By Gene Johnston
Here are some questions farmers likely have about the Corn Nitrogen Rate Calculator that uses the Maximum Return to Nitrogen (MRTN) and some answers from John Sawyer, an Iowa State University Extension soil fertility specialist.
Q: How well does this really work?
Sawyer: I’ve compared the MRTN approach with the Iowa database and each site-specific economic optimum nitrogen (N) rate (that’s perfection). The MRTN approach across the database is within approximately $13 to $16 per acre of what would be optimal for rotated and continuous corn. I think it does a good job of minimizing the risk of both over- and underapplication. It can get even better if we update data in the future, filling in gaps in geography and soils across our states.
Q: How well does it accommodate manure applications?
Sawyer: You decide how to credit manure nitrogen values. The calculator gives you a suggested nitrogen rate, and you decide how you get to that number from various nitrogen sources.
Q: How about the newer insect-protection corn hybrids; are they different in N needs?
Sawyer: No, research we’ve conducted has not shown a rate difference. I’d go with the same rate guidelines. Those hybrids are included within the database of N-rate trials.
Q: How can you take more yield off of a field and not need more N?
Sawyer: That’s a good question and there are multiple reasons. Corn plant nitrogen uptake is not as much as in the past, corn grain nitrogen concentration is lower, and plants may be more efficient at producing grain yield. When conditions are good for high corn yields, they are good for supply of crop-available nitrogen from the soil resource and good for plant growth and nutrient-use efficiency.