has shown wheat can be yellow and recover completely and it may even be a benefit. See the link for the Value of In-season Nitrogen at the end of this blog.

Cow Pox, Image courtesy Kaitlyn Nelson
Cow Pox, Image courtesy Kaitlyn Nelson

What I like most about the N-Rich Strip approach is its Simplicity.  The N-Rich Strip is applied and; Scenario 1. The N-Rich Strip becomes visible (Greener) you APPLY NITROGEN, Scenario 2.  The strip is not visible you Option A. DON’T APPLY NITROGEN Option B. Apply Nitrogen Anyways.  The conclusion to apply N or not is based on the reasoning that the only difference between the N-Rich Strip and the area 10 ft from it is nitrogen, so if the strip is greener the rest of the field needs nitrogen.  If there is no difference N is not limiting and our research shows N does not have to be applied.  However producers who decide to be risk adverse (in terms of yield) can apply N but it would be advised to do so at a reduce the rate.  Now is a good time to note that the N-Rich Strip alone provides a Yes or No, not rate recommendation.  At OSU we use the GreenSeeker optical sensor and Sensor Based Nitrogen Rate Calculator (SBNRC) to determine the rate, but that discussion will come later.  I equate the change from using yield goal N rate recs to the N-Rich Strip as to going from foam markers to light bars on a sprayer.   Not 100% accurate but a great improvement.

N-Rich Strip in no-till wheat near Hobart OK.
N-Rich Strip in no-till wheat near Hobart OK.
N-Rich Strips showing up on google earth image. You can see how the strip on the left is darker than the right suggested a greater need for nitrogen.

Now that we have covered the WHY, lets get down to the nuts and bolts HOW, WHEN, WHERE.

How the strip is applied has more to do with convenience and availability than anything else but there are a few criteria I suggest be met. The strip should be at least 10 ft wide and 300 ft long. The rate should be 40 to 50 lbs N (above the rest of the field) for grain only wheat and canola, 80 lbs N for dual purpose wheat. The normal recommendation is that when applying pre-plant either have a second, higher rate programmed into the applicator or make a second pass over an area already fertilized. Many will choose to rent a pull type spreader with urea for a day, hitting each field.


Also popular are applicators made or adapted for this specific use. ATV sprayers are the most common as they can be multi-purpose. In most cases a 20-25 gallon tank with a 1 gpm pump is placed on the ATV with an 8-10ft breakover boom. The third applicator is a ride away sprayer with a boom running along the rear of the trailer. In all cases when liquid is the source I recommend some form of streamer nozzle.
If this all sounds like to much then the easiest application method might just be a push spreader. No need for trailer or even a truck. In most cases I recommend whichever N source is the easiest, cheapest, and most convenient to apply.

Vincent N-Rich Strip Applicator, Ponca City OK
Vincent N-Rich Strip Applicator, Ponca City OK
Gard N-Rich Strip Applicator, Fairview Ok
Gard N-Rich Strip Applicator, Fairview Ok
Push spreader used by Oklahoma State Cooperative Extension service. Check with your local office. If they don’t have one, we can send one.

When the strip is applied in winter crops proper timing is regionally dependent. For the Central Great Plains, ideally the fertilizer should be applied pre-plant or soon after.  However,  in most cases as long as the fertilizer is down by December or even January everything works. Timing is more about how much the wheat is growing. If it is slow growing fall, timing can be delayed. When the N-Rich Strip approach is used on the Eastern Shore in Virginia and Maryland the strips have to be applied at green up. We have been trying this in Oklahoma and Kansas with good success.  It is always important to make the tools fit your specific regional needs and practices and not the other way around.

Where is actually the biggest unknown.  The basic answer is to place the N-Rich Strip in the area that best represents the field.  Many people question this as it doesn’t account for spatial variability in the field, and they are correct.  But my response is that in this case spatial variability is not the goal, temporal variability is.  Keeping in mind the goal is to take a field which has been receiving a flat yield goal recommendation for the last 30+ years and make a better flat rate recommendation.  My typically request is that on a field with significant variability either apply a strip long enough to cross the zones or apply smaller strips in each significant area.  This allows for in-season decisions.  I have seen some make the choice to ignore the variability in the field, made evident by the strip, and apply one rate and others choose the address the variability by applying two or more rates.  One key to the placement of N-Rich Strips is record keeping.  Either via notes or GPS, record the location of every strip.  This allows for the strips to be easily located at non-response sites.  It is also recommended to move the strip each year to avoid overloading the area with N.  

