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Based on a few recent text messages and emails I think it is time to revisit an older post about Corn and Sorghum injuries from pre-plant herbicides.
Direct link to the original post Recent Weather Causing Corn (and Sorghum) Injury From Pre-emerge Herbicides
Being educated in the realm of Soil Fertility at Oklahoma State University by the likes of Dr Gordon Johnson and Dr. Bill Raun, Brays Nutrient Mobility Concept and Mitscherlich’s Percent Sufficiency Concept are ingrained in my psyche. In class the concept of Build and Maintain for phosphorus fertilizer management was just briefly visited and not discussed as a viable option. For anyone in the corn belt, and some Okies, reading this that may seem unusual. But when I was in school on average in Oklahoma there was about 100-200 K acres of 100 120 bpa (bushel per acre) corn, 300-400 K acres of 40-50 bpa sorghum, and over 5 million acres of 20-30 bpa wheat. In a state with those average yields, replacing P removed by the crop was not a major concern.
But times are changing. There is more corn and soybean planted and the achievable yields of all crop are increasing. While the average winter wheat producer should not be worried about replacement rates of P there is a growing group of producers that should. This blog will discuss the scenarios in which sufficiency rates are best and those in which replacement should be considered. The OSU factsheet PSS-2266 goes in-depth on each of these methods.
Applying P based on sufficiency will increase soil test P levels in a low yielding environment. For example on a 20 bpa wheat field that starts out with a soil test P level of 0. Using the sufficiency recommendation each year the soil test value will reach 20 ppm (40 STP) in 20 years. A 30 bpa field would take 30 years. Yes that is a long time but the soil test value is increasing a little each year. The point of 20 ppm is important because at that level the crop is 95% sufficient, meaning if no P is added the crop will only reach 95% of the fields yield potential.
Using a mass balance approach we can determine at what point does the crop remove more than we can supply with in or near furrow starter fertilizer. Table 1 shows the values I am using for the discussion. The first column is just the average amount of P removed per bushel of grain, most of our grains fall in the .4 to .5 lbs P per bushel range. The second column is the soil test value at which P level is said to be at 90% sufficient. The reason this column is included is that the P2O5 reccomendation for this P level fits into the starter rate for all crops. The low high starter rates are the typical range of P2O5 that is delivered within the safe range (N based) and what I see as the common rates. These values may be above or below what you use.
Table 2 is pretty simple but it is the center point of this article. The one caveat I need to add is this assumes strip till or 2*2 / 3*2 is not being used. Table 2 is using the starter range and removal value to determine the yield level the starter can support. The first take on this table may provide some hint on why in a state with 5 million acres of wheat averaging 36 BPA the state soil fertility specialist didn’t focus on replacement rates. In fact for most for most the the wheat ground P application is higher than removal and P levels are slowly increasing. The big take home from this table should be is my yield level outside this window? If so do not immediately go out in crease your P rates but do take a close look at your system as a whole. Take a close look at your cropping system, not just one seasons but look at a three or four year cycle. Add up P applied and P removed, are you positive or negative net balance? If you are negative take a long hard look at your soil test over time. Some soils can supply a large amount of P even if you are removing more than you apply. Other soils will be rapidly drawn down. Regualr soil testing allows for producers to keep an eye on these values.
In the end even if the production warrants the use of replacement rates, the current market may not. For more on that read https://osunpk.com/2016/08/27/now-may-not-be-the-time-for-replacement/.
Speaking of market currently both soybeans and cotton are getting a lot of attention due to how the economics is penciling out. Soybean is a “heavy” P crop pulls .8 lbs per bpa while cotton removes 13 lbs per bales. Both of these crops are salt sensitive and the rate of inforrow is typically quite low providing only about 6 lbs when on 30″ rows. If you are growing beans or cotton make sure you account for their removal when you talley up your system.
Below is a table that I wanted to add, well because I like it. This table illustrates that buildup, and drawdown, rate is heavily impacted by existing soil test value. In short it takes a lot more fertilizer P to raise soil test p levels in a very low P testing field than it does when soil test P is closer to optimum, 19 lbs per 1 lb at STP of 10 and 5 lbs per lb when STP is 65. The exact rate changes by soil type and the same holds true to drawn down via crop removal.
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When I started writing this blog (3.13.2105) Ok grain elevator cash bids for grain sorghum aka milo was 6.61-7.70 cwt (3.7-4.31 per bushel) and corn was at 3.64-4.06 per bushel. Meaning there is currently a premium on sorghum grain. This difference among other things has increased the interest in planting sorghum. Of late I have been quite successful, at least on a small-scale, at producing sorghum yield in the 120-150 bpa range, thanks to the advice of Rick Kochenower former OSU sorghum specialist. Both of us believe that every year many producers are leaving significant bushels on the table due to one or two miss steps. I wanted to take this opportunity to share what is in my opinion the keys in producing a bumper sorghum crop. I should note that the primary key is out of our control, rain.
