Two years ago, Bob Nielsen set out to conduct nitrogen trials and collect data on response of modern hybrids. His goal was to be able to more accurately answer the question: How much nitrogen should I apply for corn in Indiana?
Jim Camberato, also a Purdue University Extension specialist, like Nielsen, soon joined him in the hunt for more accurate nitrogen-yield response data. Two years later, they're closer to their goal. While they will still collect data in the new year, Camberato recently prepared a simple chart showing how Purdue's recommended N rate varies based upon price of nitrogen, and grain price.
"We've found that the old recommendation of 1 to 1.2 pounds of N per bushel isn't holding up well," Nielsen notes. After two years of more careful study, they've determined that the figure is probably too high for more productive soils in the western half of Indiana, and perhaps too low for some conditions in eastern Indiana. The biggest, most consistent result they've seen is that the amount of N it takes to produce a bushel of corn varies widely from place to place and even year to year.
The two Extension specialists also distinguish between AONR- Agronomic Optimum nitrogen rate, based upon how much N it takes to produce top yield, and EONR- the economic optimum nitrogen rate if you're out to maximize profits. Based on data collected so far, they've concluded that the maximum agronomic rate is about 170 pounds per acre of commercial N. That's in a corn after corn rotation. Add 20-30 pounds per acre to the N rate for corn after corn.
Here's how the economic optimum recommended N rates break down, using N prices and grain prices that should apply for the '08 crop. First, if N is $0.40 per pound, and grain price is $3, $3.50, $4, or $4.50 per bushel, optimum economic rates to apply are 154, 159, 164 and 165, respectively. Note that at some point, corn price rises high enough to justify N rates that approach the maximum N rate for yield.
Suppose N is 50 cents per pound. Anhydrous ammonia is currently cheaper than that- N in liquid 28% N may be higher than that price. At 50 cents per pound for N, and $3, $3.50, $4, and $4.50 for corn per bushel, economic rates for N are 148, 152, 156 and 159 pounds per acre. Note that as N price increases, it takes higher corn prices to justify higher rates. The changes are not dramatic, but could determine how much profit you net at the end of the season.
Finally, suppose N is $0.60 per pound. At $3, $3.50, $4 and $4.50 per bushel for corn, economic rates are now 142 pounds per acre, 147, 149 and 154, respectively.
Be sure to take credit for all N that you apply, the Extension specialists notes. That includes what is applied as starter, weed and feed, pre-plant or as side-dress applications. You can usually expect at least 5% greater efficiency for side-dress applications, most agronomists note. That will vary depending upon the season and potential for N losses.