Corn Illustrated: These examples prove you need to plug in costs and prices and do what-if scenarios before settling on N rates.

Tom J Bechman 1, Editor, Indiana Prairie Farmer

May 16, 2017

ADVICE FOR RATES: Before you make your final N application this year, check out the N rate calculator on the new Corn Field Scout app. It’s available for \$5.99 at app stores.

Before you make your final nitrogen application for this season, check out the nitrogen rate calculator included in the new Corn Field Scout cellphone app, available at app stores.

Corey Gerber, director of the Purdue University Crop Diagnostic Training and Research Center, says the new app contains several helpful calculators. One determines the recommended nitrogen rate to reach optimum economic return.

Note that’s different than the optimum agronomic rate, which would produce the highest yield possible. The optimum economic rate gives you the best chance of getting the most net profit per acre. It varies based upon nitrogen cost and price of corn. Plug those choices into the calculator and get a recommendation. Then you can change the cost of N and/or price of grain and compare. Do it as many times as you like. These what-if scenarios, playing out on your cellphone instead of in the field or on paper, should help you arrive at a reasonable nitrogen recommendation for your farm.

Examples
Suppose you live in central Indiana and farm medium-textured soils. Plug those choices into the calculator. You’re applying anhydrous ammonia at \$400 per ton, and you pick \$3.50 as the corn price. Hit “evaluate” and the recommendation is 213 pounds per acre.

Seem high? It’s based on some 250 trials over 10 years conducted by Purdue’s Bob Nielsen and Jim Camberato, many of them large-scale trials on farms.

Now suppose you have to pay more for nitrogen. If N cost climbs to \$550 per ton, then the recommend rate drops slightly, to 206 pounds per acre. The recommendation is for total pounds or units of N, not pounds of anhydrous ammonia.

Leave the cost at \$550, but drop the corn price to \$3. Now how much does the calculator advise applying if your goal is reaching the economic optimum return for N applied in that field? The recommended rate drops again, but only slightly, to 202 pounds per acre.

Switch areas
Let’s run the same example in other areas, first in west-central Indiana. At \$400-per-ton anhydrous and \$3.50-per-bushel corn, the recommended N rate is 188 pounds per acre. Nielsen suggests soil type, particularly differences in natural drainage and organic matter, cause the change.

With the same example and N at \$550 per ton, you might apply less — say, 181 pounds per acre. Drop the corn price to \$3 per bushel and the recommended N rate is 177 bushels per acre.

Move to east-central Indiana to run one more set of numbers. At \$400-per-ton anhydrous and \$3.50-per-bushel corn, the recommendation is 232 pounds per acre. This is the region of the state where recommendations, based on actual results, are highest.

Raise the N cost to \$550 per ton, and your recommended rate becomes 225 pounds per acre. Leave the cost alone and drop expected corn price to \$3. Your recommended rate falls to 221 pounds per acre.