For more information on N-Rich Strips

Factsheets

https://extension.okstate.edu/fact-sheets/applying-nitrogen-rich-strips.html

https://extension.okstate.edu/fact-sheets/using-the-greenseeker-handheld-sensor-and-sensor-based-nitrogen-rate-calculator.html

https://extension.okstate.edu/fact-sheets/impact-of-sensor-based-nitrogen-management-on-yield-and-soil-quality.html

Related Blogs

YouTube Videos  

Re-Post: Sensing the N-Rich Strip and Using the SBNRC

This the recent rains across the dry wheat belt the N-Rich Strips are going to start showing up. Because I am re-posting ans older blog that walks users through the sensing process and inputting data in to SBNRC. But since post we have also release a iOS version of the Online Calculator. iOS N-Rate Calc

Original Post:
With the significant swing in temperature over the last few weeks many are chomping at the bit to get outside.  The wheat is starting to respond to the good weather and N-Rich Strips are showing up around the state.  Over the past week I have had several calls concerning the impact of the cold weather on the N-Rich Strips.  Many of the fields either are still small due to limited days of warm weather and growth or may have a good deal of damage to the foliage.  If the field of concern has only a little or no damage and the strip is visible, the time to go is NOW, but if you cannot see the strip and your field has tissue damage or is small, similar to the first two images, then you will need to wait a week or two for sensor based recommendations.  Another situation fits with the third image, the field has freeze damage but the N-Rich Strip is also visible.   In this case the predicted yield level would be reduced do to the dead tissue making the N rate recommendation a little off.  I still however recommend using the sensor and online SBNRC (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php) to make or base top-dress N rate.  Even if the recommendation is a little off it will still be much more accurate than just guessing. However you must look at the SBNRC and ensure that it makes agronomic sense, if it does not consult your county educator or myself.   This is discussed in more detail in my earlier blog about freeze damage.  Keep in mind no matter what, if you can see the N-Rich Strip, everything outside of the strip is suffering from nitrogen deficiency.  Decisions and fertilizer applications need to be made soon, to maximize yield.

Winter Wheat and Nitrogen Rich Strips.

Winter Wheat and Nitrogen Rich Strips.

Regardless of whether or not the strip is visible you should be planning to sense with the GreenSeeker Handheld very soon. Remember the sensor has the ability to detect differences before your eyes can.   To sense the N-Rich Strip and Farmer Practice the user should carry the sensor approximately 30 to 40 inches above the crop canopy while holding the sensor level over the crop.  While you are walking the two area the trigger should be held the entire time.  I recommend walking at minimum 100 paces for each.    The average NDVI value seen on the screen will only stay on the screen for a few seconds.  Therefore it is critical you have a method of recording the number for later use. The sensor has limited memory so it will time out is the trigger is held for an extended period of time.  If you wish to collect more NDVI readings just do it in multiple trigger pulls recording each.  Once you have the average NDVI for the N-Rich Strip and Farmer Practice you can go to the SBNRC site mentioned above to retrieve the N rate recommendation.   Once in the calculator, for those in Oklahoma, choose the “within Oklahoma” option in the bottom left hand corner of the screen.  This will allow the calculator to access the Oklahoma Mesonet to determine growing degree days.  After the location is picked from the options you will need to enter Planting Date and Date Prior to Sensing.  Additional information requested is the expected grain and fertilizer prices.  While these inputs will provide some economic evaluations they will not impact recommended N rate.

GreenSeeker HandHeld NDVI Sensor

GreenSeeker HandHeld NDVI Sensor

Below is a YouTube video in which I describe how to use the GreenSeeker to collect NDVI readings, describe the data needed to complete the online calculator, and how to interrupt the calculators output.