Key 1. Planting date, the optimum planting date for grain sorghum is generally when soil temperatures reach 60° F and increase after planting. For much of the region that I believe is best suited for sorghum this falls between April 1 and April 15 for south of I40 and April 15 and May 1 north of I40. graph below shows the long-term average daily 4″ soil temp (bare soil) for Apache, Blackwell, Cherokee, and Vinita. It is easy to see how your location within the state can impact soil temps.
You should not forget however that tillage practices will also impact soil temps. The two graphs below show the long-term average daily 4″ soil temp for Cherokee and Blackwell for both bare soil and under sod. Note that when the soil is covered by residue it warms slower. The two figures also show that residue will have more impact in some areas more so than others.
My best word of advise is to keep a watchful eye on the Mesonet. While the long-term average is nice to know here in Oklahoma the difference in weather from one year to the next can be huge. The figure below shows the average daily 4″ soil temp (below sod) from Blackwell for the past five years. Link to Mesonet Soil Temp page Click here.
Another great resource is a report on planting date written by Rick Kochenower presented to RMA. Link to report.
Key 2. Hybrid selection, primarily maturity group selection. Rick has created a great graphic that helps put a planting date window with maturity group. It is always important to visit with your local seed dealer to find out what has been performing best in your region and consider the importance of stay-green, standablilty and disease packages. But for me the number one key is the selection of maturity group. This should be based upon planting date and harvest strategies. Below is a great graphic created by Rick, while this may not be scientific it is a great guide created via years of experience. I also recommend that if you are planting a significant amount of acres you should diversify your maturity groups. Not only does this spread out he harvest window but it also you to spread the risk of high temps coming early or late. An additional resource is the Sorghum Performance trial summary located on the Ok Panhandle Research and Extension Center website. Click here.
Key 3. Soil Fertility, while soil pH plays a big role on sorghum productivity but it is too late in the game to do much about it this year. So the most important things to keep in mind on fertilizing sorghum are your macro-nutrients nitrogen (N), phosphorous (P) and potassium (K). It is my opinion that historically producers have underestimated the yield potential of sorghum and therefore lost yield due to under application on N. You should expect more than 60 to 80 bushel out of your crop if you put the right seed in the ground, at the right time and in the right way.
Ask around look at Rick’s yield data, producers in N. Central Ok on a good soil should be going for 125+ bpa easy. Unfortunately you are unlikely to hit these yield levels if you fertilize for a 75 bpa crop. An easy rule of thumb on N fertilization is 1.2 lbs of N per bushel, for a more exact number take a look at the image below. This comes from the corn and sorghum PeteSheet and is the same table that comes from the Soil Fertility Handbook. (If you would like some Pete Sheets just send me an email requesting them at firstname.lastname@example.org, Link to PeteSheets page).
Key 4. Weed Control With sorghum utilizing a pre-plant herbicide with residual is extremely important due to the lack of over the top options. Most times proper weed control will be accomplished by utilizing concept treated seed and use of labeled rates of a pre-emergent grass control herbicide combined with atrazine.
While I primarily focus of the four keys above there are a few other important items to consider.
Population: Prefer to think in terms of seeds per acre instead of lbs per acre. This comes into to play with the use of a planter. Rick Kochenower says “for seeding rate(on 30 inch rows), it isn’t as critical as most people think it is. Because most guys in Oklahoma tend to under plant not over plant. I always suggested 45,000 but as you look at the last slide it really don’t matter much. The way I always liked putting it is to make you sure have enough out there to not have to replant, because being late hurts more than having to few too many or too few plants.”
Row spacing: I like 30, but many may not have a planter so I suggest at least plugging every other hole in the drill to be at a 12″-20″ spacing. Make sure your population is correct for your row spacing. For this consult with your local seed dealer to match cultivar with row spacing and proper population.
Insects: Scouting for aphids and head midge is very important, these little critters are yield robbers and can gum up the works at harvest.
Harvest prep: I almost put this as the fifth key. By chemically maturing/terminating your crop you are both able to increase harvest efficiency and preserve moisture for a following winter crop of wheat or canola.
While this is a good start I suggest a visit with your local OSU Extension educator, consultant or seed dealer for information about your specific situation. Just know the crop has great potential to yield big if treated right. I like to say don’t treat your sorghum crop like the stray you adopted, treat it like your hunting dog that you traveled halfway across the country to pick up. Good luck in 2015 and I hope the rains fall when and were needed.