The How Much N Does a Corn Crop Really Need?

— Written By Tim Hambrick and last updated by
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The How Much N Does a Corn Crop Really Need?

Forsyth, Stokes, Surry, Yadkin
Tim Hambrick, Ag Ext Agent
November 15, 2021

Nitrogen is a critical nutrient for corn yield, nobody debates that. And, there is nothing harder on yield than running out of N too early. So, that begs the question, when it looks like N prices might be the highest in history, how much N does a corn crop really need? Lets start with a little calculation:

A ton of 32% weighs 2000 lbs and a gallon weighs about 11 lbs. 2000 lbs divided by 11 lbs/gal gives 181 gallons of product. At $600.00/ton divided by 181 gallons you get a cost of $3.31 per gallon of 32%.
At 3.55 units of N per gallon divided by $3.31/gal you are paying about $.93 per unit of N. That’s at todays price of about $600.00 a ton and the price hasn’t stopped rising! So, one of the factors regarding how much N is necessary has to be economics!

Crop use efficiency should be one of the factors involved as well. Today’s corn hybrids are much more efficient users of nitrogen than in the past. At an N cost of nearly a dollar a bushel, the first dollar from every bushel sold will now be paid just to nitrogen. The question becomes how little N can be applied without hurting profitability?

To help answer that question, I dug out the last 5 years of entries into the state/national corn contests – 24 entries all total. I divided the amount of N applied by the number of bushels produced to get a number that tells me how many bushels were produced per unit of N. A corn contest is an interesting thing because you only measure a yielding field – in our area, most likely a bottom that is prone to higher levels of leaching than upland soils.

Those 24 high yielding fields required an average of just under a single unit of N to produce a bushel of corn. In fact, on average, each bushel was produced at about 9/10’s of a lb of N – again, this in bottom soils that are more prone to leaching than are upland soils. If you look at the data a little closer, the most efficient field of the 24 entries produced a bu of grain on just .68 lbs of N. The least efficient produced each bushel on 1.27 units N. At a dollar per lb of N, that’s a difference of 59 cents per bushel. I’ve seen you guys wait and wait and wait to sell grain just to make an extra nickel! Of the 24 contest entries, only 6 of them required more than a lb of N to make a bushel of grain, and I would argue all day, that every one of those 6 fields was over fertilized.

NC State (Rob Austin and Deanna Osmond) has data from long term N rate studies on corn across several years in multiple counties in the eastern part of the state. Working with farmers, they determined each farmers standard N rate. From that they developed two other rates to compare to the farmer standard rate. They reduced farmer standard N by 25% to set the low rate and they increased farmer standard N by 25% to set the high rate. They did this on 100 different sites from 2013 -2017. The following chart shows average rate by average return across the 100 sites:

Average N Rate Average Yield

25% less N .83 bu per unit N
132 units (37 gal 32%) 160 bu (37 gal = $122.47)

Farmer Standard N 1.04 bu per unit N
174 units (49 gal 32%) 167 bu (49 gal = $162.19)

25% more N 1.25 bu per unit N
215 units (61 gal 32%) 172 bu (61 gal = $201.91)

Somebody is going to point out that 172 bushels is better than 160 bushels and I’d certainly agree – if you entering a corn contest. But I’ll point out that in nitrogen alone, it cost $79.44 extra dollars to make that extra 12 bushels! Just to break even, you’ve got to sell that additional 12 bushels of corn for $6.62/bu!

The results of this long term test are almost identical to the results gleaned from the 5 years of corn contest entries – you can make a bushel of corn on less than a lb of nitrogen, and in a year of historic N prices, it may certainly be more profitable to make a few less bushels! This is true even in our highest yielding bottom land soils and I’ve got farmers who are making high yields on less than a unit of N! By trusting the data, you greatly reduce some of your cost risk, especially on our more stress prone soils. Know your 5 year average yields by farm. Determine your historic N rate and consider if you can reduce that amount. There are both economic and solid agronomic reasons for making that change.

Next in this fertility series:

Are we getting all we can from manure applications?