Using the GreenSeeker after Freeze Damage

After discussions with producers in southern Kansas I felt the need to bring back this past blog.  It seems that much of (not all) the early planted wheat lost a significant amount of biomass during the winter and the N-Rich Strip GreenSeeker approach is producing what looks to be low yield potentials and N-Rate recommendations.  This should be treated much like we do grazed wheat and the planting date should be adjusted, see below.  It is also important to note that in the past year a new wheat calculator was added to the NUE Site.  http://nue.okstate.edu/SBNRC/mesonet.php. Number 1 is the original OSU SBNRC but the #2 is calculator produced by a KSU/OSU cooperative project.  This is the SBNRC I recommend for use in Kansas and much of the norther tier of counties in OK.

Original Blog on Freeze Damage and the GreenSeeker.

Dr. Jeff Edwards “OSUWheat” wrote about winter wheat freeze injury in a receive blog on World of Wheat, http://osuwheat.com/2013/12/19/freeze-injury/.  As Dr. Edwards notes injury at this stage rarely impact yield, therefore the fertility requirements of the crop has not significantly changed.  What will be impacted is how the N-Rich Strip and GreenSeeker™ sensor will be used.  This not suggesting abandoning the technology in fact time has shown it can be just as accurate after tissue damage.   It should be noted GreenSeeker™ NDVI readings should not be collected on a field that has recently been damaged.

A producer using the N-Rich Strip, GreenSeeker™, Sensor Based N-Rate Calculator approach on a field with freeze damage will need to consider a few points.  First there need to be a recovery period after significant tissue damage; this may be one to two weeks of good growth.   Sense areas that have had the same degree of damage as elevation and landscape position often impacts the level of damage.  It would be misleading to sense a area in the N-Rich strip that was not significantly damaged but an area in the Farmer Practice that took a great deal of tissue loss.

Finally we must consider how the SBNRC, available online at http://nue.okstate.edu/SBNRC/mesonet.php, works.  The calculator uses NDVI to estimate wheat biomass, which is directly related to grain yield.  This predicted grain yield is then used to calculate nitrogen (N) rate.  So if biomass is reduced, yield potential is reduced and N rate reduced.  The same issue is seen in dual purpose whet production.  So the approach that I recommend for the dual purpose guys is the same that I will recommend for those who experienced significant freeze damage.  This should not be used for wheat with just minimal tip burn.

To account for the loss of biomass, but not yield, planting date needs to be adjusted to “trick” the calculator into thinking the crop is younger and has greater potential.   Planting date should be move forward 7 or 14 days dependent  For example the first screen shot shows what the SBNRC would recommend using the real planting date.  In this case the potential yield is significantly underestimated.

The second and third screen shots show the impact of moving the planting date forward by 7 and 14 days respectively.  Note the increase in yield potential, which is the agronomically correct potential for field considering soil and plant condition, and increase in recommended N-rate recommendation.  Adjust the planting date, within the 7 to 14 day window, so that the yield potential YPN is at a level suitable to the field the yield condition and environment.  The number of days adjusted is related to the size and amount of loss.  The larger the wheat and or greater the biomass loss the further forward the planting date should be moved.  In the example below YPN goes from 37 bu ac on the true planting date to 45 bu ac with a 14 day correction.  The N-rate changes from 31 lbs to 38 lbs, this change may not be as much as you might expect.  That is because YP0, yield without additional N, also increases from 26 to 32 bushel.

freeze Zero day moveImage 1. Planting date 9/1/2013.  YPN 37 bu ac-1 and N-Rec 31 lbs ac-1.

Freeze 7 day moveImage 2. Planting date 9/8/2013.  YPN 40 bu ac-1 and N-Rec 34 lbs ac-1.

Freeze 14 day moveImage 3. Planting date 9/15/2013.  YPN 45 bu ac-1 and N-Rec 38 lbs ac-1.

This adjustment is only to be made when tissue has been lost or removed, not when you disagree with the yield potential.  If you have any questions about N-Rich Strips, the GreenSeeker™, or the online SBNRC please feel free to contact me at b.arnall@okstate.edu or 405.744.1722.

