Once upon a time, not so long ago but in a land that seems strange by today's standards, Purdue University recommended as much as 1.20 to 1.25 pounds of commercial N per bushel of corn yield goal. The time was the 1970' into the 1990's. The 'strange land' was an agricultural production system that featured conventional tillage, abusive soil compaction, and hybrids with widely-different genetics than are available today. 'Triple-stacks' were about like computers- most people couldn't even dream what they might be like stuck in 1980's Indiana agriculture.
By the late 1990s into 2000 and beyond, environmental concerns and more current research caused most universities to back down on that recommendation, suggesting less N per bushel of grain produced than before. Purdue recommendations followed suit. Recommended rates in some cases dropped closer to 0.9 pounds of commercial N per bushel of corn grown in the field.
Now, Bob Nielsen and Jim Camberato, Purdue University Extension specialists, are in their second year of trying to update and fine-tune N recommendations on Indiana soils, with Indiana's climate. Their results this year made a strong case to move away from the system of basing N recommendations on pounds epr bushel of yield goal, Nieslen notes.
Conducting trials across the state, some on actual farms, they measured both agronomic optimum yields and economic optimum rates of return in corn after soybeans, and in some cases, corn after corn. Their first observation is that corn after corn requires more nitrogen, period.
"Grain yield was not correlated with the amount of nitrogen needed to maximize grain yield," the pair concludes in an initial summary paper of this year's trials. Then they cite specific examples from various locations inside Indiana.
At the Davis Purdue Ag Center near Farmland in east-central Indiana, maximum yield for continuous corn there was only 110 bushels per acre, no matter how much N they applied. In fact, it required more than 200 pounds of N per acre to get that 110 bushel yield. A quick calculation indicates that puts the ration of pounds of nitrogen applied commercially to bushel of actual grain yield in this corn after corn situation at almost two pounds per bushel of yield.
Yet at the Throckmorton Purdue Ag Center south of Lafayette, near Romney, it took only 115 pounds of N in a corn/soybean rotation to produce 190 bushels of corn per acre. Do the math and that's about 0.6 pounds of commercial N per bushel of actual yield. Both those numbers are on the extreme end of what's expected, but they illustrate a point, Nielsen and Camberato say.
"It's why N fertilizer recommendations will no longer be tied directly to yield potential," the authors note in their paper summarizing their '07 results.