For many years, agronomic inputs were determined based on how much or how little they would influence grain yield. However, a combination of research and a revisiting of crop budgets have farmers reassessing inputs based not on how much they can boost yield, rather, how profitable they can be. A perfect example of this is nitrogen (N) fertilizer, which comprises one of the more significant variable production costs for corn. As margins grow tighter, and environmental concerns increase, the application of N as a cheap form of “insurance” is being scrutinized more and more.
The traditional rule of thumb for a corn/soybean rotation was that of 1 lb of N per bushel of expected yield, which implies that there is a linear relationship between yield and N rate. However research has been moving away from this concept as it has been found that the relationship is not linear, and you can reach a point where applied N is no longer contributing to yield increase. Thus, farms should be trying to pinpoint the “economic optimum N rate” rather than the “agronomic optimum N rate.” The economic N rate will likely change from year to year, and will decrease as N prices go up and increase as grain prices increase. Here in northwest Indiana, the average agronomic N rate has been around 212 lbs N/ac, which will vary based on soil type and expected yield. Given current N and grain prices however, the optimum economic N rate is closer to 190 lbs N/ac, a difference of around $7 per acre.
Timing and application methods have also changed over the years, with current research suggesting that the most efficient time to apply is injecting N prior to the beginning of rapid crop N uptake, which occurs around the V6 growth stage for corn. If the fertilizer program includes anhydrous ammonia, it is recommended that it be applied 10 to 14 days ahead of planting at a depth of 7 to 8 inches. If you would like to play around with different N rates and price variables, Iowa State has an excellent web based tool, the corn nitrogen rate calculator, which can be found here: http://cnrc.agron.iastate.edu/
Optimum N rates can vary greatly based on soil types. If you are interested in zeroing in on your optimum rate, Purdue researchers are always interested in looking further into nitrogen management via on farm trials. If you are interested in conducting such a trial on your farm and continuing this discussion with corn and soil fertility specialists from campus, please contact me and we will set something up. I can be reached via phone at 219-984-5115, e-mail at email@example.com, or by stopping by the White County Purdue Extension office located at the fairgrounds in Reynolds.