Sensing the N-Rich Strip and Using the SBNRC

Original Post:
With the significant swing in temperature over the last few weeks many are chomping at the bit to get outside.  The wheat is starting to respond to the good weather and N-Rich Strips are showing up around the state.  Over the past week I have had several calls concerning the impact of the cold weather on the N-Rich Strips.  Many of the fields either are still small due to limited days of warm weather and growth or may have a good deal of damage to the foliage.  If the field of concern has only a little or no damage and the strip is visible, the time to go is NOW, but if you cannot see the strip and your field has tissue damage or is small, similar to the first two images, then you will need to wait a week or two for sensor based recommendations.  Another situation fits with the third image, the field has freeze damage but the N-Rich Strip is also visible.   In this case the predicted yield level would be reduced do to the dead tissue making the N rate recommendation a little off.  I still however recommend using the sensor and online SBNRC (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php) to make or base top-dress N rate.  Even if the recommendation is a little off it will still be much more accurate than just guessing. However you must look at the SBNRC and ensure that it makes agronomic sense, if it does not consult your county educator or myself.   This is discussed in more detail in my earlier blog about freeze damage.  Keep in mind no matter what, if you can see the N-Rich Strip, everything outside of the strip is suffering from nitrogen deficiency.  Decisions and fertilizer applications need to be made soon, to maximize yield.

Winter Wheat and Nitrogen Rich Strips.

Winter Wheat and Nitrogen Rich Strips.

Regardless of whether or not the strip is visible you should be planning to sense with the GreenSeeker Handheld very soon. Remember the sensor has the ability to detect differences before your eyes can.   To sense the N-Rich Strip and Farmer Practice the user should carry the sensor approximately 30 to 40 inches above the crop canopy while holding the sensor level over the crop.  While you are walking the two area the trigger should be held the entire time.  I recommend walking at minimum 100 paces for each.    The average NDVI value seen on the screen will only stay on the screen for a few seconds.  Therefore it is critical you have a method of recording the number for later use. The sensor has limited memory so it will time out is the trigger is held for an extended period of time.  If you wish to collect more NDVI readings just do it in multiple trigger pulls recording each.  Once you have the average NDVI for the N-Rich Strip and Farmer Practice you can go to the SBNRC site mentioned above to retrieve the N rate recommendation.   Once in the calculator, for those in Oklahoma, choose the “within Oklahoma” option in the bottom left hand corner of the screen.  This will allow the calculator to access the Oklahoma Mesonet to determine growing degree days.  After the location is picked from the options you will need to enter Planting Date and Date Prior to Sensing.  Additional information requested is the expected grain and fertilizer prices.  While these inputs will provide some economic evaluations they will not impact recommended N rate.

GreenSeeker HandHeld NDVI Sensor

GreenSeeker HandHeld NDVI Sensor

Below is a YouTube video in which I describe how to use the GreenSeeker to collect NDVI readings, describe the data needed to complete the online calculator, and how to interrupt the calculators output.

Freeze Injury and N-Rich Strips,

 

Dr. Jeff Edwards “OSUWheat” wrote about winter wheat freeze injury in a receive blog on World of Wheat, http://osuwheat.com/2013/12/19/freeze-injury/.  As Dr. Edwards notes injury at this stage rarely impact yield, therefore the fertility requirements of the crop has not significantly changed.  What will be impacted is how the N-Rich Strip and GreenSeeker™ sensor will be used.  This not suggesting abandoning the technology in fact time has shown it can be just as accurate after tissue damage.   It should be noted GreenSeeker™ NDVI readings should not be collected on a field that has recently been damaged.

A producer using the N-Rich Strip, GreenSeeker™, Sensor Based N-Rate Calculator approach on a field with freeze damage will need to consider a few points.  First there need to be a recovery period after significant tissue damage; this may be one to two weeks of good growth.   Sense areas that have had the same degree of damage as elevation and landscape position often impacts the level of damage.  It would be misleading to sense a area in the N-Rich strip that was not significantly damaged but an area in the Farmer Practice that took a great deal of tissue loss.

