Peek behind curtain at N rates

Did you ever want to know what made something tick so bad that you longed for a peek inside the black box? When the unlikely crew of the timeless movie “The Wizard of Oz” finally pulled back the curtain, they discovered much less than they expected. Ask enough questions about nitrogen recommendations, and you’ll find the data backing the guidelines may not be as awe-inspiring as you thought.

The old standard of 1.1 pounds of actual N per bushel of corn yield goal doesn’t hold water, especially at higher yield goals, notes Bob Nielsen, Purdue University corn specialist. Recently, Nielsen and Jim Camberato, also a Purdue Extension agronomist, took on a massive challenge — conducting enough nitrogen trials in Indiana to determine realistic N recommendations.

“We talk in terms of agronomic optimum nitrogen rate, or AONR, because it’s simpler than the economic optimum rate,” Nielsen says. If you’re looking for rates that likely maximize net returns per dollar spent, back off the AONR by 10 to 12 bushels per acre, he says.

Key Points

• The statewide average agronomic optimum N rate is 186 pounds per acre.

• Large regional differences appear to exist in the optimum rate.

• Regional differences occur partly due to the variation in nitrogen loss risk.

State vs. regional

Statewide, the current AONR is 186 pounds of total applied N per acre. Before you decide to apply 186 pounds across the board, pull back the curtain. The Purdue pair has data from seven of Indiana’s nine crop-reporting districts. If you peek at those numbers, AONRs range from 211 in east-central Indiana to 159 pounds per acre in west-central Indiana.

“The problem when you start making too much of regional rates is that there is a limited number of trials,” Nielsen says. “There’s a much higher number of trials behind the state average.

“What averages really mean are some of the time, the true AONR will be higher than the average, and some of the time it will be lower,” Nielsen explains. “Since you can’t predict the year, you don’t know which way it will go.”

His advice? Pick rates that make sense where you live. Consider your own trial. Zero in on utilizing N most efficiently.

Help find solution to N-rate riddle

We need more trials,” Bob Nielsen says. “If we just had one on-farm N-rate trial in every county, we could make much stronger recommendations.”

The Purdue University corn specialist and Jim Camberato, a Purdue soil fertility specialist, began conducting N trials three years ago because nearly all of the N-rate data in Indiana was at least 20 years old. The pair wanted to back up recommendations made to corn growers. They want to determine where optimum agronomic nitrogen rates break in various parts of the state. If those numbers were known over a variety of years, agronomists could make more precise recommendations.

Nielsen prefers large on-farm trials vs. tiny plots. But he’s not after single-strip trials. He wants three repeats.

To help, call Nielsen at 765-494- 4802 or e-mail


WANTED, MORE TRIALS: Bob Nielsen and Jim Camberato need more nitrogen trials like this one at the Southeast Purdue Ag Center to help fine-tune nitrogen rates.

This article published in the February, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.