Finally we must consider how the SBNRC, available online at http://nue.okstate.edu/SBNRC/mesonet.php, works.  The calculator uses NDVI to estimate wheat biomass, which is directly related to grain yield.  This predicted grain yield is then used to calculate nitrogen (N) rate.  So if biomass is reduced, yield potential is reduced and N rate reduced.  The same issue is seen in dual purpose whet production.  So the approach that I recommend for the dual purpose guys is the same that I will recommend for those who experienced significant freeze damage.  This should not be used for wheat with just minimal tip burn.

To account for the loss of biomass, but not yield, planting date needs to be adjusted to “trick” the calculator into thinking the crop is younger and has greater potential.   Planting date should be move forward 7 or 14 days dependent  For example the first screen shot shows what the SBNRC would recommend using the real planting date.  In this case the potential yield is significantly underestimated.

The second and third screen shots show the impact of moving the planting date forward by 7 and 14 days respectively.  Note the increase in yield potential, which is the agronomically correct potential for field considering soil and plant condition, and increase in recommended N-rate recommendation.  Adjust the planting date, within the 7 to 14 day window, so that the yield potential YPN is at a level suitable to the field the yield condition and environment.  The number of days adjusted is related to the size and amount of loss.  The larger the wheat and or greater the biomass loss the further forward the planting date should be moved.  In the example below YPN goes from 37 bu ac on the true planting date to 45 bu ac with a 14 day correction.  The N-rate changes from 31 lbs to 38 lbs, this change may not be as much as you might expect.  That is because YP0, yield without additional N, also increases from 26 to 32 bushel.

freeze Zero day moveImage 1. Planting date 9/1/2013.  YPN 37 bu ac-1 and N-Rec 31 lbs ac-1.

Freeze 7 day moveImage 2. Planting date 9/8/2013.  YPN 40 bu ac-1 and N-Rec 34 lbs ac-1.

Freeze 14 day moveImage 3. Planting date 9/15/2013.  YPN 45 bu ac-1 and N-Rec 38 lbs ac-1.

This adjustment is only to be made when tissue has been lost or removed, not when you disagree with the yield potential.  If you have any questions about N-Rich Strips, the GreenSeeker™, or the online SBNRC please feel free to contact me at b.arnall@okstate.edu or 405.744.1722.

Nitrogen Rich Strips

The Nitrogen Rich Strip, or N-Rich Strip, is a technique/tool/process that I spend a great deal of time working with and talking about.  It is one of the most simplistic forms of precision agriculture a producer can adopt.  The concept of the N-Rich strip is to have an area in the field that has more nitrogen (N) than the rest.  Due to our fertilizer applicators this is typically a strip.  The approach maybe somewhat new but at one point most producers have had N-Rich Strips in their fields, albeit accidentally.  Before the days of auto-steer it was not uncommon, and honestly still is not, to see a area in the field that the fertilizer applicator either doubled up on or skipped.  In our pastures and dual purpose/graze out wheat every spring we can see the tell-tale signs of livestock deposits.  When over laps or “Cow Pox” become visible we can assume the rest of the field is behind in nitrogen.  I like to tell producers that the goal of the N-Rich strip is to make a really big cow pie.

Cow Pox, Image courtesy Kaitlyn Nelson
Cow Pox, Image courtesy Kaitlyn Nelson

What I like most about the N-Rich Strip approach is its Simplicity.  The N-Rich Strip is applied and; Scenario 1. The N-Rich Strip becomes visible (Greener) you APPLY NITROGEN, Scenario 2.  The strip is not visible you Option A. DON’T APPLY NITROGEN Option B. Apply Nitrogen Anyways.  The conclusion to apply N or not is based on the reasoning that the only difference between the N-Rich Strip and the area 10 ft from it is nitrogen, so if the strip is greener the rest of the field needs nitrogen.  If there is no difference N is not limiting and our research shows N does not have to be applied.  However producers who decide to be risk adverse (in terms of yield) can apply N but it would be advised to do so at a reduce the rate.  Now is a good time to note that the N-Rich Strip alone provides a Yes or No, not rate recommendation.  At OSU we use the GreenSeeker optical sensor and Sensor Based Nitrogen Rate Calculator (SBNRC) to determine the rate, but that discussion will come later.  I equate the change from using yield goal N rate recs to the N-Rich Strip as to going from foam markers to light bars on a sprayer.   Not 100% accurate but a great improvement.

N-Rich Strip in no-till wheat near Hobart OK.

N-Rich Strip in no-till wheat near Hobart OK.

Now that we have covered the WHY, lets get down to the nuts and bolts HOW, WHEN, WHERE.

How the strip is applied has more to do with convenience and availability than anything else but there are a few criteria I suggest be met. The strip should be at least 10 ft wide and 300 ft long.  The rate should be no less than 50 lbs N (above the rest of the field) for grain only wheat and canola, 80 lbs N for dual purpose wheat.  The normal recommendation is that when applying pre-plant either have a second, higher rate programmed into the applicator or make a second pass over an area already fertilized. Many will choose to rent a pull type spreader with urea for a day, hitting each field.
Becoming more popular are applicators made or adapted for use.  ATV sprayers are the most common as they can be multi-purpose.   In most cases a 20-25 gallon tank with a 1 gpm pump is placed on the ATV with an 8-10ft breakover boom.  The third applicator is a ride away sprayer with a boom running along the rear of the trailer.  In all cases when liquid is the source I recommend some form of streamer nozzle.  In most cases there is not a great deal of thought put into what source.  I recommend whichever source is the easiest, cheapest, and most convenient to apply.

Vincent N-Rich Strip Applicator, Ponca City OK

Vincent N-Rich Strip Applicator, Ponca City OK

Ok State N-Rich Applicator

Oklahoma State Univ. N-Rich Strip Applicator

Gard N-Rich Strip Applicator, Fairview Ok

Gard N-Rich Strip Applicator, Fairview Ok

When the strip is applied in winter crops proper timing is regionally dependent. For the Central Great Plains, ideally the fertilizer should be applied pre-plant or soon after.  However,  in most cases as long as the fertilizer is down by the first of November everything works.  This does not say a strip applied after this time doesn’t work but it leaves more room for error.  There is a chance the crop could already be stressed or the nitrogen tied up and not release in time.  However when the N-Rich Strip approach is used on the Eastern Shore in Virginia and Maryland the strips have to be applied at green up.  The soils in that region are very deep sands and nitrogen applied in the fall may not make it to the spring.  Also most wheat producers in the area make three or more applications of nitrogen unlike the two (pre and top) of the Great Plains.  It is always important to make the tools fit your specific regional needs and practices and not the other way around.

Where is actually the biggest unknown.  The basic answer is to place the N-Rich Strip in the area that best represents the field.  Many people question this as it doesn’t account for spacial variability in the field, and they are correct.  But my response is that in this case spatial variability is not the goal, temporal variability is.  Keeping in mind the goal is to take a field which has been receiving a flat yield goal recommendation for the last 30+ years and make a better flat rate recommendation.  My typically request is that on a field with significant variability either apply a strip long enough to cross the zones or apply smaller strips in each significant area.  This allows for in-season decisions.  I have seen some make the choice to ignore the variability in the field, made evident by the strip, and apply one rate and others choose the address the variability by applying two or more rates.  One key to the placement of N-Rich Strips is record keeping.  Either via notes or GPS, record the location of every strip.  This allows for the strips to be easily located at non-response sites.  It is also recommended to move the strip each year to avoid overloading the area with N.  

I hear a great deal of talk about how it would take to much time to put out the N-Rich Strip.  However the majority of producers that do it once on one field, end up doing it every year on every field. There is very likely someone in your area who is using the N-Rich Strips.   As top-dress grows closer keep an eye out for a blog “Using the GreenSeeker Sensor and Sensor Based Nitrogen Rate Calculator”.

For more information on N-Rich Strips check out the YouTube video below, visit http://www.npk.okstate.edu or contact me directly at b.arnall@okstate.edu.  I have lots of material I am happy to share and distribute.

See the YouTube Video  http://www.youtube.com/watch?v=kJ3DSwWYgE8